Influence of molecular structure of alcohol-water mixtures on bubble behaviour and bubble surface mobility

Abstract

The behaviour of spherical bubbles rising in water-ethanol and water-propanol mixtures was investigated experimentally for the whole range of concentrations, from pure water to pure alcohol. These two alcohols were chosen for their importance in industrial applications and for their peculiar properties resulting from the formation of organised microstructures in the non-ideal mixture at a molecular level. The effect of the composition of the alcohol-water mixtures on the physicochemical properties of the liquid was evaluated for all the solutions investigated, compared with literature values to critically access the effect of the concentration of alcohol on the bulk and interfacial properties of the mixtures. The hydrodynamic behaviour of bubbles rising in the mixtures was evaluated, by using high speed video to measure the bubble size and terminal velocity. From the corresponding drag coefficients, the mobility of the bubble’s surface was determined revealing that in alcohol-water mixtures there are three regions: (1) with very low alcohol concentrations (xE ≤ 0.01, xP ≤ 0.005), the solutions behave as surfactant solutions with increase of bubble’s drag coefficient with increase of the alcohol concentration till total immobilization of the bubble surface; (2) at intermedium concentration, the drag coefficient decreases as the concentration of alcohol increases with consequent increase of competition for location of alcohol molecules at the interface or on the bulk with formation of molecular network; (3) at high concentration (xE ≥ 0.20 and xP ≥ 0.07), the bubbles have again fully mobile surface as in a pure liquid.