Recoverable underwater superhydrophobicity from a fully wetted state via dynamic air spreading

Bioinspired Underwater Respirable Superhydrophobic Skin with a Microcone-Nanoparticle Structure for Sustainable Drag Reduction

Superhydrophobic (SH) surfaces have served as a key strategy to decrease flow resistance via gas-liquid interfaces in numerous fields such as pipeline transportation, microfluidics, the shipping industry, and so forth. However, an underwater SH surface with both good drag reduction and plastron restoration from a fully wetted state remains challenging. Inspired by the hairy structure of water spiders, herein, an underwater respirable skin (URS) with a microcone-nanoparticle structure is demonstrated. URS with different geometric parameters is achieved through laser microfabrication and chemical vapor deposition. The plastron can be completely restored from the fully wetted state after 11.6 s of air jetting, and a drag reduction rate of 15.7% ± 0.2% can be achieved. The theoretical and numerical results reveal a contradictory effect between drag reduction and plastron restoration. Our study suggests promising comprehensive perspectives for marine vehicle coatings and methodologies for sustainable drag reduction surfaces, considering both plastron restoration and the drag reduction rate.

Understanding ultrafast free-rising bubble capturing on nano/micro-structured super-aerophilic surfaces

Rapid bubble capture is essential for collecting targeted gaseous media and eliminating floating impurities across aquatic environments. While the role of nanostructures during the collision of free-rising bubbles with super-aerophilic surfaces is well established, the fundamental contribution of microtextures in promoting initial capture, even before contact, has yet to be fully understood. We report the rising bubble-induced large deformation of the entrapped gas layer, rapidly thinning the liquid film to its rupture threshold and thus achieving an ultrafast bubble capture down to about 1 ms with an array of microcones, decorated with nanoparticles as a convenient example to obtain super-aerophilicity. This rapid capture is also very stable due to the hysteresis movement of three-phase contact lines that inspired a critical pressure criterion for ensuring gas-layer stability and capture efficacy. The present nano/microstructured surface supports prolonged, loss-free gas transport in challenging shear flow as well, providing robust bubble control strategies for diverse systems.

Slippery liquid-like surfaces as a promising solution for sustainable drag reduction

Drag reduction is crucial for many industries, ranging from aerospace to microfluidics, to enhance the energy efficiency and reduce costs. This work is the first to study drag reduction enabled by novel slippery liquid-like surfaces fabricated from flexible polymers. We experimentally characterized the drag reduction performance of slippery liquid-like surfaces in the laminar flow regime. Our results indicate that liquid-like surfaces can reduce fluid drag regardless of surface wettability and have achieved nearly 20% drag reduction. Furthermore, the durability tests show that these surfaces can maintain slipperiness over a month when exposed to air or water and the drag reduction capability for at least one week under a fluid flow. These findings highlight the potential of slippery liquid-like surfaces as a promising solution for sustainable drag reduction.

Extend Plastron Longevity on Superhydrophobic Surface Using Gas Soluble and Gas Permeable Polydimethylsiloxane (PDMS)

The gas (or plastron) trapped between micro/nano-scale surface textures, such as that on superhydrophobic surfaces, is crucial for many engineering applications, including drag reduction, heat and mass transfer enhancement, anti-biofouling, anti-icing, and self-cleaning. However, the longevity of the plastron is significantly affected by gas diffusion, a process where gas molecules slowly diffuse into the ambient liquid. In this work, we demonstrated that plastron longevity could be extended using a gas-soluble and gas-permeable polydimethylsiloxane (PDMS) surface. We performed experiments for PDMS surfaces consisting of micro-posts and micro-holes. We measured the plastron longevity in undersaturated liquids by an optical method. Our results showed that the plastron longevity increased with increasing the thickness of the PDMS surface, suggesting that gas initially dissolved between polymer chains was transferred to the liquid, delaying the wetting transition. Numerical simulations confirmed that a thicker PDMS material released more gas across the PDMS-liquid interface, resulting in a higher gas concentration near the plastron. Furthermore, we found that plastron longevity increased with increasing pressure differences across the PDMS material, indicating that the plastron was replenished by the gas injected through the PDMS. With increasing pressure, the mass flux caused by gas injection surpassed the mass flux caused by the diffusion of gas from plastron to liquid. Overall, our results provide new solutions for extending plastron longevity and will have significant impacts on engineering applications where a stable plastron is desired.

Recent Advances in Superhydrophobic Materials Development for Maritime Applications

Underwater superhydrophobic surfaces stand as a promising frontier in materials science, holding immense potential for applications in underwater infrastructure, vehicles, pipelines, robots, and sensors. Despite this potential, widespread commercial adoption of these surfaces faces limitations, primarily rooted in challenges related to material durability and the stability of the air plastron during prolonged submersion. Factors such as pressure, flow, and temperature further complicate the operational viability of underwater superhydrophobic technology. This comprehensive review navigates the evolving landscape of underwater superhydrophobic technology, providing a deep dive into the introduction, advancements, and innovations in design, fabrication, and testing techniques. Recent breakthroughs in nanotechnology, magnetic-responsive coatings, additive manufacturing, and machine learning are highlighted, showcasing the diverse avenues of progress. Notable research endeavors concentrate on enhancing the longevity of plastrons, the fundamental element governing superhydrophobic behavior. The review explores the multifaceted applications of superhydrophobic coatings in the underwater environment, encompassing areas such as drag reduction, anti-biofouling, and corrosion resistance. A critical examination of commercial offerings in the superhydrophobic coating landscape offers a current perspective on available solutions. In conclusion, the review provides valuable insights and forward-looking recommendations to propel the field of underwater superhydrophobicity toward new dimensions of innovation and practical utility.

Liquid-Repellent Surfaces

Surfaces are vibrant sites for various activities with environments, especially as the transfer station for mass and energy exchange. In nature, natural creatures exhibit special wetting and interfacial properties such as water repellency and water affinity to adapt to various environmental challenges by taking advantage of air or liquid infusion media. Inspired by natural surfaces, various engineered liquid-repellent surfaces have been developed with a wide range of applications in both open and closed underwater environments. In particular, underwater conditions are characterized by high viscosity, high pressure, and complex compositions, which pose more challenges for the design of robust and functional repellent surfaces. In this Perspective, we take a parallel approach to introduce two classical liquid-repellent surfaces: an air-infused repellent surface and a lubricated liquid-repellent surface. Then we highlight fundamental challenges and design configurations of robust liquid-repellent surfaces both in air and underwater. We summarize the advantages and drawbacks of two kinds of repellent surfaces and list several applications of liquid-repellent surfaces for use in the ocean, medical care, and energy harvesting. Finally, we provide an outlook of research directions for robust liquid-repellent surfaces.