Short-time asymmetric droplet coalescence dynamics on a pre-wetted fiber

This study presents an unprecedented directional transport phenomenon during the coalescence of two droplets on a pre-wetted cylindrical fiber, where the larger droplet is pulled toward the smaller one. The magnitude of this effect often exceeds the gravitational pull, enabling coalescing droplets to climb up a vertical fiber. This occurs primarily because the viscous friction that the droplets experience is negatively correlated with the droplet size. We present a scaling relation and a mass-spring-damper model to explain the phenomenon, which shows good agreement with the experimental results. This research reveals an intriguing aspect of the coalescence dynamics of droplets on a pre-wetted fiber, offering a fresh perspective on the interfacial phenomena in droplet–fiber systems.