Dynamics of microscale droplet impact on superhydrophobic cold surfaces

Understanding the dynamics of droplet impact on superhydrophobic cold surfaces is essential for developing effective anti-icing strategies in low-temperature environments. However, most previous studies have focused on millimeter-scale droplets, and the behavior of microscale droplets, which are common in practical environments, remains largely unexplored. To address this gap, we conducted systematic experiments spanning millimeter to micrometer scales and characterized the outcomes using phase diagrams, maximum spreading factors, contact times, and restitution coefficients. The results show that the spreading stage consistently follows the classical inertia–capillarity scaling and remains unaffected by temperature reduction or droplet size. In contrast, the retraction stage is highly temperature sensitive and strongly size dependent. As surface temperature decreases, contact time is markedly prolonged due to slower retraction, restitution coefficients decline, and smaller droplets increasingly lose their rebound ability under supercooled conditions. To explain this behavior, we propose a thermal diffusion–dynamics timescale criterion that clarifies the size-dependent mechanism of freezing onset. The criterion decreases with droplet size and shows that smaller droplets complete interfacial cooling more readily within the limited dynamic window, which triggers localized freezing. This work deepens the understanding of droplet impact dynamics on cold interfaces and provides theoretical foundations for the rational design of anti-icing and de-icing surfaces in microdroplet environments.