Droplet impact and freezing dynamics on cold superhydrophobic surfaces under shear airflow

Droplet impact and freezing on cold superhydrophobic surfaces are ubiquitous phenomena in nature and engineering applications. Although this multiphysics process has been extensively studied, the coupled effects of aerodynamic shear, nucleation delay, and phase-change-induced pinning remain insufficiently understood. In this study, a numerical framework integrating the multiphase thermal lattice Boltzmann method with classical nucleation theory is constructed to systematically investigate the joint effects of the droplet Weber number, airflow Reynolds number, and substrate temperature on the phase-change timing, contact characteristics, and freezing dynamics of droplets. The results indicate that shear airflow breaks the symmetry of contact-line dynamics, leading to anisotropy in the streamwise and transverse spreading; this asymmetric evolution is further nonlinearly suppressed by freezing-induced contact-line pinning. Focusing on the icing onset time, a critical transition that arrests droplet dynamics, this study reveals a dual mechanism whereby impact inertia and aerodynamic shear accelerate nucleation by enhancing heat transfer and expanding the contact area. Accordingly, a variable-exponent power law between the icing onset time and the degree of supercooling is established. Subsequently, considering the timescale competition between nucleation delay and maximum spreading, a staged theoretical model is developed to predict the surface icing-pinning ratio, which combines an asymptotically matched unified scaling law with the modified Taylor-Culick liquid film retraction theory. Furthermore, four typical impact-freezing patterns are identified: complete rebound, partial rebound, satellite adhesion, and complete adhesion. Under strong airflow, an anomalous partial-rebound behavior induced by air-cushion thermal resistance and inertial detachment is also revealed. Finally, based on a modified Weber number and the icing-pinning ratio, a generalized regime map is constructed to classify icing-risk levels within the investigated conditions. Overall, the present results provide mechanistic insights and a quantitative framework for understanding and assessing droplet impact-freezing under coupled aerodynamic and thermal conditions.