Finite element modeling of free surface particle clustering

Within the low frequency (tens of hertz) domain the collection of particles located at the air-fluid interface has been previously demonstrated. Our work shows that such manipulation is also viable using ultrasonic actuation. In standing wave ultrasonic devices suspended particles are collected at the pressure nodes due to acoustic radiation forces. However, in more complex fields, our modeling data shows that it is necessary to consider that the collection is further confined to areas within the pressure node at which the fluid particle velocity is maximum. Experiments are conducted to demonstrate that even at a air-liquid interface, where the pressure oscillation is approximately zero, collection is possible by actuating a field which has strong particle velocity variations.