総説・解説等

In precision semiconductor cleaning, removing sub-micron particles using fluid-dynamic shear flows generated by cavitation bubbles is crucial, though violent bubble collapses risk causing surface erosion. This paper examines the mechanical role of cavitation bubbles in ultrasonic particle removal by combining bubble dynamics modeling with 28-kHz cleaning experiments in oxygen-supersaturated water. Single-bubble dynamics illustrate the acoustic localization of energy, while visualization and haze-method evaluations demonstrate that oxygen-supersaturated water significantly enhances particle removal efficiency even under low-intensity ultrasound. An optimal degree of dissolved-oxygen supersaturation exists due to a trade-off between increased bubble population and acoustic attenuation in high-void-fraction clouds. This approach achieves effective physical cleaning while suppressing erosion.

Physical cleaning based on underwater ultrasound is widely used in industry and its cleaning efficiency is known, from recent studies, to be augmented by promoting the mechanical activity of cavitation bubbles. As a side effect, ultrasound cleaning may give rise to material damage from violent collapse of cavitation bubbles. Traditionally, degassed water is favored as cleaning solution in order to reduce the probability of having cavitation (and the resulting erosion); however, there is no chance to promote cleaning efficiency with this approach. In this review, we introduce our recent effort toward the development of an erosion-free ultrasonic cleaning technique using aerated water. Under dissolved gas supersaturation in aerated water, bubbles can be created easily with low-intensity ultrasound and the resulting bubble dynamics are expected to be mild enough to avoid erosive collapse. To demonstrate this conjecture, we run a series of ultrasound cleaning tests with glass samples on which submicron SiO2 particles are spin-coated; we use a transparent cleaning bath for visualization of acoustic phenomena by a high-speed camera. Dissolved oxygen (DO) supersaturation in water (aerated by oxygen microbubbles) and ultrasound frequency (at 28 kHz or 200 kHz) are varied as parameters, while the input power to drive the ultrasound transducer is fixed and the ultrasound amplitude at the pressure antinode is set at Pe=1.0 atm (root-mean-square value) for the case of degassed water. Particle removal efficiency (PRE) is defined based on image analysis of light scattering from the residual particles (i.e., the so-called Haze method). We see that there exists an optimal DO supersaturation to maximize the PRE. We also see that the PRE is higher in the case of lower frequency (28 kHz), for its cavitation inception threshold is reduced and the number of activated bubbles is thus increased.

This review is concerned with a physical cleaning method based on ultrasound-superposed water jets that collide with cleaning targets and spread over their surface; this method is called ultrasonic water flow cleaning and can be used in semiconductor manufacturing processes. Visualization of the acoustic wave and cavitation bubbles in ultrasonic water flow is presented to examine the role of bubbles oscillating under ultrasound irradiation in the process of removing small silica particles from glass surfaces. The visualization results suggest that hydrodynamic force from single-phase water flow does not suffice for particle removal and that acoustic cavitation bubbles play a major role in particle removal as in the case of ultrasonic bath immersion cleaning.

This article outlines fluid mechanics education in the Department of Mechanical Engineering in the Faculty of Science and Technology at Keio University, emphasizing actions towards internationalization of its curriculum. The undergraduate courses cover the full scope of F.M. White’s Fluid Mechanics as a required “English” textbook. One of the undergraduate lectures has been offered in English since 2022, attracting both exchange students and Japanese students and fostering cross-cultural interaction between them. On the other hand, the graduate courses address more specialized topics including turbulent flows, compressible flows, multiphase flows, and bio-inspired fluid dynamics. To align with international standards in undergraduate and graduate curricula, some advanced topics in fluid mechanics are to shift to graduate-level teaching in the coming academic year. These initiatives are expected to contribute to the creation of an international learning environment, which is essential for cultivating global leaders for the future.

キャビテーションに起因するエロージョン現象の解明のための気泡崩壊の可視化技術を解説した。

アルミ箔を用いた水中超音波エロージョンに関する簡易試験から、溶存ガスの過飽和下では超音波キャビテーション由来のエロージョンが低減することを示した。

分散気泡流中を伝播する線形音響波のモデリングを解説した。さらに、気泡径分布を有する気泡群の力学を音響学的観点から考察した。

バネ・ダンパー・質量系の振動モデルに基づき、粘弾性媒質中の球形ガス気泡の力学を解析するためのレイリー・プリセット理論を解説した後、気泡力学に対する気泡周囲の媒質粘性の影響を定量評価するための実験および解析に関する最近の研究を紹介した。

超音波キャビテーション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による洗浄法を新たに提案した。

超音波照射による付着コンタミ粒子の除去におけるキャビテーション気泡の運動の役割を解説した。