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Discrimination of ablation, shielding, and interface layer effects on the steady-state formation of persistent bubbles under liquid flow conditions during laser synthesis of colloids

Discrimination of ablation, shielding, and interface layer effects on the steady-state formation... Over the past decade, laser ablation in liquids (LAL) was established as an innovative nanoparticle synthesis method obeying the principles of green chemistry. While one of the main advantages of this method is the absence of stabilizers leading to nano- particles with “clean” ligand-free surfaces, its main disadvantage is the comparably low nanoparticle production efficiency dampening the sustainability of the method and preventing the use of laser-synthesized nanoparticles in applications that require high amounts of material. In this study, the effects of productivity-dampening entities that become particularly relevant for LAL with high repetition rate lasers, i.e., persistent bubbles or colloidal nanoparticles (NPs), on the synthesis of colloidal gold nanoparticles in different solvents are studied. Especially under batch ablation conditions in highly viscous liquids with prolonged ablation times both shielding entities are closely interconnected and need to be disentangled. By performing liquid flow-assisted nanosecond laser ablation of gold in liquids with different viscosity and nanoparticle or bubble diffusivity, it is shown that a steady-state is reached after a few seconds with fixed individual contributions of bubble- and colloid-induced shielding effects. By analyzing dimensionless numbers (i.e., Axial Peclet, Reynolds, and Schmidt) it is demonstrated how these shielding effects strongly depend on the liquid’s transport properties and the flow-induced formation of an interface layer along the target surface. In highly viscous liquids, the transport of NPs and persistent bubbles within this interface layer is strongly diffusion-controlled. This diffusion-limitation not only affects the agglomeration of the NPs but also leads to high local densities of NPs and bubbles near the target surface, shielding up to 80% of the laser power. Hence, the ablation rate does not only depend on the total amount of shielding matter in the flow channel, but also on the location of the persistent bubbles and NPs. By comparing LAL in different liquids, it is demonstrated that 30 times more gas is produced per ablated amount of substance in acetone and ethylene glycol compared to ablation in water. This finding confirms that chemical effects contribute to the liquid’s decomposition and the ablation yield as well. Furthermore, it is shown that the highest ablation efficiencies and monodisperse qualities are achieved in liquids with the lowest viscosities and gas formation rates at the highest volumetric flow rates. . . . . Keywords Laser ablation in liquids Metal nanoparticles Liquid flow Gas formation Liquid viscosity Introduction Nowadays, nanotechnology is a rapidly developing field with increasing demand and high future potential for applications in areas such as biomedicine [1, 2], optics [3, 4], or catalysis [5, 6]. The production of nanomaterials typically takes place * Stephan Barcikowski via wet-chemical [7, 8], gas phase [9, 10], or solid-state pro- stephan.barcikowski@uni-due.de cesses [11, 12]. However, nanoparticles (NPs) produced by these methods are often subject to agglomeration and aggre- Technical Chemistry I, Center for Nanointegration Duisburg-Essen gation effects if no further stabilizing agents are added to the (CENIDE), University of Duisburg-Essen, 45141 Essen, Germany process [13]. The use of stabilizers is unwanted, in areas such Materials Science and Additive Manufacturing, School of as catalysis or biomedicine, where strict requirements are Mechanical Engineering and Safety Engineering, University of placed on the properties of the NPs, such as their size and Wuppertal, 42119 Wuppertal, Germany 774 J Flow Chem (2021) 11:773–792 purity [14]. Additionally, it is sometimes desired to have the Moreover, the influence of other liquids besides water on NPs in organic liquids, for example, for in-situ preparations of LAL-induced bubble formation has so far only been investi- nanocomposites [15–17], useful as medical devices like anti- gated for batch conditions. In this context, it was shown that microbial catheters [18, 19]. However, the synthesis of NPs in persistent bubbles form that shield up to 65% of the liquid organic liquids is difficult to realize with conventional cross-section depending on liquid viscosity, affecting produc- methods without stabilizing ligands [20]. tion rate and reproducibility of laser-generated NPs [68]. In this context, laser ablation in liquids (LAL) represents a Moreover, Hupfeld et al. observed that cavitation bubbles green [21, 22], cost-effective [23] and scalable [24, 25] alter- with quite non-symmetric, oblate-shaped geometries form in native that provides access to a variety of high-purity [26] highly viscous polyalphaolefin (PAO), whose collapse leads nanomaterials such as metals [27–29], alloys [30, 31]or ox- to long-lasting, persistent bubbles [75]. These studies under- ides [32–34], available in both aqueous [27, 28, 35] and or- line the importance of liquid viscosity for bubble formation ganic media [36–38]. Their application potential is manifold and ablation efficiency during LAL in stationary liquids. and has already found its way into fields such as biomedicine However, it is still unclear how the liquid viscosity affects [30, 39], catalysis [40–45], solar energy [46]or 3D printing the bubble and NP formation during liquid flow-assisted [47–49]. Even though the production of these materials is LAL. It is undisputed that an interface layer forms along the already possible on the gram-scale [24, 25], high-power and target surface under these ablation conditions [76], possibly expensive laser systems are required. Therefore, numerous influencing the removal of the bubbles and NPs from the approaches aimed to optimize the NP ablation efficiency ablation area. Unfortunately, its influence has hardly been based on parameters that are independent of investment costs, discussed in LAL literature so far and therefore requires more including target geometry optimization [50–52], scanning attention. strategy [24, 25, 53], and liquid height adjustment [54, 55]. For closing these evident knowledge gaps and increasing Furthermore, special ablation chambers have been de- the efficiency of this sustainable synthesis method, the influ- signed, enabling the production of NPs under batch [56]and ence of a liquid flow on the formation of NPs and persistent liquid flow conditions [24, 25]. The production of NPs in bubbles during ns-ablation in liquids of different viscosity semi-batch or batch chambers can be performed with or with- (water, acetone, and ethylene glycol) is investigated in this out stirring the liquid [56]. This method is particularly useful study and correlated with the mass ablation and gas formation for application fields where small amounts and high concen- rates. In the first section, the steady-state formation of NPs and trations of colloids are required. However, since long ablation persistent bubbles are studied depending on the liquid’svolu- times are needed to achieve high NP concentrations, the NP metric flow rate. Furthermore, the interface layer that forms production rate decreases over time due to increasing colloid- along the target surface under flow conditions is characterized induced shielding effects [57, 58]. Additionally, the probabil- to understand the influence of flow dynamics on the removal ity of NP post-irradiation effects [59] increases, leading to the of NPs and persistent bubbles from the ablation zone depend- formation of large quantities of nanobubbles [59–62]. This ing on the liquid’s viscosity. In the second section, the persis- way, the reproducibility and quality of the final products suf- tent bubbles are systematically analyzed and quantified to de- fer as fragmentation [63–65]and melting [66, 67] of the NPs termine their shielding capacity. The shielding effects are cor- alters the size distribution of the final NPs. related with the mass ablation rates in the third section of this Furthermore, LAL induces the liquid’s decomposition and work and linked with the NPs’ properties. The fourth section the formation of so-called persistent bubbles [59, 68–70]. concludes this study by evaluating the gas formation efficien- These bubbles represent a permanent shielding entity sticking cies as a function of liquid selection, shielding effects, and to the bulk target surface, screening the laser beam, and neg- mass ablation rates. atively affecting NP production rate. For this reason, LAL is often performed under dynamic flow conditions by continu- ously overflowing the target substrate [24, 25, 56]. This way, Materials and methods local NP accumulation can be significantly reduced, resulting in increased NP production rate with increasing flow rate [25, The experimental setup for performing LAL and quantifying 33, 56, 71]. Since persistent bubbles absorb, reflect, and the gas volume in a liquid flow is demonstrated in Fig. 1. defocus the laser beam, better underwater laser For all experiments, an ablation flow chamber (h: 6 mm, w: micromachining results can also be achieved using a liquid 6 mm, l: 18 mm) made of anodized aluminum was used. Side- flow [72–74]. observation windows were integrated into the chamber to Although initial work has been carried out to investigate monitor the bubble formation. A gold target (99.99%, 10 × the dynamic features of the bubbles that form during laser 5 × 0.5 mm, Allgemeine Gold) was used for ablation and cutting of silicon in flowing water [72], such studies have placed inside the ablation chamber. The thickness of the liquid not yet been performed for liquid flow assisted LAL. layer was 6 mm. The experiments were performed in J Flow Chem (2021) 11:773–792 775 Fig. 1 Experimental setup for measuring the gas volume and bubble dynamics during LAL in water, acetone, and ethylene glycol performed in a custom- made ablation flow chamber deionized water (18.2 MΩcm at 25 °C), acetone (VWR working distance corresponds to the focus placed into the Prolabo, ≥99.0%), and ethylene glycol (Sigma-Aldrich, liquid/behind the target. A working distance of zero corre- 99.8%) as carrier liquid. For ensuring a constant, pulsation- sponds to the highest ablation efficiency. Ns-LAL was per- free volumetric flow rate, a syringe pump was used. The sy- formed in each liquid for 10 min by varying the volumetric ringe was connected with the ablation chamber’s inlet by a flow rate in steps of 1, 5, 10, 15, and 20 ml/min. The ablated tube made of polytetrafluoroethylene (PTFE). A tube made of mass was measured gravimetrically after the ablation process PTFE was also connected to the outlet of the ablation cham- using a microbalance (Pesa Waagen GmbH). ber. The other end of the tube was inserted into a graduated pipette placed in a beaker filled with liquid. LAL was performed using an Nd:YAG ns-laser (Rofin Powerline E20). The laser wavelength was 1064 nm, and the pulse length 8 ns. A repetition rate of 15 kHz was used during all the experiments delivering a pulse energy of 0.33 mJ. The laser beam was guided along the target surface with a scan speed of 2 m/s using a galvanometer scanner (SCANcube 10, Scanlab). Therefore, a spiral pattern with a diameter of 5 mm was used. The laser beam was focused on the target through an F-theta lens with a focal length of 100 mm, resulting in an average spot size of 40 ± 10 μm in the focal position (mea- sured in air), which corresponds to a nominal laser fluence of 25.9 J/cm (details on the laser fluence are given in Fig. 12). Focus adjustment was performed for every liquid by varying the working distance between the F-theta lens and the ablation target in a range between −1mm and 1mm(Fig. 2). The produced colloids were characterized by UV-Vis spec- Fig. 2 Focus adjustment for ns-LAL of Au performed in water, acetone, troscopy. The extinction of the colloids at a wavelength of and ethylene glycol as a function of the extinction at a wavelength of 380 nm is proportional to the Au NP mass concentration and 380 nm (proportional to the NP mass concentration). The maximum was was plotted against the working distance. A negative/positive normalized to the relative z-position 0 776 J Flow Chem (2021) 11:773–792 The gas volume was determined by the liquid displacement As evident in the picture series, shortly after the arrival of the method. Gases produced during the ablation process were first laser pulses, persistent bubbles are formed. Gas chroma- continuously transported through the tubes into the liquid- tography measurements have shown that these bubbles contain filled graduated pipette, leading to the liquid’s displacement. permanent gases (H ,O for ns-ablation in water, and in the 2 2 By recording the amount of displaced liquid, the volume of case of ns-ablation in glycols, additional CH ,CO,CO ,C H 4 2 2 4, formed gas was obtained. The liquid flow was maintained for and C H )[68]. In addition to persistent bubbles, the formation 2 2 several minutes after the ablation process was stopped to en- of a dark cloud of NPs in the liquid is observed. The darkening sure the quantitative measurement of the gases without resi- is faster and stronger in water and acetone than in ethylene dues in the chamber or tubes. The gas cross-section imaging glycol. In the latter case, such a cloud of NPs is hardly visible was recorded with the aid of a videography system described and more located towards the target surface. in [68]. The results were evaluated with ImageJ and further The liquid’s darkening was evaluated by measuring the processed with OriginPro (version 2018b). cross-sectional light attenuation of the individual picture frames For further analysis, the temperature was measured over for each liquid and volumetric flow rate depending on time ablation time. Therefore, a thermocouple was integrated at the (Fig. 3d-f). Of course, this procedure represents only a rough outlet of the ablation chamber. Furthermore, the laser fluence estimation since the darkening of the liquid depends on the shielded by the produced colloids was measured ex-situ. For optical properties (e.g., transmission, absorption, and refraction this purpose, the produced colloids were filled into a glass cu- behavior) of the colloidal system and persistent bubbles and the vette. By placing a power meter (Coherent Inc., FieldMax > II- experimental alignment of the light source to the ablation TO) behind the glass cuvette and guiding the laser beam chamber and the camera system. For a better comparison of through the cell and colloids, thedecreaseofthe pulseenergy the results, the cross-sectional light attenuation was normalized. induced by the shielding of the NPs was determined. The col- A normalized cross-sectional light attenuation of 0% corre- loids were further characterized by measuring their size using sponds to the pure liquid. Higher percentages are due to the dynamic light scattering (Nicomp 380 DLS-ZLS). Besides, the presence of persistent bubbles and NPs. The results can be ablation profile on the Au target was determined after laser described by a hill function leading to steady-state conditions processing using confocal 3D microscopy (Nanofocus). (defined as the time after which 90% of the maximum normal- ized cross-sectional light attenuation is achieved) after a specif- ic mixing time. Hence, this procedure gives a rough idea about the mixing behavior (the time point when steady-state condi- Results and discussion tions are reached) inside the ablation chamber as a function of the ablation time and volumetric flow rate. Steady-state formation of nanoparticles and For ns-LAL of Au in water, a clear dependence on the persistent bubbles depending on the liquid flow volumetric flow rate can be observed. For the lowest volumet- dynamics ric flow rate of 1 ml/min, steady-state conditions are reached after about two seconds. The time for reaching the steady-state Recent studies have shown that NP productivity in LAL decreases steadily with increasing volumetric flow rate until strongly depends on the liquid’s viscosity, which was ex- half a second at 20 ml/min (Fig. 3d). For ns-LAL of Au in plained by the formation of persistent bubbles and their acetone, steady-state conditions are reached after one second, viscosity-dependent dwell time in the ablation zone [68, 75]. independent of the volumetric flow rate (Fig. 3e). However, Furthermore, the produced NPs can act as shielding entities for ns-LAL of Au in ethylene glycol, a normalized cross- and reduce the ablation efficiency [57–59]. These two limita- sectional light attenuation of 100% is never achieved. tions are particularly pronounced during batch processing. Typical values are about 10%, with steady-state conditions Here, the NP mass concentration increases with increasing being reached after about one second if volumetric flow rates ablation time and persistent bubbles accumulate within the of 10 ml/min and more are used (see Fig. 3f). Consequently, liquid or stick to the target surface. Liquid flow setups [25, mixing persistent bubbles and NPs within the entire chamber 33, 56, 71] are typically used to overcome these limitations volume is less efficient in ethylene glycol. and improve the removal of persistent bubbles and NPs from For further discussion, the diffusion coefficients (D)of the the ablation area. In this study, liquid flow-assisted ns-LAL of NPs and persistent bubbles were calculated according to the Au was performed in three different liquids, acetone, water, Stokes-Einstein equation [77]. and ethylene glycol, covering different liquid viscosities of 1.00, 0.33, and 20.81 mPa·s at 293 K. Fig. 3a-c shows an k  T D ¼ ð1Þ exemplary picture series for the processes occurring on the 6  π  η  R hyd millisecond time regime during ns-LAL of Au in these liquids at an exemplary volumetric flow rate of 1 ml/min. J Flow Chem (2021) 11:773–792 777 Fig. 3 a-c Exemplary images taken for water, acetone, and ethylene of time and different volumetric flow rates. The specific times, after glycol as a function of time showing the persistent bubble formation which 90% (t ) of the cross-sectional light attenuation was reached, are process and mixing of NPs in the liquid at a volumetric flow rate of included as inserts to provide reference values for the time until steady- 1 ml/min. d-f Cross-sectional light attenuation normalized to maximum state conditions are reached shielding during LAL in water, acetone, and ethylene glycol as a function −23 Here, k represents the Boltzmann constant (1.38·10 J/ performed comparisons would be lacking for a case in which K), while T is the temperature and η the liquid’s dynamic the particles and the bubbles have similar sizes. viscosity. R stands for the radius of the bubbles extracted Since the colloid is heated during LAL [78], the tempera- hyd from the shadowgraphy images or the hydrodynamic radius of ture dependency of the diffusion coefficient needs to be con- the NPs, which was measured by DLS (compare Fig. 11c-e). sidered. For this reason, the temperature increase of the colloid Please note that the calculations in following are meant to be a produced during ablation of Au in water, acetone, and ethyl- first approximation for the comparison between nanometer- ene glycol was measured at the outlet of the ablation chamber sized spherical solid particles and micrometer-sized spherical as a function of the ablation time. The results of these mea- persistent bubbles. Due to the size difference between these surements are displayed in Fig. 4a and b for volumetric flow entities, the constants determined by the Eq. 1 (as well as Eqs. rates of 1 ml/min and 20 ml/min. 2–4 introduced in the next sections) have a difference of 2–4 For a volumetric flow rate of 1 ml/min, the average heating orders of magnitude and are used for a relative comparison rate is 0.9 °C/min, and the maximum colloid temperature is between these entities generated in different solvents. The 38 °C. For a higher volumetric flow rate of 20 ml/min, the 778 J Flow Chem (2021) 11:773–792 Fig. 4 a,b Colloid temperature increase as a function of ablation time for coefficients were calculated using the Stoke-Einstein equation by consid- ns-LAL of Au in water, acetone, and ethylene glycol at volumetric flow ering the average diameter of the NPs and persistent bubbles and temper- rates of 1 and 20 ml/min. The inserts show the heating rate derived from atures of 25 °C and 35 °C the slope of the linear fit within an ablation time of 2–8min. c,d Diffusion temperature increase is lower, resulting in a maximum colloid The diffusion coefficients for the persistent bubbles temperature of 29 °C. Additionally, the heating rates are sig- (Fig. 4d) were calculated based on their average diameters nificantly lower (0.2–0.3 °C/min), indicating that the faster extracted from the cross-sectional images (the individual bub- liquid exchange reduces the heat accumulation within the col- ble size characteristics are discussed in Fig. 7d-f). The trends −14 2 loid. Note that the colloids’ absolute temperatures are the are similar to NPs with absolute values of 2.6 ± 2.5·10 m /s −14 2 highest in ethylene glycol, followed by acetone and water. for acetone, 3.5 ± 2.6·10 m /s for water, and 3.6 ± 1.9· −16 2 The different colloid temperatures are probably caused by 10 m /s for ethylene glycol. As expected, these values are the NP-induced shielding effects, contributing to the liquid’s several orders lower than the diffusion coefficients calculated heating. As discussed later in more detail, these shielding ef- for the NPs due to the significant differences in their sizes. The fects are the highest in ethylene glycol (see Fig. 12). low diffusibility in ethylene glycol is probably the reason why For calculating the diffusion coefficient, an average liquid the persistent bubbles and NPs remain very close to the target temperature of 35 °C was considered for a volumetric flow surface. In contrast, smaller bubbles are found far away from rate of 1 ml/min. Hence, also the liquid viscosity at this given the target surface in water and acetone. However, their origin temperature was used [79–81]. As displayed in Fig. 4c,the is unsure and could be attributed to gas formation effects in- diffusion coefficients are the highest for NPs produced in ac- duced by post-irradiation of the NPs [59, 82]. In this context, −10 2 etone (2.4 ± 1.0·10 m /s), followed by water (4.7 ± 1.1· Dittrich et al. recently provided indications that NPs trapped in −11 2 −12 2 10 m /s) and ethylene glycol (1.0 ± 0.5·10 m /s). Since the stationary liquid layer which is formed along the target the liquid temperature decreases by about 10 °C with increas- surface during liquid flow conditions (discussed in more detail ing volumetric flow rate (Fig. 4b), the diffusion coefficients in the following section of this work) could be the source of are also 20–40% lower (Fig. 4c). However, the general trend additional gas formation cross-effects when post-irradiated by between the individual liquids remains the same. subsequent laser pulses [83]. J Flow Chem (2021) 11:773–792 779 As pointed out before, when working in liquid flow, an Here, h stands for the liquid layer’s height, while v repre- interface layer is formed along the target surface, which char- sents the velocity of the main flow and D the diffusion coef- acteristics strongly depend on the flow-field conditions and ficient of the NPs and persistent bubbles (extracted from the liquid viscosity [76, 84]. Friction and adhesive forces be- Fig. 4c and d). Pe was calculated for the NPs and the per- ax tween the target surface and the liquid layers cause the shear- sistent bubbles and plotted against Re,as shown in Fig. 5b and ing of the liquid, resulting in a flow velocity gradient. Liquid c. Generally, for both NPs and persistent bubbles, Pe in- ax layers close to the target surface move slower if the adhesive creases with increasing Re, indicating a greater importance forces between the liquid elements and the bulk surface are of convective transport phenomena at high liquid flow veloc- larger than the cohesive forces between them. The interface ities. Within the individual liquids, the Pe are the highest for ax layer thickness is then defined as the distance at which 99% of NPs produced in ethylene glycol ranging from 4.5 ± 0.1·10 to the mainflow velocityisreached [76, 84]. Note that the flow 9.0 ± 0.1·10 . In water, the Pe values are 50 times lower ax 5 6 in the interface layer can be laminar or turbulent [85, 86]. The compared to ethylene glycol (0.9 ± 0.2·10 to 1.7 ± 0.4·10 ), type of flow can be estimated by calculating the Reynolds while in acetone, they are even 300 times lower (1.5 ± 0.1·10 number (Re) according to eq. 2 [87], which describes the ratio to 3 ± 0.1·10 ). The same trend can be observed for persistent between inertia and viscous forces in the bulk-liquid system. bubbles with Pe values about 1000–10,000 times higher ax than for the NPs (Fig. 4d). The differences in the Pe can v  l ax Re ¼ ð2Þ be assigned to the different diffusion coefficients of the NPs and persistent bubbles in the individual liquids (compare Here, v represents the main flow velocity, which can be Fig. 4c and d). calculated by dividing the volumetric flow rate by the cham- For further evaluation, the Schmidt number (Sc) was cal- ber’s flow-cross section. Furthermore, the kinematic viscosity culated according to eq. 4. ν of the corresponding liquid and the characteristic travel dis- tance l (the ablated bulk target’s length was used as reference) v Pe ax need to be considered. The results are presented in Fig. 5a. Sc ¼ ¼ ð4Þ D Re The lowest Re of 1 to 5 are obtained for ethylene glycol as liquid with the highest viscosity used in the experiments. For The Sc is defined by the ratio of the kinematic viscosity ν water, which has a 30 times lower viscosity than ethylene of the liquid to the diffusion coefficient D of the NPs or per- glycol, significantly larger Re in the range of 5 to 92 were sistent bubbles but can also be derived from the ratio of Pe to ax calculated. For low-viscosity acetone, the Re is even twice Re. The higher the Sc, the more difficult it becomes for the as large compared to water covering values from 11 to 220. NPs and persistent bubbles to cross the interface layer. This Considering the criterion for the transition into the turbulent case is particularly pronounced in ethylene glycol, where the regime under the assumption of a longitudinal flow along a Sc of NPs (1.8 ± 0.2·10 ) is 2000 times higher than in water bulk plate (Re <5·10 )[88], a laminar flow forms for all crit (1.8 ± 0.4·10 ) and even 14,000 times higher than in acetone investigated liquids and flow rates, what is probably a general (1.3 ± 0.3·10 ). For persistent bubbles, the trend is compara- characteristic of liquid flow-assisted LAL. ble, leading to the highest Sc of 5.3 ± 0.1·10 in ethylene Hupfeld et al. additionally calculated the Weber (We)and glycol followed by water (2.5 ± 1.9·10 ) and acetone (1.2 ± Capillary (Ca) number beside Re to account for the competition 1.0·10 ). Accordingly, the local concentration of NPs and per- between viscous forces, surface tension, and inertia, affecting sistent bubbles should be highest near the target surface in the dynamics and shape of the cavitation bubble, including the ethylene glycol, which is visually confirmed by Fig. 3.This interface layer height [75]. Note that the calculation of these clearly shows that the total quantity of shielding entities (per- numbers refers to the velocity of the fast-expanding cavitation sistent bubbles and NPs) is inadequate to explain ablation bubble, which is about 2000 times higher than the main liquid shielding alone in liquid flow. However, their confinement flow velocity. Hence, We and Ca are of minor importance for in the interface layer seems to rule the LAL ablation rate. In the persistent bubbles and NPs under the prevailing experimen- ethylene glycol, this confinement is strongest (Fig. 5d). tal conditions. Nevertheless, the cavitation bubble dynamics For further discussion, the velocity of persistent bubbles may influence the flow characteristics during liquid flow- formed during ns-LAL of Au in ethylene glycol was calculat- assisted LAL but are out of this study’s scope. ed, as illustrated in Fig. 6a. In the next step, the axial Peclet number (Pe ) was calcu- ax The procedure was performed for two different types of lated, which is defined as the ratio of convective to diffusive bubbles: i) bubbles directly located at the target surface, and transport phenomena in axial direction according to eq. 3. ii) bubbles located further away (≤0.3 mm) from the target surface. Two observations can be made: Firstly, persistent h  v Pe ¼ ð3Þ bubbles directly located at the target surface move slower than ax those located further away, consistent with the expectations 780 J Flow Chem (2021) 11:773–792 Fig. 5 a Calculation of the Reynolds number depending on the type of during ns-LAL of Au in water, acetone, and ethylene glycol. A liquid liquid and volumetric flow rate. b,c Axial Peclet number as a function of temperature of 298 K was considered for the calculations. d the Reynolds number calculated for NPs and persistent bubbles produced Corresponding Schmidt numbers from the flow velocity gradient. Secondly, the bubble velocity Note that the persistent bubbles may partially adhere increases with increasing flow velocity. The increase in bub- to the target surface before they are removed by the ble velocity is not unexpected since the interface layer’sthick- liquid flow [68]. Therefore, the simplified model used ness decreases with increasing flow velocity. Hence, higher to describe the forces determining the detachment of flow velocities are already achieved at lower distances to the persistent bubbles in a stationary liquid [68], needs to target surface [88]. Unfortunately, such calculations cannot be be extended. For this purpose, the additional forces act- provided for persistent bubbles formed in water and acetone ing on the bubbles in a flowing liquid in parallel and since the camera’s time resolution was too low to capture the perpendicular direction to the target surface were con- bubble movement in these liquids. However, if the theoreti- sidered. The most important forces are displayed in Fig. cally expected flow velocity profile is taken into account, 6b and can be summarized as follows: which can be approximated by a quadratic function [76], one would expect an increase in the flow velocity near the i) the surface tension force F , which is caused by the target surface with decreasing liquid viscosity. The increasing liquid’s attraction to the target surface, acting around flow velocities not only promote the removal of the bubble the perimeter of the bubble base [89, 90]. from the target surface in low viscosity liquids but also lead to ii) the buoyancy force F encompassing both gravity (F ) B g the highest bubble velocities in acetone, followed by water. and Archimedes forces [90–92]. J Flow Chem (2021) 11:773–792 781 Fig. 6 a Bubble velocity near and 3 mm above the target surface as a function of the flow velocity calculated for persistent bubbles produced during ns-LAL of Au in ethylene glycol. b Schematic of the forces acting on persistent bubbles near the bulk surface during liquid flow conditions iii) the shear lift force F , which lifts the bubble in the bubbles accumulate at the target surface and slowly sl perpendicular direction to the target surface depending slide along it. on the liquid flow velocity profile near the target sur- face [91–93]. Bubble size characteristics and shielding capacity iv) the drag force F , which acts opposite to the bubbles’ movement relative to the liquid’sflowvelocity [94]. In the next step, the bubble size characteristics are discussed in more detail. Fig. 7a-c shows exemplary picture series of the These forces can be summarized in the form of a force formation of persistent bubbles in all three liquids taken at balance (eq. 5) defining the bubbles’ detachment from the volumetric flow rates of 1, 10, and 20 ml/min. target surface and their movement in the liquid. The pictures were taken after ablation times of ten seconds to ensure steady-state conditions. Note that for better visualiza- F þ F þ F −F −F ¼ 0 ð5Þ tion of the recordings, uniform mean grey values were used for b sl d s g all pictures. As evident in the picture series, the persistent bub- When the forces that hold the bubble at the target surface bles’ average size varies from liquid to liquid. At a volumetric (negative sign) are overcompensated by the forces that pull the flow rate of 1 ml/min, the average bubble diameter (Fig. 7d)is bubble away from the target surface (positive sign), the bubble the smallest in water (28 ± 20 μm). In contrast, larger bubbles detaches and is carried away with the flow. In this context, the were found in acetone (52 ± 58 μm) and ethylene glycol (57 ± surface tension force is the most important force that prevents 43 μm). Consequently, the cross-sectional areas and volumes the bubble from detachment. Its value increases with increas- of persistent bubbles formed in acetone and ethylene glycol are ing bubble contact diameter and further depends on the bubble also larger than in water (Fig. 7e and f). wettability. In water (σ = 0.073 N/m at 293 K [464]), the The persistent bubbles’ size seems to be unaffected by the hydrophobic [422] gold target surface is more aerophilic than applied volumetric flow rate in water and acetone. In contrast, in ethylene glycol (σ = 0.048 N/m at 293 K [464]) and ace- the bubble size in ethylene glycol increases significantly at the tone (σ = 0.023 N/m at 293 K [464]) due to the higher surface l transition from 5 to 10 ml/min. A further increase in the volu- tension of the corresponding liquids [133]. Consequently, the metric flow rate does not affect the bubble size any further. This bubble wettability and capturing ability on the target surface behavior can be attributed to a combined process of bubble de- are the highest in water followed by ethylene glycol and tachment and the formation of new persistent bubbles. At the acetone. transition point, new persistent bubbles are generated faster than When the bubbles reach a critical size and the flow they can be removed. Consequently, the probability of interac- velocity is high enough, the drag force and buoyancy tion and coalescence between them increases, in particular at force overcompensate the surface tension force, and the high Sc numbers. In low-viscosity liquids, persistent bubbles bubbles detach. Note that the drag force’s direction is are removed faster than new ones are produced so that the final reversed after the bubble’s detachment since then the bubble size is less affected by coalescence effects. bubble velocity is higher than the flow velocity (Fig. In the following, the cross-sectional area for all persistent 6a). Interestingly, most bubbles, particularly in ethylene bubbles was calculated using the procedure described in [68]. glycol, are located near the target surface, indicating The results are displayed in Fig. 8a. that the liquid flow velocity is too low to lift the bub- With an average value of 0.15 mm , the cross-sectional area bles perpendicular to the target surface. As a result, the of the bubbles in water remains almost constant over the entire 782 J Flow Chem (2021) 11:773–792 Fig. 7 a-c Exemplary picture series taken after 10 s demonstrating the bubble diameter, cross-sectional area, and volume depending on the vol- distribution of persistent bubbles in water, acetone, and ethylene glycol at umetric flow rate volumetric flow rates of 1, 10, and 20 ml/min. d-f Corresponding average volumetric flow rate regime. In acetone, the bubbles’ cross- subject to great inaccuracy. Therefore, the fraction of bubbles sectional area increases to 0.34 ± 0.06 mm , while in ethylene occupying the target surface was calculated. By measuring the glycol, a value of 0.34 ± 0.06 mm was found at a maximum gray value along a defined area above the target surface, the volumetric flow rate of 20 ml/min. Consequently, 5% of the target surface occupation profile was determined (Fig. 8b). A liquid-cross section is shielded by persistent bubbles produced value of 0% corresponds to the blank target surface without during ablation in water. In contrast, this value increases to 8% in persistent bubbles, while higher values indicate that persistent acetone and 10% in ethylene glycol. bubbles are present on the target surface. This way, it is possible Since most persistent bubbles are located near the target sur- to estimate the percentage of the target area occupied with bub- face, the determination of the bubble-induced shielding effect, bles (Fig. 8c). The target surface occupation is lowest in water considering only its average in the total liquid cross-section, is (10–20%) and increases in acetone (40–50%) and ethylene J Flow Chem (2021) 11:773–792 783 Fig. 8 a Total cross-sectional area of all persistent bubbles produced rate of 1 ml/min. c Percentage of persistent bubbles occupying the target during ns-LAL of Au in water, acetone, and ethylene glycol as a function surface in the different liquids as a function of the volumetric flow rate. d of the volumetric flow rate. The percent of liquid cross-section is plotted Gas volume formation rates in cm and mmol (assuming an ideal gas with on the right y-axis. b Exemplary target occupancy profile along the target 22.4 l/mol) per hour as a function of the volumetric flow rate surface for persistent bubbles produced in acetone at a volumetric flow glycol (80%). As a result, more target surface is available for (1.07 ± 0.05 to 2.85 ± 0.14 mmol/h). In summary, the ab- ablation in water than in acetone and ethylene glycol so that the lation in acetone and ethylene glycol leads to the forma- highest NP production rate would be expected in water. tion of significantly larger amounts of gases than in water. The camera setup’s temporal and lateral resolution limits made it difficult to visualize and quantify all persistent bub- Correlation of the mass ablation rate with the bles. Therefore, the total gas volume formation rate was de- shielding effects induced by the persistent bubbles termined quantitatively by applying the liquid displacement and nanoparticles method described in the experimental section and used in previous works [69]. The results obtained after ten minutes At this point, the question arises how the mass ablation rate is of ablation were extrapolated to one hour, as displayed in Fig. influenced by the formation of persistent bubbles and NPs 8d. For all liquids, the gas volume formation rate increases depending on the liquids and volumetric flow rates. For this steadily with increasing volumetric flow rate. In water, the gas purpose, the NP production rate was determined, summarized volume formation rates range from 1.2 ± 0.1 to 3.2 ± for each liquid and volumetric flow rate in Fig. 9a. 0.2 cm /h and increase by a factor of 20 in acetone The results demonstrate the highest NP production rates for (19.2 ± 1.0 to 62.4 ± 3.1 cm /h) and ethylene glycol ns-LAL of Au in water, followed by ethylene glycol and ac- (24.0 ± 1.2 to 64.0 ± 3.2 cm /h). The same trend can be etone. Moreover, the NP production rate increases with in- observed for the molar gas formation rates, calculated by creasing volumetric flow rate. This way, NP production rate dividing the gas volume formation rates by the molar ranges from 36 ± 2 to 74 ± 4 mg/h in water, whereas in ace- volume (22.414 L/mol), assuming an ideal gas. The molar tone, it decreases by about 10% (30 ± 2 to 61 ± 3 mg/h). The gas formation rates are lowest in water ranging from 0.05 NP production rates obtained in ethylene glycol range from ± 0.01 to 0.14 ± 0.01 mmol/h and increase in acetone 33 ± 2 to 71 ± 4 mg/h, laying between water and acetone. By (0.86 ± 0.04 to 2.78 ± 0.14 mmol/h) and ethylene glycol dividing the NP production rate by the laser power, the 784 J Flow Chem (2021) 11:773–792 specific NP production rate was calculated, resulting in values described by an exponential fit leading to the lowest NP pro- of 7.6 ± 0.4 to 15.7 ± 0.8 mg/(W·h) in water, 6.3 ± 0.42 to duction rates at the highest NP mass concentrations. The 12.9 ± 0.6 mg/(W·h) in acetone, and 7.0 ± 0.4 to 15.0 ± higher the volumetric flow rates, the higher the dilution rates. 0.8 mg/(W·h) in ethylene glycol. For ns-LAL of Au in water, Consequently, the NP mass concentration decreases, and the Kohsakowski et al. found a specific NP production rate of NP production rate increases. It is worth mentioning that the 18 mg/(W·h) [95], which is in good agreement with the values NP production rate is highest in water, although the NP mass found in this study. However, note that they used 25 times concentration (62 ± 3 to 598 ± 30 mg/l) is higher than in eth- more laser power, while the NP mass concentration was three ylene glycol (58 ± 3 to 550 ± 27 mg/l) and acetone (51 ± 3 to times higher than in the present study. Therefore, their pro- 504 ± 25 mg/l). At first view, this trend is unexpected since ductivity data may not represent the upper limit of what would one would assume the same NP production rate at the same be possible if higher dilution rates (resulting in lower NP NP mass concentration. However, two points need to be con- shielding effects) were used. sidered: Firstly, the bubble shielding is the lowest in water Different productivity trends were reported for laser abla- (Fig. 8). Secondly, the colloidal system’s shielding capacity tion in acetone in literature. While Bärsch et al. found higher depends on the size characteristics of the NPs [68]. ablation efficiencies in acetone than in water [96], the opposite Information about the NP size and the agglomeration states trend was observed in other studies [29, 97, 98]. In contrast, can be extracted from the colloids’ UV-Vis extinction spectra low (specific) NP production rates were typically obtained for (Fig. 11a). ns-LAL in ethylene glycol, explained by viscosity effects [58, Generally, the surface plasmon resonance (SPR) of Au NPs 68]. However, most of these studies were performed in (typ- results in a strong absorbance band in the visible region ically horizontally orientated targets and) batch chambers around 500–600 nm [99]. The SPR band is shifted to longer without liquid flow. Therefore, shielding effects induced by wavelengths when the size of Au NPs increases [99]. persistent bubbles and NPs could have affected the ablation Furthermore, agglomeration of Au NPs leads to a redshift of results. It should be noted that shielding effects cannot be the SPR band accompanied by a broadening of the absorption completely avoided even when using a liquid flow and low- peak [100, 101]. Compared to Au NPs formed in water and viscous liquids, as the ablation profile analysis in Fig. 10 acetone, for which the SPR band is located around 520 nm, illustrates. the SPR band of Au NPs produced in ethylene glycol is The ablation pattern analysis demonstrates that the target broader and shifted to longer wavelengths at 570 nm. From front (near the chamber inlet) is ablated more efficiently than this, it can be concluded that Au NPs produced in ethylene the target end. The differences in local ablation efficiencies glycol are either larger or more agglomerated than in water can be explained by concentration gradients built up by NPs and acetone. For further evaluation, the primary particle index (PPI) was and persistent bubbles along the target surface in the liquid flow direction. calculated (Fig. 11b). The PPI is defined as the ratio of the For further discussion, the NP production rate was plotted interband absorption at a wavelength of 380 nm to the scat- against the NP mass concentration to account for the NP- tering signal of aggregates, agglomerates, and larger particles induced shielding effect (Fig. 9b). The overall trend can be at a wavelength of 800 nm [102]. This way, it is possible to Fig. 9 (a,b) (Specific) NP production rate depending on the volumetric flow rate and NP mass concentration for ns-LAL of Au in water, acetone, and ethylene glycol J Flow Chem (2021) 11:773–792 785 Fig. 10 Confocal 3D microscopy image of the ablation depth profile obtained after ns-LAL of Au in water at a volumetric flow rate of 10 ml/min estimate the degree of agglomeration of the Au NPs if imaging extinction by the colloid, the effective laser fluence techniques such as TEM are included. A PPI of 1 was calcu- available for target ablation is higher in water and ace- lated for Au NPs formed in ethylene glycol, while for Au NPs tone than in ethylene glycol. The effective laser fluence produced in water and acetone, significantly higher PPIs of 7 decreasessteadilyfrom24.2to17.1J/cm in acetone and and 15 were found. TEM measurements have shown that ns- from 23.1 to 11.7 J/cm in water. In contrast, it decreases LAL of Au in water and acetone leads to NPs with primary exponentially from 23.4 to 4.5 J/cm in ethylene glycol, particle diameters around 10 nm [68, 103], which is in good indicating that the contribution of agglomeration to agreement with the hydrodynamic diameter determined by colloid-induced shielding strongly increases with increas- DLS in the present study (Fig. 11c-e). For ethylene glycol, ing NP mass concentration and liquid viscosity. Overall, TEM measurements yielded primary particle diameters of up to 81% of the laser fluence is shielded by the colloids about 10 nm as well [68], while in this study, hydrodynamic in ethylene glycol, whereas it is 29% in acetone and 49% diameters of about 35 nm were measured by DLS. Combining in water. the particle sizes from TEM and DLS with the PPI, it can be Summarizing the shielding effects induced by the per- concluded that Au NPs tend to agglomerate in ethylene glycol, sistent bubbles and NPs, one would expect the highest whereas they are more monodisperse in acetone and water. mass ablation rates for ns-LAL of Au in water, followed The high agglomeration propensity in ethylene glycol is by acetone and ethylene glycol. Although the expectations unexpected from the first point of view since a lower particle for water can be confirmed, the trend for acetone and mobility should enhance the colloidal particle stability [104]. ethylene glycol is contradictory. Kanitz et al. stated that However, this only applies if the NPs are evenly dispersed in the ablation process alters at a stage after energy deposi- the liquid. As mentioned above, the mixing of NPs in ethylene tion [98]. They found that the ablation efficiency strongly glycol is less effective than in water and acetone due to the correlates with the light intensity emitted by the plasma slower flow velocities in the interface layer near the target formed during the first few nanoseconds after a 35 fs laser surface and low NP diffusion coefficients. Consequently, the pulse. Consequently, the highest ablation efficiencies were NPs are more concentrated towards the target surface (as achieved in those liquids where the formed plasma had deduced from Figs. 3c and 7d), which may increase the pro- the strongest light intensity (water followed by acetone pensity of the NPs to agglomerate. and toluene; ethylene glycol was not investigated). Choi Since scattering effects become more pronounced as et al. suggested that solvents with a low specific heat cool the colloidal stability decreases [57], the formation of the plasma more effectively, affecting the formation of agglomerates may also increase the colloid-induced metastable nanomaterials and perhaps also the ablation shielding effect. To verify this assumption, the fraction yield [105]. Taking into account the specific heats of wa- of the laser fluence shielded by the colloids was mea- ter (4.18 J/(g·K)), acetone (2.16 J/(g·K)), and ethylene gly- sured (see appendix Fig. 14a-c). This procedure allows col (2.5 J/(g·K)) [106], their order would fit the trend in the calculation of the laser fluence available for target NP production rate. However, before an exact statement ablation, as shown in Fig. 12. Considering the attenua- can be made about the liquid’s influence on the cooling of tion of the laser intensity in the liquid and the laser light the plasma, further experiments, and modeling are 786 J Flow Chem (2021) 11:773–792 Fig. 11 a Exemplary UV-Vis extinction spectra of colloids produced by by E /E c-e Number-weighted hydrodynamic diameter of Au 380nm 800nm ns-LAL of Au in water, acetone, and ethylene glycol (EG) at a volumetric NPs produced by ablation in water, acetone, and ethylene glycol at a flow rate of 5 ml/min. The UV-Vis spectra are normalized on the volumetric flow rate of 20 ml/min. The size measurements were per- interband absorption of Au at 380 nm. b Primary particle index calculated formed using dynamic light scattering necessary. Furthermore, the liquid’s chemical reactivity during (the early phase) of LAL could be important, as discussed in the next section. Correlation of the NP-induced shielding effects and the mass ablation rate with the gas formation efficiency In the last section of this work, the gas formation pathway is discussed by linking the laser power used for ablation, the NP- induced shielding effects, and the gas formation and mass ablation rates to each other. For this purpose, the total specific gas volume was first calculated by dividing the gas volume formation rate by the NP production rate and the total applied laser power, as shown in Fig. 13a. The results indicate that the total specific gas volume is slightly higher at low nanoparticle mass concentrations below Fig. 12 Effective laser fluence available for target ablation as a function 200 mg/l. In water, total specific gas volumes of 0.01 cm / of the NP mass concentration, by measuring the laser power attenuation at these concentrations (mg·W) were obtained, while in acetone and ethylene glycol, J Flow Chem (2021) 11:773–792 787 Fig. 13 Specific gas volume formed during ns-LAL of Au in water, laser power available for bulk ablation. c Molar ratio of the amount of acetone, and ethylene glycol (EG) depending on the NP mass concentra- formed gas to the amount of ablated mol at a nanoparticle mass concen- tion: a normalized to the total applied laser power and b normalized to the tration of 200 mg/l. 20 times higher values were found (~0.2 cm /(mg·W)). With above a specific NP mass concentration, the gas formation increasing nanoparticle mass concentration, the total specific process is dominated by post-irradiation of NPs and the for- gas volume decreases by 40–50%, reaching values of mation of nanobubbles while the target-ablation-related spe- 3 3 0.005 cm /(mg·W) in water and ~ 0.15 cm /(mg·W) in ace- cific gas volume remains rather constant [59]. Consequently, tone and ethylene glycol. Extrapolating these specific values gas formation cross-effects induced by post-irradiation of NPs to 121 W and 2200 mg/h [95] would amount to gas volume are also dominant under liquid flow conditions when high NP formation rates of 1.3 (water) or 39.9 (organic liquids) liter per mass concentrations >200 mg/l are reached. hour during high-power ns-LAL in liquid flow. The specific gas volumes depend not only on the NP mass Note that the NPs shield the target from the incoming laser concentration but also on the type of liquid. To further em- beam depending on their concentration in the liquid, thus re- phasize this dependence, the molar gas volume formation ducing the laser fluence available for target ablation (compare rates were correlated with the molar NP production rate, as Fig. 12), which was not included in the calculations so far. shown in Fig. 13c, exemplarily for a NP mass concentration of Therefore, the NP production rate was correlated with the gas 200 mg/l. It is evident that the gas formation process is 30 volume formations rates and the effective laser power avail- times more efficient in acetone and ethylene glycol than water. able for target ablation, resulting in the specific gas volume In a first approach, the differences in the gas formation effi- (Fig. 13b). The specific gas volumes are highest at the lowest ciencies can be explained by considering the molecular bonds NP mass concentrations, leading to values of 0.15 ± 0.02 cm / in the vapor phase, which are weaker for acetone and ethylene (mg·W) in water, 0.22 ± 0.02 cm /(mg·W) in acetone, and glycol than water. The decomposition of acetone proceeds by 0.69 ± 0.07 cm /(mg·W) in ethylene glycol. With increasing a unimolecular reaction and leads to methane and acetyl rad- NP mass concentration, the specific gas volume decreases and icals, which recombine and decompose further to gas products shows a threshold-like behavior at NP mass concentrations like molecular hydrogen, methane, ethane, or carbon monox- around 200 mg/l. Above this threshold, the specific gas vol- ide and dioxide [107–109]. The most important step is break- ume does not decrease as quickly as before. These findings are ing the C-C bond (3.6 eV), which is significantly weaker than consistent with our previous results, where it was shown that the O-H bond in water vapor (4.8 eV) [110]. Ethylene glycol Fig. 14 Radar chart summarizing determinants (i.e., shielding effects) (a) and read-outs (b) during ns-LAL of Au in water, acetone, and ethylene glycol. The logarithms of the Schmidt numbers for NPs and persistent bubbles (PBs) are plotted for better visibility in the graph 788 J Flow Chem (2021) 11:773–792 behaves similarly, supported by the gas formation efficiencies, gas volume formation rates are 20 times lower. Accompanied which are in the same order of magnitude as acetone. by a lower diffusion-limitation, bubbles shield only 15–20% The proposed decomposition pathways are thermal, elec- of the target surface. Acetone represents a border case. Here, tron-, or photon-induced reactions triggered by the liquid’s the gas volume formation rates are comparable to ethylene interaction with the laser-induced plasma [111]. However, glycol, but the diffusion-limitation is the lowest. chemical reactions between the target material and the liquid Consequently, the bubble shielding (40–50%) lies between molecules may also be important for the decomposition of the water and ethylene glycol (Fig. 14a). liquid [69, 112]. For ns-ablation of Au in ethylene glycol, the Due to the low diffusivity in ethylene glycol, it is also more formation of persistent bubbles, consisting of molecular hy- difficult for the NPs to leave the interface layer. Hence, high drogen and carbon monoxide as main decomposition products NP concentration gradients are built up close to the target besides smaller amounts of carbon dioxide, methane, acety- surface, promoting the agglomeration of the NPs which is also lene, ethylene, and ethane was confirmed by gas chromato- showcased in Fig. 14b where LAL in ethylene glycol leads to graphic measurements [68]. Evangelista et al. also reported the highest average hydrodynamic particle size. Linked with the formation of these gas products when heating ethylene the stability of the NPs, the formation of agglomerates, and the glycol in a metal tube to ~1400 K. They suggested that ethyl- high colloid-induced shielding, the effective laser fluence ene glycol is catalytically decomposed when the metal tube is available for bulk ablation decreases by 81% for ethylene coated with a catalytic material such as platinum [113]. The glycol, compared to 50% in water and 30% in acetone, where decomposition of ethylene glycol (and acetone) during ns- lower agglomeration degrees were observed. Hence, within LAL of Au may also have been catalyzed. Such reaction path- the radar chart shown in Fig. 14a it is recommended to choose ways should be further investigated in future experiments by a liquid with determinants closer to the center. Accordingly, varying the target material and liquid type. low viscosity liquids with low gas formation rates are recom- mended in combination with high flow rates to achieve max- imum production rate with a high primary particle index and Conclusion small average nanoparticle size (Fig. 14). Nevertheless, the clear solvent-molecular reason for the NP Commonly, batch conditions are used for lab-scale LAL pro- production rate differences within the individual liquids sum- cessing. However, depending on the ablation time and the marized in Fig. 14b remains unclear. It was shown that during liquid’s transport properties, shielding effects induced by ns-LAL of Au in acetone and ethylene glycol, 30 times more NPs and persistent bubbles increase steadily over time, lead- gas is produced per ablated amount of substance, indicating ing to cross-effects that require their disentanglement in a the importance of chemical reactions for the ablation yield during the early phase of LAL. Analyzing the plasma charac- systematic study. By performing ns-LAL of Au in liquids (water, acetone, and ethylene glycol) of different viscosity teristics and performing downstream gas chromatography and diffusivity and applying a liquid flow above the target analysis could help clarify the underlying mechanism. surface, it was shown that the complexity of the time- Supplementary Information The online version contains supplementary dependent shielding effects can be significantly reduced, material available at https://doi.org/10.1007/s41981-021-00144-7. resulting in a steady-state with constant individual contribu- tions after 0.5 to 2 s. The liquid’s ns-LAL key determinants Acknowledgements The authors gratefully acknowledge funding from (i.e., shielding effects) and important synthesis read-outs char- the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) within the projects GO 2566/8-1 (Project ID 440395856) acterized in this study are summarized in Fig. 14. and GO 2566/10-1 (Project ID 445127149). In Fig. 14a it is evident that all discussed shielding effects are more pronounced for ns-LAL in ethylene glycol where Funding Open Access funding enabled and organized by Projekt DEAL. shielding effects strongly depend on the interface layer that forms along the target surface under liquid flow conditions, as Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adap- indicated by the Schmidt number. By calculating dimension- tation, distribution and reproduction in any medium or format, as long as less numbers (i.e., Reynolds, axial Peclet, and Schmidt num- you give appropriate credit to the original author(s) and the source, pro- ber), it was shown that the flow profile is fully laminar (as vide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included usual during LAL in liquid flow) and that the viscosity of the in the article's Creative Commons licence, unless indicated otherwise in a liquid controls the accumulation of both NPs and persistent credit line to the material. If material is not included in the article's bubbles within an interface layer. Creative Commons licence and your intended use is not permitted by Linked with the highest gas volume formation rates of up to statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this 60 cm /h, it was demonstrated that the bubble population con- licence, visit http://creativecommons.org/licenses/by/4.0/. centrates near the target surface in ethylene glycol, shielding up to 80% of the target surface. For ns-ablation in water, the J Flow Chem (2021) 11:773–792 789 urinary tract infection in hospitalized patients. 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Chem Eng Sci 58:55–69 792 J Flow Chem (2021) 11:773–792 Mark-Robert Kalus studied Stephan Barcikowski studied chemistry at the University of chemistry in Braunschweig and Duisburg-Essen and graduated Hannover, and received his PhD with a master’s degree in 2015. in Mechanical Engineering Afterwards, he started as an exter- (Materials). At the Laser nal doctoral student at Particular Zentrum Hannover, Barcikowski GmbH in the position as head of built up the Nanomaterials group, sales. During his doctoral studies and later led the institute’s from 2015 to 2020 at the Institute Materials P roces sing of Chemical Technology I at the Department. In 2010, he co- University of Duisburg-Essen, he founded the company Particular investigated the physical and GmbH. Since 2011, he chairs the chemical effects occurring during Institute of Chemical Technology laser synthesis of colloids under I at the University of Duisburg- the supervision of Prof. Stephan Essen. Stephan Barcikowski has Barcikowski. In 2021, he moved to the Jülich Research Centre operating more than 250 reviewed papers and patent files, cited over 7,000 times. as a scientific project manager in the field of corporate development. He launched the scientific video channel ‘nanofunction’ on youtube with more than 78,000 viewings. He serves editing the Journal Applied Surface Science and guest editing several Journals. Riskyanti Lanyumba studied en- vironmental engineering at the University of Technology in Bilal Gökce studied physics and Yogyakarta, Indonesia, from received his “Diplom” degree 2009 to 2013, where she recieved from RWTH Aachen University her bachelor’s degree. From 2014 in 2008. From 2007 to 2009, he to 2019, she studied water chem- worked on laser material process- istry at the University of ing of metals at Fraunhofer Duisburg-Essen, where she com- Institute for Laser Technology. pleted her master’s thesis at the During his Ph.D. studies at North Institute of Chemical Technology Carolina State University from I, focusing on the fundamentals of 2009 to 2012, he studied funda- laser synthesis of colloids. mental phenomena in condensed matter through ultrafast laser spectroscopy. Afterwards, he worked as a researcher on laser applications for semiconductors at the company T-Systems International. In 2014 he joined the Faculty of Chemistry at the University of Duisburg-Essen as a group leader to establish his own group focusing on functionalization of laser-generated nanoparticles and developing materials for additive manufacturing. 2021 he became a full professor at the University of Wuppertal and since then heads the Chair of Materials Science and Additive Manufacturing. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Flow Chemistry Springer Journals

Discrimination of ablation, shielding, and interface layer effects on the steady-state formation of persistent bubbles under liquid flow conditions during laser synthesis of colloids

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10.1007/s41981-021-00144-7
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Abstract

Over the past decade, laser ablation in liquids (LAL) was established as an innovative nanoparticle synthesis method obeying the principles of green chemistry. While one of the main advantages of this method is the absence of stabilizers leading to nano- particles with “clean” ligand-free surfaces, its main disadvantage is the comparably low nanoparticle production efficiency dampening the sustainability of the method and preventing the use of laser-synthesized nanoparticles in applications that require high amounts of material. In this study, the effects of productivity-dampening entities that become particularly relevant for LAL with high repetition rate lasers, i.e., persistent bubbles or colloidal nanoparticles (NPs), on the synthesis of colloidal gold nanoparticles in different solvents are studied. Especially under batch ablation conditions in highly viscous liquids with prolonged ablation times both shielding entities are closely interconnected and need to be disentangled. By performing liquid flow-assisted nanosecond laser ablation of gold in liquids with different viscosity and nanoparticle or bubble diffusivity, it is shown that a steady-state is reached after a few seconds with fixed individual contributions of bubble- and colloid-induced shielding effects. By analyzing dimensionless numbers (i.e., Axial Peclet, Reynolds, and Schmidt) it is demonstrated how these shielding effects strongly depend on the liquid’s transport properties and the flow-induced formation of an interface layer along the target surface. In highly viscous liquids, the transport of NPs and persistent bubbles within this interface layer is strongly diffusion-controlled. This diffusion-limitation not only affects the agglomeration of the NPs but also leads to high local densities of NPs and bubbles near the target surface, shielding up to 80% of the laser power. Hence, the ablation rate does not only depend on the total amount of shielding matter in the flow channel, but also on the location of the persistent bubbles and NPs. By comparing LAL in different liquids, it is demonstrated that 30 times more gas is produced per ablated amount of substance in acetone and ethylene glycol compared to ablation in water. This finding confirms that chemical effects contribute to the liquid’s decomposition and the ablation yield as well. Furthermore, it is shown that the highest ablation efficiencies and monodisperse qualities are achieved in liquids with the lowest viscosities and gas formation rates at the highest volumetric flow rates. . . . . Keywords Laser ablation in liquids Metal nanoparticles Liquid flow Gas formation Liquid viscosity Introduction Nowadays, nanotechnology is a rapidly developing field with increasing demand and high future potential for applications in areas such as biomedicine [1, 2], optics [3, 4], or catalysis [5, 6]. The production of nanomaterials typically takes place * Stephan Barcikowski via wet-chemical [7, 8], gas phase [9, 10], or solid-state pro- stephan.barcikowski@uni-due.de cesses [11, 12]. However, nanoparticles (NPs) produced by these methods are often subject to agglomeration and aggre- Technical Chemistry I, Center for Nanointegration Duisburg-Essen gation effects if no further stabilizing agents are added to the (CENIDE), University of Duisburg-Essen, 45141 Essen, Germany process [13]. The use of stabilizers is unwanted, in areas such Materials Science and Additive Manufacturing, School of as catalysis or biomedicine, where strict requirements are Mechanical Engineering and Safety Engineering, University of placed on the properties of the NPs, such as their size and Wuppertal, 42119 Wuppertal, Germany 774 J Flow Chem (2021) 11:773–792 purity [14]. Additionally, it is sometimes desired to have the Moreover, the influence of other liquids besides water on NPs in organic liquids, for example, for in-situ preparations of LAL-induced bubble formation has so far only been investi- nanocomposites [15–17], useful as medical devices like anti- gated for batch conditions. In this context, it was shown that microbial catheters [18, 19]. However, the synthesis of NPs in persistent bubbles form that shield up to 65% of the liquid organic liquids is difficult to realize with conventional cross-section depending on liquid viscosity, affecting produc- methods without stabilizing ligands [20]. tion rate and reproducibility of laser-generated NPs [68]. In this context, laser ablation in liquids (LAL) represents a Moreover, Hupfeld et al. observed that cavitation bubbles green [21, 22], cost-effective [23] and scalable [24, 25] alter- with quite non-symmetric, oblate-shaped geometries form in native that provides access to a variety of high-purity [26] highly viscous polyalphaolefin (PAO), whose collapse leads nanomaterials such as metals [27–29], alloys [30, 31]or ox- to long-lasting, persistent bubbles [75]. These studies under- ides [32–34], available in both aqueous [27, 28, 35] and or- line the importance of liquid viscosity for bubble formation ganic media [36–38]. Their application potential is manifold and ablation efficiency during LAL in stationary liquids. and has already found its way into fields such as biomedicine However, it is still unclear how the liquid viscosity affects [30, 39], catalysis [40–45], solar energy [46]or 3D printing the bubble and NP formation during liquid flow-assisted [47–49]. Even though the production of these materials is LAL. It is undisputed that an interface layer forms along the already possible on the gram-scale [24, 25], high-power and target surface under these ablation conditions [76], possibly expensive laser systems are required. Therefore, numerous influencing the removal of the bubbles and NPs from the approaches aimed to optimize the NP ablation efficiency ablation area. Unfortunately, its influence has hardly been based on parameters that are independent of investment costs, discussed in LAL literature so far and therefore requires more including target geometry optimization [50–52], scanning attention. strategy [24, 25, 53], and liquid height adjustment [54, 55]. For closing these evident knowledge gaps and increasing Furthermore, special ablation chambers have been de- the efficiency of this sustainable synthesis method, the influ- signed, enabling the production of NPs under batch [56]and ence of a liquid flow on the formation of NPs and persistent liquid flow conditions [24, 25]. The production of NPs in bubbles during ns-ablation in liquids of different viscosity semi-batch or batch chambers can be performed with or with- (water, acetone, and ethylene glycol) is investigated in this out stirring the liquid [56]. This method is particularly useful study and correlated with the mass ablation and gas formation for application fields where small amounts and high concen- rates. In the first section, the steady-state formation of NPs and trations of colloids are required. However, since long ablation persistent bubbles are studied depending on the liquid’svolu- times are needed to achieve high NP concentrations, the NP metric flow rate. Furthermore, the interface layer that forms production rate decreases over time due to increasing colloid- along the target surface under flow conditions is characterized induced shielding effects [57, 58]. Additionally, the probabil- to understand the influence of flow dynamics on the removal ity of NP post-irradiation effects [59] increases, leading to the of NPs and persistent bubbles from the ablation zone depend- formation of large quantities of nanobubbles [59–62]. This ing on the liquid’s viscosity. In the second section, the persis- way, the reproducibility and quality of the final products suf- tent bubbles are systematically analyzed and quantified to de- fer as fragmentation [63–65]and melting [66, 67] of the NPs termine their shielding capacity. The shielding effects are cor- alters the size distribution of the final NPs. related with the mass ablation rates in the third section of this Furthermore, LAL induces the liquid’s decomposition and work and linked with the NPs’ properties. The fourth section the formation of so-called persistent bubbles [59, 68–70]. concludes this study by evaluating the gas formation efficien- These bubbles represent a permanent shielding entity sticking cies as a function of liquid selection, shielding effects, and to the bulk target surface, screening the laser beam, and neg- mass ablation rates. atively affecting NP production rate. For this reason, LAL is often performed under dynamic flow conditions by continu- ously overflowing the target substrate [24, 25, 56]. This way, Materials and methods local NP accumulation can be significantly reduced, resulting in increased NP production rate with increasing flow rate [25, The experimental setup for performing LAL and quantifying 33, 56, 71]. Since persistent bubbles absorb, reflect, and the gas volume in a liquid flow is demonstrated in Fig. 1. defocus the laser beam, better underwater laser For all experiments, an ablation flow chamber (h: 6 mm, w: micromachining results can also be achieved using a liquid 6 mm, l: 18 mm) made of anodized aluminum was used. Side- flow [72–74]. observation windows were integrated into the chamber to Although initial work has been carried out to investigate monitor the bubble formation. A gold target (99.99%, 10 × the dynamic features of the bubbles that form during laser 5 × 0.5 mm, Allgemeine Gold) was used for ablation and cutting of silicon in flowing water [72], such studies have placed inside the ablation chamber. The thickness of the liquid not yet been performed for liquid flow assisted LAL. layer was 6 mm. The experiments were performed in J Flow Chem (2021) 11:773–792 775 Fig. 1 Experimental setup for measuring the gas volume and bubble dynamics during LAL in water, acetone, and ethylene glycol performed in a custom- made ablation flow chamber deionized water (18.2 MΩcm at 25 °C), acetone (VWR working distance corresponds to the focus placed into the Prolabo, ≥99.0%), and ethylene glycol (Sigma-Aldrich, liquid/behind the target. A working distance of zero corre- 99.8%) as carrier liquid. For ensuring a constant, pulsation- sponds to the highest ablation efficiency. Ns-LAL was per- free volumetric flow rate, a syringe pump was used. The sy- formed in each liquid for 10 min by varying the volumetric ringe was connected with the ablation chamber’s inlet by a flow rate in steps of 1, 5, 10, 15, and 20 ml/min. The ablated tube made of polytetrafluoroethylene (PTFE). A tube made of mass was measured gravimetrically after the ablation process PTFE was also connected to the outlet of the ablation cham- using a microbalance (Pesa Waagen GmbH). ber. The other end of the tube was inserted into a graduated pipette placed in a beaker filled with liquid. LAL was performed using an Nd:YAG ns-laser (Rofin Powerline E20). The laser wavelength was 1064 nm, and the pulse length 8 ns. A repetition rate of 15 kHz was used during all the experiments delivering a pulse energy of 0.33 mJ. The laser beam was guided along the target surface with a scan speed of 2 m/s using a galvanometer scanner (SCANcube 10, Scanlab). Therefore, a spiral pattern with a diameter of 5 mm was used. The laser beam was focused on the target through an F-theta lens with a focal length of 100 mm, resulting in an average spot size of 40 ± 10 μm in the focal position (mea- sured in air), which corresponds to a nominal laser fluence of 25.9 J/cm (details on the laser fluence are given in Fig. 12). Focus adjustment was performed for every liquid by varying the working distance between the F-theta lens and the ablation target in a range between −1mm and 1mm(Fig. 2). The produced colloids were characterized by UV-Vis spec- Fig. 2 Focus adjustment for ns-LAL of Au performed in water, acetone, troscopy. The extinction of the colloids at a wavelength of and ethylene glycol as a function of the extinction at a wavelength of 380 nm is proportional to the Au NP mass concentration and 380 nm (proportional to the NP mass concentration). The maximum was was plotted against the working distance. A negative/positive normalized to the relative z-position 0 776 J Flow Chem (2021) 11:773–792 The gas volume was determined by the liquid displacement As evident in the picture series, shortly after the arrival of the method. Gases produced during the ablation process were first laser pulses, persistent bubbles are formed. Gas chroma- continuously transported through the tubes into the liquid- tography measurements have shown that these bubbles contain filled graduated pipette, leading to the liquid’s displacement. permanent gases (H ,O for ns-ablation in water, and in the 2 2 By recording the amount of displaced liquid, the volume of case of ns-ablation in glycols, additional CH ,CO,CO ,C H 4 2 2 4, formed gas was obtained. The liquid flow was maintained for and C H )[68]. In addition to persistent bubbles, the formation 2 2 several minutes after the ablation process was stopped to en- of a dark cloud of NPs in the liquid is observed. The darkening sure the quantitative measurement of the gases without resi- is faster and stronger in water and acetone than in ethylene dues in the chamber or tubes. The gas cross-section imaging glycol. In the latter case, such a cloud of NPs is hardly visible was recorded with the aid of a videography system described and more located towards the target surface. in [68]. The results were evaluated with ImageJ and further The liquid’s darkening was evaluated by measuring the processed with OriginPro (version 2018b). cross-sectional light attenuation of the individual picture frames For further analysis, the temperature was measured over for each liquid and volumetric flow rate depending on time ablation time. Therefore, a thermocouple was integrated at the (Fig. 3d-f). Of course, this procedure represents only a rough outlet of the ablation chamber. Furthermore, the laser fluence estimation since the darkening of the liquid depends on the shielded by the produced colloids was measured ex-situ. For optical properties (e.g., transmission, absorption, and refraction this purpose, the produced colloids were filled into a glass cu- behavior) of the colloidal system and persistent bubbles and the vette. By placing a power meter (Coherent Inc., FieldMax > II- experimental alignment of the light source to the ablation TO) behind the glass cuvette and guiding the laser beam chamber and the camera system. For a better comparison of through the cell and colloids, thedecreaseofthe pulseenergy the results, the cross-sectional light attenuation was normalized. induced by the shielding of the NPs was determined. The col- A normalized cross-sectional light attenuation of 0% corre- loids were further characterized by measuring their size using sponds to the pure liquid. Higher percentages are due to the dynamic light scattering (Nicomp 380 DLS-ZLS). Besides, the presence of persistent bubbles and NPs. The results can be ablation profile on the Au target was determined after laser described by a hill function leading to steady-state conditions processing using confocal 3D microscopy (Nanofocus). (defined as the time after which 90% of the maximum normal- ized cross-sectional light attenuation is achieved) after a specif- ic mixing time. Hence, this procedure gives a rough idea about the mixing behavior (the time point when steady-state condi- Results and discussion tions are reached) inside the ablation chamber as a function of the ablation time and volumetric flow rate. Steady-state formation of nanoparticles and For ns-LAL of Au in water, a clear dependence on the persistent bubbles depending on the liquid flow volumetric flow rate can be observed. For the lowest volumet- dynamics ric flow rate of 1 ml/min, steady-state conditions are reached after about two seconds. The time for reaching the steady-state Recent studies have shown that NP productivity in LAL decreases steadily with increasing volumetric flow rate until strongly depends on the liquid’s viscosity, which was ex- half a second at 20 ml/min (Fig. 3d). For ns-LAL of Au in plained by the formation of persistent bubbles and their acetone, steady-state conditions are reached after one second, viscosity-dependent dwell time in the ablation zone [68, 75]. independent of the volumetric flow rate (Fig. 3e). However, Furthermore, the produced NPs can act as shielding entities for ns-LAL of Au in ethylene glycol, a normalized cross- and reduce the ablation efficiency [57–59]. These two limita- sectional light attenuation of 100% is never achieved. tions are particularly pronounced during batch processing. Typical values are about 10%, with steady-state conditions Here, the NP mass concentration increases with increasing being reached after about one second if volumetric flow rates ablation time and persistent bubbles accumulate within the of 10 ml/min and more are used (see Fig. 3f). Consequently, liquid or stick to the target surface. Liquid flow setups [25, mixing persistent bubbles and NPs within the entire chamber 33, 56, 71] are typically used to overcome these limitations volume is less efficient in ethylene glycol. and improve the removal of persistent bubbles and NPs from For further discussion, the diffusion coefficients (D)of the the ablation area. In this study, liquid flow-assisted ns-LAL of NPs and persistent bubbles were calculated according to the Au was performed in three different liquids, acetone, water, Stokes-Einstein equation [77]. and ethylene glycol, covering different liquid viscosities of 1.00, 0.33, and 20.81 mPa·s at 293 K. Fig. 3a-c shows an k  T D ¼ ð1Þ exemplary picture series for the processes occurring on the 6  π  η  R hyd millisecond time regime during ns-LAL of Au in these liquids at an exemplary volumetric flow rate of 1 ml/min. J Flow Chem (2021) 11:773–792 777 Fig. 3 a-c Exemplary images taken for water, acetone, and ethylene of time and different volumetric flow rates. The specific times, after glycol as a function of time showing the persistent bubble formation which 90% (t ) of the cross-sectional light attenuation was reached, are process and mixing of NPs in the liquid at a volumetric flow rate of included as inserts to provide reference values for the time until steady- 1 ml/min. d-f Cross-sectional light attenuation normalized to maximum state conditions are reached shielding during LAL in water, acetone, and ethylene glycol as a function −23 Here, k represents the Boltzmann constant (1.38·10 J/ performed comparisons would be lacking for a case in which K), while T is the temperature and η the liquid’s dynamic the particles and the bubbles have similar sizes. viscosity. R stands for the radius of the bubbles extracted Since the colloid is heated during LAL [78], the tempera- hyd from the shadowgraphy images or the hydrodynamic radius of ture dependency of the diffusion coefficient needs to be con- the NPs, which was measured by DLS (compare Fig. 11c-e). sidered. For this reason, the temperature increase of the colloid Please note that the calculations in following are meant to be a produced during ablation of Au in water, acetone, and ethyl- first approximation for the comparison between nanometer- ene glycol was measured at the outlet of the ablation chamber sized spherical solid particles and micrometer-sized spherical as a function of the ablation time. The results of these mea- persistent bubbles. Due to the size difference between these surements are displayed in Fig. 4a and b for volumetric flow entities, the constants determined by the Eq. 1 (as well as Eqs. rates of 1 ml/min and 20 ml/min. 2–4 introduced in the next sections) have a difference of 2–4 For a volumetric flow rate of 1 ml/min, the average heating orders of magnitude and are used for a relative comparison rate is 0.9 °C/min, and the maximum colloid temperature is between these entities generated in different solvents. The 38 °C. For a higher volumetric flow rate of 20 ml/min, the 778 J Flow Chem (2021) 11:773–792 Fig. 4 a,b Colloid temperature increase as a function of ablation time for coefficients were calculated using the Stoke-Einstein equation by consid- ns-LAL of Au in water, acetone, and ethylene glycol at volumetric flow ering the average diameter of the NPs and persistent bubbles and temper- rates of 1 and 20 ml/min. The inserts show the heating rate derived from atures of 25 °C and 35 °C the slope of the linear fit within an ablation time of 2–8min. c,d Diffusion temperature increase is lower, resulting in a maximum colloid The diffusion coefficients for the persistent bubbles temperature of 29 °C. Additionally, the heating rates are sig- (Fig. 4d) were calculated based on their average diameters nificantly lower (0.2–0.3 °C/min), indicating that the faster extracted from the cross-sectional images (the individual bub- liquid exchange reduces the heat accumulation within the col- ble size characteristics are discussed in Fig. 7d-f). The trends −14 2 loid. Note that the colloids’ absolute temperatures are the are similar to NPs with absolute values of 2.6 ± 2.5·10 m /s −14 2 highest in ethylene glycol, followed by acetone and water. for acetone, 3.5 ± 2.6·10 m /s for water, and 3.6 ± 1.9· −16 2 The different colloid temperatures are probably caused by 10 m /s for ethylene glycol. As expected, these values are the NP-induced shielding effects, contributing to the liquid’s several orders lower than the diffusion coefficients calculated heating. As discussed later in more detail, these shielding ef- for the NPs due to the significant differences in their sizes. The fects are the highest in ethylene glycol (see Fig. 12). low diffusibility in ethylene glycol is probably the reason why For calculating the diffusion coefficient, an average liquid the persistent bubbles and NPs remain very close to the target temperature of 35 °C was considered for a volumetric flow surface. In contrast, smaller bubbles are found far away from rate of 1 ml/min. Hence, also the liquid viscosity at this given the target surface in water and acetone. However, their origin temperature was used [79–81]. As displayed in Fig. 4c,the is unsure and could be attributed to gas formation effects in- diffusion coefficients are the highest for NPs produced in ac- duced by post-irradiation of the NPs [59, 82]. In this context, −10 2 etone (2.4 ± 1.0·10 m /s), followed by water (4.7 ± 1.1· Dittrich et al. recently provided indications that NPs trapped in −11 2 −12 2 10 m /s) and ethylene glycol (1.0 ± 0.5·10 m /s). Since the stationary liquid layer which is formed along the target the liquid temperature decreases by about 10 °C with increas- surface during liquid flow conditions (discussed in more detail ing volumetric flow rate (Fig. 4b), the diffusion coefficients in the following section of this work) could be the source of are also 20–40% lower (Fig. 4c). However, the general trend additional gas formation cross-effects when post-irradiated by between the individual liquids remains the same. subsequent laser pulses [83]. J Flow Chem (2021) 11:773–792 779 As pointed out before, when working in liquid flow, an Here, h stands for the liquid layer’s height, while v repre- interface layer is formed along the target surface, which char- sents the velocity of the main flow and D the diffusion coef- acteristics strongly depend on the flow-field conditions and ficient of the NPs and persistent bubbles (extracted from the liquid viscosity [76, 84]. Friction and adhesive forces be- Fig. 4c and d). Pe was calculated for the NPs and the per- ax tween the target surface and the liquid layers cause the shear- sistent bubbles and plotted against Re,as shown in Fig. 5b and ing of the liquid, resulting in a flow velocity gradient. Liquid c. Generally, for both NPs and persistent bubbles, Pe in- ax layers close to the target surface move slower if the adhesive creases with increasing Re, indicating a greater importance forces between the liquid elements and the bulk surface are of convective transport phenomena at high liquid flow veloc- larger than the cohesive forces between them. The interface ities. Within the individual liquids, the Pe are the highest for ax layer thickness is then defined as the distance at which 99% of NPs produced in ethylene glycol ranging from 4.5 ± 0.1·10 to the mainflow velocityisreached [76, 84]. Note that the flow 9.0 ± 0.1·10 . In water, the Pe values are 50 times lower ax 5 6 in the interface layer can be laminar or turbulent [85, 86]. The compared to ethylene glycol (0.9 ± 0.2·10 to 1.7 ± 0.4·10 ), type of flow can be estimated by calculating the Reynolds while in acetone, they are even 300 times lower (1.5 ± 0.1·10 number (Re) according to eq. 2 [87], which describes the ratio to 3 ± 0.1·10 ). The same trend can be observed for persistent between inertia and viscous forces in the bulk-liquid system. bubbles with Pe values about 1000–10,000 times higher ax than for the NPs (Fig. 4d). The differences in the Pe can v  l ax Re ¼ ð2Þ be assigned to the different diffusion coefficients of the NPs and persistent bubbles in the individual liquids (compare Here, v represents the main flow velocity, which can be Fig. 4c and d). calculated by dividing the volumetric flow rate by the cham- For further evaluation, the Schmidt number (Sc) was cal- ber’s flow-cross section. Furthermore, the kinematic viscosity culated according to eq. 4. ν of the corresponding liquid and the characteristic travel dis- tance l (the ablated bulk target’s length was used as reference) v Pe ax need to be considered. The results are presented in Fig. 5a. Sc ¼ ¼ ð4Þ D Re The lowest Re of 1 to 5 are obtained for ethylene glycol as liquid with the highest viscosity used in the experiments. For The Sc is defined by the ratio of the kinematic viscosity ν water, which has a 30 times lower viscosity than ethylene of the liquid to the diffusion coefficient D of the NPs or per- glycol, significantly larger Re in the range of 5 to 92 were sistent bubbles but can also be derived from the ratio of Pe to ax calculated. For low-viscosity acetone, the Re is even twice Re. The higher the Sc, the more difficult it becomes for the as large compared to water covering values from 11 to 220. NPs and persistent bubbles to cross the interface layer. This Considering the criterion for the transition into the turbulent case is particularly pronounced in ethylene glycol, where the regime under the assumption of a longitudinal flow along a Sc of NPs (1.8 ± 0.2·10 ) is 2000 times higher than in water bulk plate (Re <5·10 )[88], a laminar flow forms for all crit (1.8 ± 0.4·10 ) and even 14,000 times higher than in acetone investigated liquids and flow rates, what is probably a general (1.3 ± 0.3·10 ). For persistent bubbles, the trend is compara- characteristic of liquid flow-assisted LAL. ble, leading to the highest Sc of 5.3 ± 0.1·10 in ethylene Hupfeld et al. additionally calculated the Weber (We)and glycol followed by water (2.5 ± 1.9·10 ) and acetone (1.2 ± Capillary (Ca) number beside Re to account for the competition 1.0·10 ). Accordingly, the local concentration of NPs and per- between viscous forces, surface tension, and inertia, affecting sistent bubbles should be highest near the target surface in the dynamics and shape of the cavitation bubble, including the ethylene glycol, which is visually confirmed by Fig. 3.This interface layer height [75]. Note that the calculation of these clearly shows that the total quantity of shielding entities (per- numbers refers to the velocity of the fast-expanding cavitation sistent bubbles and NPs) is inadequate to explain ablation bubble, which is about 2000 times higher than the main liquid shielding alone in liquid flow. However, their confinement flow velocity. Hence, We and Ca are of minor importance for in the interface layer seems to rule the LAL ablation rate. In the persistent bubbles and NPs under the prevailing experimen- ethylene glycol, this confinement is strongest (Fig. 5d). tal conditions. Nevertheless, the cavitation bubble dynamics For further discussion, the velocity of persistent bubbles may influence the flow characteristics during liquid flow- formed during ns-LAL of Au in ethylene glycol was calculat- assisted LAL but are out of this study’s scope. ed, as illustrated in Fig. 6a. In the next step, the axial Peclet number (Pe ) was calcu- ax The procedure was performed for two different types of lated, which is defined as the ratio of convective to diffusive bubbles: i) bubbles directly located at the target surface, and transport phenomena in axial direction according to eq. 3. ii) bubbles located further away (≤0.3 mm) from the target surface. Two observations can be made: Firstly, persistent h  v Pe ¼ ð3Þ bubbles directly located at the target surface move slower than ax those located further away, consistent with the expectations 780 J Flow Chem (2021) 11:773–792 Fig. 5 a Calculation of the Reynolds number depending on the type of during ns-LAL of Au in water, acetone, and ethylene glycol. A liquid liquid and volumetric flow rate. b,c Axial Peclet number as a function of temperature of 298 K was considered for the calculations. d the Reynolds number calculated for NPs and persistent bubbles produced Corresponding Schmidt numbers from the flow velocity gradient. Secondly, the bubble velocity Note that the persistent bubbles may partially adhere increases with increasing flow velocity. The increase in bub- to the target surface before they are removed by the ble velocity is not unexpected since the interface layer’sthick- liquid flow [68]. Therefore, the simplified model used ness decreases with increasing flow velocity. Hence, higher to describe the forces determining the detachment of flow velocities are already achieved at lower distances to the persistent bubbles in a stationary liquid [68], needs to target surface [88]. Unfortunately, such calculations cannot be be extended. For this purpose, the additional forces act- provided for persistent bubbles formed in water and acetone ing on the bubbles in a flowing liquid in parallel and since the camera’s time resolution was too low to capture the perpendicular direction to the target surface were con- bubble movement in these liquids. However, if the theoreti- sidered. The most important forces are displayed in Fig. cally expected flow velocity profile is taken into account, 6b and can be summarized as follows: which can be approximated by a quadratic function [76], one would expect an increase in the flow velocity near the i) the surface tension force F , which is caused by the target surface with decreasing liquid viscosity. The increasing liquid’s attraction to the target surface, acting around flow velocities not only promote the removal of the bubble the perimeter of the bubble base [89, 90]. from the target surface in low viscosity liquids but also lead to ii) the buoyancy force F encompassing both gravity (F ) B g the highest bubble velocities in acetone, followed by water. and Archimedes forces [90–92]. J Flow Chem (2021) 11:773–792 781 Fig. 6 a Bubble velocity near and 3 mm above the target surface as a function of the flow velocity calculated for persistent bubbles produced during ns-LAL of Au in ethylene glycol. b Schematic of the forces acting on persistent bubbles near the bulk surface during liquid flow conditions iii) the shear lift force F , which lifts the bubble in the bubbles accumulate at the target surface and slowly sl perpendicular direction to the target surface depending slide along it. on the liquid flow velocity profile near the target sur- face [91–93]. Bubble size characteristics and shielding capacity iv) the drag force F , which acts opposite to the bubbles’ movement relative to the liquid’sflowvelocity [94]. In the next step, the bubble size characteristics are discussed in more detail. Fig. 7a-c shows exemplary picture series of the These forces can be summarized in the form of a force formation of persistent bubbles in all three liquids taken at balance (eq. 5) defining the bubbles’ detachment from the volumetric flow rates of 1, 10, and 20 ml/min. target surface and their movement in the liquid. The pictures were taken after ablation times of ten seconds to ensure steady-state conditions. Note that for better visualiza- F þ F þ F −F −F ¼ 0 ð5Þ tion of the recordings, uniform mean grey values were used for b sl d s g all pictures. As evident in the picture series, the persistent bub- When the forces that hold the bubble at the target surface bles’ average size varies from liquid to liquid. At a volumetric (negative sign) are overcompensated by the forces that pull the flow rate of 1 ml/min, the average bubble diameter (Fig. 7d)is bubble away from the target surface (positive sign), the bubble the smallest in water (28 ± 20 μm). In contrast, larger bubbles detaches and is carried away with the flow. In this context, the were found in acetone (52 ± 58 μm) and ethylene glycol (57 ± surface tension force is the most important force that prevents 43 μm). Consequently, the cross-sectional areas and volumes the bubble from detachment. Its value increases with increas- of persistent bubbles formed in acetone and ethylene glycol are ing bubble contact diameter and further depends on the bubble also larger than in water (Fig. 7e and f). wettability. In water (σ = 0.073 N/m at 293 K [464]), the The persistent bubbles’ size seems to be unaffected by the hydrophobic [422] gold target surface is more aerophilic than applied volumetric flow rate in water and acetone. In contrast, in ethylene glycol (σ = 0.048 N/m at 293 K [464]) and ace- the bubble size in ethylene glycol increases significantly at the tone (σ = 0.023 N/m at 293 K [464]) due to the higher surface l transition from 5 to 10 ml/min. A further increase in the volu- tension of the corresponding liquids [133]. Consequently, the metric flow rate does not affect the bubble size any further. This bubble wettability and capturing ability on the target surface behavior can be attributed to a combined process of bubble de- are the highest in water followed by ethylene glycol and tachment and the formation of new persistent bubbles. At the acetone. transition point, new persistent bubbles are generated faster than When the bubbles reach a critical size and the flow they can be removed. Consequently, the probability of interac- velocity is high enough, the drag force and buoyancy tion and coalescence between them increases, in particular at force overcompensate the surface tension force, and the high Sc numbers. In low-viscosity liquids, persistent bubbles bubbles detach. Note that the drag force’s direction is are removed faster than new ones are produced so that the final reversed after the bubble’s detachment since then the bubble size is less affected by coalescence effects. bubble velocity is higher than the flow velocity (Fig. In the following, the cross-sectional area for all persistent 6a). Interestingly, most bubbles, particularly in ethylene bubbles was calculated using the procedure described in [68]. glycol, are located near the target surface, indicating The results are displayed in Fig. 8a. that the liquid flow velocity is too low to lift the bub- With an average value of 0.15 mm , the cross-sectional area bles perpendicular to the target surface. As a result, the of the bubbles in water remains almost constant over the entire 782 J Flow Chem (2021) 11:773–792 Fig. 7 a-c Exemplary picture series taken after 10 s demonstrating the bubble diameter, cross-sectional area, and volume depending on the vol- distribution of persistent bubbles in water, acetone, and ethylene glycol at umetric flow rate volumetric flow rates of 1, 10, and 20 ml/min. d-f Corresponding average volumetric flow rate regime. In acetone, the bubbles’ cross- subject to great inaccuracy. Therefore, the fraction of bubbles sectional area increases to 0.34 ± 0.06 mm , while in ethylene occupying the target surface was calculated. By measuring the glycol, a value of 0.34 ± 0.06 mm was found at a maximum gray value along a defined area above the target surface, the volumetric flow rate of 20 ml/min. Consequently, 5% of the target surface occupation profile was determined (Fig. 8b). A liquid-cross section is shielded by persistent bubbles produced value of 0% corresponds to the blank target surface without during ablation in water. In contrast, this value increases to 8% in persistent bubbles, while higher values indicate that persistent acetone and 10% in ethylene glycol. bubbles are present on the target surface. This way, it is possible Since most persistent bubbles are located near the target sur- to estimate the percentage of the target area occupied with bub- face, the determination of the bubble-induced shielding effect, bles (Fig. 8c). The target surface occupation is lowest in water considering only its average in the total liquid cross-section, is (10–20%) and increases in acetone (40–50%) and ethylene J Flow Chem (2021) 11:773–792 783 Fig. 8 a Total cross-sectional area of all persistent bubbles produced rate of 1 ml/min. c Percentage of persistent bubbles occupying the target during ns-LAL of Au in water, acetone, and ethylene glycol as a function surface in the different liquids as a function of the volumetric flow rate. d of the volumetric flow rate. The percent of liquid cross-section is plotted Gas volume formation rates in cm and mmol (assuming an ideal gas with on the right y-axis. b Exemplary target occupancy profile along the target 22.4 l/mol) per hour as a function of the volumetric flow rate surface for persistent bubbles produced in acetone at a volumetric flow glycol (80%). As a result, more target surface is available for (1.07 ± 0.05 to 2.85 ± 0.14 mmol/h). In summary, the ab- ablation in water than in acetone and ethylene glycol so that the lation in acetone and ethylene glycol leads to the forma- highest NP production rate would be expected in water. tion of significantly larger amounts of gases than in water. The camera setup’s temporal and lateral resolution limits made it difficult to visualize and quantify all persistent bub- Correlation of the mass ablation rate with the bles. Therefore, the total gas volume formation rate was de- shielding effects induced by the persistent bubbles termined quantitatively by applying the liquid displacement and nanoparticles method described in the experimental section and used in previous works [69]. The results obtained after ten minutes At this point, the question arises how the mass ablation rate is of ablation were extrapolated to one hour, as displayed in Fig. influenced by the formation of persistent bubbles and NPs 8d. For all liquids, the gas volume formation rate increases depending on the liquids and volumetric flow rates. For this steadily with increasing volumetric flow rate. In water, the gas purpose, the NP production rate was determined, summarized volume formation rates range from 1.2 ± 0.1 to 3.2 ± for each liquid and volumetric flow rate in Fig. 9a. 0.2 cm /h and increase by a factor of 20 in acetone The results demonstrate the highest NP production rates for (19.2 ± 1.0 to 62.4 ± 3.1 cm /h) and ethylene glycol ns-LAL of Au in water, followed by ethylene glycol and ac- (24.0 ± 1.2 to 64.0 ± 3.2 cm /h). The same trend can be etone. Moreover, the NP production rate increases with in- observed for the molar gas formation rates, calculated by creasing volumetric flow rate. This way, NP production rate dividing the gas volume formation rates by the molar ranges from 36 ± 2 to 74 ± 4 mg/h in water, whereas in ace- volume (22.414 L/mol), assuming an ideal gas. The molar tone, it decreases by about 10% (30 ± 2 to 61 ± 3 mg/h). The gas formation rates are lowest in water ranging from 0.05 NP production rates obtained in ethylene glycol range from ± 0.01 to 0.14 ± 0.01 mmol/h and increase in acetone 33 ± 2 to 71 ± 4 mg/h, laying between water and acetone. By (0.86 ± 0.04 to 2.78 ± 0.14 mmol/h) and ethylene glycol dividing the NP production rate by the laser power, the 784 J Flow Chem (2021) 11:773–792 specific NP production rate was calculated, resulting in values described by an exponential fit leading to the lowest NP pro- of 7.6 ± 0.4 to 15.7 ± 0.8 mg/(W·h) in water, 6.3 ± 0.42 to duction rates at the highest NP mass concentrations. The 12.9 ± 0.6 mg/(W·h) in acetone, and 7.0 ± 0.4 to 15.0 ± higher the volumetric flow rates, the higher the dilution rates. 0.8 mg/(W·h) in ethylene glycol. For ns-LAL of Au in water, Consequently, the NP mass concentration decreases, and the Kohsakowski et al. found a specific NP production rate of NP production rate increases. It is worth mentioning that the 18 mg/(W·h) [95], which is in good agreement with the values NP production rate is highest in water, although the NP mass found in this study. However, note that they used 25 times concentration (62 ± 3 to 598 ± 30 mg/l) is higher than in eth- more laser power, while the NP mass concentration was three ylene glycol (58 ± 3 to 550 ± 27 mg/l) and acetone (51 ± 3 to times higher than in the present study. Therefore, their pro- 504 ± 25 mg/l). At first view, this trend is unexpected since ductivity data may not represent the upper limit of what would one would assume the same NP production rate at the same be possible if higher dilution rates (resulting in lower NP NP mass concentration. However, two points need to be con- shielding effects) were used. sidered: Firstly, the bubble shielding is the lowest in water Different productivity trends were reported for laser abla- (Fig. 8). Secondly, the colloidal system’s shielding capacity tion in acetone in literature. While Bärsch et al. found higher depends on the size characteristics of the NPs [68]. ablation efficiencies in acetone than in water [96], the opposite Information about the NP size and the agglomeration states trend was observed in other studies [29, 97, 98]. In contrast, can be extracted from the colloids’ UV-Vis extinction spectra low (specific) NP production rates were typically obtained for (Fig. 11a). ns-LAL in ethylene glycol, explained by viscosity effects [58, Generally, the surface plasmon resonance (SPR) of Au NPs 68]. However, most of these studies were performed in (typ- results in a strong absorbance band in the visible region ically horizontally orientated targets and) batch chambers around 500–600 nm [99]. The SPR band is shifted to longer without liquid flow. Therefore, shielding effects induced by wavelengths when the size of Au NPs increases [99]. persistent bubbles and NPs could have affected the ablation Furthermore, agglomeration of Au NPs leads to a redshift of results. It should be noted that shielding effects cannot be the SPR band accompanied by a broadening of the absorption completely avoided even when using a liquid flow and low- peak [100, 101]. Compared to Au NPs formed in water and viscous liquids, as the ablation profile analysis in Fig. 10 acetone, for which the SPR band is located around 520 nm, illustrates. the SPR band of Au NPs produced in ethylene glycol is The ablation pattern analysis demonstrates that the target broader and shifted to longer wavelengths at 570 nm. From front (near the chamber inlet) is ablated more efficiently than this, it can be concluded that Au NPs produced in ethylene the target end. The differences in local ablation efficiencies glycol are either larger or more agglomerated than in water can be explained by concentration gradients built up by NPs and acetone. For further evaluation, the primary particle index (PPI) was and persistent bubbles along the target surface in the liquid flow direction. calculated (Fig. 11b). The PPI is defined as the ratio of the For further discussion, the NP production rate was plotted interband absorption at a wavelength of 380 nm to the scat- against the NP mass concentration to account for the NP- tering signal of aggregates, agglomerates, and larger particles induced shielding effect (Fig. 9b). The overall trend can be at a wavelength of 800 nm [102]. This way, it is possible to Fig. 9 (a,b) (Specific) NP production rate depending on the volumetric flow rate and NP mass concentration for ns-LAL of Au in water, acetone, and ethylene glycol J Flow Chem (2021) 11:773–792 785 Fig. 10 Confocal 3D microscopy image of the ablation depth profile obtained after ns-LAL of Au in water at a volumetric flow rate of 10 ml/min estimate the degree of agglomeration of the Au NPs if imaging extinction by the colloid, the effective laser fluence techniques such as TEM are included. A PPI of 1 was calcu- available for target ablation is higher in water and ace- lated for Au NPs formed in ethylene glycol, while for Au NPs tone than in ethylene glycol. The effective laser fluence produced in water and acetone, significantly higher PPIs of 7 decreasessteadilyfrom24.2to17.1J/cm in acetone and and 15 were found. TEM measurements have shown that ns- from 23.1 to 11.7 J/cm in water. In contrast, it decreases LAL of Au in water and acetone leads to NPs with primary exponentially from 23.4 to 4.5 J/cm in ethylene glycol, particle diameters around 10 nm [68, 103], which is in good indicating that the contribution of agglomeration to agreement with the hydrodynamic diameter determined by colloid-induced shielding strongly increases with increas- DLS in the present study (Fig. 11c-e). For ethylene glycol, ing NP mass concentration and liquid viscosity. Overall, TEM measurements yielded primary particle diameters of up to 81% of the laser fluence is shielded by the colloids about 10 nm as well [68], while in this study, hydrodynamic in ethylene glycol, whereas it is 29% in acetone and 49% diameters of about 35 nm were measured by DLS. Combining in water. the particle sizes from TEM and DLS with the PPI, it can be Summarizing the shielding effects induced by the per- concluded that Au NPs tend to agglomerate in ethylene glycol, sistent bubbles and NPs, one would expect the highest whereas they are more monodisperse in acetone and water. mass ablation rates for ns-LAL of Au in water, followed The high agglomeration propensity in ethylene glycol is by acetone and ethylene glycol. Although the expectations unexpected from the first point of view since a lower particle for water can be confirmed, the trend for acetone and mobility should enhance the colloidal particle stability [104]. ethylene glycol is contradictory. Kanitz et al. stated that However, this only applies if the NPs are evenly dispersed in the ablation process alters at a stage after energy deposi- the liquid. As mentioned above, the mixing of NPs in ethylene tion [98]. They found that the ablation efficiency strongly glycol is less effective than in water and acetone due to the correlates with the light intensity emitted by the plasma slower flow velocities in the interface layer near the target formed during the first few nanoseconds after a 35 fs laser surface and low NP diffusion coefficients. Consequently, the pulse. Consequently, the highest ablation efficiencies were NPs are more concentrated towards the target surface (as achieved in those liquids where the formed plasma had deduced from Figs. 3c and 7d), which may increase the pro- the strongest light intensity (water followed by acetone pensity of the NPs to agglomerate. and toluene; ethylene glycol was not investigated). Choi Since scattering effects become more pronounced as et al. suggested that solvents with a low specific heat cool the colloidal stability decreases [57], the formation of the plasma more effectively, affecting the formation of agglomerates may also increase the colloid-induced metastable nanomaterials and perhaps also the ablation shielding effect. To verify this assumption, the fraction yield [105]. Taking into account the specific heats of wa- of the laser fluence shielded by the colloids was mea- ter (4.18 J/(g·K)), acetone (2.16 J/(g·K)), and ethylene gly- sured (see appendix Fig. 14a-c). This procedure allows col (2.5 J/(g·K)) [106], their order would fit the trend in the calculation of the laser fluence available for target NP production rate. However, before an exact statement ablation, as shown in Fig. 12. Considering the attenua- can be made about the liquid’s influence on the cooling of tion of the laser intensity in the liquid and the laser light the plasma, further experiments, and modeling are 786 J Flow Chem (2021) 11:773–792 Fig. 11 a Exemplary UV-Vis extinction spectra of colloids produced by by E /E c-e Number-weighted hydrodynamic diameter of Au 380nm 800nm ns-LAL of Au in water, acetone, and ethylene glycol (EG) at a volumetric NPs produced by ablation in water, acetone, and ethylene glycol at a flow rate of 5 ml/min. The UV-Vis spectra are normalized on the volumetric flow rate of 20 ml/min. The size measurements were per- interband absorption of Au at 380 nm. b Primary particle index calculated formed using dynamic light scattering necessary. Furthermore, the liquid’s chemical reactivity during (the early phase) of LAL could be important, as discussed in the next section. Correlation of the NP-induced shielding effects and the mass ablation rate with the gas formation efficiency In the last section of this work, the gas formation pathway is discussed by linking the laser power used for ablation, the NP- induced shielding effects, and the gas formation and mass ablation rates to each other. For this purpose, the total specific gas volume was first calculated by dividing the gas volume formation rate by the NP production rate and the total applied laser power, as shown in Fig. 13a. The results indicate that the total specific gas volume is slightly higher at low nanoparticle mass concentrations below Fig. 12 Effective laser fluence available for target ablation as a function 200 mg/l. In water, total specific gas volumes of 0.01 cm / of the NP mass concentration, by measuring the laser power attenuation at these concentrations (mg·W) were obtained, while in acetone and ethylene glycol, J Flow Chem (2021) 11:773–792 787 Fig. 13 Specific gas volume formed during ns-LAL of Au in water, laser power available for bulk ablation. c Molar ratio of the amount of acetone, and ethylene glycol (EG) depending on the NP mass concentra- formed gas to the amount of ablated mol at a nanoparticle mass concen- tion: a normalized to the total applied laser power and b normalized to the tration of 200 mg/l. 20 times higher values were found (~0.2 cm /(mg·W)). With above a specific NP mass concentration, the gas formation increasing nanoparticle mass concentration, the total specific process is dominated by post-irradiation of NPs and the for- gas volume decreases by 40–50%, reaching values of mation of nanobubbles while the target-ablation-related spe- 3 3 0.005 cm /(mg·W) in water and ~ 0.15 cm /(mg·W) in ace- cific gas volume remains rather constant [59]. Consequently, tone and ethylene glycol. Extrapolating these specific values gas formation cross-effects induced by post-irradiation of NPs to 121 W and 2200 mg/h [95] would amount to gas volume are also dominant under liquid flow conditions when high NP formation rates of 1.3 (water) or 39.9 (organic liquids) liter per mass concentrations >200 mg/l are reached. hour during high-power ns-LAL in liquid flow. The specific gas volumes depend not only on the NP mass Note that the NPs shield the target from the incoming laser concentration but also on the type of liquid. To further em- beam depending on their concentration in the liquid, thus re- phasize this dependence, the molar gas volume formation ducing the laser fluence available for target ablation (compare rates were correlated with the molar NP production rate, as Fig. 12), which was not included in the calculations so far. shown in Fig. 13c, exemplarily for a NP mass concentration of Therefore, the NP production rate was correlated with the gas 200 mg/l. It is evident that the gas formation process is 30 volume formations rates and the effective laser power avail- times more efficient in acetone and ethylene glycol than water. able for target ablation, resulting in the specific gas volume In a first approach, the differences in the gas formation effi- (Fig. 13b). The specific gas volumes are highest at the lowest ciencies can be explained by considering the molecular bonds NP mass concentrations, leading to values of 0.15 ± 0.02 cm / in the vapor phase, which are weaker for acetone and ethylene (mg·W) in water, 0.22 ± 0.02 cm /(mg·W) in acetone, and glycol than water. The decomposition of acetone proceeds by 0.69 ± 0.07 cm /(mg·W) in ethylene glycol. With increasing a unimolecular reaction and leads to methane and acetyl rad- NP mass concentration, the specific gas volume decreases and icals, which recombine and decompose further to gas products shows a threshold-like behavior at NP mass concentrations like molecular hydrogen, methane, ethane, or carbon monox- around 200 mg/l. Above this threshold, the specific gas vol- ide and dioxide [107–109]. The most important step is break- ume does not decrease as quickly as before. These findings are ing the C-C bond (3.6 eV), which is significantly weaker than consistent with our previous results, where it was shown that the O-H bond in water vapor (4.8 eV) [110]. Ethylene glycol Fig. 14 Radar chart summarizing determinants (i.e., shielding effects) (a) and read-outs (b) during ns-LAL of Au in water, acetone, and ethylene glycol. The logarithms of the Schmidt numbers for NPs and persistent bubbles (PBs) are plotted for better visibility in the graph 788 J Flow Chem (2021) 11:773–792 behaves similarly, supported by the gas formation efficiencies, gas volume formation rates are 20 times lower. Accompanied which are in the same order of magnitude as acetone. by a lower diffusion-limitation, bubbles shield only 15–20% The proposed decomposition pathways are thermal, elec- of the target surface. Acetone represents a border case. Here, tron-, or photon-induced reactions triggered by the liquid’s the gas volume formation rates are comparable to ethylene interaction with the laser-induced plasma [111]. However, glycol, but the diffusion-limitation is the lowest. chemical reactions between the target material and the liquid Consequently, the bubble shielding (40–50%) lies between molecules may also be important for the decomposition of the water and ethylene glycol (Fig. 14a). liquid [69, 112]. For ns-ablation of Au in ethylene glycol, the Due to the low diffusivity in ethylene glycol, it is also more formation of persistent bubbles, consisting of molecular hy- difficult for the NPs to leave the interface layer. Hence, high drogen and carbon monoxide as main decomposition products NP concentration gradients are built up close to the target besides smaller amounts of carbon dioxide, methane, acety- surface, promoting the agglomeration of the NPs which is also lene, ethylene, and ethane was confirmed by gas chromato- showcased in Fig. 14b where LAL in ethylene glycol leads to graphic measurements [68]. Evangelista et al. also reported the highest average hydrodynamic particle size. Linked with the formation of these gas products when heating ethylene the stability of the NPs, the formation of agglomerates, and the glycol in a metal tube to ~1400 K. They suggested that ethyl- high colloid-induced shielding, the effective laser fluence ene glycol is catalytically decomposed when the metal tube is available for bulk ablation decreases by 81% for ethylene coated with a catalytic material such as platinum [113]. The glycol, compared to 50% in water and 30% in acetone, where decomposition of ethylene glycol (and acetone) during ns- lower agglomeration degrees were observed. Hence, within LAL of Au may also have been catalyzed. Such reaction path- the radar chart shown in Fig. 14a it is recommended to choose ways should be further investigated in future experiments by a liquid with determinants closer to the center. Accordingly, varying the target material and liquid type. low viscosity liquids with low gas formation rates are recom- mended in combination with high flow rates to achieve max- imum production rate with a high primary particle index and Conclusion small average nanoparticle size (Fig. 14). Nevertheless, the clear solvent-molecular reason for the NP Commonly, batch conditions are used for lab-scale LAL pro- production rate differences within the individual liquids sum- cessing. However, depending on the ablation time and the marized in Fig. 14b remains unclear. It was shown that during liquid’s transport properties, shielding effects induced by ns-LAL of Au in acetone and ethylene glycol, 30 times more NPs and persistent bubbles increase steadily over time, lead- gas is produced per ablated amount of substance, indicating ing to cross-effects that require their disentanglement in a the importance of chemical reactions for the ablation yield during the early phase of LAL. Analyzing the plasma charac- systematic study. By performing ns-LAL of Au in liquids (water, acetone, and ethylene glycol) of different viscosity teristics and performing downstream gas chromatography and diffusivity and applying a liquid flow above the target analysis could help clarify the underlying mechanism. surface, it was shown that the complexity of the time- Supplementary Information The online version contains supplementary dependent shielding effects can be significantly reduced, material available at https://doi.org/10.1007/s41981-021-00144-7. resulting in a steady-state with constant individual contribu- tions after 0.5 to 2 s. The liquid’s ns-LAL key determinants Acknowledgements The authors gratefully acknowledge funding from (i.e., shielding effects) and important synthesis read-outs char- the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) within the projects GO 2566/8-1 (Project ID 440395856) acterized in this study are summarized in Fig. 14. and GO 2566/10-1 (Project ID 445127149). In Fig. 14a it is evident that all discussed shielding effects are more pronounced for ns-LAL in ethylene glycol where Funding Open Access funding enabled and organized by Projekt DEAL. shielding effects strongly depend on the interface layer that forms along the target surface under liquid flow conditions, as Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adap- indicated by the Schmidt number. By calculating dimension- tation, distribution and reproduction in any medium or format, as long as less numbers (i.e., Reynolds, axial Peclet, and Schmidt num- you give appropriate credit to the original author(s) and the source, pro- ber), it was shown that the flow profile is fully laminar (as vide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included usual during LAL in liquid flow) and that the viscosity of the in the article's Creative Commons licence, unless indicated otherwise in a liquid controls the accumulation of both NPs and persistent credit line to the material. If material is not included in the article's bubbles within an interface layer. Creative Commons licence and your intended use is not permitted by Linked with the highest gas volume formation rates of up to statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this 60 cm /h, it was demonstrated that the bubble population con- licence, visit http://creativecommons.org/licenses/by/4.0/. centrates near the target surface in ethylene glycol, shielding up to 80% of the target surface. For ns-ablation in water, the J Flow Chem (2021) 11:773–792 789 urinary tract infection in hospitalized patients. 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Chem Eng Sci 58:55–69 792 J Flow Chem (2021) 11:773–792 Mark-Robert Kalus studied Stephan Barcikowski studied chemistry at the University of chemistry in Braunschweig and Duisburg-Essen and graduated Hannover, and received his PhD with a master’s degree in 2015. in Mechanical Engineering Afterwards, he started as an exter- (Materials). At the Laser nal doctoral student at Particular Zentrum Hannover, Barcikowski GmbH in the position as head of built up the Nanomaterials group, sales. During his doctoral studies and later led the institute’s from 2015 to 2020 at the Institute Materials P roces sing of Chemical Technology I at the Department. In 2010, he co- University of Duisburg-Essen, he founded the company Particular investigated the physical and GmbH. Since 2011, he chairs the chemical effects occurring during Institute of Chemical Technology laser synthesis of colloids under I at the University of Duisburg- the supervision of Prof. Stephan Essen. Stephan Barcikowski has Barcikowski. In 2021, he moved to the Jülich Research Centre operating more than 250 reviewed papers and patent files, cited over 7,000 times. as a scientific project manager in the field of corporate development. He launched the scientific video channel ‘nanofunction’ on youtube with more than 78,000 viewings. He serves editing the Journal Applied Surface Science and guest editing several Journals. Riskyanti Lanyumba studied en- vironmental engineering at the University of Technology in Bilal Gökce studied physics and Yogyakarta, Indonesia, from received his “Diplom” degree 2009 to 2013, where she recieved from RWTH Aachen University her bachelor’s degree. From 2014 in 2008. From 2007 to 2009, he to 2019, she studied water chem- worked on laser material process- istry at the University of ing of metals at Fraunhofer Duisburg-Essen, where she com- Institute for Laser Technology. pleted her master’s thesis at the During his Ph.D. studies at North Institute of Chemical Technology Carolina State University from I, focusing on the fundamentals of 2009 to 2012, he studied funda- laser synthesis of colloids. mental phenomena in condensed matter through ultrafast laser spectroscopy. Afterwards, he worked as a researcher on laser applications for semiconductors at the company T-Systems International. In 2014 he joined the Faculty of Chemistry at the University of Duisburg-Essen as a group leader to establish his own group focusing on functionalization of laser-generated nanoparticles and developing materials for additive manufacturing. 2021 he became a full professor at the University of Wuppertal and since then heads the Chair of Materials Science and Additive Manufacturing.

Journal

Journal of Flow ChemistrySpringer Journals

Published: Dec 1, 2021

Keywords: Laser ablation in liquids; Metal nanoparticles; Liquid flow; Gas formation; Liquid viscosity

References