Acoustic radiation force and torque on spheroids inside an ideal cylindrical cavity

Abstract

We perform a theoretical analysis of the radiation forces and torques produced on a spheroid particle by the acoustic modes of an ideal (with a rigid top and bottom and with soft or hard lateral walls) cylindrical chamber. Exact analytical expressions are obtained for a rigid particle and assuming the fluid inviscid approximation. A particular emphasis is given to the resonator's fundamental mode for which the axial and radial trap stiffnesses are calculated. The axial trap stiffness depends inversely with the cavity's height squared. We also predict that the radiation torque induces the particle to oscillate around its minor axis. The corresponding oscillation frequency is then determined. The theory is showcased for studying the dynamics of an Au microrod (modeled as an elongated spheroid) with a 20 μm-length and 2 μm-width in a water medium. In doing so, we use typical parameters of acoustofluidic devices (cavity's dimensions and energy density). The obtained results show terminal velocities of few micrometers per second, while the oscillation frequency is in the kilohertz range. In addition, a discussion on the dynamics of micro-/nanorobots of spheroidal shape propelled by ultrasound is outlined.