Infinite Lifetime of Underwater Superhydrophobic States

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 rearranged 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

Design, Fabrication, and Evaluation of Superhydrophobic (SHPo) Surfaces for Drag Reduction in Turbulent Boundary Layer Flows

Sustaining a gas layer on them in liquid, superhydrophobic (SHPo) surfaces have attracted enormous attention due to the possibility of reducing friction drag in numerous flow applications. Although many SHPo surfaces proved to reduce drag significantly (e.g., > 10%) in microchannel flows and certain SHPo surfaces proved to have an unprecedentedly large slip length (e.g., > 100 microns), a significant drag reduction is still elusive in turbulent flows that reflect most applications, such as watercraft in marine environment. Recognizing the gas layer (called plastron) as the key and studying its robustness under water of varying depths, we first conclude that the SHPo surfaces capable of a significant drag reduction cannot maintain the plastron indefinitely if submerged deeper than a few centimeters. By developing a high-resolution shear sensor for centimeters-size sample surfaces and using silicon SHPo surfaces that keep plastron more robust than others, we obtain up to ~25% drag reduction in turbulent boundary layer flows at Reynolds numbers up to 1.1x107. Obtained at a high-speed water tunnel and a high-speed tow tank, the results also indicate that the drag reduces more with increasing Reynolds number, corroborating the numerical studies in the literature. Moreover, we develop and conduct SHPo drag experiments using a real boat in marine conditions for the first time, achieving ~20% drag reduction. Finally, a scalable fabrication process is developed for scale-up manufacturing of both passive and semi-active SHPo surfaces. For the semi-active SHPo surfaces, i.e., SHPo surfaces with self-regulating gas restoration capability, we propose and demonstrate a gas generation mechanism that does not require any external power input

Self-Powered Plastron Preservation and One-Step Molding of Semiactive Superhydrophobic Surfaces.

Gas-trapping-typically 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 semiactive SHPo surface based on self-regulated electrolysis. Here, we report two major advances: (i) a self-powered plastron restoration mechanism that obviates the need for external power; (ii) a one-step molding process to mass-manufacture semiactive SHPo surfaces. The advances clear major hurdles for real-world implementation

Infinite lifetime of underwater superhydrophobic states.

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 rearranged 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

Brightness of Microtrench Superhydrophobic Surfaces and Visual Detection of Intermediate Wetting States

For a superhydrophobic (SHPo) surface under water, the dewetted or wetted states are easily distinguishable by the bright silvery plastron or lack of it, respectively. However, to detect an intermediate state between the two, where water partially intrudes the surface roughness, a special visualization technique has been needed. Focusing on SHPo surfaces of parallel microtrenches and considering drag reduction as a prominent application, we (i) show the reliance on surface brightness alone may seriously mislead the wetting state, (ii) theorize how the brightness is determined by water intrusion depth and viewing direction, (iii) support the theory experimentally with confocal microscopy and CCD cameras, (iv) present how to estimate the intrusion depth using optical images taken from different angles, and (v) showcase how to detect intermediate states slightly off the properly dewetted state by simply looking. The proposed method would allow monitoring SHPo trench surfaces without bulky instruments - especially useful for large samples and field tests