Force on a compressible sphere and the resonance of a bubble in standing surface acoustic waves

In this paper, the theory for acoustic radiation force exerted by standing surface acoustic waves (SSAWs) is extended to a compressible sphere in inviscid fluids. The conventional theory, developed in plane standing waves, fails to predict the radiation force incident on particles in the SSAW. Our extended formulas reveal that, in the direction normal to the piezoelectric substrate, the acoustic radiation force cannot only push the sphere away, but also pull it back towards the substrate. In the direction parallel to the substrate, both the magnitude of the radiation force and the equilibrium positions for particles can be actively tuned by changing the Rayleigh angle. This is exceedingly meaningful for particle positioning by tilted angle SSAWs and SSAWs. Furthermore, the Rayleigh angle can also be used to actively regulate the resonance point and peak for bubbles acted on by SSAWs. The extended theory can thus be used in the design of SSAWs for manipulating particles and bubbles.