Determination of pivotal points during salty droplet freezing under various conditions

Abstract

As a typical solid-liquid phase transition phenomenon, water droplet freezing has attracted intensive attention in recent decades. However, droplets in natural environments are rarely pure but usually contain dissolved solutes, making their freezing behavior more complex. Among various solutions, saline water has attracted particular attention due to its ubiquity and environmental relevance, yet existing studies remain limited. Here, we systematically investigate the freezing characteristics of NaCl solution droplets with different concentrations under substrate temperatures above and below the eutectic point, and identify the key stages and pivotal points that characterize saline droplet freezing. For saline droplets with lower concentrations (ω < 23.3%) below the eutectic temperature, the freezing process involves the ice crystals growth and the hydrate precipitation, with both onset and completion precisely identified. In higher-concentration droplets (ω > 23.3%), the process proceeds via sequential NaCl crystallization and subsequent hydrate formation, and a temperature rise is observed on the temperature–time curve during the hydrate precipitation stage. The time fraction associated with each freezing stage are quantitatively compared among different freezing types. Then the factors governing hydrate-precipitation kinetics are examined under substrate temperatures below the eutectic point. Both decreasing temperature and increasing salinity accelerate the hydrate precipitation time. This study reveals the distinct freezing characteristics of saline droplets under different conditions and provides new insights into the development of anti-icing and cryogenic engineering technologies.