Figure 4. Selected snapshots showing the frost morphology evolution over time on three different substrates.

(a) Flat hydrophobic surface: After one hour of condensation frosting experiment, the whole hydrophobic surface is covered by thick frost layer (time: 0 s). The frost is fractured into many irregular pieces during the defrosting stage, which can be clearly seen in zoom-in images at 65 s. With the time progression, the fractures avalanche in a domino-like process, leading to the formation of enlarged cracks, as shown in the image at 80 s. Finally, the hydrophobic surface is covered by many scattered sticky water droplets (~110 s). (b) Nanograssed superhydrophobic surface: During the defrosting process, the nanograssed surface exhibits a relative low fracture density and the melting frost becomes a large spherical water droplet at the culmination of defrosting (~95 s). (c) Hierarchical superhydrophobic surface: During the defrosting stage, the frost preserves its integrity without the formation of visible cracks and eventually detaches from the surface within a relatively shorter time (~85 s) in the first frosting/defrosting cycle. The unique defrosting behavior is sustained in the repetitions of frosting/defrosting experiments (see the defrosting cycles 2 and 3). Moreover, there is no obvious degradation in the defrosting time, suggesting the remarkable stability of the hierarchical surface in sustaining its efficient defrosting behavior.