Progress in Ultrasonic Cleaning Research

Abstract

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.