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 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.

Particle Removal in Ultrasonic Water Flow Cleaning Role of Cavitation Bubbles as Cleaning Agents

“…When it comes to achieving particle removal, it is essential to create wall shear stress that is strong enough to defeat particle adhesion such as van der Waals force [1]. Recent studies on conventional cleaning with ultrasonic cleaning baths [2,3,4] suggest that instantaneous liquid flow acceleration induced by ultrasound propagation (without cavitation) plays a minor role in ultrasonic cleaning but near-wall liquid flow promoted by dynamics of acoustic cavitation bubbles at or near cleaning targets plays an essential role as main cleaning agents. We speculate that acoustic bubble dynamics come into play also in the case of ultrasonic water flow cleaning.…”

Optimal Injection Distance in Ultrasonic Water Flow Cleaning

“…In this technique, ultrasound-superposed water jets are set into collision against cleaning targets and the hydrodynamic force acting on attached particles is expected to contribute to particle removal [1]. From visualization of 0.43 MHz or 0.95 MHz ultrasound superposed water jets and the subsequent film flow spreading over glass surfaces [2,3], cavitation bubbles are found to play a key role in particle removal as in the conventional method with ultrasound cleaning baths [4,5,6]. To be specific, motion of near-wall cavitation bubbles can produce strong wall shear stress [7] and thus augment the cleaning performance.…”