Directional Transport of Coalesced Droplets on Superhydrophilic Wires

For various applications, including water harvesting, anti-icing, and condensation heat transfer, it is essential to achieve directional droplet transport on solid surfaces. A vast number of prior studies have reported that a smaller droplet moves towards a larger droplet when they coalesce into a single droplet on non-wetting flat surfaces. Our recent findings reveal the opposite-directional droplet transport following coalescence on superhydrophilic, prewetted wires. When the droplets contact each other on the wire, the merged droplet moves to the direction where the small droplet was placed. This counter-intuitive droplet transport is attributed to the difference between the shear stress in the two parent droplets. The shear stress difference is generated from the liquid flow due to the local pressure difference between each droplet and the liquid bridge linking the two droplets. As such stress is inversely correlated to the thickness of two parent droplets, the smaller droplet remains relatively pinned during the coalescence while the larger one easily migrates. On vertically positioned superhydrophilic wires, merged droplets can move upward based on the mechanism, overcoming the gravitational force when the Bond number is smaller than unity.