Dynamics of a single Newtonian droplet in a turbulent microscopic cross-slot flow of a shear thinning fluid
Résumé
An experimental characterization of the dynamics of a single Newtonian droplet within a non-Newtonian fluid in a cross-slot microchannel is presented. The dynamics of the droplet are quantitatively assessed by high-speed video imaging which allows one to measure the statistics of droplet sizes and shapes and to describe droplet breakup process. To assess the role of non-Newtonian shear thinning behavior on the droplet dynamics, xanthan solutions with various concentrations (0.05% − 0.2%) have been used as a continuous phase. The imposed flow rates are of the order of mL/min which are substantially higher than those used in conventional droplet-based microfluidics which renders the flow inertially unstable and, for the highest flow rates investigated, inertially turbulent. The experimental results indicate that the specific swirl motion observed at the impact of the two fluid streams in the cross-slot micro-channel increases the probability for droplets of getting trapped within the collision region. The oscillatory dynamics of the droplet shape within the trapping region is gradually inhibited when the shear thinning effects are gradually increased. The shear thinning rheology of the aqueous phase equally influences the breakup dynamics of the droplet. At intermediate Reynolds number the shear thinning behavior leads to the formation of very thin and long filaments. A decrease of the characteristic breakup time of droplet is observed when the polymer concentration increases. As the Reynolds number is increased and the flow turns turbulent, the shear-thinning effects delays the droplet breakup process. A full phase diagram of the dynamical modes observed by varying both the Reynolds number and the rheology of the continuous phase is presented.