Study of gas diffusion for underwater super-hydrophobic surfaces*

Gas diffusion could cause a wetting transition for underwater super-hydrophobic surface (SHS). Here, we studied this problem through a combination of numerical and experimental approaches. In the numerical study, we solved the spatial-temporal evolution of the gas concentration in the liquid in COMSOL. We found that the profiles of gas concentration at different times are self-similar, and the mass flux reduces with time (t) at a rate of 1/t^0.5. In addition, we examined the impact of texture parameters, including pitch, gas fraction, texture height, and advancing contact angle, on the diffusion process. We proposed simple analytical models that can predict the longevity for SHS with various texture geometries. Experimentally, we fabricated SHSs with regular patterns on transparent PDMS (Polydimethylsiloxane) surface and studied their longevity in a pressurized chamber. Reflection Interference Contrast Microscopy was used to investigate the shape of gas-liqiuid interface as well as the plastron longevity. We found that the patterned PDMS surfaces have much shorter longevities than the numerical predictions. This disagreement might be due to the presence of other wetting mechanisms in the experiments, such as thermodynamic energy minimization.