Importance of Bubble Size Control in Ultrasonic Surface Cleaning by Pulsed High-Frequency Sound Fields

The occurrence of acoustic cavitation is incorporating a multitude of interdependent effects that strongly depend on the bubble size. Therefore, bubble size control would be beneficial for industrial processes that rely on acoustic cavitation. A pulsed acoustic field can result in bubble size control and bubble recycling, which strongly enhances cavitation activity. In this paper, we present experimentally determined bubble size distributions for different power densities, which are interpreted in the frame of numerical calculations of the oscillatory responses of the bubbles to the intermittent driving sound field. The distributions are found to be shaped by the size dependent interplay between bubble pulsations, rectified diffusion, coalescence and the development of parametrically amplified shape instabilities. Furthermore, a relation between cleaning and acoustic cavitation activity is established. The results show that the changed acoustic conditions bubbles experience in close vicinity to a solid surface, need to be taken into account. The influence of the inertial type of cavitation activity on the structural damage that is occurring during the cleaning process, is evaluated. It is found that the damage to the fragile structures that ought to be cleaned, is significantly reduced, when the applied power is kept below values, for which inertial cavitation is observed.