Acoustic radiation force on a spherical particle near a planar boundary in a weakly thermosviscous fluid.

Abstract

A rigorous formalism is presented for the time-averaged acoustic radiation force on a spherical particle near an infinite planar boundary subjected to a plane wave. The background medium is assumed to be a weakly thermoviscous fluid, with the viscous and thermal boundary layers much smaller than either the acoustic wavelength or the particle radius. The total acoustic field, composed of the incident, scattered, and reflected fields, is derived using the finite series expansion theory and the image method. Based on the approximate solution to the governing equations of acoustic pressure, velocity, and temperature, we analyze the vorticity mode, the acoustic mode, and the thermal mode in the surrounding fluid, further yielding the boundary conditions at the surface of rigid or nonrigid particles from continuity of velocity. Subsequently, the radiation force function in terms of scattering coefficients is obtained neglecting the contributions of the acoustic streaming effect, followed by a variety of numerical examples. A significant deviation of the force function curves from the ideal-fluid results occurs especially at low reflection coefficients and high incident frequencies. In the Rayleigh frequency range, the first three terms are sufficiently accurate to describe the radiation force, with the critical value of dimensionless frequency decreasing along with the boundary layers. Potential applications include the development of acoustic tweezers involving precise manipulation of microparticles in a bounded and nonideal fluid.