Surface cleaning from laser-induced cavitation bubbles

APPLIED PHYSICS LETTERS 89, 074102 2006Surface cleaning from laser-induced cavitation bubblesClaus-Dieter Ohl, a Manish Arora, Rory Dijkink, Vaibhav Janve, and Detlef LohseFaculty of Science, Physics of Fluids, University of Twente, 7500 AE Enschede, The NetherlandsReceived 26 April 2006; accepted 28 June 2006; published online 14 August 2006When bubbles expand and collapse close to boundaries, a shear flow is generated which is able toremove particles from the surface, thus locally cleaning it. Here the authors demonstrateexperimentally with microparticle tracking velocimetry that the strongest forcing of particles occursduring a very brief time interval of the bubble oscillation period. During this interval a jet flowimpacts and spreads radially along the surface, thus transporting the particles with it. © 2006American Institute of Physics. DOI: 10.1063/1.2337506The study of the flow induced from cavitation bubblesoscillating close to a surface is motivated by the ability of thebubbles to remove attached dirt particles, thus cleaning thesurface. An example of the cleaning ability of a bubble ispresented in Fig. 1. Here, the surface is covered with a thinlayer of grease Vaseline mixed with colored particles servingas dirt particles. The left image in Fig. 1a depicts thesurface before it is exposed to a single cavitation bubble.After the bubble oscillation has ceased, a disk shaped areawith a diameter of approximately 0.4 mm is cleared from thegrease-particle mixture, see center image in Fig. 1a. After asecond bubble has collapsed at the same location, thecleaned area is enlarged and most of the small grease islandshaving remained after the first bubble collapse are now alsoremoved right image of Fig. 1.Although cavitation bubble dynamics close to surfaces isstudied now for decades numerically 1 and experimentally, 2yet the flow along on the surface has not been investigated ingreater detail. In this letter the mechanism of acceleration ofparticles close to the surface is experimentally determined bycorrelating the bubble dynamics with the particle motion.The bubbles in the experiments are generated by focusingan intense laser pulse from a Q-switched neodymiumdopedyttrium aluminium garnet laser 1064 nm wavelengthwith 6 ns duration and 15 mJ laser energy with an aberrationminimized lens system from above into a water filledcuvette. A microscope slide built into the bottom of the cuvetteserves as the material surface.The bubble dynamics close to a boundary is recordedfrom the side with a high speed camera at a framing rate of112 500 frames/s resolution of 12864 pixels and selectedframes are depicted in Fig. 2a. The boundary is locatedat the bottom of each frame. We can classify the bubblemotion into four dynamical stages which are represented bythe individual rows of Fig. 2a. The four stages from top tobottom are bubble expansion, bubble collapse, jetting, andthe reexpansion of the bubble. Initially, the bubble expandswithin 100 s to its maximum diameter of 2 mm. Due to theproximity to the boundary the bubble flattens at the bottom.Thus it obtains a nonspherical shape at maximum expansion.During shrinkage the upper more curved part of the bubbledevelops a liquid jet flow 3,4 through the center of the bubbletowards the rigid boundary causing the mushroomlike shapeat time of 213 s in Fig. 2. This jet flow impacts betweena Electronic mail: c.d.ohl@utwente.nl204 and 213 s onto the boundary. The minimum volume ofthe bubble is reached around 216 s after bubble creation.The bubble then reexpands with a roughened surface and astructure consisting of microbubbles pointing upwards. 5The jet flow inside the bubble originates from a flowfocusing mechanism. 3–10 In Fig. 2b the jet is visualizedwith diffuse stroboscopic illumination. The left frame depictsthe bubble at time of 204 s with the jet crossing through thecenter of the bubble. The jet impact on the boundary is photographedin the right frame of Fig. 2b. Here, the jet haspenetrated through the lower bubble wall and is surroundedby an annular ring, presumably a vapor and gas mixture. Thevelocity of the jet at impact is estimated from these twoframes to be approximately 40 m/s.The flow tangential and very close to the surface is studiedwith streak images from fluorescent particles 8 m indiameter, red fluorescent polymer microspheres, Cat. No.36-3, Duke Scientific Corp., Palo Alto, CA, USA, density of1.1910 3 kg/m 3 . Here, the particles are not embedded in agrease matrix but sediment freely onto the microscope slide.Therefore, they are injected gently into the liquid about aminute before the experiment to allow for their settling.The fluorescence is excited with a 50 W mercury lampfrom below. Excitation and imaging of the particles are donewith a 5 microscope objective on an inverted microscopeAxiovert CF40, Zeiss GmbH, Göttingen, Germany. The excitationlight green is seperated from the fluorescent imagered with a dichroic mirror and a long pass filter. An additionallaser line filter just below the microscope slide protectsthe microscope optics. The particle trajectories are recordedwith a sensitive camera Sensicam QE, PCO GmbH,Kelheim, Germany, 12801024 pixels. Therefore, the camerais opened for a variable time at specific moments of thebubble dynamics. Due to the low light level four neighboringpixels are summed up, thus reducing the optical resolution to5 m per pixel. By this method we obtain for a single cavitationbubble streak images from approximately 100–500particles. A timing unit controls the start of the recording, theexposure duration, and the trigger of the laser.Figure 3 depicts an example of particle streaks for thefour stages corresponding to the rows of Fig. 2. The brightspot in the center is due to the plasma emission during thebubble creation and marks the center of the bubble projectedonto the surface. During expansion the particles move approximatelya similar distance over the exposure time of100 s outwards Fig. 3a as they move inwards Fig. 3bduring shrinkage. Thus volume pulsation does not produce0003-6951/2006/897/074102/3/$23.0089, 074102-1© 2006 American Institute of PhysicsDownloaded 18 Aug 2006 to 130.89.86.60. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

074102-2 Ohl et al. Appl. Phys. Lett. 89, 074102 2006FIG. 1. Color online Layer of grease mixed with coloredparticles before left frame and after centerframe it has been exposed to a cleaning bubble. Therightmost frame depicts the surface after a secondbubble has been generated at the same location.Thereby, some remains of the grease from the first shotare removed and the size of the cleaned area isenlarged.considerable transport of the particles. However, after theimpact of the jet Fig. 3c all particles become largely displacedand the center region is cleared completely from particles,see Fig. 3d.The averaged velocities during the different stages of thebubble dynamics are quantitatively analyzed by measuringthe streak lengths from the images. Figure 4 presents themodulus of the averaged velocity plotted as a function of theinitial distance from the center r=0. In the expansion andcollapse stages from 0 to 100 s and from 100 to 200 s,respectively the maximum of the averaged radial velocitiesis around ±1 m/s with positive velocities during expansionand negative during shrinkage. The velocity drops to zero forr=0 which is plausible as the center is a stagnation point.Only during the time interval of the jet impact and radialspreading 200−230 s considerably higher particle velocitiesof 10 m/s are found. The highest velocities are reachedvery close to the region of jet impact. In the streak images30 s later 230−260 s all particles within a distance of0.9 mm from the center are washed away, demonstrating thatit is the induced jet flow which is responsible for the cleaningeffect.In conclusion, we have found that the most prominentboundary layer flow on the substrate occurs when the jetflow spreads radially on the surface. It is this relatively shorttime interval where the highest shear stress is exerted and thesurface becomes cleaned. Possibly, a similar cleaning mechanismmight be responsible in an ultrasonic water bath. There,many bubbles are driven into resonant pulsations by continuousacoustic waves. When they are driven to large enoughoscillations they can undergo a violent collapse, and in thepresence of a boundary they develop a jet. 11 Although, thelifetime of the bubble studied in this letter would correspondto a driving frequency of only a few kilohertz in an ultrasoundbath, we suggest that the cleaning mechanism is similarfor smaller bubbles, thus for higher and more relevantfrequencies. Still, further research has to be undertaken tocompare the importance of the jet induced shear flow withother proposed cleaning mechanisms. 12–16This work was supported by grants from the “Stichtingvoor Fundamenteel Onderzoek der Materie FOM” and the“Nederlandse Organisatie voor Wetenschappelijk OnderzoekNWO.”FIG. 2. Color online a Selected frames from a highspeedseries of a laser-induced cavitation bubble near aboundary. The time in the upper part of each frame isthe framing time of the bubble after generation with thelaser. The four rows relate to four stages of the bubbledynamics: initial bubble expansion, bubble collapse, jetimpact, and reexpansion. b Stroboscopic pictures visualizingthe liquid jet within the bubble left and impactingon the boundary right.Downloaded 18 Aug 2006 to 130.89.86.60. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp