Burr-like structures growth and diffuse freezing front during saltwater droplet impact freezing

The underlying segregative freezing mechanism of saltwater is widely relevant to metallurgy, materials processing, environmental systems, and biomaterials. Under the effect of ion exclusion, the freezing process of saltwater may significantly differ from that of pure water due to variations in dendritic growth. Moreover, differences in droplet supercooling significantly influence dendrite formation at the freezing front, thereby affecting heat transfer during crystallization.

We prepared NaCl solutions covering the salinity range of marine conditions and utilized hydrophobically microstructured surfaces. Under controlled temperature and humidity, we conducted experiments on the impact freezing of saltwater droplets and observed the morphology and growth of the freezing front.

The freezing interface displayed distinct burr-like structures-irregular, short, and sharp protrusions-which evolved into more typical dendritic patterns under enhanced supercooling. Based on these observations, we proposed a new heat transfer calculation method using the Lattice Boltzmann Method, incorporating the effect of ion exclusion. Simulation results confirmed that the presence of burr-like structures enhances thermal conduction, and the model is also applicable to pure water freezing. Furthermore, we found that thickened phase change regions, slowed freezing at the droplet top, and reduced freezing depth lead to a blurred interface in later stages and disappearance of the freezing tip (singularity). Finally, we quantified the influence of salinity on freezing time, showing that salinity governs freezing depth and plays a critical role in the reduced mechanical hardness of saltwater ice.