Modeling Directional Droplet Transport on Wettability-Confined Wedge Tracks

Wettability-confined tracks facilitate the spontaneous pumpless transport of droplets on open-surface microfluidic devices. In recent experimental and computational studies, the speed of the advancing front of such self-propelled droplets on the hydrophilic or superhydrophilic wedges has been shown to vary with the wedge angle, volume of the droplet, and the shear-thinning nature of the liquid. However, the influence of wettability difference between the two domains, as described by the difference between the contact angle of the hydrophilic wedge and the surrounding hydrophobic surface, is less well understood. In this study, we analyze the effect of such wettability difference on the droplet transport under consideration. To do so, we setup the FEM-based multiphase simulations in COMSOL Multiphysics software and show the variation in the speed of the advancing front of the droplets with respect to different combinations of hydrophobic and hydrophilic contact angles. The multiphase flow simulations are modelled using the Phase-Field method. The results are validated against published experiments.