Dynamics of a Newtonian droplet in the turbulent flow of a shear thinning fluid in a microchannel

A systematic experimental investigation of the dynamics of a Newtonian drop in microscopic cross-slot of a shear thinning and inelastic fluid (aqueous solutions of xanthan) is presented. The flows are driven at large Reynolds numbers and are relevant to efficient high-throughput emulsification in microchannels. Depending on the initial size of the drop, the driving flow rates and the rheological behavior of the continuous phase, two fundamental dynamic modes are observed. The first dynamic mode relates to the trapping of the drop. By time-resolved tracking of both the positions and the deformations of the drop over 100 distinct drops a comprehensive statistical description of the trapping events is provided. The probability of trapping when xanthan solutions are used as a continuous phase follow a common trend when the effective strength of the swirling flow motion within the impingement region is gradually increased by tuning both the flow rates and the polymer concentration, suggesting that the trapping events emerge via an imperfect bifurcation. A second phenomenon that is of particular relevance to the emulsification process relates to the breakup of drops. The dynamics of the breakup process are quantitatively described in terms of the characteristic breakup times, number of emerging daughter droplets, and drop morphology are equally dependent on both the driving flow rates and the polymer concentration. Further physical insights into the intricate coupling between the flow conditions, the shear thinning rheology of the continuous phase, and the single-drop dynamics are obtained in terms of a quantitative description of the kinematics of drop deformation. This analysis was performed using a novel tool that allows one to assess the velocity distributions along the drop contours and extract the rates of deformation, the strains corresponding to the breakup process, and the kinematic print of the flow (shear or extension). Finally, a full diagram mapping all the modes of the single-droplet dynamics is presented.