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

3.2.1 Silica Nanoparticles
In the creation of superhydrophobic cotton fabrics, silica nanoparticles are a significant element. The process starts with the synthesis of silica nanoparticles via a sol-gel process. These particles, typically spherical with diameters of 143 and 378 nm, are then applied to the cotton fabric along with a cost-effective water-repellent (WR) agent.
However, it is important to note that the silica nanoparticle treatment alone does not change the hydrophilic surface of the cotton fabric. Water droplets are still adsorbed into the fabrics because of the presence of hydroxyl groups on both the cotton and silica nanoparticle surfaces.
The breakthrough comes when both silica nanoparticles and the WR agent are combined in the treatment of the cotton fabrics. Even at a very low WR agent concentration of 0.1 wt%, a contact angle above 130 degrees can be achieved. Thus, superhydrophobic cotton fabrics can be created via this combined treatment of silica nanoparticle and WR agent, an approach that is more cost-effective compared to fluorinated silane treatments​ [16]​.
3.2.2 Carbon Nanotubes
Carbon nanotubes (CNTs) are an intriguing option for superhydrophobic coatings due to their unique structure, high surface area, and mechanical strength. They are capable of imparting a relatively uniform nanostructure to surfaces, which is key to achieving superhydrophobicity. However, the application of CNTs in creating superhydrophobic coatings is not without challenges. Due to their delicate nature, with diameters often in the tens of nanometers for multi-walled CNTs, these structures may require the addition of a thin-film hydrophobic coating to render them superhydrophobic. Additionally, fine control over the diameter of the as-grown CNTs or the overall nanostructure is necessary to optimize superhydrophobic properties​ [17]​.
A technique that has shown promise in overcoming these challenges involves the use of carbon infiltration. By layering amorphous carbon on multi-walled nanotubes, researchers have been able to improve the structural integrity of the CNTs and achieve superhydrophobic behavior with tunable geometry. This carbon-infiltrated carbon nanotube (CICNT) surfaces exhibit an increased number of contact points between neighboring tubes, resulting in a composite structure with improved mechanical stability. Furthermore, the native surface of these CICNTs can be rendered superhydrophobic with a vacuum pyrolysis treatment, achieving contact angles as high as 160° and contact angle hysteresis on the order of 1°. The ability to tune the geometry while maintaining superhydrophobic behavior has been found to be highly beneficial for various applications such as in condensation heat transfer, anti-icing, microfluidics, anti-microbial surfaces, and other bio-applications​ [17]​.
These findings underscore the significant potential of carbon nanotubes in the development of superhydrophobic coatings. However, further research is necessary to refine these techniques and to fully explore the benefits and limitations of using CNTs in superhydrophobic cotton fabric production.