Slippery when stretched: New soft, stretchable material provides tunable wetting characteristics

The typical wetting behavior of water droplets at a solid surface can be avoided by modifying a surface’s microtexture: a lotus leaf, for example, has waxy microscopic protrusions that cause water to bead and have little contact with the leaf, so droplets simply roll off. Now, researchers from France’s National Center for Scientific Research have developed an elastic material whose surface texture – and thus wetting properties – dynamically change as it is stretched and relaxed. They describe the design in the 19 June 2017 issue of Applied Physics Letters.

The researchers fabricated a mold out of rigid silicon – a simple square with a matrix of microscopic, evenly spaced ‘micropillars’ whose size and spacing approximated the bumps on lotus leaves. Then they produced flexible replicates out of the common polymer polydimethylsiloxane and coated these with the hydrophobic elastomer vinylpolysiloxane. The researchers submitted these to various tests, employing water droplets in both the Cassie and Wensel states.

To test repellency, they released water droplets from a distance above samples. Like a lotus leaf, the material’s tiny pillars caused the water to bead and roll off. But with stretched samples, the droplets stuck to the material instead. Scanning electron microscopy revealed the underlying cause: stretching the samples increased the spacing among pillars, allowing falling droplets with momentum to squeeze past tips and impact the solid base. The droplets became pinned among pillars.

Other tests showed that the material could also be made more or less slippery. All effects scaled with stretching, performed equally under biaxial and uniaxial extension, and were completely reversible. Author David Quéré says that their work proves the concept possible and hopes to extend the design to softer materials and fabrics.

Source: “Soft, elastic, water-repellent materials,” by Martin Coux, Christophe Clanet & David Quéré, Applied Physics Letters (2017). The article can be accessed at https://doi.org/10.1063/1.4985011 .