Impact and icing of a single water droplet on smooth and superhydrophobic surfaces: Decoupled modeling of interfacial thermodynamics and bulk solidification

Elucidating the transient impact and icing dynamics of a single water droplet is critical for improving the efficacy of anti-icing agents, but the complex coupling of fluid flow, phase change, and micro-scale interfacial phenomena poses a significant challenge. This study presents a decoupled numerical modeling framework that mathematically separates microscopic interfacial thermodynamics from the physical resistance of bulk solidification. Interfacial thermodynamics are integrated into the macroscopic solver via experimentally prescribed temperature-dependent dynamic contact angles, while bulk solidification is governed by a universal mushy zone constant. The framework is validated against droplet impact experiments on two substrates (a smooth hydrophilic surface and a superhydrophobic surface) across varying thermal regimes (18, −20, −25, and −30 °C). The model accurately reproduces isothermal hydrodynamics and captures the anomalous spreading and early contact line pinning observed on the smooth hydrophilic surface under sub-zero conditions. On the superhydrophobic surface, macroscopic pinning at −25 and −30 °C is successfully predicted. However, at −20 °C, the model predicts pinning instead of the experimentally observed complete rebound. By maintaining this decoupled framework and avoiding arbitrary parameter tuning, this study objectively demonstrates that this discrepancy originates from the absence of a microscopic thermal contact resistance model, which accounts for the mitigated cooling rate caused by the superhydrophobic micro-textures.