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.

超声波清洗浴中会出现许多声音和流体动力学现象，并有助于对浸入表面进行物理清洗。超声波在清洗槽中的传播和反复反射会形成类似驻波的声场。当超声强度梯度出现在声场中时，原则上它可以引起稳定的流动。当超声强度足够大时，会发生空化，并且振荡的空化气泡会被捕获在声场中或被平流到流中。据信，这些现象产生了机械作用，以除去附着在材料表面的污染物颗粒。最近的研究表明，空泡的机械作用是超声清洗中去除颗粒的主要因素，但是高强度超声导致的气泡破裂可能会剧烈到足以引起表面腐蚀的程度。在本文中，我们旨在仔细检查超声波清洗试验中空化气体在水中的浓度变化时空化气泡的作用。在我们使用28 kHz超声波进行的清洁测试中，氧气微气泡曝气会产生氧气过饱和的水，并将其用作清洁溶液，并使用旋涂有微米/亚微米尺寸二氧化硅颗粒的载玻片来定义清洁效率。高速相机记录和在压力波腹处的压力振荡幅度为1.4 atm的粒子图像测速分析表明，随着溶解氧过饱和度的增加，空化气泡的数量增加，浴液内部的流动得到促进。发现主要通过空化气泡的作用来实现颗粒的去除，但是存在最佳的气体过饱和以最大化去除效率。我们的发现表明，在清洁溶液的最佳气体过饱和条件下进行的低强度超声辐照可产生温和的气泡动力学特性，而不会剧烈崩塌，从而清洁表面而不会出现气蚀现象。最后，据报道，对单个气泡动力学的观察以及由此产生的颗粒去除效果进一步证明了空化气泡作为清洁剂的作用。我们的发现表明，在清洁溶液的最佳气体过饱和条件下进行的低强度超声辐照可产生温和的气泡动力学特性，而不会剧烈崩塌，从而清洁表面而不会出现气蚀现象。最后，据报道，对单个气泡动力学的观察以及由此产生的颗粒去除效果进一步证明了空化气泡作为清洁剂的作用。我们的发现表明，在清洁溶液的最佳气体过饱和条件下进行的低强度超声辐照可产生温和的气泡动力学特性，而不会剧烈崩塌，从而清洁表面而不会出现气蚀现象。最后，据报道，对单个气泡动力学的观察以及由此产生的颗粒去除效果进一步证明了空化气泡作为清洁剂的作用。