Seminar. Benoit Scheid

We present a non-embedded co-flow-focusing configuration for single and double-emulsification in microfluidics. Experimental results demonstrate the universality of the geometry in terms of the variety of fluids that can be emulsified, as well as the range of droplet radii that can be obtained, without neither the need of surfactant nor coating. In a first part, a quasi-static model is developed for the generation of droplets and shows excellent predictive capability for their size. Our results demonstrate that the mechanism for the dripping-to-jetting transition in this quasi-static limit is intrinsically nonlinear and differs from the absolute-to-convective transition of the Rayleigh-Plateau instability for a jet. In a second part, we focus on the dynamics of droplets and bubbles in microchannels, and specifically on their equilibrium position and velocity, while assessing the influence of their deformability. We then present stability maps for the centred position showing that non-deformable objects dominated by inertial effects are only stable if large enough, whereas deformable objects dominated by capillary effects can be stable for much smaller sizes, except for intermediate viscosity ratios for which deformability also plays a destabilizing role, as for inertia.  We then specifically explore the fascinating behaviour of bullet bubbles, among which the super-fast ones that go faster than the maximum velocity of the carrying liquid phase. We finally show some practical examples of micro-encapsulation with double emulsions.