Gas permeability-dependent plastron longevity on superhydrophobic surfaces: opposite trends under hydrostatic pressure and degassed water conditions.

HYPOTHESIS: The plastron trapped on an underwater superhydrophobic surface plays a crucial role in various applications, such as drag reduction, but its longevity may be limited due to gas diffusion. A gas permeable substrate may significantly affect plastron longevity due to its capability to store or release gas. However, the effect of gas permeability on plastron longevity has received little attentions. EXPERIMENTS: In this work, surfaces with varying gas permeabilities from ∼0.05 to ∼1600 Barrers were fabricated using polydimethylsiloxane (PDMS) and polyepoxide (Epoxy), as well as by varying the mixing ratio of the base and curing agents of PDMS. These surfaces featured an array of micro-holes, where isolated plastrons formed when the samples were submerged in water. A nonintrusive optical method was employed to quantify the plastron longevity. FINDINGS: We found that when high pressure was applied to the samples, with increasing gas permeability, plastron longevity significantly decreased, since air was dissolved not only into the water but also into the gas-permeable material. In contrast, when the samples were exposed to degassed water, with increasing gas permeability, plastron longevity notably increased, due to the release of gas from the substrate into the plastron, effectively increasing the plastron volume. A numerical model, based on solving the diffusion equation and mass flux at interfaces, captured the trends observed in the experiments. Our work provides guideline on the implementation of gas-permeable materials for fabricating superhydrophobic surface.