Motion of microscopic grains in a periodic potential of plate acoustic waves

We report on a novel method of charge particle transport. It is based on the application of a moving electrostatic potential and an oscillating friction force to the particle which occur due to acoustic waves travelling in a piezoelectric medium. ZnS grains placed onto the surface of a Y-cut, Z-propagating LiNbO3 plate experience a sequence of jumps in the forward and in the backward directions with respect to the phase velocity of the plate wave. The jump probability appears to change with the Lamb mode supported by the plate and the sign of the grain charge. We present computed spatial profiles of the piezoelectric potential and the displacement components on the surface of the plate for the two lowest resonant modes of Lamb waves. The occurrence of the grain motions is then explained by the presence of the electrical and friction forces, and good qualitative correspondence of the theory and experiment is found.