Interfacial dynamics of NaCl solution droplets impact on cold surfaces: spreading and salt-induced phase transition

Abstract

Understanding the interfacial behavior of saline droplets impacting cold surfaces is critical for a wide range of thermal, environmental, and crystallization-related applications. This study experimentally investigates the spreading and phase separation dynamics of NaCl solution droplets on temperature-controlled hydrophilic silicon surfaces. Using a custom-built platform with high-speed imaging, we captured the full evolution of droplet morphology, including inertial spreading, salt-induced phase separation, and final solidification. The influences of impact height, surface temperature, and salt concentration were systematically investigated. A modified empirical correlation for the maximum spreading factor was developed based on hydrophilic surface conditions, while retaining the theoretical Weber number scaling while adjusting fitting parameters to reflect salinity-induced changes in viscosity and surface tension. Building on this, a thermal response time model was adapted to account for the influence of concentration-dependent thermal diffusivity and freezing point depression. By introducing a dimensionless temperature parameter, an empirical correlation was proposed to predict the visual phase separation time as a function of both inertial and thermal effects. This unified framework shows strong agreement with experimental data across a broad range of conditions. The findings provide new insights into multicomponent droplet impact behavior and offer predictive tools for engineering systems involving freeze desalination, anti-icing coatings, and phase-controlled thermal microfluidics.