Understanding the frosting mechanisms on solid surfaces is crucial to a broad range of industrial sectors such as aerospace, power transmission, and refrigeration. During the last few decades, extensive studies have been conducted on fundamental frosting phenomena, including ice nucleation, growth, bridging, and frost propagation, with few studies focusing on frost halo formation which has been shown to affect frosting dynamics on hydrophilic substrates. The role of frost halo dynamics formation on superhydrophobic surface remains unclear due to limited characterization in the past. Here, in order to study frost propagation dynamics, particularly freezing-induced vapor diffusion and frost halo formation, condensation frosting on highly-reflective nanostructured superhydrophobic surfaces (θ ≈170º) was visualized using high-speed top-view optical microscopy. Condensation frosting was initiated by cooling the surface to -20 ± 0.5°C in atmospheric conditions (relative humidity ≈50% and air temperature ≈25°C). We show that the wave front reaches neighboring supercooled droplets along the path of frost propagation, resulting in supercooled droplet freezing within ~100 ms and numerous microscale (~1 µm) condensing droplets forming around the primary freezing droplet. The microscale droplets form a condensate halo stretching two times the freezing droplet radius. The condensate halo was formed by the rapid evaporation of the supercooled recalescent freezing droplet due to the fast (~100 ms) release of latent heat, resulting in the heating of the freezing droplet and thus outwards diffusion of vapor. Further diffusion of vapor led to the subsequent evaporation of the halo condensate droplets within ~4 s. Interestingly, accompanied by the freezing of the primary droplet and condensate halo formation, the neighboring satellite droplets in the halo zone were observed to oscillate directionally and dramatically, indicative of the presence of a strong flow field disturbance due to rapid vapor diffusion. The visualizations presented here not only help to quantify the physics of condensate halo formation during frost wave propagation on superhydrophobic surfaces, but also provide insights into the role of freezing-induced vapor diffusion during frost dynamics.

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