Microstreaming induced by two interacting acoustic bubbles

Abstract

Microbubbles submitted to a sufficiently strong ultrasound field can show nonspherical oscillations called surface modes. These oscillations induce a slow mean flow named microstreaming in the vicinity of the bubble interface. We present experimental and theoretical investigations of the microstreaming induced by a bubble pair as an elementary modelling of a dense bubble cloud. Two bubbles are trapped in a dual-frequency acoustic chamber that allows studying their interface dynamics. The secondary radiation force they experience and make them attract under increasing acoustic forcing is quantified through the relative location of the bubble within the acoustic field. Each bubble can exhibit several nonspherical modes that are either axisymmetric or not. The typical flow signatures induced by these nonspherical modes are identified. When the bubble shape oscillations are axisymmetric, the extension of microstreaming pattern does not allow fluid interaction in the region between the two bubbles. In case of asymmetric oscillations, large-scale microstreaming is obtained. These experimental findings are discussed by means of a theoretical modelling of the fluid flow induced by two nonspherically-oscillating microbubbles.