Surfactant effects on pressure-driven drop motion in a slit and Y-shaped channel

The effect of insoluble surfactants on deformable drop motion in a pressure-driven flow through straight and Y-shaped microchannels at low Reynolds numbers is investigated using three-dimensional boundary-integral and flow-biased least squares methods. Generally, drop steady-state velocity decreases and drop deformation increases with increasing drop diameter, though non-monotonic behavior of axial velocity versus droplet size was observed in the limit of small drop viscosity. In a straight channel, at high surface Péclet number, Marangoni stresses partially immobilize the drop surface, significantly reducing its steady velocity, while surfactant accumulation at the rear tips of the droplets lowers interfacial tension and induces larger deformations to satisfy the normal stress balance. Numerical results predict the droplet cross-stream migration speed to be dependent on surfactant concentration, with the contaminated droplet migrating faster than the non-contaminated one when the capillary number is O(1), while at Ca≪1, the opposite effect is observed. Finally, we study the effect of surfactant contamination on the volume partition of a droplet near the junction of a symmetric Y-channel for different flow ratios into the branches of the channel.