Coalescence of Bubbles and Stability of Foams in Brij Surfactant Systems

Abstract

This work presents coalescence of air bubbles and stability of foams in aqueous solutions of polyoxyethylene (4) lauryl ether (Brij 30), polyoxyethylene (23) lauryl ether (Brij 35), and polyoxyethylene (20) oleyl ether (Brij 98). Adsorption of these surfactants at the air-water interface was also studied. The surface tension profiles were fitted by the Szyskowski equation. It was found that the surface tensions at the critical micelle concentrations of these surfactants followed the sequence Brij 35 > Brij 98 > Brij 30. The air bubbles were most stable to coalescence in Brij 35 solution and least stable in Brij 30 solution. Coalescence in Brij 35 solution was hindered beyond a certain concentration of the surfactant. Stochastic distributions of coalescence time were observed in all surfactant systems. These distributions were fitted well by the stochastic model. Seven film-drainage models were used to predict the coalescence time. However, their predictions did not agree well with the mean values of the coalescence time distributions. Stability of foams was analyzed by the Ross-Miles test. The initial and residual foam heights were measured at each surfactant concentration. The foam heights followed the trends that are expected from the coalescence times of the bubbles.

Physics of Electrolytic Gas Evolution

“…In contrast to these equilibrium measurements, studies of the dynamics of gas evolution showed that the bubble formed by the coalescence of two large bubbles would jump off the electrode and sometimes even return [15]. Other authors [27,28,[33][34][35] have also reported that bubble coalescence often precedes their detachment from the electrode surface. It was concluded that the expanding boundaries of the new bubble mechanically forced it off the electrode.…”

Factors affecting the coalescence stability of microbubbles

“…Due to its practical importance, the coalescence stability of foams was extensively studied in the literature for many years [11][12][13][14][15][16][17][18][19][20]. In the course of these studies it was found that there is a critical capillary pressure, above which the foams become unstable [11][12], as well as the existence of threshold air volume fraction, above which the rate of bubble coalescence dramatically increases [13].…”

“…For foams formed from solutions with relatively low surfactant concentration, around or below the CMC, the coalescence between bubbles is very pronounced even at low air volume fraction and have to be taken into account when analyzing the foamability of such solutions. In several recent papers Ghosh et al [16][17][21][22] studied the bubble stability at different surfactant concentrations, for various surfactants and surfactant mixtures. It was shown that the coalescence time has stochastic distribution for all studied surfactants.…”

Comparison of Bubble Size Distributions Inferred from Acoustic, Optical Visualisation, and Laser Diffraction

“…The crux of the hypothesis lies in the smaller population of the bubbles in order to test whether they 'hide' among the larger ones, which can be achieved using the fluidic oscillator. Whilst larger bubbles are simple to generate, smaller bubbles generated with a ceramic sparger are not as simple unless noncoalescent media [61] or surfactants [62,63] are added, which affect the sizing and visualisation paradigms.…”