Cotton fabrics and textiles have become a major part of the world’s consumption, becoming popular in civilian and military applications. Cotton-based fabrics have found more use in the textile industry, clothing, paper, and the fishing industry, especially in outdoor wear and protective fabrics; yet, cotton is very prone to absorbing and retaining water. Though soft, comfortable, and breathable, cotton’s functionality simply is not adept, as its uses are limited due to poor waterproofing; thus, the hydrophobic effect demonstrated by a multitude of polymers has become a practical application and focus in the realm of textile engineering. Engineering a polymer, with an efficient synthesis and application process, would allow cotton fabrics to become water repellent while maintaining its original structure and breathability. Consequently, the same polymers that provide hydrophobic properties also generate flame retardant, self-cleaning, self-healing, and improved mechanical properties. Superhydrophobic cotton textiles, then, have a greater scope of use. Awnings and umbrellas can easily shed water and be easily stored. Cleaning clothes, or more generally surfaces, can become easier and less expensive; furthermore, a shirt or blouse could have ketchup or wine slide off like a spill never happened. To make cotton more practical, five of many methods are described: sol-gel processing, fluorinated block copolymerization, electrospinning, mist copolymerization, and pad-dry-cure application.
Superhydrophobic surfaces have two characteristics: a static water angle, θCA, greater than 150o and a surface sliding angle, θSA, less than 10o. Any surface satisfying these conditions is labeled as a superhydrophobic surface. If the contact angle is anything less than 150o, the surfaces is labeled hydrophilic. Surfaces change, both chemically and physically on the micro-scale; moreover, because the surface changes, the contact angle between liquid and solid changes. Contact angle hysteresis (CAH) is used to characterize, though challenging to measure, hydrophobic surfaces by determining the difference between the advancing and receding contact angles of a droplet of liquid. CAH describes the stability of the Cassie-Baxter Model, where a stable superhydrophobic surface has a high static contact angle and a low contact angle hysteresis. Furthermore, stability relies on the surface morphology of the substrate.
Superhydrophobic properties were first exhibited in nature and first studied in Nelumbo nucifera, lotus leaves. Botanists Wilhelm Barthlott and Christoph Neinhuis first discovered the effect using a scanning electron microscope, photographing the microscopic structure of the leaves. Simply, the leaf acts like a wooden board with a blanket of nails, and because the pointy nails decrease the contact area, any particle or molecule would hover on the points of the nails. These leaves, as a result of this structure have ultra-low water adhesion and self-cleaning properties. Aptly named the “Lotus Effect, ” the self-cleaning mechanism is able to remove dust and small particles, leaving a clear, clean trail behind a moving water droplet. These plants showed that the stability of superhydrophobic surfaces is a consequence of dual surface structure: microscopic convex cell papilla and nanoscopic epicuticular wax.
Young attempted to model surface wettability by assuming a smooth surface [Young]. Essentially, chemical make-up of the solid allows a surface tension to give rise to a large static contact angle, the unchanging contact angle between the liquid and the surface. This model excels in simplicity; however, it fails to described the non-smooth surfaces of today’s world.
No surface is ever smooth; thus, models were developed for rough surfaces. In both the Wenzel and the Cassie-Baxter model, water droplets sit on top of microprotrusions on the surface of the textile. These microprotrusions, or grooves, are comprised further of a solid and an air composite. Different combinations of the air and solid composites allow the surface to fall into two roughness categories: homogeneous and heterogeneous. Homogeneous surfaces allow the water droplet to fill the space between the microprotrusions so that there are no air pockets underneath the droplet. On the other hand, heterogeneous surfaces allow the droplet to sit on top of both the microprotrusions and the air pockets. These models are much more complex than Young’s model. Wenzel’s model focuses with homogenous surfaces. Notably, both models represent extreme states; thus, superhydrophobic surfaces demonstrate states of both models, going through transitions of both models when an external pressure is applied.
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