Pinning-Induced Microdroplet Self-Transport.

Droplets are prone to adhere or "pin" on solid surfaces which contain unavoidable micro- and nanoscale surface defects formed through chemical and topographical heterogeneity. To initiate droplet motion, potential energy gradients, surface energy gradients, or external energy input are needed. Here, in contrast to established wisdom, we show that properly designed surface heterogeneity can promote microdroplet self-transport without any external force or anisotropy. In the presence of topological defects, microdroplets can take advantage of contact line pinning to generate contact line and corresponding contact angle asymmetry, leading to spontaneous motion over distances 10-20 times larger than the droplet radius. The outcomes of this work present an alternative pathway for taking advantage of intrinsic surface heterogeneity to achieve droplet mobility in a range of applications, where passive droplet motion is desired.