Bubble Coalescence during Acoustic Cavitation in Aqueous Electrolyte Solutions

Abstract

Bubble coalescence behavior in aqueous electrolyte (MgSO(4), NaCl, KCl, HCl, H(2)SO(4)) solutions exposed to an ultrasound field (213 kHz) has been examined. The extent of coalescence was found to be dependent on electrolyte type and concentration, and could be directly linked to the amount of solubilized gas (He, Ar, air) in solution for the conditions used. No evidence of specific ion effects in acoustic bubble coalescence was found. The results have been compared with several previous coalescence studies on bubbles in aqueous electrolyte and aliphatic alcohol solutions in the absence of an ultrasound field. It is concluded that the impedance of bubble coalescence by electrolytes observed in a number of studies is the result of dynamic processes involving several key steps. First, ions (or more likely, ion-pairs) are required to adsorb at the gas/solution interface, a process that takes longer than 0.5 ms and probably fractions of a second. At a sufficient interfacial loading (estimated to be less than 1-2% monolayer coverage) of the adsorbed species, the hydrodynamic boundary condition at the bubble/solution interface switches from tangentially mobile (with zero shear stress) to tangentially immobile, commensurate with that of a solid-liquid interface. This condition is the result of spatially nonuniform coverage of the surface by solute molecules and the ensuing generation of surface tension gradients. This change reduces the film drainage rate between interacting bubbles, thereby reducing the relative rate of bubble coalescence. We have identified this point of immobilization of tangential interfacial fluid flow with the "critical transition concentration" that has been widely observed for electrolytes and nonelectrolytes. We also present arguments to support the speculation that in aqueous electrolyte solutions the adsorbed surface species responsible for the immobilization of the interface is an ion-pair complex.

High-pressure emulsion formation in cylindrical coaxial orifices: Influence of cavitation induced pattern on oil drop size

“…In acoustical cavitation experiments Sunartio et al (2007) show that the average cavitation bubble size is reduced by adding sodium dodecyl sulfate, simultaneously less coalescence is observed at lower surface tensions (Lee et al, 2007). By addition of an electrolyte like sodium chloride a similar reduced coalescence behavior of cavitation bubbles is reported (Browne et al, 2011). Further on, acoustic cavitation damage caused by collapsing bubbles decreases with lowered surface tension (Iwai and Li, 2003), which indicates a reduced energy dissipation during the collapse of cavities.…”

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

“…For small values of viscosity (~1 -3 cp) there is a varied relation to coalescence time, however for a large viscosity it is clear that the coalescence time increases. This could imply that highly viscous fluids inhibit bubble surface motion and film drainage between the bubbles in contact, which is known to reduce coalescence [213]. However, the time for coalescence to occur becomes relevant in an ultrasound environment due to time dependant forces on the bubbles [36].…”

“…Several authors have recognised the ability of electrolyte solutions to influence the size of bubbles during sonication, the severity of collapse and coalescence [162,[211][212][213]251]. There are two suggestions as to the electrolytes' effect.…”

“…There are two suggestions as to the electrolytes' effect. Firstly, ion absorption at the bubble surface that influences coalescence via reduction in film drainage between bubbles in contact and steric repulsion [213]. Secondly, a reduction in the gas content of the liquid due to a "salting out" effect that both reduces the amount of nucleation, clustering of bubbles and would decrease attenuation of the sound field [162,211,251].…”

An experimental study on the motion of water droplets in oil under ultrasonic irradiation

“…Meanwhile, they indicated that acoustic intensity was more important than other factors. As to the microcosmic mechanism, current researches are mainly about the characteristics of cavitation bubbles under ultrasonic field, while few works reported the mechanism of ultrasonic demulsification [20][21][22][23][24][25][26][27]. However, the physical properties between oil-water system and gas-liquid system are significantly different.…”