Acoustic microstreaming around a gas bubble.

The problem of acoustic microstreaming that develops around a gas bubble in an ultrasound field is considered. It is shown that the solutions obtained previously by Wu and Du [(1997). J. Acoust. Soc. Am. 101, 1899-1907], which are based on the assumption that viscous effects are essential only within a thin boundary layer while beyond the boundary layer the liquid can be considered to be inviscid, lead to a severe underestimation of the power of acoustic streaming. An improved theory is suggested that corrects the errors of the previous theory and extends its limits. The proposed theory treats the entire bulk of the liquid outside the bubble and the gas inside the bubble as viscous heat-conducting fluids. No restrictions are imposed on the size of the bubble relative to the viscous, thermal, and sound wavelengths in the ambient liquid and those in the internal gas medium. All modes of the bubble's motion (volume pulsation, translation, and shape oscillations) are taken into account. Expressions for the radial and tangential stresses produced by the acoustic streaming are also derived. Numerical examples for parameters of interest are presented.