Study of the Coalescence of Acoustic Bubbles as a Function of Frequency, Power, and Water-Soluble Additives

Abstract

The effect that surface-active solutes, such as aliphatic alcohols and sodium dodecyl sulfate (SDS), have on the extent of bubble coalescence in liquids under different sonication conditions has been investigated by measuring the volume change of the solution following a period of sonication. In general, the adsorption of surface-active solutes onto the bubble surface retards bubble coalescence. Within the limitations of the measurement method and the systems studied, bubble coalescence does not appear to be dependent on the applied acoustic power. Also, varying the applied acoustic frequency has a minimal effect on the extent of bubble coalescence in systems where long-range electrostatic repulsion between bubbles, imparted by the adsorbed surface-active solutes, dominates. However, when short-range steric repulsion (or other short-range repulsive forces) is the primary factor in inhibiting bubble coalescence, the dependence on the applied acoustic frequency becomes apparent, with less coalescence inhibition at higher frequencies. It is also concluded that SDS does not reach an equilibrium adsorption level at the bubble/solution interface under the sonication conditions used. On the basis of this conclusion, a method is proposed for estimating nonequilibrium surface excess values for solutes that do not fully equilibrate with the bubble/solution interface during sonication. For the case of SDS in the presence of excess NaCl, the method was further employed to estimate the maximum lifetime of bubbles in a multibubble field. It was concluded that an acoustic bubble in a multibubble field has a finite lifetime, and that this lifetime decreases with increasing applied frequency, ranging from up to 0.35 +/- 0.05 ms for 213 kHz to 0.10 +/- 0.05 ms for 1062 kHz. These estimated lifetimes equate to a bubble in a multibubble field undergoing an upper limit of 50-200 oscillations over its lifetime for applied ultrasound frequencies between 200 kHz and 1 MHz.

Ultrasonic Cavitation at Solid Surfaces

“…It is also concluded that sodium dodecyl sulfate does not reach an equilibrium adsorption level at the bubble/solution interface under the sonication conditions used. [ 16 ] Low-density bubble clusters were observed in the presence of 1 m M sodium dodecyl sulfate and 0.1 M n-propanol. [ 17 ] When 0.1 M sodium chloride was added to 1 m M sodium dodecyl sulfate solution, the extent of clustering and the density of the clusters enhanced signifi cantly; a similar observation was made at a higher sodium dodecyl sulfate concentration (10 m M ).…”

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

“…However, some direct diffusion of highly soluble products from the bubble can occur [52]. The maximum lifetime of a bubble in a multibubble environment has been found to range from 0.35 ± 0.05 ms in a 213 kHz field and 0.10 ± 0.05 ms in a 1062 kHz field [90]. If the lifetime of active bubbles is shorter than the lifetime of the radicals produced, then the radicals are released into the liquid (via fragmentation or dissolution) where they may trigger chemical reactions [91][92][93].…”

“…The ionic surfactant adsorption of a hydrophobic solute can cause a reduction in coalescence via steric and electrostatic repulsion particularly when surface-active solutes are charged [31,90]. This is evident when salt, which acts to inhibit electrostatic effects, is present in dodecyl trimethyl ammonium chloride (DTAC) solutions; the associated electrostatic effects of the surfactant are no longer observed [141].…”

Influence of surface tension on cavitation noise spectra and particle removal efficiency in high frequency ultrasound fields

“…On the contrary, for charged surfactants such as sodium dodecyl sulfate (SDS) and dodecyl trimethyl-ammonium chloride (DTAC), electrostatic repulsion plays a major role in inhibiting bubble coalescence. 24,25 This in turn affects bubble growth and bubble size distribution. Therefore, by using a non-ionic agent, the stress is put mainly on the investigation of the effect of bulk surface tension reduction.…”