Published papers

Shock propagation through a bubbly liquid filled in a deformable cylindrical tube is considered. A steady shock theory that accounts for the compressibility of the structure, the liquid and the bubbles is reviewed. Experiments are conducted in which a free-falling steel projectile impacts the top of an air/water mixture in a polycarbonate tube, and stress waves in the tube material are measured. The experimental data indicate that the linear theory cannot properly predict the propagation speeds of shock waves in mixture-filled tubes; the shock theory is found to more accurately estimate the measured wave speeds.

This paper demonstrates the dynamic stressing of viscous liquids in microfluidic channels. An infrared laser pulse is focused within the testing liquid in a microfluidic channel and a spherical shock wave near the interface is created. The shock is reflected by the free surface due to acoustic impedance mismatch. The displacement of the free surface in hundred nanoseconds is captured by a developed double-exposure optical system. The tensile strength can be estimated by using a series of results from different distances between the bubble and the free surface. This study has a great potential in the optical breakdown of biomaterials.

This paper presents a liquid jet caused by a laser-induced cavitation bubble in thin films (height varies from micro to nanometers). The dynamics is highly depending on the thin film’s dimension and viscosity. The penetrating axial jet can be enhanced by increasing the laser energy or decreasing the distance between the laser focus and the targeted gas bubble surface. In the microchannel, strong shear stress ruptures part of the gas bubble and Rayleigh-Plateau instability further shattered it into small bubbles. While in the nanochannel, the nanojets can accelerate femtoliter liquids with thickness of hundreds of nanometers.

This paper demonstrates the dynamic stressing of viscous liquids in microfluidic channels. An infrared laser pulse is focused within the testing liquid in a microfluidic channel and a spherical shock wave near an air-liquid interface is created. The shock is reflected as a tension wave by the free surface due to the acoustic impedance mismatch. The displacement of the free surface within hundred nanoseconds is captured by a developed double-exposure optical system. The tensile strength can be estimated by extrapolating a series of shock pressure approximation at several different distances between the bubble and the free surface to the measurement accuracy. This study has a great potential in the optical breakdown of biomaterials.

The rupture of liquids with decreasing pressure (so-called cavitation) has been studied by many researchers. The cavitation inception strongly depends on the purity of liquids; the inception process with minimal effects of contaminants in the liquid phase is termed homogeneous nucleation. However, the measured tensile strength of water is very scattered because of difficulties in carefully controlling the water quality. In this work, we experimentally study the homogeneous nucleation of water, with careful control of its purity, contained in a microfluidic system. In the system, an air-water interface is created with which a laser-induced shock wave collides. The shock reflection at the interface yields tensile waves, which can cause cavitation. Photographic observations of the resulting cavitation bubbles in the neighborhood of the interface will be presented.

In underwater explosions, the interaction of a shock wave with submarine structure often leads to strong tension in the liquid that can induce cloud cavitation. As a canonical example, we numerically study the dynamics of cloud cavitation that arises in one-dimensional, shock-structure interaction problems; an underwater shock is replicated using Cole’s empirical formula and its interaction with a movable rigid plate (i.e., Taylor’s free plate) is considered. The flow is modeled using homogeneous bubbly flow conservation equations in which interaction between bubbles and the averaged flow is incorporated through the mixture-averaged pressure field. To capture unsteadiness associated with bubble dynamics, the averaged equations are closed with a Rayleigh-Plesset-type equation. The resulting cloud cavitation is assumed to result from heterogeneous cavitation inception with single-sized, gas bubble nuclei. In solving the equations, a finite volume method with shock capturing is used and a time-step splitting technique is adopted to resolve stiffness owing to bubble-dynamic sources. One-dimensional cloud cavitation induced by the shock-structure interaction is simulated to clarify the fundamental mechanisms of the inception and the subsequent collapse of the cloud cavitation.

Shock propagation through a bubbly liquid filled in a deformable cylindrical tube is considered. A steady shock theory that accounts for the compressibility of the structure, the liquid and the bubbles is reviewed. Experiments are conducted in which a free-falling steel projectile impacts the top of an air/water mixture in a polycarbonate tube, and stress waves in the tube material are measured. The experimental data indicate that the linear theory cannot properly predict the propagation speeds of shock waves in mixture-filled tubes; the shock theory is found to more accurately estimate the measured wave speeds.

When a droplet impacts a solid surface at high speed, the contact periphery expands very quickly and liquid compressibility plays an important role in the initial dynamics and the formation of lateral jets. The high speed impact results in high pressures that can account for the surface erosion. In this study, we numerically investigated a high speed droplet impacts on a solid wall. The multicomponent Euler equations with the stiffened equation of state are computed using a FV-WENO scheme with an HLLC Riemann solver (Johnsen & Colonius 2006) that accurately captures shocks and interfaces. In order to compare the available theories and experiments, 1D, 2D and axisymmetric solutions are obtained. The generated pressures, shock speeds, and the lateral jetting generation are investigated. In addition, the effect of target compliance is evaluated.

A method for on-demand droplet coalescence and mixing is realized using laser-induced cavitation bubbles. Water droplets are formed using a microchannel with the coupled T-junction structure. Laser-induced cavitation bubbles are generated inside water droplets to deform the droplets, which break up the continuous phase between the droplets and trigger the fusion process. The diffusion dominated mixing process after the fusion is observed. For two 0.11 pl droplets, they have been mixed in 800 µs to a high mixing quality.

An experimental technique is developed to measure the rupture strength of water in a microfluidic channel. A transparent channel is filled with distilled water, partially leaving an air-water interface. An infrared laser pulse is focused within the liquid to create a spherical shock wave near the interface. The shock reflects, due to acoustic impedance mismatch, as a tension wave. The liquid becomes stretched and at the critical tension ruptures with the nucleation of vapor bubbles. The bubble nucleation is captured using a CCD camera with an optical delay and short exposure times. Reproducible observations of the bubble nucleation are obtained, confirming the realization of homogeneous nucleation. Euler flow simulation estimates a tensile stress threshold of -60 MPa, which is the largest reported tension among dynamic measurements.

Cavitation phenomena in real world are typically confined by one or more boundaries. Confining cavitation in small channels allows to study their interaction with cells, the formation of emulsions, and even sonochemical reactions in far greater detail as it would be possible in the bulk. However, it was expected that boundary layers will hinder bubble collapse more and more as the structure sizes are reduced. In this presentation the channel size is reduced even further, thus from microfluidic to nanofluidic channels. In microfluidic channels cavitation bubbles are generated with focused laser pulses and with acoustic waves. Acoustic cavitation in micrometer sized allows the formation of homogeneous emulsions, rapid rupture of cells (yeats and bacterias), and the dispersion of nanoparticles. While laser induced cavitation bubbles allow the study of bubble dynamics and bubble interaction in nanofluidic channels. In particular we will present experimental results on the dynamics of single bubbles and bubble-bubble interaction in nanochannels.

This paper presents the study of cavitation bubbles created by focusing a pulsed laser into an nanochannel filled with a light-absorbing liquid. The resulting bubbles are two-dimensional and sandwiched between the channel walls at the top and bottom. The bubble collapse is found to have two stages: (1) the collapse of a vapor bubble and (2) a rather slow decay associated with the mass diffusion of non-condensible gases. The bubble dynamics is examined with different channel heights and surface hydrophobicity. The measured bubble evolution can be reproduced by the Hele-Shaw-type model that accounts for the effect of shear stresses from the channel wall.

We report on observations of a laser-induced shock in a sessile water drop and the interaction of the shock and material interfaces. A short laser pulse is focused into a water drop on a glass surface and images of the shock-interface interaction are captured using a CCD camera with very short exposure times. It is interesting to observe that small vapor bubbles nucleate, on the glass surface, near the shock front at which the liquid pressure is definitely positive; these bubbles disappear shortly after the nucleation. We explain this as shear-induced cavitation; shear stress within the boundary layer that forms after the shock passage may rupture the liquid at the surface. Surface cavitation due to wave interaction with air-water interfaces is also studied.

High-speed droplet impact is a phenomenon of importance in physical cleaning using jetting liquid flow. Here, we numerically study high-speed droplet impact against a deformable wall in order to evaluate the effect of cavitation that possibly occurs within the droplet. First, we simulate the interaction between the droplet and the wall based on multicomponent Euler equations. Second, we examine the dynamics of a cavitation bubble heterogeneously nucleated from a preexisting bubble nucleus initially located in the droplet center. Then, Rayleigh-Plesset-type calculations are performed in one-way-coupling manner; the simulated pressure evolution within the droplet is used as far-field pressure in the bubble dynamics calculations. The simulation shows that cavitation can possibly occur within the droplet impacting on the wall. It is also found that radiated pressure from the collapsing cavitation bubble can be dominating over the water hammer pressure due to the droplet impact. This implies that cavitation may possibly play a role in enhancing the efficiency of physical cleaning based on droplet impact.

Experiments are performed to examine bubble growth driven by mass diffusion in tap water in which oxygen is dissolved beyond its saturation level; the water is called “supersaturated water” in this paper. To prepare such supersaturated water, we use a commercial fine bubble generator of spiral flow type. Tap water in a tank open to atmosphere is circulated through the generator that injects oxygen gas in the water circulation. The injected oxygen results in micron-sized bubbles, which are expected to effectively dissolve into the water with Laplace pressure augmented. After the aeration, large-sized bubbles escape due to buoyancy from the water tank. Interestingly, the resulting water, even though it looks transparent like usual tap water, exhibits a supersaturation state detected by a dissolved oxygen meter; such a supersaturation state lasts for hours (or even a couple of days). To further support the detected supersaturation, we perform experiments in which a microscope glass slide is inserted into the water in order to intentionally trigger heterogeneous nucleation on the glass surface (i.e., gaseous cavitation). The nucleated bubble attached on the surface is found to show gradual growth that agrees with the classical theory of bubble growth driven by mass diffusion. This means that the water is indeed supersaturated with oxygen even at one atmosphere. The relevance to the existence of bulk nanobubbles is also discussed.

Viscoelastic effects on spherical bubble oscillations under ultrasound irradiation are studied with experiments and numerical simulations. In the experiments of high-speed camera visualization, a setup has been designed to observe radial motion of a single spherical bubble in porcine skin gel. A spherical bubble is created by focusing an infrared laser pulse into the gel whose viscoelasticity is controlled by changing its mass concentration. The peak amplitude of radial oscillations of the bubble is found to be suppressed as the gel concentration increases under its resonant condition. The experimental results are used to validate the viscoelastic constitutive equation; numerical simulations are performed using a Rayleigh–Plesset-type model of spherical bubble dynamics coupled with the Voigt model, which is a linear viscoelastic constitutive equation to numerically replicate gel properties. It follows from comparisons between the experiments and the simulations that the present model cannot reproduce both amplitudes and periods of the measured oscillations simultaneously. This means that effect of relaxation or nonlinearity in the constitutive relation of the gel, which the Voigt model ignores, needs to be incorporated to more properly predict the bubble dynamics.

Erosion induced by high-speed droplet impact is one of substantial issues in fluid machineries such as steam turbines which are exposed to liquid spray or jet flow. The erosion mechanism of droplet impact is often considered to result from water-hammer events. Moreover, recent studies explain the roll of cavitation in the erosion problem. Cavitation within the droplet can in principle occur due to acoustic impedance mismatch between the water-hammer shock and the droplet interface. Cavitation accompanied by droplet impact has been observed in previous experiments. In this study, we numerically study the roll of cavitation within a cylindrical droplet colliding with a deformable wall, which is modeled by multicomponent Euler equations. The dynamics of cavitation bubbles heterogeneously triggered from the droplet impact are determined in one-way-coupling manner according to the far-field liquid pressure that is obtained from the Euler flow simulation. It is confirmed in the simulations that the droplet impact leads to cavitation bubble nucleation from micron-sized gas bubble nuclei. Furthermore, it is found that the bubble collapse radiates strong acoustic waves whose amplitude overwhelms the water-hammer pressure at the impact. This suggests that there is a need to account for loading from cavitation bubble collapse as well as water-hammer events when it comes to studying material erosion in high-speed droplet impact problems.

This paper is concerned with observations of the translation of a microbubble (80 μm or 137 μm in radius) in a viscoelastic medium (3 w% gelatin), which is induced by acoustic radiation force originating from 1 MHz focused ultrasound. An optical system using a high-speed camera was designed to visualize the bubble translation and deformation. If the bubble remains its spherical shape under the sonication, the bubble translation we observed can be described by theory based on the Voigt model for linear viscoelastic solids; mechanical properties of the gelatin are calculated from measurements of the terminal displacement under the sonication.

We experimentally study a shock-bubble interaction problem in a viscoelastic solid, which is relevant to shock wave lithotripsy. A gas bubble is produced by focusing an infrared laser pulse into gelatin. A spherical shock is then created, through rapid expansion of plasma that results from the laser focusing, in the vicinity of the gas bubble. The shock-bubble interaction is recorded by a CCD camera with flash illumination of a nanosecond green laser pulse. The observation captures cavitation inception in the gelatin under tension that results from acoustic impedance mismatching at the bubble wall. Namely, the shock reflects at the bubble interface as a rarefaction wave, which induces the nucleation of cavitation bubbles as a result of rupturing the gelatin.

Physical cleaning techniques are of great concern to remove particulate contamination because of its efficiency and low environmental impact. One of the promising candidates is based on water jets that often involve fission into droplet fragments and collide with target surfaces to which contaminant particles stick. Particle removal is believed to be achieved by droplet-impact-induced shear flow at the surfaces. However, it is very difficult to clarify such a phenomenon from experiments. Here, we simulate high-speed droplet impact on wet surfaces in order to investigate shear flow created at the surfaces after the impact. The problem is modeled by the compressible Navier-Stokes equations where bulk viscosity is omitted and is numerically solved by a finite volume method that is designed to capture both shocks and interfaces in accurate and robust fashion. As an example, we present the result for the impact of a spherical water droplet (200 μm in diameter) at velocity 50 m/s against a (no-slip) rigid wall that is covered with a water film (100 μm in thickness). Contaminant particles are assumed to stick to the wall via van der Waals force and their size is assumed too small to disturb the fluid flow. This allows us to derive a particle removal criterion in a one-way-coupling manner. To be specific, we discuss the particle removal from a comparison between particle adhesion and the fluid dynamics force obtained from the simulation.