Born approximation of acoustic radiation force used for acoustofluidic separation

Abstract

Acoustofluidic separation often involves biological targets with specific acoustic impedance similar to that of the host fluid, and with dimensions up to the order of a wavelength. This parameter range, combined with the use of standing waves to separate the targets, lends itself to use of the Born approximation for calculating the acoustic radiation force. Considered here is the configuration analyzed by Peng et al. [J. Mech. Phys. Solids 145, 104134 (2020)], in which two intersecting plane waves radiated into the fluid by a standing surface acoustic wave exert a force on a eukaryotic cell modeled as a multilayered sphere. The acoustic field in this case has the structure of a standing wave parallel to the substrate and a traveling wave perpendicular to the substrate. For all parameter values considered by Peng et al., including spheres several wavelengths in diameter, the Born approximation of the acoustic radiation force parallel to the substrate is in good agreement with a full theory based on spherical wave expansions of the acoustic field. The importance of incorporating the internal structure of the cell is demonstrated by examining the counterexample of a homogenized sphere with uniform material properties obtained by volume averaging.