Droplet fragmentation in microfluidic devices

International audienceWe study both experimentally and theoretically the fragmentation of droplets colliding with a microfluidic obstacle placed in a conduct, a phenomenon which occurs when the capillary number at play exceeds a critical value. When the distance between droplets is large enough so that they are isolated hydrodynamically, we identify the seven dimensionless parameters of the problem that control fragmentation. We introduce a simple theoretical framework that allows one to well predict the no-breakup/breakup transition and the volumes of the two daughter droplets produced when breakup occurs in terms of all possible experimental variables [1,2]. Our findings outline the crucial role played by the viscosity contrast between disperse and continuous phases on the breakup of confined objects. By working with one-dimensional periodical assemblies of drops, we also show the possible emergence of complex fragmentation dynamics for which the sizes of the daughter droplets created upon breaking of mother drops may become a periodic function of time. We demonstrate that this phenomenon results from time-delayed feedbacks between successive breakup events induced by drop-to-drop hydrodynamic interactions. We derive the existence of numerous bifurcations between periodic regimes and we establish diagrams mapping the various regimes observed as a function of the governing (physicochemical, hydrodynamic and geometrical) parameters. Refs: [1] L