LAUSR

Inertial effects in microfluidics afford an interesting set of tools for the control of particle positions. The gradients of steady channel flows, as well as the gradients of acoustic field amplitudes, have been used prominently to this purpose, the latter in acoustofluidics. Here, we investigate directly the effect of an oscillating interface on the fluid surrounding it and particles suspended in the fluid. The fast oscillatory motion gives rise to strong inertial effects, while the method allows for versatile force actuation because of the variety of flow fields, frequencies, and length scales under the experimentalist's control. We show in experiment and theory that oscillating bubbles simultaneously (i) guide particles close to the bubble interface by streaming flow, and (ii) exert strong lift forces that can be used to sort the particles by size or density. The lift forces and the ensuing particle displacement constitute an effect separate from streaming and can be understood analytically on the time...