Combined experimental and theoretical investigation of the gas bubble motion in an acoustic field

•Experimental and theoretical methods are used to study gas bubbles in an acoustic field.•Single gas bubble oscillates due to the equilibrium of the primary Bjerknes and buoyancy force.•The resultant of forces acting on two bubbles contributes to the bubble coalescence. The objective of this paper is to apply the combined experimental and theoretical method to investigate the various behaviors of gas bubbles in an acoustic field. In the experiments, high-speed video and ultrasonic processor are used to capture the transient evolution of gas bubble patterns, as well as velocity profiles. In the theoretical analysis, the theories of primary and secondary Bjerknes forces and buoyancy force are introduced to accurately demonstrate the variations of bubble volume and motion. Results are presented for gas bubbles with the radius of 1.4mm under an acoustic field with a frequency of 18kHz, for three cases, namely single bubble rising in a quiescent liquid, acoustic single bubble oscillation and two bubbles coalescence conditions. The results show that the fragments around the single gas bubble presents the periodical behaviors, namely, splitting, attraction, and secondary splitting motion. The centroid of the single gas bubble almost oscillates without motion upwards or downwards, because of the equilibrium of the primary Bjerknes force caused by acoustic waves and the effect of the buoyancy force. For the two coalescing bubbles, the resultant of buoyancy, primary and secondary Bjerknes forces acting on two bubbles are same in magnitude, but in opposite direction, which indicates that two gas bubbles attract each other and and coalesce into one.