Ultimate Stable Underwater Superhydrophobic State

Underwater metastability hinders the durable application of superhydrophobic surfaces. In this work, through thermodynamic analysis, we theoretically demonstrate the existence of an ultimate stable state on underwater superhydrophobic surfaces. Such a state is achieved by the synergy of mechanical balance and chemical diffusion equilibrium across the entrapped liquid-air interfaces. By using confocal microscopy, we in situ examine the ultimate stable states on structured hydrophobic surfaces patterned with cylindrical micropores in different pressure and flow conditions. The equilibrium morphology of the meniscus is tuned by the dissolved gas saturation degree within a critical range at a given liquid pressure. Moreover, with fresh lotus leaves, we prove that the ultimate stable state can also be realized on randomly rough superhydrophobic surfaces. The finding here paves the way for applying superhydrophobic surfaces in environments with different liquid pressure and flow conditions.

Figure 1

Schematics of meniscus morphologies on submerged superhydrophobic surfaces with (a) arbitrary and (b) pore-patterned structures. Dashed lines in (b) show the pinned CB (1), depinned recession (2), and expansion (3) states, respectively. (c) Prediction of the equilibrium curvature ($\kappa$) dependence on the dissolved gas saturation degree ($s$) for different liquid pressures ($p_L$) on the pore-structured surface. Solid lines: stable states. Dashed lines: unstable states. Here, ${\theta }_a$ and ${\theta }_r$ are chosen as 120° and 95°, respectively, consistent with our experiments below.

Figure 2

(a) Schematics of the experiment setup. Confocal microscope is employed to examine the meniscus morphology on the specimen under different liquid pressure and flow rate. The gas dissolution in the water is assisted by a bubbling device. Scanning electronic microscopic (SEM) images show the surface structures of pore-patterned samples (b) and lotus leaf samples (c), respectively.

Figure 3

(a) Variation of curvature ($\kappa$) as a function of time ($t$) under air pressure ranging from 0.5 to 2 bar with or without flow rate ($Q=23\mathrm{mL}/\min$) at two different water height, $H_w=19$ and -13 cm. Solid dots: experiments in flow. Open dots: experiments in quiescent water. (b) Dependence of curvature ($\kappa$) on water height ($H_w$) at a constant air pressure, $p_a=1.2$ bar. Dots: experiments. Solid line: theoretical prediction. Dashed lines: predicted upper and lower curvature bounds, respectively. (c) Contour plot of ${\gamma }_{LG}\kappa$ as a function of liquid pressure ($p_L$) and dissolved gas saturation degree ($s$) in the pinned CB state according to Eq. (4). Numbers with unit [kPa] indicate the values of ${\gamma }_{LG}\kappa$ for the corresponding lines or dots. Solid lines: theoretical prediction. Solid and open dots: experiments in pinning stable and depinned unstable states, respectively. The dashed line shows a series of experiments carried out by maintaining the air pressure in the tank.

Figure 4

Snapshots of confocal microscopy images showing the long-term stable (a), (b) and unstable (c), (d) lotus leave surfaces under a liquid pressure of 1.5 bar and a flow rate of $23\mathrm{mL}/\min$. In (a) and (b), the dissolved gas saturation degree, $s=1$. And in (c) and (d), $s=0.67$. The blue region indicates water. The planar red surface is the water-air interface. The red rough part beneath is the microstructure of the lotus leave.