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

2. Mechanisms of Superhydrophobicity
Superhydrophobicity is a property characterized by extreme water repulsion, where water droplets bead up and roll off the surface, leaving it dry. The mechanisms behind superhydrophobicity can be traced back to the interplay of two key factors: surface roughness and chemical composition.
2.1 The Lotus Effect
The Lotus Effect is named after the lotus plant, whose leaves exhibit remarkable water repellency. The lotus leaf surface is composed of a series of micro and nano-sized bumps, creating a rough texture that reduces the contact area between the water droplet and the leaf surface. In addition to this unique structure, the leaf is coated with a layer of hydrophobic waxes, making it chemically repellent to water. As a result, water droplets on a lotus leaf take on a nearly spherical shape and roll off the leaf surface, taking dirt particles with them, a process known as self-cleaning or the Lotus Effect. This natural phenomenon has inspired the design of synthetic superhydrophobic surfaces, where the dual roughness hierarchy and hydrophobic chemistry observed in the lotus leaf are mimicked to achieve similar water repellent properties.
2.2 Surface Roughness and Contact Angle
Surface roughness is a crucial factor in superhydrophobicity. A rough surface can trap air in the valleys between the surface structures, reducing the contact area between the water and the surface, and thereby increasing water repellency. The water repellency of a surface is typically quantified by the contact angle - the angle formed at the point of contact between a water droplet and the surface. Superhydrophobic surfaces are characterized by a high contact angle of over 150 degrees, signifying a low degree of wettability.
The relationship between surface roughness and contact angle can be explained using two models - the Wenzel model and the Cassie-Baxter model. The Wenzel model describes a scenario where the liquid fills the valleys of a rough surface, increasing the contact area and leading to higher wettability. On the other hand, the Cassie-Baxter model describes a situation where air remains trapped in the valleys of a rough surface, reducing the contact area and leading to higher water repellency. Superhydrophobic surfaces typically operate in the Cassie-Baxter state.
The mechanisms of superhydrophobicity revolve around the combination of surface roughness and hydrophobic chemistry. By replicating these features in synthetic materials like cotton fabrics, we can engineer superhydrophobic surfaces with wide-ranging potential applications.