Bjerknes force and bubble levitation under single-bubble sonoluminescence conditions

Bubble levitation in an acoustic standing wave is re-examined here for the case of single-bubble sonoluminescence. The equilibrium position of the bubble is calculated by equating the average Bjerknes force with the average buoyancy force. The predicted values, as a function of pressure amplitude, are compared with experimental measurements. Our measurements indicate that the equilibrium position of the bubble shifts away from the pressure antinode as the drive pressure increases, in qualitative agreement with calculations, but unexpected when only linear theory is considered [A. Eller, J. Acoust. Soc. Am. 43, 170 (1968)]. The Bjerknes force also provides an upper limit to the drive pressure in which a bubble can be levitated near (and above) a pressure antinode, even in the absence of shape instabilities.