Jumping of Saline Droplets from the Heated Hydrophobic Surface Coinduced by Evaporation and Crystallization

Spontaneous droplet jumping on surfaces is a fascinating phenomenon with significant applications in self-cleaning, anti-icing, and anticorrosion. However, existing research has primarily focused on pure water droplets under nonheated or condensation conditions, with limited attention to the behavior of heated, evaporating saline droplets commonly encountered in industrial processes such as seawater desalination and high-salinity wastewater treatment. This study reports, for the first time, two novel types of crystallization-induced jumping behaviors of saline (NaCl solution) droplets on superhydrophobic heated surfaces (static contact angle ≈ 157°, contact angle hysteresis ≈ 5°). Through systematic experiments with droplets of initial mass fractions ranging from 3.5% to 26.5% on surfaces heated between 60 to 260 °C, we identified two distinct jumping modes: explosive leaping and sustained small-amplitude bouncing. Our findings reveal that the competition between outward capillary flow and Marangoni flow governs the macroscopic morphology of the salt shells (fully sealed bowl-like vs partially open spherical-cap-like). Furthermore, the "vapor pressure vs shell strength" competition leads to explosive leaping, while the "vapor thrust vs droplet weight" competition results in sustained small-amplitude bouncing. A phase diagram based on surface temperature and initial droplet mass fraction is constructed, filling a research gap in spontaneous jumping of heated saline droplets and providing new insights for the design of intelligent antifouling equipment and process optimization in related industrial applications.