Coalescence and Self-Propelled Dynamics of Micron-Sized Oil Droplets on a Fiber with Wettability Gradient

This work investigates the coalescence and directional movement of microsized oil droplets on a fiber with a wettability gradient using numerical simulations. The volume of fluid (VOF) method and an improved dynamic contact angle model were employed to examine the effects of the wettability gradient on droplet coalescence and motion. The results indicate that as the wettability gradient increases, the droplet movement velocity accelerates, with a maximum velocity of approximately 0.2 m/s. The peak droplet velocity occurs when the trailing edge crosses the wettability transition, and the larger the contact angle on the high-contact angle side, the higher the velocity. The coalescence and self-propelled motion of micron-sized viscous droplets on fibers with wettability gradient occur in the low-Reynolds number regime. Dimensionless analysis shows that as the droplet-to-fiber diameter ratio increases, the velocity increases, but the rate of increase diminishes. The curvature of the fiber surface restricts droplet spreading, causing the maximum velocity to be 25–30% lower than that on a flat surface. During coalescence, microsized oil droplets release nearly 7–12% of their surface energy.