Sustainability of superhydrophobicity under pressure

Abstract

Prior studies have demonstrated that superhydrophobicity of submerged surfaces is influenced by hydrostatic pressure and other environmental effects. Sustainability of a superhydrophobic surface could be characterized by both how long it maintains the trapped air in its surface pores, so-called “longevity,” and the pressure beyond which it undergoes a global wetting transition, so-called “terminal pressure.” In this work, we investigate the effects of pressure on the performance of electrospun polystyrene fibrous coatings. The time-dependent hydrophobicity of the submerged coating in a pressure vessel is optically measured under elevated pressures. Rheological studies are also performed to determine the effects of pressure on drag reduction and slip length. The measurements indicate that surface longevity exponentially decays with increasing pressure in perfect agreement with the studies reported in the literature at lower pressures. It is found, however, that fibrous coatings could resist hydrostatic pressures significantly higher than those of previously reported surfaces. Our observations indicate that superhydrophobic fibrous coatings could potentially be used for underwater applications.

Infinite Lifetime of Underwater Superhydrophobic States

“…2(b), the meniscus movement shown in Fig. 2 and previous experimental results [7,13,15]. However, when the immersion depth was smaller than a certain value, the trench retained the air pocket for a very long time.…”

“…However, most of the studies only reported statistical information, such as average wetting time; more direct knowledge such as air-depletion dynamics or the effect of roughness geometries is needed to design the SHPo surface to be more robust against wetting. While some underwater insects boast a long-term or even indefinite (tested up to 120 days [12]) plastron, all artificial SHPo surfaces retained their plastron for much shorter length of time (mostly less than 2 an hour; rarely for days [7,9,[13][14][15] This paper aims to understand the air depletion process on submerged SHPo surfaces and to determine if an indefinite plastron is achievable. In order to systematically study the effect of geometric parameters of the surface structures for an intended goal, SHPo surfaces made of regular structures would be far more informative than those of random structures that produce only statistical data.…”

“…2 an hour; rarely for days [7,9,[13][14][15]). To date, no artificial SHPo surface has been demonstrated to retain an air layer indefinitely unless assisted [16].…”

Polymeric Slippery Coatings: Nature and Applications

“…Additionally, the figure indicates that the terminal pressure for this particular coating could go up to 9.5 bar, which is two orders of magnitudes higher than those observed in other studies [8,11] for other coatings. Samaha et al [76] interpreted that the scale of microroughness (the distances between the fibers) was about five microns at some locations and approached nanoscales at others, while the corresponding scales in the studies [8,11] were of the order of hundred microns. The significantly smaller scales of the fibrous coatings increase the sustainability of the air-water interface against pressure, indicating that such coatings could potentially be used for deep underwater applications.…”

“…They studied the effect of hydrostatic pressure [76], water flow [108], and salinity of water [109]. Figure 38 shows an example of the longevity measurements versus pressure conducted for the fibrous coatings.…”