Impact Dynamics and Heat Exchange of Cold Droplet on Supercooled Macrotextured Nonwettable Surfaces

When a water droplet impacts a supercooled surface, it may freeze upon contact and adhere to it. Superhydrophobic surfaces, which repel water, are often used in anti-icing applications due to their ability to reduce ice adhesion by minimizing the contact time and contact area between the droplet and the substrate. However, they can often be ineffective, as ice nucleation may still occur when either the droplet, the surface, or both are supercooled. Here, we demonstrate that adding macrotextures to nonwettable substrates significantly influences droplet surface interactions and alters the freezing dynamics, enabling a shift from full droplet adhesion to partial or even complete rebound, even under supercooled conditions, which is favorable for ice nucleation. By capturing impact dynamics from both side and top views using high-speed imaging, we show that increasing the number of spokes reduces the fraction of the droplet adhering to the surface by shortening both contact time and contact area. In addition to impact dynamics, our results reveal that the thermal properties of the surface material play a key role in determining the dominant heat transfer mechanism, ranging from finite to effectively infinite heat exchange scenarios. A comparison between theoretical models for these two regimes and experimental observations highlights how thermal conductivity influences droplet freezing behavior. Moreover, heat transfer between the droplet and the surface, estimated through contact area measurements, decreases significantly as the number of spokes increases. These findings provide mechanistic insights into how both the macrotexture design and thermal properties of nonwettable surfaces can be optimized to mitigate ice accretion by modulating contact time, interfacial area, and freezing dynamics.