Twisted capillary flow: Sliding drops on twisted fibers

Droplet capture by complex fiber networks such as fog nets can greatly increase water security in communities with little to no rainfall. Here we investigate the sliding motion of a single droplet dispensed onto two adjacent vertical fibers that are either parallel or twisted together to an increasing degree. Post deposition the droplet assumes either an axisymmetric (barrel) or asymmetric (clamshell) shape around the fiber bundle. At long times the droplet may lose mass as it coats the fiber. Interestingly, we find that irrespective of viscosity, the groove created by the two adjacent fibers can stabilize the asymmetric shape, allowing for greater downward transport speeds than the axisymmetric configuration. The twisted fibers induce a wide range of sliding dynamics influencing the drop's shape and downward velocity, which are controlled by the drop's surface tension, viscosity, volume, and the fiber's radius and amount of twisting.