Directional Motion of Coalesced Viscous Droplets on Fibers with Wettability Gradients

The coalescence and directional migration dynamics of oil droplets on wettability gradient fibers were investigated using the volume of fluid (VOF) method combined with an improved dynamic contact angle model. We clarified the effects of initial configuration, wettability gradient, and liquid viscosity on droplet morphology, migration velocity, and viscous dissipation. The results indicate that after coalescence, droplets migrate directionally along the wettability gradient, and a larger gradient leads to a higher migration velocity. Low viscosity droplets exhibit noticeable oscillations during acceleration, while high viscosity droplets move more smoothly due to increased energy dissipation. As viscosity increases from 0.024 Pa s to 0.093 Pa s, normal strain dissipation dominates the total viscous dissipation, accounting for about 63% at the peak stage, corresponding to liquid bridge formation and strong droplet deformation. The average sensitivity of maximum velocity to viscosity is approximately 7.5%, with stronger competition between driving and resistive forces in the low viscosity regime and a transition to viscosity dominated behavior at higher viscosities. A stronger wettability-driven force slightly weakens the suppressive effect of viscous resistance and increases kinetic energy conversion efficiency. These findings provide insight into the migration dynamics of microdroplets driven by wettability gradients.