Published papers

Many applications such as ultrasonic cleaning or sonochemistry use the ability of bubbles to oscillate and drive liquid flow. But bubbles have also received attention in porous media, where drying may cause cavitation, a phenomenon occurring in plant tissues. Here we explore the dynamics of cavitation bubbles when the liquid is fully entrapped in an elastic solid, using light scattering, laser strobe photography and high speed camera recordings. Our experiments show unexpectedly fast bubble oscillations in volume. They depend on the confinement size and elasticity, which we explain with a simple model where liquid compressibility is a key parameter. We also observe rich non-spherical dynamics, with ejection away from the walls and bubble fragmentation, which reveal extreme fluid motion at short timescales.

Here we report the first demonstration on droplet generation from the transformation of a single bubble in a nanofluidic channel by a laser-induced jet. A viscous two-dimensional Rayleigh–Plesset-type model is derived to describe the bubble dynamics in the nanofluidic channel, which accounts for the effect of shear stresses from the channel wall. The droplet generation (number and volume) is investigated experimentally by controlling the jet velocity via laser energy and distance. This study expands the understanding of jetting in the nanofluidic channel and demonstrates a novel method for femtoliter-volume single or multiple droplet formation. It is envisioned that this work will open new doors in on-demand generation of nanodroplets.

In this paper, for the first time, the tensile strength of water is directly measured using an optofluidic chip based on the displacement of air–water interface deformation with homogeneous nucleation. When water in a microchannel is stretched dynamically via laser-induced shock reflection at the air–water interface, the shock pressures are determined by measuring the displacements of the deformed interface. Observation of the vapor bubbles is used as a probe to identify the cavitation threshold with a critical distance, and the tensile strength of water at 20 °C is measured to be −33.3 ± 2.8 MPa. This method can be extended to investigate the tensile strength of other soft materials such as glycerol, which is measured to be −59.8 ± 10.7 MPa at 20 °C.

Acoustically forced oscillation of spherical gas bubbles in a viscoelastic material is studied through comparisons between experiments and linear theory. An experimental setup has been designed to visualize bubble dynamics in gelatin gels using a high-speed camera. A spherical gas bubble is created by focusing an infrared laser pulse into (gas-supersaturated) gelatin gels. The bubble radius (up to 150 μm) under mechanical equilibrium is controlled by gradual mass transfer of gases across the bubble interface. The linearized bubble dynamics are studied from the observation of spherical bubble oscillation driven by low-intensity, planar ultrasound driven at 28 kHz. It follows from the experiment for an isolated bubble that the frequency response in its volumetric oscillation was shifted to the high frequency side and its peak was suppressed as the gelatin concentration increases. The measurement is fitted to the linearized Rayleigh–Plesset equation coupled with the Voigt constitutive equation that models the behavior of linear viscoelastic solids; the fitting yields good agreement by tuning unknown values of the viscosity and rigidity, indicating that more complex phenomena including shear thinning, stress relaxation, and retardation do not play an important role for the small-amplitude oscillations. Moreover, the cases for bubble-bubble and bubble-wall systems are studied. The observed interaction effect on the linearized dynamics can be explained as well by a set of the Rayleigh–Plesset equations coupled through acoustic radiation among these systems. This suggests that this experimental setup can be applied to validate the model of bubble dynamics with more complex configuration such as a cloud of bubbles in viscoelastic materials.

We investigate the motion of a gas–liquid interface in a test tube induced by a large acceleration via impulsive force. We conduct simple experiments in which the tube partially filled with a liquid falls under gravity and hits a rigid floor. A curved gas–liquid interface inside the tube reverses and eventually forms a so-called focused jet. In our experiments, there arises either vibration of the interface or an increment in the velocity of the liquid jet, accompanied by the onset of cavitation in the liquid column. These phenomena cannot be explained by a considering pressure impulse in a classical potential flow analysis, which does not account for finite speeds of sound or phase changes. Here we model such water-hammer events as a result of the one-dimensional propagation of a pressure wave and its interaction with boundaries through acoustic impedance mismatching. The method of characteristics is applied to describe pressure-wave interactions and the subsequent cavitation. The model proposed is found to be able to capture the time-dependent characteristics of the liquid jet.

Erosion due to high-speed droplet impact is a crucial issue in industrial applications. The erosion is caused by the water-hammer loading on material surfaces and possibly by the reloading from collapsing cavitation bubbles that appear within the droplet. Here, we simulate the dynamics of cavitation bubbles accompanied by high-speed droplet impact against a deformable wall in order to see whether the bubble collapse is violent enough to give rise to cavitation erosion on the wall. The evolution of pressure waves in a single water (or gelatin) droplet to collide with a deformable wall at speed up to 110 m/s is inferred from simulations of multicomponent Euler flow where phase changes are not permitted. Then, we examine the dynamics of cavitation bubbles nucleated from micron/submicron-sized gas bubble nuclei that are supposed to exist inside the droplet. For simplicity, we perform Rayleigh–Plesset-type calculations in a one-way-coupling manner, namely, the bubble dynamics are determined according to the pressure variation obtained from the Euler flow simulation. In the simulation, the preexisting bubble nuclei whose size is either micron or submicron show large growth to submillimeters because tension inside the droplet is obtained through interaction of the pressure waves and the droplet interface; this supports the possibility of having cavitation due to the droplet impact. It is also found, in particular, for the case of cavitation arising from very small nuclei such as nanobubbles, that radiated pressure from the cavitation bubble collapse can overwhelm the water-hammer pressure directly created by the impact. Hence, cavitation may need to be accounted for when it comes to discussing erosion in the droplet impact problem.

Translational motion of an oscillating bubble cluster under sound irradiation is studied experimentally and is modeled in the framework of the classical approach of Bjerknes. An experimental technique is proposed to observe bubble cluster formation and its translational dynamics interacting with wall boundaries due to the secondary Bjerknes force. The translational motion observed in the experiment is modeled by extending the classical theory of Bjerknes on a single bubble; a bubble cluster is treated as a single bubble. The extended Bjerknes theory is shown to allow us to predict the overall trajectory of the cluster translating toward a wall of finite acoustic impedance by tuning acoustic energy loss at the wall. The drag force turns out to be unimportant for the translation of a millimeter-sized cluster that we observed.

An optical visualization technique is developed to study cavitation bubble nucleation that results from interaction between a laser-induced shock and a preexisting gas bubble in a 10 wt. % gelatin gel; images of the nucleated cavitation bubbles are captured and the cavitation inception pressure is determined based on Euler flow simulation. A spherical gas cavity is generated by focusing an infrared laser pulse into a gas-supersaturated gel and the size of the laser-generated bubble in mechanical equilibrium is tuned via mass transfer of the dissolved gas into the bubble. A spherical shock is then generated, through rapid expansion of plasma induced by 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 gel under tension that results from acoustic impedance mismatching at the bubble interface interacting with the shock. We measure the probability of cavitation inception from a series of the repeated experiments, by varying the bubble radius and the standoff distance. The threshold pressure is defined at the cavitation inception probability equal to one half and is calculated, through comparisons to Euler flow simulation, at −24.4 MPa. This threshold value is similar to that from shock-bubble interaction experiments using water, meaning that viscoelasticity of the 10 wt. % gelatin gel has a limited impact on bubble nucleation dynamics.

Physical cleaning techniques are of great concern to remove particulate contamination because of their low environmental impact. One of the promising candidates is based on water jets that often involve fission into droplet fragments. Particle removal is believed to be achieved by droplet-impact-induced wall shear flow. Here, we simulate a high-speed droplet impact on a dry/wet rigid wall to investigate the wall shear flow as well as water hammer after the impact. The problem is modeled by the axisymmetric compressible Navier–Stokes equations and solved by a finite volume method that can capture both shocks and material interface. As an example, we consider the impact of a spherical water droplet (200 µm in diameter) at velocity from 30 to 50 m/s against a dry/wet rigid wall. In our simulation, we can reproduce both acoustic and hydrodynamic events. In the dry wall case, the strong wall shear appears near the moving contact line at the wetted surface. On the other hand, once the wall is covered with the liquid film, the wall shear stress gets weaker as the film thickness increases—a similar trend holds for the water-hammer shock loading at the wall. According to the simulated base flow, we compute hydrodynamic force acting on small particles that are assumed to be attached at the wall, in a one-way-coupling manner. The hydrodynamic force acting on the particles is estimated under Stokes’ assumption and compared to particle adhesion of van der Waals type, enabling us to derive a simple criterion of the particle removal.

The growth rate of vegetables in plant factories can be regulated by environmental factors including light, temperature, and chemicals, which might give rise to mutation in leaf health. Here, we aim to devise a new way that allows for controlling the growth rate of plants in hydroponics as well as maintaining the product quality; we apply underwater ultrasound and dissolved oxygen supersaturation as external stimuli to plants. As an example, we examine the growth of leaf lettuce in hydroponics with exposure to 28-kHz ultrasound and dissolved oxygen supersaturation up to 36 mg/L at 20 °C. Our results show that exposure to the ultrasound of peak-to-peak pressure at 20 kPa or larger works as the growth inhibitor of the leaves and the roots, while the oxygen supersaturation as the growth promoter, without any degradation of chlorophyll in the leaves. This suggests that these external stimuli can be used in the growth control system of plant factories.

A number of acoustic and fluid-dynamic phenomena appear in ultrasonic cleaning baths and contribute to physical cleaning of immersed surfaces. Propagation and repeated reflection of ultrasound within cleaning baths build standing-wave-like acoustic fields; when an ultrasound intensity gradient appears in the acoustic fields, it can in principle induce steady streaming flow. When the ultrasound intensity is sufficiently large, cavitation occurs and oscillating cavitation bubbles are either trapped in the acoustic fields or advected in the flow. These phenomena are believed to produce mechanical action to remove contaminant particles attached at material surfaces. Recent studies suggest that the mechanical action of cavitation bubbles is the dominant factor of particle removal in ultrasonic cleaning, but the bubble collapse resulting from high-intensity ultrasound may be violent enough to give rise to surface erosion. In this paper, we aim to carefully examine the role of cavitation bubbles from ultrasonic cleaning tests with varying dissolved gas concentration in water. In our cleaning tests using 28-kHz ultrasound, oxygen-supersaturated water is produced by oxygen-microbubble aeration and used as a cleaning solution, and glass slides spin-coated with silica particles of micron/submicron sizes are used to define cleaning efficiency. High-speed camera recordings and Particle Image Velocimetry analysis with a pressure oscillation amplitude of 1.4 atm at the pressure antinode show that the population of cavitation bubbles increases and streaming flow inside the bath is promoted, as the dissolved oxygen supersaturation increases. The particle removal is found to be achieved mainly by the action of cavitation bubbles, but there exists optimal gas supersaturation to maximize the removal efficiency. Our finding suggests that low-intensity ultrasound irradiation under the optimal gas supersaturation in cleaning solutions allows for having mild bubble dynamics without violent collapse and thus cleaning surfaces without cavitation erosion. Finally, observations of individual bubble dynamics and the resulting particle removal are reported to further support the role of cavitation bubbles as cleaning agents.

We theoretically and experimentally study the quasistatic growth of bubbles in a gelatin gel under dissolved-gas supersaturation in order to examine the role of the gel elasticity in the mass-diffusion-driven process. First, we model the diffusion-driven bubble growth with the classical Epstein-Plesset approach for quasistatic bubble growth, accounting for elasticity of the medium surrounding the bubbles. Next, we devise an experimental technique to visualize the bubble growth in an air-supersaturated gel of different gelatin concentrations and to obtain the growth rate of the bubble. We show, from comparisons between the theory and experiments, that the bubble growth is hindered by the gel elasticity.

Visualization experiments were performed to study the relation between free-surface motion and bubble translation in a 1-MHz ultrasonic cleaning bath. From the visualization with a video camera, the characteristic frequencies of the free-surface oscillation (under the acoustic radiation force) and the translational velocity of cavitation bubbles (trapped via the primary Bjerknes force) were extracted, showing that there is a strong correlation between the free-surface oscillation and bubble translation. From the context of megasonic cleaning, such free-surface oscillation is expected to contribute to uniform cleaning performance with cavitation bubbles.

In order to study the role of growing cavitation bubbles in the context of ultrasonic cleaning, we perform two-dimensional, axisymmetric Navier-Stokes simulation for compressible, multicomponent flow and examine the so-called Rayleigh growth of an air bubble (with initial radius 33 µm and pressure 10 MPa) near a rigid wall. The simulation suggests that strong shear stress, which is important in physical cleaning such as particle removal, appears as a result of the bubble-growth-induced shock passage. The parametric study with varying a standoff distance of the bubble to the wall shows that the wall shear stress linearly decreases against the standoff distance.

Visualization experiments were performed to examine whether acoustic bubbles play a role in ultrasonic water flow cleaning, as in convention cleaning with ultrasonic baths. Schlieren visualization confirmed the standing-wave-like acoustic field in ultrasonic water flow that collides with a glass surface. Backlight visualization showed that cavitation bubbles appear in the water flow spreading over the glass surface. These bubbles are found to oscillate in volume and move inside film flow and thus expected to play a role as active cleaning agents.

Visualization experiments are performed to examine the role of acoustic cavitation bubbles that appear in 0.43-MHz ultrasonic water flow spreading over glass surfaces in the context of physical cleaning. The cleaning performance is evaluated using glass samples on which small silica particles are spin-coated. The visualization suggests that acoustic cavitation bubbles play a major role in particle removal as in the case of conventional cleaning with ultrasonic cleaning baths.

Large-scale cell suspension culture technology opens up opportunities for numerous medical and bioengineering applications. For these purposes, scale-up of the culture system is paramount. For initial small-scale culture, a simple static suspension culture (SSC) is generally employed. However, cell sedimentation due to the lack of agitation limits the culture volume feasible for SSC. Thus, when scaling up, cell suspensions must be manually transferred from the culture flask to another vessel suitable for agitation, which increases the risk of contamination and human error. Ideally, the number of culture transfer steps should be kept to a minimum. The present study describes the fabrication of an ultrasonic suspension culture system that stirs cell suspensions with the use of acoustic streaming generated by ultrasound irradiation at a MHz frequency. This system was applied to 100-mL suspension cultures of Chinese hamster ovary cells—a volume ten-fold larger than that generally used. The cell proliferation rate in this system was 1.88/day when applying an input voltage of 40 V to the ultrasonic transducer, while that of the SSC was 1.14/day. Hence, the proposed method can extend the volume limit of static cell suspension cultures, thereby reducing the number of cell culture transfer steps.

We experimentally observe and theoretically analyse oscillations of a spherical bubble in a gelatin gel under ultrasound irradiation to quantify viscoelastic effects on the nonlinear bubble dynamics. A bubble nucleus is generated by focusing a laser pulse into a 6 wt% gelatin gel supersaturated with dissolved air, which enables us to control the bubble radius at mechanical equilibrium via influx of the gas air into the bubble. Linearized and finite-amplitude oscillations of the bubble are driven by 28 kHz ultrasound and recorded by a high-speed camera; the resonance curves of the oscillation amplitude as a function of the equilibrium radius are constructed for different ultrasound intensities. First, the viscosity and shear modulus of the gel are obtained by fitting the resonance curve (for the lowest ultrasound intensity) to the linearized solution of the Rayleigh–Plesset model that accounts for the gel's nonlinear elasticity of neo-Hookean type and diffusive effects on the bubble dynamics. Next, finite-amplitude oscillations of the bubble are compared with the nonlinear Rayleigh–Plesset calculations. The comparison suggests a need to include the gel's elasticity in the calculations to more accurately reproduce the nonlinear bubble dynamics. Another important finding is that the so-called spring softening feature appears in the experimentally determined resonance curve as the oscillation amplitude increases, which can be predicted by the Rayleigh–Plesset model. Furthermore, our experiment with the highest ultrasound intensity shows non-spherical oscillation of mode 1 that does not appear in the case of water but can be predicted by shape instability theory.

Acoustic cavitation is used for megasonic cleaning in the semiconductor industry, especially of wafers with fragile pattern structures. Control of transient cavitation is necessary to achieve high particle removal efficiency (PRE) and low pattern damage (PD). In this study, the cleaning performance of solutions with different concentrations of dissolved gas (H2) and anionic surfactant (sodium dodecyl sulfate, SDS) in DIW (DI water) on silicon (Si) wafers was evaluated in terms of PRE and PD. When only DIW was used, PRE was low and PD was high. An increase in dissolved H2 gas concentration in DIW increased PRE; however, PD also increased accordingly. Thus, we investigated the megasonic cleaning performance of DIW and H2-DIW solutions with various concentrations of the anionic surfactant, SDS. At 20 ppm SDS in DIW, PRE reached a maximum value and then decreased with increasing concentration of SDS. PRE decreased slightly with increasing concentrations of SDS surfactant when dissolved in H2-DIW. Furthermore, PD decreased significantly with increasing concentrations of SDS surfactant in both DIW and H2-DIW cases. A high-speed camera setup was introduced to analyze bubble dynamics under a 0.96 MHz ultrasonic field. Coalescence, agglomeration, and the population of multi-bubbles affected the PRE and PD of silicon wafers differently in the presence of SDS surfactant. We developed a hypothesis to explain the change in bubble characteristics under different chemical environmental conditions.

Ultrasound facilitates the penetration of macromolecular compounds through the skin and offers a promising non-invasive technique for transdermal delivery. However, technical difficulties in quantifying ultrasound-related parameters have restricted further analysis of the sonophoresis mechanism. In this study, we devise a bolt-clamped Langevin transducer-based sonophoresis device that enables us to measure with a thin lead zirconate titanate (PZT) sensor. One-dimensional acoustic theory accounting for wave interaction at the skin interface indicates that the acoustic pressure and cavitation onset on the skin during sonophoresis are sensitive to the subcutaneous support, meaning that there is a strong need to perform the pressure measurement in an experimental environment replacing the human body. From a series of the experiments with our new device, the transdermal penetration of polystyrene, silica and gold nanoparticles is found to depend on the size and material of the particles, as well as the hardness of the subcutaneous support material. We speculate from the acoustic pressure measurement that the particles' penetration results from the mechanical action of cavitation.