Enhancement of Focused Liquid Jets by Surface Bubbles - Langmuir

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Enhancement of focused liquid jets by surface bubbles Ryosuke Yukisada, Akihito Kiyama, Xuehua Zhang, and Yoshiyuki Tagawa Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00246 • Publication Date (Web): 17 Mar 2018 Downloaded from http://pubs.acs.org on March 18, 2018

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Enhancement of focused liquid jets by surface bubbles Ryosuke Yukisada,† Akihito Kiyama,† Xuehua Zhang,∗,‡,¶,§ and Yoshiyuki Tagawa∗,† †Department of Mechanical Systems Engineering, Tokyo University of Agriculture and Technology, Nakacho 2-24-16 Koganei, Tokyo 184-8588, Japan ‡Department of Chemical and Materials Engineering, Faculty of Engineering, University of Alberta, Edmonton, Alberta T6G1H9, Canada ¶Soft Matter & Interfaces Group, School of Engineering, RMIT University, Melbourne, VIC 3001, Australia §Institue of Global Innovation Research, Tokyo University of Agriculture and Technology, Harumicho 3-8-1, Fuchu, Tokyo 183-8538, Japan. E-mail: [email protected]; [email protected]

Abstract We investigate enhancement of the velocity of focused liquid jets by surface bubbles pre-formed on the inner surface of the container. The focused jets are created from impact on a liquid-filled cylindrical tube at cavitation numbers of 0.37 (strong impact where cavitation is likely to occur on unprocessed surfaces) and 2.1 (weak impact where cavitation does not occur from the impact). The bubbles with base diameter up to hundreds of micrometers were formed via the process of solvent exchange using air-equilibrated ethanol and water. Our measurements by high-speed imaging show that at both cavitation numbers the jet velocities with the pre-formed bubbles are

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significantly higher than those without pre-formed bubbles. Furthermore, our results show that after the process of solvent exchange, a large number of expanding bubbles are observed at cavitation number of 0.37, indicating that possibly both sub-millimeter and sub-micrometer bubbles on the surface contribute to the jet velocity enhancement. At cavitation number of 2.1 surface bubbles are observed to oscillate immediately after the impact. The measurements of the liquid pressure after the impact reveal that at both cavitation numbers the negative pressure is damped by the pre-formed surface bubbles, contributing to the increase of the jet velocity. This work sheds light on the crucial role of surface bubbles on the impulsive motion of liquids. Our findings have significant implications for the focusing jet technology, opening the opportunities for jetting fragile samples such as biological samples.

Keywords Focused liquid jet, Jet velocity, Solvent exchange, Micro/Nano surface bubbles, Cavitation, High-speed imaging.

Introduction Focused liquid jet is generated from a concave liquid free surface exposed to an impulsive external forces. 1–5 This fluid dynamical phenomenon has potential to eject highly viscous liquid, 6 promising important applications in inkjet printing, fabrication of medical devices, microcircuits, functional automobile coatings and many others. A simple method for generating focused jets is known as “Pokrovski’s experiment”, 7–9 in which a test tube filled with a wetting liquid falls freely under gravity. Liquid surface in the free-falling tube forms concave shape due to the contribution of surface tension. Once the tube impacts a floor, violent reverse motion of the liquid induces formation of focused jets. Recent researches in Pokrovski’s experiment revealed that cavitation induced by an im-

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pulsive force is able to double the jet velocity. 10 The mechanism of this velocity enhancement is explained as follows: Water-hammer pressure induced by the impact keeps being trapped inside the liquid column through reflections at boundaries of air-water and solid-water. The propagating pressure wave changes liquid pressure between positive water-hammer pressure and negative one periodically with frequency ∼ 4L/c, where L and c are respectively the length of the liquid column and the speed of sound. Positive pressure accelerates the motion of liquid surface (i.e. jets) while negative pressure decelerates it. Once cavitation bubbles in liquid emerge under negative pressure, expansion of the bubbles relaxes surrounding pressure, suppressing the deceleration effect on jet velocity. The effects from cavitation bubbles lead to a jet faster than that without cavitation occurrence. Thus mutual interaction between pressure wave and bubble dynamics plays a crucial role on the jet velocity. Note that cavitation inception is expected if Cavitation number Ca is smaller than unity, 11–13 where Ca = (pr − pv )/(ρLa), pr , pv , ρ and a are respectively the reference pressure, the vapor pressure of the liquid, the density, and the imposed acceleration. However the cavitation-enhanced jet velocity is hard to be controlled since the onset of cavitation depends on serendipitous nuclei pre-existing in the system. Although in the field of acoustics bubble behavior under periodic pressure change has been extensively studied, 14–18 it remains largely unexplored how to take advantage of bubble dynamics for the relaxation of the surrounding pressure in focusing liquid jet. In this work we investigate the dynamics of pre-formed surface bubbles after the impact and their effect on the liquid pressure and the jet velocity. Surface bubbles of typically micro/nano meters at solid-liquid interfaces can be produced by a process called solvent exchange. 19–23 We refer the “pre-formed surface bubble via solvent exchange” as “SB” in this paper. In this process, the substrate surface is in contact with air-equilibrated ethanol that is then displaced by water under controlled flow conditions. 24 A transient gas oversaturation is created during the solvent exchange, due to the reduction in gas solubility with dilution by water. The as-produced SBs are stabilized by the pinning effect at the contact line and

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the mild oversaturation in water. One may also expect that on a hydrophobic surface it is more likely to produce SBs. The similar process of the solvent exchange has recently been developed for the controlled formation of surface nanodroplets where the droplet liquid oversaturation is determined by flow and solution conditions and physical and chemical properties of the surface. 25,26 In this study the timescale and length scale of jets are on respectively tens of miliseconds and tens of milimeters. These jets will be controlled by microscopic phenomena of expanding cavitation with typical timescale and length scale of respectively sub-milisecond and hundreds of micrometers. We conduct Pokrovski’s experiments with and without SBs. We measure jet velocity, pressure, and bubble dynamics to quantify the relaxation of negative pressure at both strong impact (Ca 1) to see the net effect of surface bubble dynamics. Our results reveal the crucial role of SBs on the impulsive motion of the liquids. Our findings are important for the development of focusing jet technology for deposition of fragile samples at reduced impact.

Experiments Hydrophobilization of inner surface of the tubes and surface bubble formation by solvent exchange To make SBs on the bottom part of the glass tubes and meanwhile obtain symmetric meniscus of the liquid surface, we prepared the glass tubes with bottom part hydrophobic and the top part hydrophilic. The inner wall of the bottom of the cylindrical glass tubes was hydrophobilized by two-step procedure of coating and cleaning. In brief, the entire glass tubes were soaked in 10 w% NaOH aqueous solution for 20-30 mins at 75o C, and then rinsed thoroughly with water (Mill-Q, 18.2 MΩ). The tubes were dried under a stream of air. The silane solution was prepared by dissolving choloro(dimenthyl)octadecylsilane (OTS) in hexadecane (Sigma-Aldrich, 95% ) at the concentration of 1 w%. The solution was put inside 4


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the dry glass tube. After 15 mins at room temperature, the silane solution was vacated from the tube. The inner wall of the tube was rinsed with ethanol and the entire wall of the glass tubes became hydrophobic. To make the top part of the inner wall hydrophilic, part of the coated tube was placed upside down into the solution of NaOH to clean off the coating on the top section completely while the coating on the bottom part of the tube remained intact. The partially-coated tubes were finally cleaned by water before use for impact experiments. In the liquid jet experiments, effects from surface roughness was negligible since the liquid jet generation is dominated by the motion of liquid located near the tube axis. 27 To make SBs as sketched in Figure 1 (a), we placed 1 mL of air-equilibrated ethanol in the bottom of the tube and then added water drop by drop into ethanol. As water and ethanol mixed, SBs were produced on the inner wall of the glass tube. As the level of liquid went up, the ethanol aqueous solution was taken out from the tube and more water was added till the total volume of added water reached ∼ 100 mL. By the end, the residue of ethanol in the liquid was negligible. With this upper limit of 1% ethanol, the influence from ethanol residue (if any) on surface tension and the viscosity of the liquid is safely neglected. The level of water was adjusted to 90 mm for strong impact and 25 mm for weak impact.

Experimental setups for the measurements of jet velocity, bubble dynamics, and liquid pressure The set-ups for the measurement of the velocity of jets and bubble dynamics are shown in Figure 1 (b) for strong impact and (c) for weak impact. We used a high-speed camera (Photron, Fastcam SA-X) to capture motion of the liquid jet, the entire liquid column, SBs, and cavitation bubbles simultaneously. The frame rate and spatial resolution of the camera were set at 50,000 - 90,000 frames per second and 0.1 - 0.2 mm/pixel, respectively. The tube was filled with a certain level of water and illuminated by white-light sources (LedW-BL-400×200-LLUB-Q-IR-24V, Phlox). For strong impact (Figure 1 (b)), the tube of 16 mm in diameter was suspended by an electromagnet via metal cap of the tube. When 5


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the electromagnet was turned off, the tube falled freely from the height H (defined as the distance between the bottom of the tube and the metal plate) and eventually impacted the rigid floor, resulting in the emergence of the jet from the curved free surface. To obtain weak impact by this free-falling method, duration of free fall is too short for air-water interface to obtain hemispherical shape. Thus we used another set-up for weak impact condition shown in Figure 1 (c). The tube of 10 mm in diameter was held stationary above the piston. We imposed an impact on the tube bottom by colliding the piston, leading to the formation of the jet. In this measurement, the meniscus was asymmetric after the impact, possibly due to the hydrophobic residue of the wall surface, since the the meniscus is close to hydrophobic coating area compared to the strong impact condition. Figure 1 (d) shows the set-up for the measurement of the pressure fluctuation inside a liquid for both weak and strong impact. A submerged needle-shaped hydrophone probe (Mueller Instruments, Mueller-Platte Needle Probe) was used to measure the pressure in a liquid. The probe was placed along the axis of the tube. The distance from the probe tip to the tube bottom was ∼70 mm. The pressure was recorded via an oscilloscope (Iwatsu co., ViewGo II, DS 5554-A) and analyzed by using PC. We also used a high-speed camera (Photron, Fastcam SA-X) to capture the motion of the bubbles in liquid column. The frame rate and spatial resolution of camera was set at 90,000 fps and 0.2 mm/pixel, respectively. Difference in the two setups in our experiments on weak impact case is not expected to affect the correlation between the pressure and the jet velocity. Pan, et al. 11 has shown that Ca describes universally the onset of cavitation, which is closely related to pressure fluctuation in the liquid column without influence from different experimental setups. With the same Ca values, the results of pressure measurement obtained from two different setups in our measurements can be directly compared. It is the same for liquid velocity, as shown in several papers 10,27,28 that jet velocity measurement is not influenced by the exact impacting method.

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Experimental parameters In this section we show conditions for strong or weak impact based on the cavitation number Ca =(pr − pv )/(ρLa), where pr , pv , ρ, L and a are respectively the reference pressure (i.e., atmospheric pressure), the vapor pressure of water, the density of water ∼ 1000 kg/m3 , the liquid height that we explain later, and the imposed acceleration. 11 It is noted that the vapor pressure of water was varied from 1.3 kPa to 2.3 kPa due to variation in room temperature. Such level of vapor pressure is sufficiently smaller than the atomspheric pressure pr (= 101 kPa), therefore the influence from temperature did not have significant influence on Ca = (pr − pv )/ρLa. The imposed acceleration is calculated as a ∼ U0 /∆t, 29 where U0 and ∆t are the impact velocity and the duration of the impact. Cavitation is likely to occur for Ca < 1 (strong impact) while cavitation is unlikely for Ca > 1 (weak impact). In our experiments, we varied two relevant parameters, namely the liquid height L and the impact velocity U0 . In the measurements of the pressure wave in weak impact (Ca > 1), we increased Ca by decreasing the impact velocity U0 without decreasing the liquid height L. The detailed parameter ranges for all series of experiments are listed in table 1. Table 1: Experimental parameters in the measurements of jet velocity and pressure Experiments impact L (mm) H (mm) U0 (m/s) ∆t (ms) Ca (-) Jet velocity strong 90 60 1.5 ± 0.05 0.5 0.37 ± 0.01 Pressure strong 90 60 1.5 ± 0.05 0.5 0.37 ± 0.01 Jet velocity weak 20-25 1.5 ± 0.4 0.6 2.1 ± 0.4 Pressure weak 90 4-8 0.39 ± 0.07 0.5 1.14 ± 0.02

Data analysis From analysis of high-speed images, the liquid height L, the impact velocity U0 , and the jet velocity Vj were measured. We calculated Vj from the displacement of free surface divided by the measuring duration (10 ms in strong condition, 1 ms in weak condition) and U0 from the displacement of the tube using MATLAB image processing toolbox. Non-dimensional jet velocity was the measured jet velocity Vj normalized by U0 . To analyze the bubble size 7


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as a function of time, we calculated the area-equivalent radius of the bubble from binarized images taken from the side view of the tube. The image distortion due to the tube curvature was not considered. The R∗ was the bubble radius normalized by initial bubble radius. For pressure measurement, we apply the bandpass filter (1 kHz - 20 kHz) to raw data from the hydrophone to capture the signal of the pressure wave expected to be c/4L ∼ 4.2 kHz. p∗ was the pressure normalized by the first peak pressure. Signal processing is also performed by using MATLAB.

Results and discussion We present results and discussion first for strong impact and then for weak impact. We classify phenomena in a liquid after impact into three cases; no cavitation, natural cavitation, and SB cavitation. Here no cavitation means the absence of cavitation onset after the impact; natural cavitation, the onset of cavitation without SBs; and SB cavitation, the onset of cavitation with SBs.

Strong impact Enhanced jet velocity from strong impact Figure 2 shows representative high-speed images of liquid in the tube recorded in the measurements of strong impact. Very defined menisci were present in all three cases, which is essential for symmetric shape of the jet. The cavitation number in all the experiments is 0.37 ± 0.01. When no SB pre-exist on the inner wall, we observed two possibilities: no cavitation and natural cavitation after impact. At t∗ = 41.7(t = 10 ms) after impact, the jet velocity normalized by U0 was measured to be 2.3 ± 0.14 in no cavitation case, and to be 2.7 ± 0.11 in natural cavitation case. Here t∗ is elapsed time normalized by acoustic time scale (4L/c) where L is liquid heights. Clearly the cavitation creates increment in the jet velocity, consistent with the previous report. 10 However, the occurrence of cavitation appeared to 8


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Figure 1: (a) Schematics for surface bubbles (SBs) formation by solvent exchange. (i) the inner surface of the glass tube was coated by a hydrophobic layer. (ii) the tube was filled with a small volume of ethanol. (iii) water was injected into the tube, leading to the formation of SBs on the wall. (b) Sketch of experimental set-up for measurements of jet velocity created by strong impact. Liquid was inside a cylindrical glass tube with a steel cap. The distance from the bottom of tube to the metal plate was controlled by the jack. The L and H are the liquid height and the impact distance respectively. The liquid jet created by the impact was recorded by a high-speed camera. (c) Sketch of experimental set-up for measurements of jet velocity created by weak impact. Liquid filled cyclindrical glass tube is suspended by a holder. After the collision of the piston to the tube bottom, the tube starts to move upward. (d) A sketch of experimental set-up for pressure measurements. Bubble cavitation was triggered by impact on a metal plate. The onset of cavitation was recorded by a high speed camera. The pressure during the impact and after the impact were measured by hydrophone immersed in water. The signal from hydrophone was recorded by oscilloscope.

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be serendipitous. 30 The position of bubbles is unpredictable, either in water or on the wall, while the number of bubbles is usually very low. Significant difference was observed when SBs were on the inner wall of the tube prior to the impact. Cavitation was observed reproducibly after each impact at the cavitation number of 0.37 ± 0.01. At t∗ = 41.7, the jet velocity was measured to be 3.1 ± 0.30, significantly higher than fastest jet velocity of 2.9 created in natural cavitation case. The plot in Figure 2 (d) shows the jet velocity from 10 repeating measurements under the same impact conditions i.e., the same impact velocity U0 and the same liquid height L. Although, in some runs the jet velocity in presence of SBs is just marginally faster than the velocity from natural cavitation, very importantly, the formation of SB cavitation is observed in all runs of measurements with high reproducibility. Overall the jet velocity is clearly enhanced by SBs.

Pressure generated from strong impact The pressure at Ca = 0.37 ± 0.01 for no cavitation, natural cavitation, and SB cavitation was measured by the hydrophone as shown in Figure 3. In all three cases, the pressure first increases and then decreases. In the case of no cavitation (Figure 3 (a), black), the oscillating pressure wave propagates for an extended period. The pressure over a longer period is shown in Supporting Information. The dominant frequency for no cavitation case is fairly consistent with the theoretical prediction. 10,31 In the case of natural cavitation (Figure 3 (a), blue), the negative pressure is damped during the pressure reduction, resulting in the negative peak pressure less than that in the case of no cavitation. Remarkably, SB cavitation (Figure 3 (a), red) further damps the negative pressure. Similar trends are observed in all 10 trials (Figure 3 (c)). Averaged negative peak pressure in SB cavitation is 31% smaller than that in natural cavitation case and 46% smaller than that in no cavitation case (Figure 3 (c)). This indicates that the SBs assist the pressure relaxation significantly. The magnitude of pressure relaxation contributes to the enhancement of jet

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Figure 2: High speed images of a liquid jet induced by strong impact. (a)(b) no pre-existing surface bubbles (SBs) on the inner wall. Two scenarios could happen at this cavitation number. (a) without cavitation from the impact jet velocity is 2.3 ± 0.14. (b) cavitation occurs during the impact. The jet velocity is 2.7 ± 0.11. (c) SB on the inner wall, cavitation occurs from the pre-formed bubbles as well as uncontrolled nucleation sites. The jet velocity is 3.1 ± 0.30. At given time the jet in (a) is much lower, (c) is much higher. The zoom-in images show representative inner surface of the tube before the impact. The bubbles in (c) are ∼ 0.1 mm in the diameter. (d) Plot for jet velocity in different rounds of experiments. 11


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velocity, similar to the mechanism how natural cavitation enhances the jet velocity reported in recent literature. 10 The above result suggests that (1) the pressure relaxation mechanism of natural cavitation holds for SB cavitation and (2) SB has excellent reproducibility and high efficiency to assist jet velocity enhancement compared to natural cavitation. As seen in Figure 2, typical radius of SB is much larger than that of nuclei for natural cavitation. A larger R leads to smaller Laplace pressure (2σ/R), resulting in smaller pressure required for bubble expansion. Thus it is expected that installing SB causes the pressure relaxation at smaller negative pressure, leading higher jet velocity. Importantly, the SBs do not damp the positive pressure wave at the beginning, because the volumetric reduction of SBs under compression is extremely limited due to their small initial size.

Surface bubble dynamics after strong impact In this section, we focus on the interesting aspect of SB dynamics immediately after impact. Figure 4 (a) shows the snapshots within t∗ = 8.1 after strong impact. Compared to natural cavitation case, we notice the following distinct features: 1. More cavitation bubbles in the tube after solvent exchange are located in the region with hydrophobic coatings (see also Figure 2 (c)). 2. Extremely high number of bubbles were observed in the tube after the solvent exchange (Figure 4 (b)). These two features suggest that many bubbles with size below the resolution limit (∼200 µm) may have also formed by the solvent exchange, especially in the area with hydrophobic coatings. Those invisible microscopic bubbles grow in response to the impact and become visible. Note that some natural cavitation bubbles may emerge since overall cavitation number is already sufficiently smaller than unity. Figure 4 (a) also shows that large SBs first shrank, and then rapidly expanded while many other bubbles form in the liquid. The expansion of the large SBs are synchronized with the formation of small bubbles. At around t∗ =3.8, small bubbles disappeared, meanwhile the large SBs still keep growing. Sometime after the disappearance of small bubbles, the large SBs collapsed before the next round of new cavitation bubbles form around t∗ =7.8. This

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Figure 3: (a) Plot of pressure as a function of time. Non-dimensional pressure is the measured pressure normalized by the first peak pressure after impact. Non-dimensional time is the elapsed time normalized by time scale (4L/c) where L is liquid heights, c is speed of sounds. Cavitation leads to relaxation of pressure in water after impact. Negative pressure is less for SB cavitation. (b) is the zoom-in of the dotted region in (a). (c) Plot for non-dimensional negative pressure in different rounds of experiments.

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phenomenon suggests that the large SBs dominate the lifetime of all cavitation bubbles. In natural cavitation case, the timing for rebound of all bubbles and the timing for radiating strong shockwaves is synchronized. In SB cavitation case, all cavitation bubbles rebound after the collapse of large-SB-sourced cavitation. It indicates that the dynamics of pre-formed cavitation bubbles, which have longer lifetime, determines the dynamics of all cavitation bubbles, as well as the pressure inside a liquid.

Weak impact Enhanced jet velocity from weak impact The SB enhancement of focused jets becomes even more pronounced at a larger cavitation number of 2.1 ± 0.4. With this cavitation number, the impact is sufficiently weak so that no cavitation in the liquid could be created by impact as shown in Figure 5 (a). However, SB cavitation was observed every time after impact as shown in Figure 5 (b), which was desirable for enhanced jet velocity. Remarkably at t∗ = 41.7, the SB-enhanced jet velocity normalized by U0 was measured to be 2.4 ± 0.2 significantly faster than that in no cavitation case (1.8 ± 0.1). The plot in Figure 5 shows the jet velocity created from weak impact in repeating measurements. Overall weak impact with SB created 15 to 55 % faster jet. The deviation in the SB-enhanced jet velocity from each measurement was 0.19, possibly due to variation in size and number of cavitation bubbles, which is obviously originated from those of initial SBs.

Pressure and surface bubble dynamics generated from weak impact Time course snapshots in Figure 6 (a) shows the change of bubbles with time after weak impact. The bubbles first shrank at t∗ = 0.67, and then expanded rapidly, reaching their maximal size at t∗ = 1.7. We observed that, unlike the case of strong impact, visible bubbles of different sizes oscillate in the synchronized manner but rarely collapse (Figure 6 (a)). The temporal characteristics of pressure at Ca = 2.1 ± 0.4 for no cavitation and SB 14


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Figure 4: (a) Time course snapshots of bubbles upon strong impact (Ca = 0.34 ). The initial size of pre-formed SB before impact is shown at t = 0 ms. These SB first shrink and clearly become smaller at t∗ = 0.38. Then they expand at t∗ = 0.75. All the bubbles grow and expand as shown at t∗ = 1.3 and 2.5, while clear coalescence occurs among neigbouring SB. Some SB cavitation bubbles start to disappear from the top of the tube from t∗ = 3.8 ms. By the time of 7.8 all of them vanish, except the biggest SB. At the start of the second cycle of t∗ = 8.1, bubbles at this location fragment to a cloud of many tiny bubbles, presumably due to strong pinning effect, at the meantime many other cavitation bubble form again. (b) Plot of the number of bubbles formed after the impact in 10 runs of experiments. On average, there are more bubbles with presence of SB. 15 ACS Paragon Plus Environment

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Figure 5: High speed images of liquid jet from weak impact. (a) no pre-existing SBs on the inner wall. No cavitation was observed after the impact. The jet velocity is 1.8 ± 0.1. (b) pre-formed SBs on the inner wall, cavitation occurred from the pre-formed bubbles. The jet velocity is 2.4 ± 0.2. At given time the jet position from the bottom of the tube in (a) is lower than that in (b). The zoom-in images show representative inner surface of the tube before the impact. The bubbles in (b) could be observed. (c) The jet velocity created from weak impact with and without pre-formed SBs. The velocity is clearly faster in the former case.

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cavitation (Figure 6 (b)) is overall similar to that observed for strong impact case (Ca = 0.37 ± 0.01). Regardless of the onset of cavitation, the pressure first increases and then decreases. In the case of no cavitation (Figure 6 (b), black), the oscillating pressure wave propagates for an extended period. In the case of SB cavitation (Figure 6 (b), red), the negative pressure is damped during the pressure reduction, resulting in the negative peak pressure less than that in the case of no cavitation. The temporal evolution of pressure corresponds to the change in the radius of one selected bubble in Figure 6 (c) suggesting that the pressure and volumetric change of bubble are synchronized. The negative peak pressure is damped due to the expansion of the bubbles. Appearance of positive pressure peaks corresponds to violent volumetric change at the bubble rebound. The lifetime of a bubble shown in Figure 6 (c) was approximately 0.65 ms. In this case, overall volume equivalent radius R = (3V /4π)1/3 ∼ 1.4 mm. Estimated lifetime of bubbles p based on Rayleigh’s equation (= 1.83R ρ/pr ) is 0.25 ms. Although this estimation is for very simplified situation, the order of estimated value matches the experimental value. It should be emphasized that the dynamics of SBs can trigger the pressure relaxation: Once the pressure in the liquid drops negative, the pre-formed air-contained bubble expands and relax surrounding pressure as observed in strong impact case. Such pressure relaxation causes the jet velocity enhancement (Figure 5 (c)) as well. Therefore the installation of SB leads to the enhancement of focused liquid jets even if the impact is sufficiently weak (Ca > 1), promising potential applications for jetting fragile samples such as biological samples with less impact. Alternatively, to achieve the same jet velocity, pre-formed SBs may reduce the energy input required to create the jet. Further control of the volume of SBs could be achieved by employing micropatterned inner surface. 32 33

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Figure 6: (a) Time course snapshots of bubbles upon weak impact (Ca = 1.3). The initial size of pre-formed SBs before impact is shown at t∗ = 0. These SBs shrink and become invisible at t∗ = 0.67. Then the bubbles grow and become much larger at t∗ = 1.7. The bubbles shrink again at t∗ = 3.4 and expand at t∗ = 4.4. The cycle of bubble oscillation repeat in the same fashion till the end of the jet measurement. (b) The non-dimensional pressure as a function of non-dimensional time. Non-dimensional pressure p∗ is the measured pressure normalized by the first peak after the impact. Non-dimensional time t∗ is the elapsed time normalized by time scale (4L/c). (c) The non-dimensional bubble radius measured by the number of pixel and non-dimensional pressure measured by hydrophone in SB cavitation case as a function of non-dimensional time. Non-dimensional bubble radius R∗ is the radius normalized by the initial bubble radius.

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Conclusion In summary, we experimentally investigated the enhancement of focused liquid jets by surface bubbles. These surface bubbles were introduced on the inner wall of glass tubes using solvent exchange technique. Remarkably, the jet velocity is faster when the surface bubbles are present on the wall after both strong and weak impact. Our pressure measurements together with bubble size analysis suggested that the enhanced jet velocity is attributed to damping effect from surface bubbles on the pressure wave in the liquid generated by the impact. The negative peak pressure was significantly relaxed by the presence of bubbles. The oscillation of bubble size in high speed images was synchronized with the temporal evolution of pressure at weak impact. This study is among the first to highlight altered bulk liquid behavior as a consequence of surface nanobubbles. Our findings demonstrate that pre-formed surface bubbles can be potentially applied for jetting fragile samples at reduced impact.

Acknowledgement This work is financially supported by JSPS KAKENHI Grant Numbers 26709007, 16J08521, and 17H01246. The authors thank Brendan Dyett in RMIT University for assistance in surface modification.

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Enhancement of Focused Liquid Jets by Surface Bubbles - Langmuir

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