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

3. Techniques for Making Superhydrophobic Cotton
Various techniques have been employed to render cotton fabrics superhydrophobic. These methods primarily focus on creating a dual roughness hierarchy on the fabric surface while also modifying the surface chemistry to increase its hydrophobicity.
3.1 Surface Modification Techniques
One of the pioneering methods in this field involves the use of Octadecyltrichlorosilane (OTS), a long-chain alkylsilane, which, when reacted with water and hexane, forms a superhydrophobic coating. In this method, water is added to OTS in a mole ratio of 1:2, followed by immediate mechanical mixing. The resulting mixture is then diluted with hexane before being applied to the fabric surface. This coating not only imparts superhydrophobicity but also creates a micro-to-nanoscale roughness on the fabric surface, mimicking the hierarchical morphology observed on lotus leaves. The result is a fabric with an ultrahigh water contact angle and ultralow sliding angle, embodying the excellent water-repellency associated with superhydrophobic surfaces [​10]​.
3.1.1 Sol-Gel Methods
While the OTS-based method has been successful, further optimization of the reaction conditions was required to enhance the performance. It was found that the key factor for achieving the best performance was the amount of water added to the OTS. Less or more water than the optimal amount would result in a decrease in the water contact angle of the treated surface. The reaction between OTS and water creates an acidic condition, leading to the formation of spherical nanoparticles. These nanoparticles then aggregate to form micro-size particles, which are capable of covalently bonding to the fabric surface, assuming the presence of hydroxyl groups​ [10]​.
3.1.2 Layer-by-Layer (LBL) assembly
Layer-by-Layer (LBL) assembly is a powerful thin film fabrication technique utilized in the development of superhydrophobic cotton fabrics. LBL forms films by depositing alternating layers of oppositely charged materials, separated by wash steps. This can be achieved using various techniques such as immersion, spin, spray, electromagnetism, or fluidics​1​. In a simplified representation, LbL technique is presented as a multilayer build-up based solely on electrostatic attraction. However, the process also involves other interactions such as hydrophobic attraction. Multilayer build-up is facilitated by multiple attractive forces acting cooperatively, while electrostatic repulsion provides self-limitation of the absorption of individual layers. This versatility makes it possible to extend the LbL technique to numerous systems, including hydrogen-bonded films, nanoparticles, similarly charged polymers, hydrophobic solvents, and more​ [11]​.
The preparation method significantly impacts the properties of the resultant films and opens up various applications. LbL is simple, potentially inexpensive, and allows a high degree of control over thickness. It provides easy control over the thickness with 1 nm resolution since each bilayer can be as thin as 1 nm. This is due to the variable growth profile of the films, which directly correlates to the materials used, the number of bilayers, and the assembly technique​ [11]​.
LbL has been applied in various areas, such as protein purification, corrosion control, (photo)electrocatalysis, biomedical applications, ultrastrong materials, and many others. For instance, LbL composites from graphene oxide ushered the appearance of numerous graphene and graphene oxide composites, and the first use of reduced graphene oxide composites for lithium batteries was also demonstrated with LbL multilayers​ [11​].
Given its versatility, the LbL method is indeed a promising technique in creating superhydrophobic cotton fabrics, offering a unique combination of efficiency, control, and potential for innovation.