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

Abstract

In this study, the coalescence time between two contacting sub-resonance size bubbles was measured experimentally under an acoustic pressure ranging from 10kPa to 120kPa, driven at a frequency of 22.4kHz. 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.

A parametric review of sonochemistry: Control and augmentation of sonochemical activity in aqueous solutions

“…Therefore at some point rebound will be more favourable as opposed to coalescence. However, coalescence is also influenced by the applied ultrasonic frequency that determines the time period available for it to occur [38,39]. If the pressure amplitude has sufficient intensity and the sound field nears the resonance frequency, bubbles of similar size have been observed not to coalesce or rebound, remaining in contact and side by side [34].…”

3-D Network Pore Structures in Copper Foams by Electrodeposition and Hydrogen Bubble Templating Mechanism

“…19 Moreover, it could be remarkably influenced by cathode current density, concentrations of Cu 2+ and H + ions in the electrolyte solutions and, especially, the additives for adjusting electrodeposition process. [20][21][22] For example, the addition of CH 3 COOH may notably decrease the average sizes of hydrogen bubbles because of the suppression on the coalescence of bubbles, [23][24][25][26][27] while the incorporation of HCl can effectively reduce the dimension of dendritic copper deposit particles due to acceleration of copper deposition by building chloride bridges. 28 In addition, Tan et al reported that the effect of bromide ions was similar to chloride ions, while PEG (polyethylene glycol) changed the foam structure oppositely.…”

Supercooling of Water Controlled by Nanoparticles and Ultrasound

“…Bubble coalescence means that some smaller cavitation bubbles coalesce and form a larger bubble. Unlike the cavitation bubbles formed by rectified diffusion, bubbles formed by bubble coalescence do not undergo the cavitation cycle and do not collapse [ 28 , 29 ]. We therefore infer that cavitation bubbles in the positive and negative zones may be formed by rectified diffusion and bubble coalescence, respectively.…”