Effect of surfactant on flow fields in droplets generated in microfluidics devices

We study the effect of surfactants on flow patterns inside a drop moving inside a rectangular microchannel both experimentally and numerically. In our experiments, we use Ghost Particle velocimetry to visualize flows. The numerical simulations are based on solving the full three-dimensional Navier-Stokes equations, coupled with two transports equations for surfactants in the aqueous bulk phase and on the interface. The results of this work demonstrate that surfactant dynamics has a crucial effect on flow patterns inside the drop. If surfactant equilibration rate is faster than the interface deformation rate then surfactant addition results in suppression of corner flow effect and can be described by change in the capillary number. In the opposite case, Marangoni stresses result in faster surface motion at corners and flow reversal in the central part of drop.