Delaying Frost Formation by Controlling Surface Chemistry of Carbon Nanotube-Coated Steel Surfaces.

Superhydrophobic surfaces are appealing as anti-icing surfaces, given their excellent water repellent performance. However, when water condenses on the surface due to high humidity, the water becomes pinned, and superhydrophobic surfaces fail to perform. Here we studied how the stability of the superhydrophobicity affected water condensation and frost formation. We created rough surfaces with the same surface structure, but with a variety of surface chemistries, and compared their antifrost properties as a function of intrinsic contact angle. Frost initiation was significantly delayed on surfaces with higher intrinsic contact angles. We coupled these macromeasurements with environmental scanning electron microscopy of water droplet initiation under high humidity conditions. These provide experimental evidence toward previous hypotheses that for a lower intrinsic-angle rough surface, Wenzel state is thermodynamically favorable, whereas the higher intrinsic-angle surface maintains a Cassie-Baxter state. Surfaces with a thermodynamically stable Cassie-Baxter state can then act both as antisteam and antifrost surfaces. This research could answer the persistent question of why superhydrophobic surfaces sometimes are not icephobic; anti-icing performance depends on the surface chemistry, which plays a critical role in the stability of the superhydrophobic surfaces.