The role of primary and secondary acoustic radiation forces on the contactless manipulation of liquid droplets in mid-air

The primary and the secondary acoustic radiation forces acting on water droplets are investigated in a multifocal point acoustic levitator consisting of an array of 256 ultrasonic transducers on top of a plane reflector. Two water droplets are levitated separately and manipulated horizontally until they coalesce. The motion of each droplet is captured by a high-speed camera and a tracking algorithm is used for determining the position of each droplet over time. From positions of each droplet over time, a numerical procedure is used for determining the acoustic radiation force acting on each droplet. In addition, the primary force is simulated by a numerical model that combines a matrix method with the Gor'kov equation and the secondary force is calculated through simulations using the Finite Element Method.