Evaporation and crystallization behaviors of saline droplets on heated hydrophobic nanoporous substrates

Salt crystallization on membrane surfaces leads to the formation of solidified crusts that critically compromise desalination performance, necessitating effective strategies for crust removal. Although the self-ejection or self-lifting behavior of salt crystals during evaporation of saline droplets has been examined on various smooth and rough substrates, evaporative crystallization on nanoporous substrates remains largely unexplored. Here, we systematically investigate the evaporation and crystallization dynamics of sodium chloride solution droplets on heated hydrophobic nanoporous substrates. Our results reveal that both the evaporation rate and the crystal growth rate exhibit a relatively weak dependence on the initial saturation ratio for saline droplets, yet increase substantially with substrate temperature. When the initial saturation ratio exceeds a critical value (S0≳0.8) and/or the substrate temperature surpasses a threshold (Ts≳60 °C), the probability of crystal leg formation reaches 100%. Moreover, a closed-pore structure promotes leg growth, which might be attributed to elevated pore pressure. These findings provide mechanistic insights into controlling evaporative salt crystallization and offer practical applications in high-salinity wastewater treatment and the development of antifouling desalination technologies.