Collapse Behavior of Saltwater Droplets during Icing on Solid Surfaces

The icing behavior of saltwater droplets on solid surfaces is common in both natural environments and industrial applications. Its complex heat and mass transfer behaviors have significant implications for structural safety and freeze desalination technologies. In this study, the collapse phenomena of saltwater droplets during icing on solid surfaces were experimentally investigated in the condition when ambient air was oversaturated relative to the substrate temperatures. Three representative collapse patterns were identified and classified, and their underlying mechanisms and evolution processes were elucidated. Moreover, the intrinsic relationship between collapse behavior and surface thermal conductivity was revealed. On surfaces with low thermal conductivity, complex collapse behaviors are more frequently observed. Specifically, the spontaneous nucleation-recalescence of the droplet releases water vapor, which either promotes the growth of frost branches or condenses into droplets within the dry region─both pathways ultimately resulting in droplet collapse. In contrast, on high thermal conductivity surfaces, the collapse tends to follow a simpler pattern in which frost branches directly contact and trigger the collapse of the saltwater droplet. The collapse process may involve hybrid patterns, wherein multiple collapse mechanisms coexist, thereby accelerating the collapse. Additionally, collapse behaviors accelerate ice accumulation and promote the degradation of superhydrophobic coatings. These findings enhance the fundamental understanding of saltwater droplet icing dynamics on solid surfaces and provide insights for the design of freeze desalination systems and anti-icing technologies.