Influence of acoustic pressure and bubble sizes on the coalescence of two contacting bubbles in an acoustic field

• Film drainage time between two contacting bubbles was measured in an acoustic field.
• Drainage time was increased when bubbles were oscillating out of phase.
• An alignment exists between drainage time and secondary Bjerknes force.
• A correlation exists between drainage time and surface approach velocity.

In this study, the coalescence time between two contacting sub-resonance size bubbles was measured experimentally under an acoustic pressure ranging from 10 kPa to 120 kPa, driven at a frequency of 22.4 kHz. The coalescence time obtained under sonication was much longer compared to that calculated by the film drainage theory for a free bubble surface without surfactants. It was found that under the influence of an acoustic field, the coalescence time could be probabilistic in nature, exhibiting upper and lower limits of coalescence times which are prolonged when both the maximum surface approach velocity and secondary Bjerknes force increases. The size of the two contacting bubbles is also important. For a given acoustic pressure, bubbles having a larger average size and size difference were observed to exhibit longer coalescence times. This could be caused by the phase difference between the volume oscillations of the two bubbles, which in turn affects the minimum film thickness reached between the bubbles and the film drainage time. These results will have important implications for developing film drainage theory to account for the effect of bubble translational and volumetric oscillations, bubble surface fluctuations and microstreaming.