Mechanisms of removal of micron-sized particles by high-frequency ultrasonic waves

In this paper, theories of particle removal by high-frequency ultrasonic waves are discussed and tested against recent experimental data. First, the principal adhesion forces such as van der Waals forces are briefly reviewed and the typical uncertainties in their size in particle-surface systems are assessed. The different ultrasound-induced forces-linear forces such as added mass, drag, lift, and Basset forces and nonlinear ones due to radiation pressure, and drag exerted by acoustic streaming-are discussed and their magnitudes are evaluated for typical cleaning operations. It is shown that high-frequency ultrasound can clean particles most effectively in media with properties like water because: (1) the wavelength can be made comparable to the particle radius to promote effective sound-particle interaction; (2) the viscous boundary layer is thin, minimizing particle "hide-out;" and (3) both the added mass and radiation pressure forces exceed typical adhesion forces at high frequencies. Based on these analyses, possible mechanisms of particle removal are discussed and interpreted in terms of experimental observations of particle cleaning.<>