Dynamic Defrosting on Scalable Superhydrophobic Surfaces

Abstract

Recent studies have shown that frost can grow in a suspended Cassie state on nanostructured superhydrophobic surfaces. During defrosting, the melting sheet of Cassie frost spontaneously dewets into quasi-spherical slush droplets that are highly mobile. Promoting Cassie frost would therefore seem advantageous from a defrosting standpoint; however, nobody has systematically compared the efficiency of defrosting Cassie ice versus defrosting conventional surfaces. Here, we characterize the defrosting of an aluminum plate, one-half of which exhibits a superhydrophobic nanostructure while the other half is smooth and hydrophobic. For thick frost sheets (>1 mm), the superhydrophobic surface was able to dynamically shed the meltwater, even at very low tilt angles. In contrast, the hydrophobic surface was unable to shed any appreciable meltwater even at a 90° tilt angle. For thin frost layers (≲1 mm), not even the superhydrophobic surface could mobilize the meltwater. We attribute this to the large apparent contact angle of the meltwater, which for small amounts of frost serves to minimize coalescence events and prevent droplets from approaching the capillary length. Finally, we demonstrate a new mode of dynamic defrosting using an upside-down surface orientation, where the melting frost was able to uniformly detach from the superhydrophobic side and subsequently pull the frost from the hydrophobic side in a chain reaction. Treating surfaces to enable Cassie frost is therefore very desirable for enabling rapid and low-energy thermal defrosting, but only for frost sheets that are sufficiently thick.

Droplet Retention on Superhydrophobic Surfaces: A Critical Review

“…Moreover, superhydrophobic surfaces demonstrate a faster removal of the melted thick frost than a smooth and hydrophobic surface. [ 182 ] This is because there exists an air layer underneath the formed frost on superhydrophobic surfaces so that the water droplets after melting are in the low‐retention Cassie–Baxter state.…”

Frost Self-Removal Mechanism during Defrosting on Vertical Superhydrophobic Surfaces: Peeling Off or Jumping Off

“…[25][26][27] Compared with conventional surfaces such as bare Aluminum surfaces and hydrophobic surfaces, the retention water mass or fraction presents a much lower value. [28][29][30] However, there are still questions unclear for the defrosting on superhydrophobic surface. For example, how does the frost depart from a vertical superhydrophobic surface during defrosting, slide down, peel off or jump off?…”

Improvement of Wettability of Photocatalytic TiO₂–Coated Wafers by Microwave/UV Pre-treatment

“…The contact angle of water on a clean TiO 2 surface can be repeatedly cycled between approximately zero after UV irradiation 1 8 and 50-60 after irradiation with visible light or storage in the dark 9 . This phenomenon is exploited in commercially available materials with properties which include antifouling and anti-fogging of glass 10,11 . To improve the performance of the self-cleaning effect, Kazuo Yamaguchi was successful in increasing the contact angle of the surface using a self-assembled monolayer method SAM 12,13 .…”