Design and Development of Superhydrophobic Textile Surfaces

Abstract

The relationship between contact angles, surface tensions and surface roughness is reviewed. The various numerical formulae related to contact angles were used to predict the surface tension and wetting behavior of polymer surfaces. The apparent contact angle of a droplet deposited on a textured surface is presented, and the characteristics required for a superhydrophobic surface are described.

The numerical formulae related to superhydrophilic and superhydrophobic polymer rough surfaces are shown using two approaches, Wenzel and Cassie-Baxter models. Using these models as a guide, artificial superhydrophilic or superhydrophobic surfaces were created. Rough nylon surfaces mimicking the Lotus leaf were created by coating polyester surface with nylon 6,6 short fibers using the flocking process. Poly(acrylic acid) (PAA) chains were grafted onto nylon 6,6 surfaces followed by grafting 1H, 1H-perfluorooctylamine to the PAA chains. Water contact angles as high as 178° were achieved.

For a woven superhydrophobic surface, the original Cassie-Baxter model better describes the wetting of rough surfaces. Using mechanical and chemical surface modification of nylon 6,6 woven fabric, artificial Lotus leaves having water contact angles as high as 168° were prepared. Good agreement between the predictions based on the original Cassie-Baxter model and experiments was obtained. However, the version of the Cassie-Baxter model in current use could not explain the wetting behavior of woven fabrics since the surface area fractions in this form is valid only when the liquid is in contact with a flat porous surface.

The angle at which a water droplet rolls off the surface has also been used to define a superhydrophobic surface. It is shown that the roll-off angle is highly dependent on droplet size. For our samples, the advancing contact angles of the 1H, 1H-perfluorooctylamine-grafted or octadecylamine-grafted multifilament fabric surface become very close to 180° when the droplet begins to move. However, the receding contact angles are affected by the local structures of fabric such as protruding yarns, yarn size and yarn spacing on the surface. Although the receding contact angles are as small as 90°, the roll-off angles of these superhydrophobic surfaces were less than 5° when a 0.5 mL water droplet was applied.