Cyclic diphasic flows for well-controlled foam generation -Thibaut Gaillard (LPS, Orsay)

Major challenges in two-phase flows are concerned with fluid dynamic instabilities which result in the dispersion of one fluid in another. Such kind of instabilities can arise under a wide range of flow conditions – each producing characteristic size distributions of the dispersed phase. Here we consider gas/liquid dispersions which are generated when a gas and a liquid are pushed simultaneously and repeatedly through a constriction which connects two syringes .We use this technique to study the generation of bubbles stabilised by surface active agents. After motorization of the set-up we investigate how the different processing conditions affect the properties of the obtained bubble size distributions. We show that this technique is unique in producing foams with very small bubbles (radius of the order of 10 mm) having very reproducible size distributions over a wide range of device and flow conditions (tubing, fluid velocities, etc.). The only parameters which seem to influence the obtained size distributions are related to the physical chemistry of the system (fluid viscosity and type of foam stabiliser). Using a high-speed camera we directly observe the conformation of the gas and liquid at different position in the constriction, and report two different foaming mechanisms producing tiny bubbles. To improve our understanding of the associated stresses which drive these mechanisms we study the dimensionless numbers of the flow within the set-up, and compare our experimental results with drop size distributions in liquid-jet fragmentation processes. Finally, we investigate separately these two mechanisms using purpose-designed millifluidic experiments with different geometries.