Numerical Verification of Gross Restoring Force on Connected Multiple Particles Acoustically Levitated in Standing Wave Nodes

Acoustic levitation is a technique that traps and manipulates objects in the air by controlling the acoustic radiation force in a spatiotemporal manner. One major strategy is to trap an object inside one of the nodes of standing ultrasonic waves using phased arrays of air-coupled ultrasonic transducers. However, the maximum weight of the levitated object is very small because the restoring force inside a single node is weak. Here we propose a method applicable for a heavier object using a levitating anchor composed of rigidly connected multiple lightweight beads, which are simultaneously trapped in multiple adjacent nodes of two-dimensionally stretching standing waves. In this study, the standing wave field is supposed to be generated using four opposing phased arrays whose phases are controlled to form a focus in the center, and the anchor is assumed to have five spherical beads placed on the nodes. We numerically confirmed that the gross restoring force acting on the anchor increased compared to that on a single particle. Thus, the proposed strategy is expected to contribute to a more stable acoustic levitation of a heavier object.