Reconfigurable Slippery Gradient Structure for Programmable Droplet Self-Propulsion

Transporting millimeter-sized droplets on a surface is fundamental for droplet fluidics, which requires minimization of droplet sticking and energy input. One promising strategy is tethering the self-propelling ability to droplets on nonsticking surfaces. Self-propulsion is achieved by forming a gradient structure on the substrate. Typically, the gradient is highly directional and nonreprogrammable, making the regulation of transport challenging. Here, we report that droplets are self-propelled from the edge to the center of the substrate. The substrate surface is covered with a stabilized lubricant that exhibits droplet slipperiness. Near the substrate edge, a lubricant thickness gradient forms, which self-propels the droplet. This property enabled the regulated self-propulsion of droplets by spatiotemporal edge-shape design. The edge shape determines the direction of the droplet's self-propulsion. Moreover, the in situ edge appearance/disappearance, achieved by separating/connecting two neighboring coated substrates, enabled the regulation of self-propelling timing because the lubricant gradient structure is reconfigurable. We demonstrate droplets' self-climbing, upside-down self-propulsion, in situ stop-and-go motion, two-droplet attractive motion, and reconfigurable two-dimensional droplet motion. We also demonstrate that the sliding droplet on the zigzag-shaped substrate moves along with the substrate's shape due to edge self-propulsion. These droplet manipulations require no special devices, making them increasingly accessible.