When it comes to designing extremely water-repellent surfaces, shape and size matter. That’s the finding of a group of scientists at the U.S. Department of Energy’s Brookhaven National Laboratory, who investigated the effects of differently shaped, nanoscale textures on a material’s ability to force water droplets to roll off without wetting its surface. These findings and the methods used to fabricate such materials—published online October 21, 2013, in Advanced Materials (“Robust Superhydrophobicity in Large-Area Nanostructured Surfaces Defined by Block-Copolymer Self Assembly”)—are highly relevant for a broad range of applications where water-resistance is important, including power generation and transportation.”The idea that microscopic textures can impart a material with water-repellent properties has its origins in nature,” explained Brookhaven physicist and lead author Antonio Checco. “For example, the leaves of lotus plants and some insects’ exoskeletons have tiny-scale texturing designed to repel water by trapping air. This property, called ‘superhydrophobicity’ (or super-water-hating), enables water droplets to easily roll off, carrying dirt particles along with them.”Mimicking this self-cleaning mechanism of nature is relevant for a wide range of applications, such as non-fouling, anti-icing, and antibacterial coatings. However, engineered superhydrophobic surfaces often fail under conditions involving high temperature, pressure, and humidity—such as automotive and aircraft windshields and steam turbine power generators—when the air trapped in the texture can be prone to escape. So scientists have been looking for schemes to improve the robustness of these surfaces by delaying or preventing air escape.Side view scanning electron microscope image of a silicon surface textured with (a) cylindrical pillars and (b) nanocones.
Creating nanoscale textures“In principle, the high robustness required for several applications could be achieved with texture features as small as 10 nanometers because the pressure needed for liquid to infiltrate the texture and force the air out increases dramatically with shrinking texture size,” explained Checco. “But in practice, it is difficult to shrink the surface texture features while maintaining control over their shape.””For this work, we have developed a fabrication approach based on self assembly of nanostructures, which lets us precisely control the surface texture geometry over as large an area as we want—in principle, even as large as square meters,” Checco said.The procedure for creating these superhydrophobic nanostructured surfaces, developed in collaboration with scientists at Brookhaven’s Center for Functional Nanomaterials (CFN), takes advantage of the tendency of “block copolymer” materials to spontaneously self-organize through a mechanism known as microphase separation. The self-assembly process results in polymer thin films with highly uniform, tunable dimensions of 20 nanometers or smaller. The team used these nanostructured polymer films as templates for creating nanotextured surfaces by combining with thin-film processing methods more commonly used in fabricating electronic devices, for example by selectively etching away parts of the surface to create textured designs.”This new approach leverages our thin-film processing methods, in order to precisely tailor the surface nanotexture geometry through control of processing conditions,” said Brookhaven physicist and co-author Charles Black.