Metastable States and Wetting Transition of Submerged Superhydrophobic Structures

Superhydrophobicity on structured surfaces is frequently achieved via the maintenance of liquid-air interfaces adjacent to the trapped air pockets. These interfaces, however, are subject to instabilities due to the Cassie-Baxter-to-Wenzel transition and total wetting. The current work examines in situ liquid-air interfaces on a submerged surface patterned with cylindrical micropores using confocal microscopy. Both the pinned Cassie-Baxter and depinned metastable states are directly observed and measured. The metastable state dynamically evolves, leading to a transition to the Wenzel state. This process is extensively quantified under different ambient pressure conditions, and the data are in good agreement with a diffusion-based model prediction. A similarity law along with a characteristic time scale is derived which governs the lifetime of the air pockets and which can be used to predict the longevity of underwater superhydrophobicity.

Figure 1

(a) Schematics of the experiment setup. Hydrostatic pressure is supplied by compressing nitrogen up to 1.5 atm and is monitored by a pressure transmitter at the sample surface. The tube connecting chambers 1 and 2 is 1 m in length and 4 mm in diameter. The meniscus profile is measured by confocal microscopy. (b) Scanning electron microscopic images showing the pore-patterned surface. Each pore has a radius of $r=25\mu \mathrm{m}$ and a depth of $H=40\mu \mathrm{m}$. The scale bar is $200\mu \mathrm{m}$. (c) Schematics of the pinned Cassie-Baxter and depinned metastable states.

Figure 2

Confocal microscopy images showing the meniscus 5 min after fresh immersion under 0 kPa (a),(e), 14 kPa (b),(f), 50 kPa (c),(g), and 15 min after immersion under 50 kPa (d),(h). Images in the first row were acquired with fluorescence-labeled water; the second row images were acquired without the dye. All images are corrected following a calibration method outlined in the Supplemental Material [37].

Figure 3

Wetting transition induced by pressurization. Graphs show variations of (a) contact angle $\theta$ and (b) sagging depth $h$ versus applied pressure $\mathrm{\Delta }p$. Symbols: experimental data. Lines: theoretical predictions. The experimental measurements were performed 5 min after fresh immersion.

Figure 4

(a) Experimental measurement of the dynamic evolution of the normalized sagging depth under different liquid pressures. The dot-dashed line shows the averaged collapse depth, $0.72H$. (b) The collapse time $t_c$, as a function of $\mathrm{\Delta }p$. Symbols: experimental data. Solid: theoretical prediction.

Figure 5

Normalized sagging depth as a function of normalized time in the metastable states.