Microbubble acoustic surface cleaning

Abstract

Surface cleaning is accomplished by fluid mechanical forcing, often assisted by chemical and acoustical activation. Examples include the removal of nanoparticles from IC semiconductor substrates and the disruption of bacterial biofilms in dentistry. The presence of bubbles is known to greatly enhance the cleaning efficiency as it promotes mixing of the chemicals and it yields higher stresses through acoustic streaming and jetting following asymmetric collapse of the bubbles. With smaller nanofabricated structures and more delicate surgical therapies, there is a growing demand for precision cleaning with minimum damage to the surrounding media. Here we explore the concept of micro-machined cylindrical pits acting as cavitation nuclei for a continuous source of microbubbles, thereby localizing the cavitation phenomena and suppressing its inherent chaotic nature. The micropit bubble was found to be stable against dissolution, and the resonance behavior of surface mode vibrations of the cap was modeled with the unsteady Stokes equation combined with a Fourier-Bessel expansion. Above a pressure threshold, destabilization of the micromeniscus results in bubble pinch-off which was studied using high-speed imaging down to nanoseconds timescales. It was also found that the acoustic coupling and merging of cavitation clouds from neighboring micropits increasingly promote acoustic surface cleaning.