Longevity of superhydrophobic surface in undersaturated liquid*

We experimentally studied the longevity of superhydrophobic surface (SHS) when it exposed to undersaturated liquid. A wetting transition was induced due to the diffusion of gas from SHS to ambient liquid. SHSs with two types of texture geometries, holes and posts, fabricated on polydimethylsiloxane (PDMS) were studied. The status of gas on the SHS was observed by a non-intrusive optical technique based on light reflection. Due to the non-uniform diffusion rate across the surface, SHS longevity was determined based on the status of gas on the entire surface. We found that the SHS longevity (t_f) followed a scaling relation: t_f~(1−s)⁻², where s was the ratio of gas concentration in liquid to that in the plastron. This scaling relation implied that as the gas was dissolved into the liquid, the mass flux J reduced with time as: J~t^−0.5. Furthermore, we found that the diffusion length L_D reduced as the undersaturation level increased, following a scaling relation: L_D~(1−s)⁻¹. Last, we found that the SHS longevity in undersaturated liquid increased as increasing the texture depth and as increasing the thickness of the gas permeable material (here, the thickness of PDMS surface). Our results provide a better understanding of SHS longevity in undersaturated liquid.