Wet-a-materials: designing hairy surfaces for droplet manipulation

Many insects evolve hairy pads to interact with wet or mucus-covered environments. In turn, the mechano-geometry of the soft substrates determines the fate of liquids attaching to the hairs. Here, we elucidate such complex behaviors by investigating the process of one liquid droplet migrating between two elastic fibers. We show that the splitting proportion of a droplet between two fibers has a simple and sensitive dependence on the hair orientations. We then couple the substrate deformation with such fiber orientations, facilitating a novel functionality of sustained droplet transport among arrays of fibers. Our work points to a new regime of material design: combining elasticity and wetting properties through soft texture arrangements. Our results could be adapted to understanding biological behaviors or aiding designs in pharmaceutical manufacturing.