Beyond acoustophoresis: Attractive and repulsive forces on particles

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 experimentalistu0027s control. We show in experiment and theory that the forces on particles can be evaluated analytically, on both the oscillatory and the steady, time-averaged time scales. The latter formalism generalizes streaming flow computations to particle motion, and reveals new potential strategies for manipulating particles with tunable attractive or repulsive forces, depending not only on characte...