Microfluidics investigation of foam stability

Aqueous foams are dispersions of gas bubbles in a liquid continuous phase and they are important for various applications from biological systems to Enhanced Oil Recovery. Foam stability relies on the use of surfactants/nanoparticles and on the rheological properties of the foaming liquid. One of nowadays challenges is to solve key foam industrial issues while considering their environmental impact. This duality is addressed by employing microfluidics to study static and dynamic foam stability. In this thesis the foam stability is first studied in static conditions by investigating the use of highly stable bubbles termed bulk nanobubbles as potential surface-active agent on non-ionic and anionic surfactant-stabilised foams. This work has shown that the presence of nanobubbles in solution impacted significantly the foam stability and foamability. These findings suggest that nanobubbles attracted surfactant molecules at their interface and that nanobubbles adsorbed at the foam gas-liquid interface. At low surfactant concentration, nanobubbles attraction removed surfactants molecules from the gas-liquid interface. This effect increased destabilisation of non- ionic surfactant foam. Oppositely, for anionic surfactant foams, the presence of nanobubbles enhanced foam stability. Indeed, nanobubbles enhanced the disjoining pressure within the thin liquid film due to an increase in the electrostatic repulsive forces between the two contiguous interfaces forming the film. Subsequently, the rheological properties of the foaming liquid are considered. In particular, the influence on the “dynamic” foam stability or the property of the foam to resist and/or recover after the deformation was evaluated in a microchannel. This work proposed a microfluidics approach to determine qualitatively the foam hysteretic behaviour after an induced deformation. This study has found that the viscosity and the bulk elasticity reduced the foam recovery. The result of this investigation showed that two mechanisms led to foam structural hysteresis after deformation: a retardation effect linked to the increase in viscosity and a tension thickening effect arising from the bulk elasticity