Dynamic acoustic fields for size selective particle separation on centimeter scale

Dynamic acoustic fields offer an interesting alternative for acoustic standing-wave fields in acoustic separation applications. This paper reports on an investigation of two methods for generating dynamic acoustic fields and their applicability for selective particle separation. The first method applies a dual-frequency excitation to generate a standing-wave field that in which the pressure nodes travel at constant velocity. The second method uses frequency-ramping, where the velocity of the nodes in the resulting standing-wave field depends on both time and position. Both methods were investigated analytically and with computational models, yielding a dimensionless number that predicts particle behavior without having to solve the differential equations of motion. This dimensionless number can also be used to estimate the acoustic pressure in practical applications. Experiments carried out with polyethylene particles and the two prototypes confirmed the theoretical and numerical predictions. Both methods are suitable for selective particle separation applications.