Superhydrophobicity in Nature

Superhydrophobic surfaces possess extreme water-repellent properties, causing water droplets to bead up and easily roll off the surface. The lotus leaf is a well-known example of a natural superhydrophobic surface, often used as a benchmark for comparison with other characters.

Likewise, the leaves of the Taro plant exhibit superhydrophobic properties due to a combination of epicuticular wax crystals and microscale papillae, contributing to water repellency.

Cabbage leaves have a rough surface texture with microscale bumps and even smaller nanoscale papillae. This roughness traps air pockets, preventing water droplets from spreading across the surface. The wax coating on the papillae acts as a water-repellent barrier, reducing the adhesion of water droplets and making cabbage a superhydrophobic character.

The carnauba plant's superhydrophobic properties are attributed to its unique nanostructure, with tiny wax crystals containing hydrophobic long-chain fatty acids and fatty alcohols. The arrangement of smaller nanoscale protrusions on larger microscale bumps creates a complex roughness that traps air pockets, preventing water droplets from spreading out on the surface.

Rose petals exhibit superhydrophobicity due to hierarchical structures at both the nanoscale and microscale levels. Hydrophobic trichomes, covered with wax-like compounds, enhance water resistance at the nanoscale, while numerous ridges and bumps at the microscale reduce the contact area with water droplets, causing them to bead up and roll off the surface.

Insects also demonstrate superhydrophobic effects. Water striders have superhydrophobic legs with a hierarchical structure featuring both nanoscale and microscale characteristics. The legs appear rough with setae (fine hairs) coated in wax, reducing the surface area in contact with water droplets, along with microscale ridges and grooves that enhance surface roughness. This combination reduces the contact area between the leg surface and water droplets, causing the droplets to bead up and roll off, allowing water striders to glide on the water's surface.

Butterfly wings possess micro and nanostructures, including tiny ridges, bumps, and scales arranged in a specific pattern, resulting in a complex and rough surface. This irregular surface prevents water droplets from adhering to the wings, maintaining a nearly perfect spherical shape due to the water droplets' surface tension being greater than the gravitational force, which would otherwise cause them to slide off. The superhydrophobic property of butterfly wings not only repels water but also helps keep the wings clean by preventing dirt and impurities from adhering to the surface.

Researchers have developed novel materials and coatings inspired by these natural examples, with potential applications ranging from self-cleaning surfaces to waterproof coatings for electronic devices and gadgets.

Reference

• Emiliano Lepore, M. G. Mimicking water striders’ legs superhydrophobicity and buoyancy with cabbage leaves and nanotube carpets. Journal of Material Research, 976 983.2013
• Hans J Ensikat, c. a.-K. Superhydrophobicity in perfection: the outstanding properties of the lotus leaf. BEILSTEIN journal of Nanotechnology, 152-161.2011
• Ilsa Basson, a. E. An investigation of the structures and molecular dynamics of natural waxes. II. Carnauba wax. Journal of Physics D Applied Physics , 1429. 2000
• Nosonovsky, B. B. The rose petal effect and the modes of superhydrophobicity. The Royal Society, 4713-4728. 2010.
• Zhiwu Han, J. F. Long-term durability of superhydrophobic properties of butterfly wing scales after continuous contact with water. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 139 -144. 2017.
• Henry Lambley, T. M. S. a. D. P.,. Superhydrophobic surfaces for extreme environmental conditions. Engineering , 117 ((44)), pp. 27188-27194. 2020.

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