Self-propelled droplet dynamics on superhydrophobic surfaces under low-pressure conditions

The self-removal of sessile droplets under low-pressure conditions is critical for extreme-environment surface engineering. Combining high-speed photography with quantitative analysis, we reveal the unique dynamics of ethylene glycol-water solution droplets on irregular micro-nanostructured superhydrophobic surfaces. The experimental results demonstrate asymmetric depinning driven by evaporation-induced local overpressure gradients, generating a recoil momentum toward the pinned side that propels the droplet to move along the surface at a steady acceleration. This acceleration follows a quadratic scaling law with the evaporation flux. When the local overpressure exceeds the triple-phase line pinning forces, the droplets transition to the levitation phase. However, the unevenness of the surface micro-nanostructures leads to the instability of levitation, causing the droplet to rapidly recontact the surface and enter the secondary steady acceleration phase. Subsequently, the droplet maintains a dual-phase periodic transition mode of acceleration-levitation until it disappears from the field of view. In this study, we elucidate the synergy between recoil momentum and dynamic overpressure, advancing the fundamental understanding of low-pressure complex droplet dynamics and enabling surface morphology design strategies for droplet control in extreme environments.