研究発表

This study investigates the spontaneous generation of Marangoni flow during the solvent evaporation of cosmetic suspensions containing surface-treated functional powders (mica and silica) and a UV-absorbing oil (ethylhexyl methoxycinnamate, EHMC). Using optical microscopy, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS), and UV absorbance analyzers, the research examines how Marangoni contraction dynamic droplets affect the spatial distribution of components and the resulting UV-protecting performance. The findings reveal that while Marangoni contraction occurs across all formulations during drying, powder migration and structural inhomogeneity depend heavily on powder type, concentration, and applicator gap size. Inhomogeneous powder distributions induce spatial heterogeneity in the UV-absorbing oil, which significantly degrades the UV protection of the applied layer. Conversely, increasing powder content suppresses particle migration, promoting higher spatial uniformity and enhanced UV-shielding efficacy. Ultimately, the work underscores that managing far-from-equilibrium hydrodynamic phenomena during solvent evaporation is crucial for optimizing the functional performance of suspension-type cosmetics.

This study examines how volatile solvent evaporation and particle sedimentation influence the structure and UV-protecting performance of sunscreen films containing titanium dioxide (TS) microparticles and ethylhexyl methoxycinnamate (EHMC). Using digital microscopy, scanning electron microscopy with energy-dispersive X-ray spectrometry (SEM-EDS), and SPF analysis, the research evaluates the dynamic behavior of applied suspension layers on quartz substrates. The findings reveal that solvent evaporation triggers Marangoni contractions and "tears of wine" phenomena, which dictate whether fine pores or smoothed ring structures form in the deposited layer. Specifically, TS particles weaken Marangoni flow and promote pore creation at greater film thicknesses, whereas the addition of EHMC inhibits pore formation, smoothing the surface and enhancing UV protection. Additionally, bulk sedimentation tests demonstrate that particle suspensions spontaneously generate localized upward and downward flows that alter internal microparticle distribution. Ultimately, controlling far-from-equilibrium hydrodynamic patterns and sedimentation dynamics during application is critical for optimizing the spatial uniformity and efficacy of sunscreen products.

In this study, the Epstein–Plesset equation was extended by incorporating both linear and nonlinear neo–Hookean elasticity to clarify the effect of elasticity on bubble growth in gelatin gels. Porcine skin-derived gelatin gels with concentrations of 3 wt%, 6 wt%, and 9 wt% were used. Bubbles were nucleated by laser irradiation, and their growth was monitored over time as gas diffused from the supersaturated gel. The results showed that the effect of elasticity was negligible in the 3 wt% gel. In gels with concentrations of 6 wt% or higher, the nonlinear elastic model more accurately reproduced the experimental results, indicating a significant contribution of nonlinear elasticity. Furthermore, in the highly concentrated 9 wt% gel, the bubble growth could not be fully described by either model. These findings indicate that nonlinear elasticity and softening effects are essential for accurately modeling bubble growth, especially in highly concentrated gels.

An experimental technique was developed for two-dimensional Particle Image Velocimetry (PIV) measurement of wall shear flow around a bubble growing under dissolved gas supersaturation. Carbonated water was used as dissolved-gas-supersaturated liquid, and its flow was created in a small container with a tube pump. An isolated CO2 bubble nucleated from an intentionally created scratch on the glass surface was placed in the flow. The mass-diffusion-driven growth of the bubble (from nucleation to detachment from the surface) was recorded using a video camera with backlighting; the radius of the detached bubble was below 1 mm in the present experimental conditions. The velocity field without and with the wall-attached bubble was obtained through PIV with the water (seeded with fluorescent particles) and with a planar laser sheet, which enables one to obtain local shear flow. The proposed PIV measurement technique allows for careful examination of bubble detachment dynamics and convective mass transfer around attached bubbles.

A fundamental understanding of acoustic bubbles in soft matter is of great importance in medical applications of high-intensity focused ultrasound (HIFU) for lithotripsy and histotripsy. In these applications, cavitation often appears as a cloud of cavitation bubbles that are nucleated from gas bubble nuclei existing randomly in human tissue. However, it is yet a challenge to model and simulate bubble cloud dynamics. Here, as a fundamental study, we consider the simpler case with a single bubble in gelatin gels under ultrasound irradiation and show the potential use of acoustic bubbles for rheological measurement of their surrounding media at spatially and temporally small scales. A single spherical bubble nucleus can be created by focusing a laser pulse into gels under dissolved-gas supersaturation. The size of nucleated bubbles can be controlled in quasistatic manner by their mass-diffusion-driven growth. With ultrasound irradiation, one can observe oscillations of the single spherical bubble with a high-speed video camera. From comparisons of the observed bubble dynamics to Rayleigh-Plesset-type simulation, one can obtain viscosity and elasticity of the gel surrounding the bubble. In my talk, I will summarize previous studies on acoustic bubbles in soft matter from my research group and then present recent studies on thermal effects on dynamics of a bubble in a gel under long-term ultrasound irradiation.

Ultrasonic cleaning is widely used in industry and its cleaning effect is known, from recent experimental and simulation studies, to result from wall shear stress induced by dynamics of ultrasonic cavitation bubbles. Resonant motion of near-wall cavitation bubbles can emphasize the hydrodynamic effect but possibly give rise to erosion at target surfaces to be cleaned. For ultrasonic cleaning to be more efficiently and less erosive, we favor mild dynamics of cavitation bubbles (with no violent collapse) under lower-intensity ultrasound irradiation. The key is a reduced cavitation inception threshold in gas-supersaturated water in which bulk and/or particle-attached nanobubbles may work as cavitation nuclei. In my talk, I introduce my research group’s recent studies on ultrasonic cleaning of immersion type and water jet type and discuss the role of dissolved gas supersaturation and cavitation bubble dynamics.

From the perspective of ultrasound-based medical applications, it is important to investigate the temperature rise of the viscoelastic medium around a bubble oscillating under long-term ultrasound irradiation. In this study, a Rayleigh–Plesset-type model of a spherical gas bubble and its numerical modeling are proposed, which take the temperature rise of the surrounding medium into account. We also devise an experimental system that allows for measuring the temperature rise of the gelatin gel around a 100-μm-order bubble oscillating under 28-kHz ultrasound with a thermocouple for 2 seconds together with the visualization of bubble oscillations. It is observed in experiments that the decrease in viscoelasticity of the surrounding medium due to temperature rise causes the increase in fluidity of the medium, thus leading to translational motions and fissions of the bubble. It follows from comparisons of the experimental and simulation results that the temperature rise depends significantly on the behavior of bubbles (spherical/non-spherical, with/without bubble fission).

We devise a micron-scale rheological measurement technique for gels deformed by free oscillation of laserinduced bubbles. As a gel sample, we select Tetra-PEG, which can be produced by the reaction of two types of four-armed prepolymers and has a network similar to diamond structure. Experimentally, we focus an infrared (1064 nm) nanosecond laser pulse into the gel sample to generate a micronsized bubble (i.e., microbubble). We record the evolution of free oscillation of the bubble with a high-speed camera and compare it to a spherical bubble dynamics model equipped with the neo-Hookean model for the gel’s elasticity. This comparison allows for locally calculating the rigidity and viscosity of the gel under very high strain rates.

Acoustic bubbles come into play in therapeutic ultrasound applications. Since human tissues are viscoelastic, it is desirable to clarify the role of viscoelasticity in acoustic bubble dynamics. Although many theoretical and numerical studies on bubble dynamics in soft matter have been reported, experimental data are yet lacking, and thus limiting model validation as well as quantitative evaluation of the viscoelasticity effects on bubble dynamics. In my talk, we examine the role of viscoelasticity in oscillations of spherical bubbles in gelatin gels. First, I introduce an experimental technique to generate spherical gas bubble nuclei in gels and visualize ultrasound-induced oscillations of bubbles with a high-speed camera. Then, we compare the bubble oscillation data to the Rayleigh-Plesset-type equation that accounts for the gel's elasticity and viscosity on spherical bubble dynamics. From the comparison, we can measure the gel rheology locally in the bubble’s neighborhood at ultrahigh strain rates.

Underwater ultrasound whose frequency is on the order of 10 kHz is conventionally used to degrease glass and metallic parts. It is practically important to reveal its cleaning mechanism in order to achieve more efficient cleaning. In this study, we performed visualization of acoustic cavitation bubbles in ultrasonic degreasing processes. Glass plates on which an oil drop was deposited were used as a cleaning sample. The sample was soaked in a water container and subjected to 28-kHz underwater ultrasound. The ultrasound-induced cavitation bubbles near the glass surface were recorded by a high-speed camera. It follows from the visualization that cavitation bubbles nucleate preferentially in less viscous water. These nucleated bubbles subsequently penetrate, via the Marangoni effect, into the oil whose surface tension is lower than that of water. This suggests that the dynamics of bubble clouds in the oil play an essential role in degreasing.

Shock wave damage from the collapse of laser-induced bubbles is utilized in the removal of kidney stones (laser lithotripsy). It is important to understand how bubble dynamics are affected by neighboring boundaries and viscoelasticity in media such as human tissue. In this study, an Nd-YAG laser pulse is used to create bubbles in the near-wall region of 6 wt.% gelatin gel samples in acrylic casing. A high-speed camera combined with a pulse-laser backlighting system is used to record the inception, non-spherical collapse, and subsequent rebounds as well as shock waves emitted at bubble inception and violent collapses. It was observed that the neighboring wall had a damping effect on bubble oscillations, especially during rebounds. After an incident shock wave is emitted at the initial bubble collapse, there is an reflected shock front, whose compressing effects could be inhibiting bubble growth in rebound stages. Additionally, image analysis of the bubble centroid found translational motion towards and away from the wall, in phase with volume oscillation. In collapses closer to the wall, which tend to show larger nonsphericities, such motion was greatly damped. This is possibly due to greater viscous effects, as the flow field around the bubble becomes rotational in nonspherical cases. More on such dynamics will be presented in the upcoming conference.

In post-CMP cleaning in semiconductor industry, there is a need to remove very small contaminant particles from silicon wafers. One of the promising candidates for effective post-CMP cleaning is based on ultrasound-superposed water jets. Here, we call this technique ultrasonic water flow cleaning. To examine whether acoustic bubbles play a role in ultrasonic water flow cleaning, visualization experiments and cleaning tests were performed for the case of ultrasound frequency at 0.43 MHz. Schlieren visualization confirmed standing acoustic wave formation in ultrasonic water flow that collides with a glass surface. Backlight visualization showed the existence of cavitation bubbles in water film flow spreading over the glass surface. Together with the cleaning tests with glass plate spin-coated with small silica particles as contamination, ultrasound-induced motion of bubbles is found to play a major role in particle removal in the process of ultrasonic water flow cleaning, as in the case of conventional cleaning with ultrasonic cleaning baths. From a practical point of view, our experimental results suggest that it is favorable to avoid the formation of standing acoustic fields in the water film flow over target surfaces (say, by changing the water jet collision location during the cleaning process), which allows for uniform cleaning performance with cavitation bubbles being populated uniformly in space.

In the context of underwater explosions, much attention has been put on acoustic phenomena (including shockwaves and cavitation) in the water. However, to the authors' knowledge, little is studied regarding shockwave transmission from underwater explosions to the atmosphere. The transmitted shock dynamics will be of importance when it comes to examining ignition in the case of flammable gases over the water interface. Here, we perform visualization of shock transmission from laser-induced underwater explosions to the atmosphere. Nano-second pulse laser at 1064 nm is focused into water to create a spherical shockwave. The shock interaction with an air-water interface is recorded by a high-speed camera with pulse laser backlighting. The evolution of the transmitted shockwave in the air is examined with the aid of the Ranking-Hugoniot relation and Euler flow simulation.

減圧脱気した水中における気泡の収縮を実験的に観察し、その収縮速度を修正エプスタイン・プリセット理論と比較することにより、溶存窒素濃度を算定する手法を提案した。

水中に浸したポリプロピレン板に付着する気泡の物質拡散駆動による収縮運動を可視化した。気泡から付着固体壁面へのガスの物質移動を考慮しないエプスタイン・プリセット型の気泡収縮モデルとの比較から、ポリプロピレン板中の溶存空気の物質移動は溶解気泡の収縮速度に顕著な影響を与えることを明らかにした。

液膜で覆われた剛体壁に対する複数個の液滴の衝突を再現する圧縮性ナビエ・ストークスの式に基づくシミュレーションを行った。複数液滴の衝突により生成される水撃圧および壁面せん断応力は、単一液滴の場合のそれらを凌駕することを示した。

緑色ナノ秒パルスレーザーの集束照射により生じる球形衝撃波の時空間発展を解像する超高速度可視化のための実験系を構築した。グリセリン水溶液中の球形衝撃波の可視化から取得される衝撃波伝播速度から、ランキン・ユゴニオ衝撃関係式に基づき衝撃圧を計算した。粘性をパラメータとする実験を行い、粘性による衝撃波の減衰を評価した。

周波数1 MHzのメガソニック洗浄槽における自由界面および水中超音波キャビテーション気泡の運動を可視化するための実験系を構築した。音響放射力により変形する自由界面の非定常運動を、矩形容器内に形成される表面重力波に関するポテンシャル理論に加え、第一ビャクネス力により駆動されるキャビテーション気泡の非定常並進運動の実験観察に基づき考察した。

超音波洗浄のエロージョンフリー化への取り組みとして酸素過飽和水を洗浄液とする低音圧超音波照射を提案し、物理洗浄における超音波キャビテーション気泡の運動の役割を、可視化実験および数値シミュレーションに基づき解説した。

28 kHzの超音波照射下のゼラチンゲル内の気泡の振動を高速度カメラで撮影した。微小振幅の結果と粘弾性の影響を考慮した拡張レイリー・プリセットの式の線形解の比較からゲルの粘性および弾性係数を取得した。実験より取得した有限振幅の球形振動は、各種散逸効果および非線形弾性を考慮した拡張レイリー・プリセットの式の数値解により再現できることを示した。さらに、有限振幅振動の場合、非球形の形状振動モードが励起し（弾性のないニュートン流体中では生じない）1次の球面調和関数のモードが発現することを示した。