Spontaneous self-dislodging of freezing water droplets and the role of wettability

Abstract

Spontaneous removal of liquid, solidifying liquid and solid forms of matter from surfaces, is of significant importance in nature and technology, where it finds applications ranging from self-cleaning to icephobicity and to condensation systems. However, it is a great challenge to understand fundamentally the complex interaction of rapidly solidifying, typically supercooled, droplets with surfaces, and to harvest benefit from it for the design of intrinsically icephobic materials. Here we report and explain an ice removal mechanism that manifests itself simultaneously with freezing, driving gradual self-dislodging of droplets cooled via evaporation and sublimation (low environmental pressure) or convection (atmospheric pressure) from substrates. The key to successful self-dislodging is that the freezing at the droplet free surface and the droplet contact area with the substrate do not occur simultaneously: The frozen phase boundary moves inward from the droplet free surface toward the droplet-substrate interface, which remains liquid throughout most of the process and freezes last. We observe experimentally, and validate theoretically, that the inward motion of the phase boundary near the substrate drives a gradual reduction in droplet-substrate contact. Concurrently, the droplet lifts from the substrate due to its incompressibility, density differences, and the asymmetric freezing dynamics with inward solidification causing not fully frozen mass to be displaced toward the unsolidified droplet-substrate interface. Depending on surface topography and wetting conditions, we find that this can lead to full dislodging of the ice droplet from a variety of engineered substrates, rendering the latter ice-free.

Interfacial Materials for Anti‐Icing: Beyond Superhydrophobic Surfaces

“…Reprinted (adapted) with permission from Ref. . e) Optical microscopy images showing the off‐surface growth (OSG) mode of ice on a hydrophobic surface ( θ =107.3°), and f) the along‐surface growth (ASG) mode of ice on a hydrophilic surface ( θ =14.5°).…”

“…The surfaces with CAs from 70 to 110° meet the above criteria, and the mean dislodging time decreases with increasing CA . In addition, the substrate thermal conductivity, diffusivity, and heat transfer need to be minimized . This may provide an extremely novel method for designing anti‐icing materials.…”

“…[45].The microscope images demonstrating the self-dislodging behavior of water droplets is reprinted (adapted)with permission from Ref. [44].…”

“…[43] Very interestingly, the self-dislodging of icy droplets waso bservedb yP oulikakos et al in an atmospheric-pressure environment as long as the droplet-substrate interface remains a liquid throughout the process and freezes last (Figure 6d). [44] That is, the frozen phase boundary moves inwardf rom the droplet-free surfacest owardt he droplet-substrate interface, thereby leading to ag radual reduction in the droplet-sub- strate contact, which triggers the self-dislodging process. [44] The surfaces with CAs from 70 to 1108 meet the above criteria, and the mean dislodging time decreases with increasing CA.…”

“Open‐Mouth” Mesoporous Hollow Micro/Nano Coatings Based on POSS/PDMS: Fabrication, Mechanisms, and Anti‐Icing Performance

“…This fact turned attention of researchers to nature‐made hierarchical surfaces such as super‐hydrophobic structure (SHS) of lotus leaves which could spontaneously shed‐off any water deposits, with anticipation that these would also be able, by analogy, to prevent ice adhesion by preventing attachment of water to the surface, leaving nothing to freeze on the surface at subzero temperatures . A broad body of researches on SHS provided encouraging results regarding their potential for anti‐icing coatings .…”