Monodisperse fragmentation in emulsions: Mechanisms and kinetics

How can a crude polydisperse emulsion be transformed into a monodisperse one? Mason and Bibette (Phys. Rev. Lett., 77 (1996) 3481) have experimentally discovered this phenomenon by applying a shear step on a crude emulsion. In this paper, we examine how this transformation occurs. Our strategy is to prepare calibrated emulsions and to examine the fragmentation kinetics as a function of the initial droplet size. We show that the fragmentation process involves two distinct regimes. At short time (shorter than one second), the droplet diameter decreases abruptly. The droplets deform into long threads that undergo a Rayleigh instability. The obtained diameter is mainly determined by the applied stress and weakly depends on the viscosity ratio between the dispersed and continuous phases. After this first step, the resulting droplets can, once again, break up into daughter droplets. This second mechanism is much slower with a characteristic time of several hundred seconds. Depending on the initial size, the first step can vanish and only the second slow step subsists.