Uphill directional passive transport of water droplets on axisymmetric surfaces

Spontaneous directional transport of droplets by a surface curvature gradient, adopted by many biological species such as cactus and sand moss, is particularly suitable for applications including anti-icing, self-cleaning, and water harvesting, which eliminates the need for external energy input. However, this directional droplet transport is limited to short transport distance and no maneuverability, i.e., droplets can only migrate toward a flatter region and gradually stop. Fixed structures that can regulate droplet movement, if they could be created, would significantly advance their applications in a variety of areas. In this work, we propose a method to regulate the spontaneous motion of droplets on solid surfaces using surface curvature gradients. Molecular dynamics simulations show that droplets on general bowl-shaped axisymmetric surfaces can travel in the uphill direction (from the base to the apex) and move continuously to the apex with almost a constant speed. The mechanisms governing opposite directional transport of droplets are explained, and the conditions required to guarantee the transport in the reversed direction are discussed.