Experimental study on anti-frost property and edge effect of superhydrophobic surface with millimeter-scale geometries

Frost suppression ability and edge effect are crucial for the frost formation on engineering surfaces. However, certain fundamental aspects remain ambiguous, particularly in the context of superhydrophobic surfaces featuring millimeter-scale structures.. In this study, condensation, frosting, and defrosting experiments were conducted on superhydrophobic surfaces with multiple millimeter-scale geometries. The results indicate that condensate droplets tend to grow on the top of right-angled, semi-circular protrusions, and conical structures. In the course of frost formation, superhydrophobic surfaces with millimeter-scale structures exhibit certain anti-frost properties when compared to the non-structured superhydrophobic surface. Condensation state of droplets can be maintained for a long time on the bottom area of the grooves, creating a frost-free zone. By observing the growth of frost crystals in various regions of the millimeter structure, it has been revealed that the manipulation of water vapor contact spaces can lead to the design of anti-frost surfaces with adjustable rates of frost crystal growth. Frost formation process on superhydrophobic surfaces can generate the so-called "edge effect" and is influenced by the edge structure. Frost crystals tend to form first on conical edges due to their sharp structural features. The average height of frost crystals on 90° edges, obtuse edges, and semicircular edges is significantly lower than that on conical edges, with the reductions of 31.8 %, 28.4 %, and 14.3 %, respectively. During the defrosting process, two spontaneous methods were observed to remove the frost layer: a "hand-in-hand" manner lifting off the groove and an "airfoil retraction" contraction on the protruding structures. Regardless of the structure shape, defrosting water floats on top and falls off, allowing for rapid drying of the surface and groove. This work has broadened the understanding of frost formation and defrosting on millimeter-structured superhydrophobic surfaces, and holds potential for applications in anti-frost engineering.