How roughness controls the water repellency of woven fabrics

The coming ban on substances of high environmental concern used to provide water repellency to fabrics creates new challenges for the textile industry. Here we show that the design of the texture of woven fabrics is an important parameter to play with in order to achieve higher water repellency, which could help relax the limitations set by more environmentally-friendly coating materials. A mathematical framework is first developed to describe the roughness of woven fabrics and its relationship with water repellency, and metrological aspects of the characterization of the roughness of such fabrics by profilometry are discussed. The roughness of eight different fabric surfaces is then characterized, together with their water repellency after having been coated by wax-based, silicone-based or perfluorobutyl-based commercial polymer formulations. Fabrics of measured roughness lower than a critical value are in a partially-wet state with a substantial pinning of the droplets on their surface and an absence of roll-off whatever the type of hydrophobic coating. Above this critical value, the fabrics enter the superficially-wet state, in which the contact angle becomes controlled by the amount of air trapped in the texture. In this regime, the roll-off angle strongly depends on the wetting hysteresis of the coating material.