Concave Structures Boost Wetting Resistance

Abstract

Water-repellent superhydrophobic surfaces are ubiquitous in nature. The fundamental understanding of bio/bio-inspired structures facilitates practical applications surmounting metastable superhydrophobicity. Typically, the hierarchical structure and/or reentrant morphology have been employed hitherto to suppress the Cassie-Baxter to Wenzel transition (CWT). Herein, a new design concept is reported, an effect of concave structure, which is vital for the stable superhydrophobic surface. The thermodynamic and kinetic stabilities of the concave pillars are evaluated by continuous exposure to various hydrostatic pressures and sudden impacts of water droplets with various Weber numbers (We), comparing them to the standard superhydrophobic normal pillars. Specifically, the concave pillar exhibits reinforced impact resistance preventing CWT below a critical We of ≈27.6, which is ≈1.6 times higher than that of the normal pillar (≈17.0). Subsequently, the stability of underwater air film (plastron) is investigated at various hydrostatic pressures. The results show that convex air caps formed at the concave cavities generate downward Laplace pressure opposing the exerted hydrostatic pressure between the pillars, thus impeding the hydrostatic pressure-dependent underwater air diffusion. Hence, the effects of trapped air caps contributing to the stable Cassie-Baxter state can offer a pioneering strategy for the exploration and utilization of superhydrophobic surfaces.

By mimicking the structure of a leaf beetle, a super water-repellent surface that is resistant to water droplet impact and water pressure has been developed. This technology is expected to increase efficiency and contribute to reducing maintenance costs in various industries such as marine, aviation, and energy.

A research team, led by Professor Dong Woog Lee in the School of Energy and Chemical Engineering at UNIST imitated the concave structure found in living things, such as leaf beetle species. Based on this structure, a concave column-shaped surface that can maintain extreme water repellency even in harsh environments was implemented.

By borrowing the original structure found in nature, the surface was prevented from getting wet and the super water repellency was secured more than before. This concave column structure has proven to be more resistant to impact and water pressure than the existing super water-repellent surface.

Extreme water repellency refers to the property that water does not permeate the surface and falls easily. This property is used in various fields such as self-cleaning, ice prevention, and pollution prevention.

The existing super water-repellent surfaces had limitations in that water droplets were easily wetted when shock or water pressure was applied. To overcome this, a stable anti-wetting function is required, and super-water repellency must be maintained even in harsh environments.

The research team got the hint from the concave structure of leaf beetles and the soil-dwelling springtail species. Based on this structure, the team created a column-shaped surface with concave pores. This surface showed stable and extreme water repellency even when water droplets collide at high speed or underwater environments with high water pressure.

As a result of the experiment, the concave column structure was not wet by about 1.6 times higher impact than the general column structure. In an environment of high water pressure, about 87% of the general column structure was wet, while only 7% of the concave column structure was wet.

The concave voids formed an air cushion when water droplets touched the surface. This cushion acted like a spring and prevented water from permeating the surface. Thanks to this, the surface of the concave column was able to stably maintain super water repellency for more than 24 hours.

Professor Lee said, "We have presented a new direction for stable super-water repellent surface design," and added, "If this design is put into practical use, it is expected to make an important contribution in various industrial sites."

The research has been participated by Jinhoon Lee and Jinwoo Park, as the first authors and carried out with the support by Basic Science Research Program and the Nano & material Technology Development program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT (MSIT). The findings were published in the online version of Advanced Materials on October 2, 2024.

Journal Reference

Jinhoon Lee, Jinwoo Park, Kwang Hui Jung, et al., "Enhancing Resistance to Wetting Transition through the Concave Structures," Advanced Materials, (2024).