Infinite Lifetime of Underwater Superhydrophobic States

Abstract

Submerged superhydrophobic (SHPo) surfaces are well known to transition from the dewetted to wetted state over time. Here, a theoretical model is applied to describe the depletion of trapped air in a simple trench and re-arranged to prescribe the conditions for infinite lifetime. By fabricating a microscale trench in a transparent hydrophobic material, we directly observe the air depletion process and verify the model. The study leads to the demonstration of infinite lifetime (> 50 days) of air pockets on engineered microstructured surfaces under water for the first time.Environmental fluctuations are identified as the main factor behind the lack of a long-term underwater SHPo state to date.The stability of the air layer on superhydrophobic (SHPo [1]) surfaces fully submerged in water is of critical importance because their key anticipated applications, such as drag reduction [2][3][4][5] and anti-biofouling [6], are under water. Unfortunately, the air film on the underwater SHPo surface, often called a plastron, is fragile [2,7]. Over time, the air initially trapped on the SHPo surface diffuses away into the surrounding water, collapsing the air-water interface and causing a transition from the dewetted to wetted state [8]. Recent experimental studies showed that the lifetime of the underwater SHPo state is influenced by various environmental parameters [7,[9][10][11]. 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. Furthermore, for drag-reduction application in particular, SHPo surfaces with parallel trenches [3,4,17,18] have been found to outperform those with random structures [19,20], making SHPo surfaces with trenches a good candidate to study. Since multiple trenches are in parallel and isolated from each other, a single trench can represent the whole SHPo surface as far as the stability of the trapped air is concerned. The potential over-estimation of the depletion speed on single trench compared with the parallel trenches due to the edge effect [21,22] is considered minor to our goal. Importantly for our purpose instead, a sample with single trench would allow clear images of one air-water meniscus; in comparison, multiple menisci in multiple trench...

Superhydrophobic drag reduction in laminar flows: a critical review

“…Unfortunately, the surfaces that produce larger slips (i.e., large pitch, large air fraction) are more susceptible to the wetting transition, as explained in the previous section. A recent study demonstrated that SHPo surfaces with a useful slip length (i.e., ~100 µm) cannot maintain the plastron if placed under water deeper than several centimeters even if the environment is made perfectly stable (Xu et al 2014), confirming the theories formed for such an ideal environment. In contrast, under a usual laboratory environment with temperature and pressure fluctuating, reliable data were not even attainable, explaining why all other stability experiments were only statistical studies.…”

“…Many have reported persistent plastron on natural and artificial SHPo surfaces made with nanoscale structures. Since gas pockets in nanoscale cavities are several orders of magnitude more stable than in microscale cavities, they can stay indefinitely at even a relatively high liquid pressure (e.g., meters of water depth) according to the theoretical predictions proven experimentally by Xu et al (2014). However, we do not discuss the nanostructured SHPo surfaces here because they do not provide slip lengths large enough to be useful for most engineering applications.…”

Superhydrophobic surfaces for corrosion protection: a review of recent progresses and future directions

“…In fact, almost none of the surfaces developed so far have shown an acceptably long underwater superhydrophobicity for practical applications. 117 Most of them failed within hours and rarely for days. Xu et al have shown that the air film trapped under water in a specially designed trench can be retained for >1200 h if maintained at a shallow position and with minimal environmental fluctuation.…”

“…Obviously, the effectiveness of superhydrophobic surfaces to protect immersed structures largely depends on the underwater stability of the air films which rapidly decays with increasing hydraulic pressure, [115][116][117] flow, 118 or salinity 119 of the surrounding fluids. In fact, almost none of the surfaces developed so far have shown an acceptably long underwater superhydrophobicity for practical applications.…”

Unraveling the Salvinia Paradox: Design Principles for Submerged Superhydrophobicity

“…Most applications of superhydrophobicity to date have concentrated on drops deposited on surfaces, both statically and dynamically. On the other hand, there is a growing interest in the properties of submerged surfaces entrapping gas: [10][11][12] in this case, superhydrophobicity is a means to reduce the liquidsolid contact which, in turn, diminishes drag [13][14][15] and prevents (bio)fouling. Given their relevance for global industry and transportation, even small improvements in the fuel effi ciency and maintenance costs of watercrafts and marine structures could have a signifi cant impact on society.…”