Self-Propelling Water Droplets on Conical Spikes with Sawtooth Surface Structure.

Directional fluid transport is critical for water, energy, and biomedical applications, including passive fog harvesting. The unique shape gradient of conical structures can induce capillary pressure and drive the self-propulsion of droplets as the droplets settle on wettable sharp cones and move toward the cone base as they grow. In this work, we achieve passive droplet transport by fabricating conical spikes with sawtooth and imbricated (reversed-sawtooth) surface structures via high-resolution 3D printing. Fog harvesting experiments on various spikes indicate that the sawtooth structure exhibits the most efficient droplet mobilization toward the spike base, while the imbricated surface structure promotes isolated droplet formation with delayed transport and the smooth spikes would keep droplets stationary unless coalescences occur. Further droplet motion analysis reveals that the flat surface with imbricated structure exerts 3.5 times more hysteresis force than the sawtooth one under dry conditions and nearly twice under wet conditions. During fog harvesting, microdroplets in fog fill the teeth gaps along the water-wet sawtooth spike, and the resulting big barrel droplet exhibits a series of stop-and-go motions when it continues growing. Our quantitative analysis reveals that the interplay between the capillary and hysteresis forces is responsible for the droplet self-propulsion. Our experiments with the conical sawtooth spike array further demonstrate that the fog water harvesting rate with 10 μm teeth spacing is twice that with 20 μm spacing and triple that with 40 μm spacing.