Self-propelling droplets on fibres subject to a crosswind

In many situations in which droplets wet fibres, wind is present. Large nets are used to harvest fog transported by coastal breezes from the ocean 1 , 2 and noxious aerosols are contained in chemical plants by driving them across fibrous filters 3 , 4 . In glass wool factories, thin fibres are subjected to airflows as they are simultaneously sprayed with resin to glue them together 5 , 6 . The control and reconfiguration of the liquid in these situations is essential. It can be set geometrically, as is the case for assemblies of non-parallel fibres 6 , 7 or tapered cylinders 8 – 11 , but the wind itself may also be exploited for this purpose 4 , 12 , 13 . Here, we show that a transverse wind can induce directional motion of droplets along horizontal fibres—even upwind if the fibre is tilted—and generate strong repulsive interactions between droplets. All of these effects are interpreted as consequences of asymmetric wakes behind the liquid. A transverse wind is shown to be capable of inciting a droplet to move along a horizontal fibre due to the presence of an asymmetric wake behind the droplet. Such a perturbation can even induce repulsive interactions between droplets.