Pinning-Induced Evaporating Droplet Self-Propulsion*

Droplet formation and removal dynamics are ubiquitous processes found in nature and have significant impact on engineering applications. Droplets are prone to adhere on solid surfaces which contain unavoidable surface defects stemming from chemical and topographical heterogeneity. Although powerful, state-of-the-art techniques to overcome the contact line (CL) pinning are spatially limited due to surface energy gradients or require external energy to initiate droplet motion. Here, we show that intrinsic CL pinning on defects can be utilized to generate spontaneous droplet motion without any external energy input. Through experimental and theoretical analysis, we demonstrated that droplets can harness uniformly distributed surface defects to create CL curvature asymmetry and corresponding contact angle variation, which lead to a rapid motion of droplet for a wide range of conditions. Furthermore, we experimentally and numerically demonstrated that self-propelled droplets can enhance evaporation and carry contaminants inside as an alternate self-cleaning mechanism. In contrast to conventional understanding, the insights gained here reveal opportunities for taking advantage and tailoring CL pinning to achieve high droplet mobility.