Superhydrophobicity in Cotton Fabrics: Mechanisms, Techniques, and Future Directions Abstract

Definition and properties of superhydrophobic materials

Superhydrophobicity is a property that endows surfaces with a high degree of water repellency. It can be observed in a variety of natural settings, such as the leaves of the lotus plant, and has inspired a wealth of scientific research and innovation in the creation of water-resistant materials. Superhydrophobicity is quantitatively defined by the static water contact angle being above 150 degrees and contact angle hysteresis being less than 5 degrees [5]​. When a water droplet lands on a superhydrophobic surface, it retains a spherical shape and can easily roll off, leaving the surface dry.
The underlying mechanism of superhydrophobicity involves a combination of surface roughness and low surface energy materials. Superhydrophobic surfaces are typically characterized by a micro or nanostructure that increases surface roughness and promotes water repellence. However, these very tiny structures are delicate and can be easily damaged by wear, cleaning, or any sort of friction. If the structure is damaged even slightly, it loses its superhydrophobic properties​ [6]​.
Superhydrophobic surfaces rely on composite materials where one component provides the roughness and the other provides low surface energy. Superhydrophobic coatings can be made from many different materials, including manganese oxide polystyrene, zinc oxide polystyrene, precipitated calcium carbonate, carbon nanotube structures, silica nano-coating, and fluorinated silanes​2​. Silica-based coatings are currently the most cost-effective option. They are gel-based and can be easily applied either by dipping the object into the gel or via aerosol spray.
Superhydrophobic coatings have a range of applications, including ultra-dry surface applications, maritime industry, vehicle windshields, surgical tools, medical equipment, and textiles, among others. They have the ability to increase fuel efficiency in ships, aid in harvesting minerals from seawater brine, prevent rain droplets from clinging to vehicle glass, and make removal of salt deposits possible without using fresh water​ [6]​.
However, there are limitations to the application of superhydrophobic coatings due to their fragility. They are most useful in environments that are not exposed to wear or cleaning, such as electronic components and air conditioning heat transfer fins, to protect from moisture and prevent corrosion​ [6]​.
Recent research has proposed a new class of waxgel material that can self-reconstruct its surface layer both chemically and structurally after being damaged or removed. This waxgel material is durable, can recover its superhydrophobicity, and withstands severe mechanical challenges. Importantly, it does not require any external input for recovery, such as heat or UV light, which distinguishes it from previous self-healing superhydrophobic systems​ [7]​.
While superhydrophobic materials have promising applications, their durability and ability to withstand wear and tear remain major challenges. As research progresses, new methods and materials are being developed to enhance the resilience and self-healing capabilities of superhydrophobic coatings.

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