If you were to soak even your best raincoat underwater for two months it would be wet through at the end of the experience. But a new waterproof material developed by Swiss chemists would be as dry as the day it went in. Lead researcher Stefan Seeger at the University of Zurich says the fabric, made from polyester fibres coated with millions of tiny silicone filaments, is the most water-repellent clothing-appropriate material ever created. Drops of water stay as spherical balls on top of the fabric (see image) and a sheet of the material need only be tilted by 2 degrees from horizontal for them to roll off like marbles. A jet of water bounces off the fabric without leaving a trace. The secret to this incredible water resistance is the layer of silicone nanofilaments, which are highly chemically hydrophobic. The spiky structure of the 40-nanometre-wide filaments strengthens that effect, to create a coating that prevents water droplets from soaking through the coating to the polyester fibres underneath.
“The combination of the hydrophobic surface chemistry and the nanostructure of the coating results in the super-hydrophobic effect,” Seeger explained to New Scientist. “The water comes to rest on the top of the nanofilaments like a fakir sitting on a bed of nails,” he says. A similar combination of water-repelling substances and tiny nanostructures is responsible for many natural examples of extreme water resistance, such as the surface of Lotus leaves. The silicone nanofilaments also trap a layer of air between them, to create a permanent air layer. Similar layers – known as plastrons – are used by some insects and spiders to breathe underwater. This fine layer of air ensures that water never comes into contact with the polyester fabric. It can be submerged in water for two months and still remain dry to the touch, says Seeger.
In addition, the plastron layer can also reduce drag when moving from water by up to 20% according to preliminary experiments conducted by Seeger. “This could be very interesting for athletic swimwear applications,” he suggests, raising the possibility of future swimsuits that never get wet. The new coating is produced in a one-step process, in which silicone in gas form condenses onto the fibres to form nanofilaments. The coating can also be added to other textiles, including wool, viscose and cotton, although polyester currently gives the best results. Experiments also showed that the new coating is durable. Unlike some water-resistant coatings, it remains more-or-less intact when the fabric is rubbed vigorously, although it didn’t survive an everyday washing machine cycle. For Steven Bell, director of the Innovative Molecular Materials Group at Queen’s University Belfast, it is this durability that represents the really exciting aspect of this work. “Although the textiles did show some degradation in the mechanical abrasion tests, their performance was very impressive,” he says. “The era of self-cleaning clothes may be closer than we think.”