Bubble surface mobility regimes in mixed aqueous solutions of short chain alcohols and fluorosurfactant.

Studies of bubble behavior in mixed surfactant solutions attract rising interest due to growing industrial applications. In this work, we study the mobility of a bubble surface in aqueous solutions of short chain alcohols (ethanol, propanol) mixed with a stronger fluorosurfactant with lower surface tension thus enabling the observation of changes of surface tension. Understanding interfacial processes on the rising bubble relied on analysis of equilibrium and dynamic surface tension of individual surfactant solutions and their mixtures. The surface of bubble rising in surfactant solution without alcohol additive is immobile across the entire range of concentrations studied. At low alcohol concentrations bubble surface remains immobile, with both alcohol and surfactant adsorbing at the interface. With increasing alcohol concentration, alcohol molecules replace surfactant in the surface layer. Fitting the surface tension data for the mixture using co-surfactant model shows that such replacement can be due to strong interaction between alcohol and surfactant resulting in a considerable decrease in surfactant activity. As a consequence, at propanol molar fraction 0.3 and ethanol molar fraction 0.5, surfactant adsorption is completely hindered while alcohol dominates the adsorption layer. The gradients of surface tension are diminished resulting in remobilization of the bubble surface, which manifests itself in an increase in terminal bubble rising velocity. The study revealed that surface remobilization in mixed solutions occurs at higher alcohol concentrations than in surfactant-free solutions.