Droplet boomerang on hot surfaces

Droplets on hot surfaces can skid, spin, swing, or even hop, yet their motion is often stochastic. Despite progress in achieving directional droplet transport under specific boiling states, translating such stochastic dynamics into reconfigurable transport that adapts to evolving boiling states is still elusive. Here, we establish a boiling-state-mediated transport strategy in which the evolving boiling state switches the dominant driving mechanism on a surface featuring both wettability and topographical gradients. During contact boiling, droplets slide toward hydrophilic regions driven predominantly by wettability gradients. In the Leidenfrost regime, vapor-mediated propulsion dominates, directing droplets toward hydrophobic zones with pronounced rotation. At intermediate substrate temperatures, a boomerang-like transport mode emerges, where droplets initially migrate in the backward direction before spontaneously reversing to move forward. The reversal develops during an evaporation-mediated boiling-state transition, in which droplet shrinkage lowers the Leidenfrost temperature and the evolving interfacial state shifts the dominant driving mechanism from wettability-driven contact motion to vapor-mediated propulsion. A force-balance model further captures the transition boundaries among the three transport regimes. These findings position phase-change-mediated droplet steering as a route toward adaptive thermal and fluidic control.