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

Abstract

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.

Figures (4)

• Figure 1: Melting processes of droplets on different substrates: (a) Cu substrate; (b) HMN substrate; (c) SN substrate. Experimental parameters include a heating temperature of 30 $^\circ$C and a droplet volume of 55.6 $\mu$l in panels (b) and (c). (Multimedia available online) (d) Melting modes of droplets on different substrates: floating mode with ice at the top (e.g., on the Cu and HMN substrates); deposited mode with ice at the bottom (e.g., on the SN substrate).
• Figure 2: Melting modes (a) and melting times (b) of frozen droplets on different substrates and under different heating temperatures. In panel (a), the maximum droplet volume used is 55.6 $\mu$l on the Cu substrate (which corresponds to the maximum achievable apparent contact angle of 132$^\circ$), and the droplet volume used is 37.1 $\mu$l on the HMN and SN substrates. In panel (b), the blue triangles show the melting times on the SN substrate, among which the solid triangles above the polyline represent the melting times of droplets in the floating mode, and the open triangles below the polyline represent the melting times of droplets in the deposited mode. The points on the polyline (half-filled triangles at the heating temperatures of 25 and 30 $^\circ$C) represent the melting times of droplets in a composite mode (with a transition from the deposited mode to the floating mode, described in Section S3 of the Supplementary Material).
• Figure 3: Flow during the melting of frozen droplets on (a) the HMN substrate and (b) the SN substrate. The experiment is conducted with a fixed heating temperature of 30 $^\circ$C and a controlled droplet volume of 37.1 $\mu$l. By overlaying 40 consecutive images acquired at 100 fps, the flow field information inside the droplet can be intuitively obtained.
• Figure 4: Schematic diagrams of the melting modes and force analysis. (a) Schematics diagram of the floating mode, including the early stage (a1), the force analysis (a2), and the stable stage (a3). (b) Schematics diagram of the deposited mode, including the early stage (b1), the force analysis (b2), the liquid film (b3), the Marangoni effect (b4), and the stable stage (b5).