A melting mode of frozen sessile droplets with unmelted ice layer deposited at the bottom

Water-repellent properties of superhydrophobic surfaces make them promising for anti-icing and deicing applications. Through experimental visualization of frozen sessile droplets undergoing melting on superhydrophobic surfaces, we identify a melting mode with the unmelted ice layer deposited at the bottom of the melting droplet, even though the density of ice is lower than that of water. In the deposited mode of the melting process, the time required for the frozen droplet to melt completely is much shorter than that in the floating mode. Force analysis shows that the melted fluid flows along the gas-liquid interface toward the top of the melting droplet, thereby exerting force and then suppressing the upward movement of the unmelted ice layer. Moreover, the flow within the liquid film formed between the unmelted ice layer and the heating wall is dominated by the viscous force, which has a lubrication effect and maintains the deposition of the unmelted ice layer. High heating temperature, large contact angle, and low particle concentration are helpful for the occurrence of the deposited mode.

💡 Research Summary

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This paper investigates the melting behavior of frozen sessile droplets placed on super‑hydrophobic surfaces and identifies two distinct melting modes: a conventional “floating” mode, in which the unmelted ice layer rises to the top of the droplet, and a newly observed “deposited” mode, in which the ice layer remains at the bottom of the droplet despite ice being less dense than water. The authors performed high‑speed visualizations of droplets (volumes 37–56 µL) on three types of substrates: a copper (Cu) plate, a hierarchical micro‑nano structured (HMN) super‑hydrophobic surface, and a single‑scale nano‑structured (SN) super‑hydrophobic surface. All droplets were heated from 20 °C to 80 °C, and the evolution of the ice–water–air three‑phase region was recorded.

Key observations include: (1) On Cu and HMN surfaces the ice layer floats as expected; on the SN surface the ice layer deposits at the substrate, producing a markedly shorter overall melting time (≈ 20 s versus ≈ 45 s for the floating mode, a reduction of about 56 %). (2) Increasing the substrate temperature, enlarging the apparent contact angle, or reducing the concentration of particles detached from the surface all promote the deposited mode. At intermediate temperatures (25–30 °C) on the SN surface a composite mode appears, where the ice initially deposits, then detaches and rises as the flow reorganizes.

The authors attribute the mode selection to a competition between thermocapillary (Marangoni) driving, buoyancy of the ice layer, and viscous lubrication in a thin liquid film that forms between the ice and the heated wall. Temperature gradients along the gas‑liquid interface generate surface‑tension gradients that drive liquid either upward (Q₁) or downward (Q₂) relative to the ice layer. In the deposited mode, the Marangoni flow is strong enough that Q₁ > Q₂, producing a net downward force (F₁) that, together with buoyancy, overcomes the upward force (F₂) and pushes the ice toward the substrate. Simultaneously, a nanometric liquid film of thickness h ≪ d (film diameter) creates a lubrication regime where the Reynolds number multiplied by h/d is far below unity. Consequently, viscous stresses dominate over inertial effects, generating a resistance that suppresses any upward motion of the ice.

To formalize these ideas, two dimensionless groups are introduced: (i) a thermocapillary‑to‑buoyancy ratio Π = |dσ/dT| ΔT / (Δρ g L²), where ΔT is the temperature difference between the substrate and the melting front, L a characteristic droplet length, and Δρ the density difference between ice and water; (ii) a viscous‑to‑buoyancy ratio Λ = μ ū L / (h² Δρ g H), where μ is the liquid viscosity, ū an average flow speed in the film, and H the film height. Large Π favors strong interfacial flow that bypasses the ice (deposited mode), while large Λ indicates that viscous lubrication can stabilize the ice at the bottom. Experimental trends—higher heating temperature increasing Π, larger contact angle increasing L, and lower particle concentration reducing flow damping—are consistent with the model predictions.

The paper concludes that the deposited melting mode provides a more efficient heat‑transfer pathway because the ice directly contacts the heated substrate and the thin film enables rapid removal of meltwater. This insight has practical implications for anti‑icing and de‑icing technologies in aviation, wind turbines, power transmission, and phase‑change energy storage, where rapid melting can reduce downtime and energy consumption. The authors note that their experiments were conducted under quiescent air and controlled heating; future work should explore the influence of external airflow, variable humidity, and dynamic loading to assess the robustness of the deposited mode in real‑world conditions.