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...

Despite two decades of research, drag reduction with superhydrophobic (SHPo) surfaces has never been experimentally confirmed under the most coveted condition, i.e., high Reynolds number flows in open water, thus casting doubt on its ultimate impact. Here, we present large drag reductions (approximately 30%) by SHPo surfaces tested on the sea at Re x estimated to be as high as 6.5 × 10 6. To achieve success, we prepare 4 × 7 cm 2 silicon samples of re-entrant microtrenches for maximum plastron retention and test them as a portion of hull surface underneath a motorboat using a custom-developed comparative shear sensor. The state of plastron on the sample surfaces are observed throughout the tests via a custommade miniature underwater camera. The successful results attest the importance of microscopic nuances of SHPo surfaces for plastron retention and drag reduction, guiding directions for future applications.

Abstract

Accurate measurement of shear stress on a solid surface is a crucial but challenging task in fluid mechanics. Different sensors are usually used for different experimental settings: water channel, wind tunnel, towing tank, watercraft, aircraft, etc. This paper presents a direct shear sensor designed to work for varying test objects and flow conditions. Designed to compare two different sample surfaces, the shear sensor is comprised of two floating elements, whose displacement is proportional to the shear stress they experience, and two optical encoders, which measure the displacements precisely, right under the floating elements. The main plate includes two identical sets of floating elements and flexure beams machined monolithically from a thick piece of metal, allowing displacements in only one in-plane direction. The sideby-side arrangement allows the two floating elements to experience essentially the same flow conditions, regardless of test condition, enabling the comparative sensing. The method of machining these folded-beam flexures, whose width is on the scale of micrometers, while thickness and length are in millimeters and centimeters, respectively, is presented. The main plate is designed with the help of finite element analysis to ensure dynamic response of the floating elements is appropriate for target flow conditions. The utility of the shear sensor is verified in three different flow settings, i.e., water tunnel, boat in open water, and wind tunnel. A miniature underwater camera system is also developed to observe the sample surfaces during testing on a moving object, such as a boat.

Gas-trapping, mostly superhydrophobic (SHPo), surfaces are useful for underwater applications only while their plastron lasts. Because the plastron unfortunately disappears under most practical conditions, various active approaches to supply ample gas have been reported, including the semi-active SHPo surface based on selfregulated electrolysis. Here, we report two major advances: (i) self-powered plastron restoration mechanism that obviates the need for external power; (ii) one-step molding process to mass-manufacture semi-active SHPo surfaces. The advances clear major hurdles for real-world implementation.