Unidirectional Droplet Transport on the Biofabricated Butterfly Wing.

Water droplet unidirectional transport on the asymmetric superhydrophobic surface has attracted much interest in theory analysis and applications, such as self-cleaning, antifogging, anti-icing, heat transfer, and so on. Different from the symmetrical performance on the uniform topographies, the droplets acting on the asymmetric surface exhibit an anisotropic state and easily roll off the surface along the special direction. This phenomenon is indicated by natural butterfly wings. The flexible asymmetrically arranged microstep induces the droplet to release along the outside radial (RO) direction and to pin against the RO direction. Here, inspired by butterfly wings, a kind of surface for superhydrophobic and unidirectional droplet transport is achieved by integrating the methods of soft lithography and enhanced crystal growth. The water droplet shows the anisotropic state on the biofabricated surface, and it rolls off easily along the step direction. The droplet is unidirectionally driven off the surface by the asymmetric surface tension force generated by the microstep topography. This experiment is significant for designing self-cleaning surfaces.