When it comes to designing extremely water-repellent surfaces, shape and size matter. That's the finding of a group of scientists 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 are highly relevant for a broad range of applications where water-resistance is important, including power generation and transportation.

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.

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. 

The scientists created and tested new materials with different nanoscale textures—some decorated with tiny straight-sided cylindrical pillars and some with angle-sided cones. They were also able to control the spacing between these nanoscale features to achieve robust water repellency.

After coating their test materials with a thin film of wax-like material, the scientists measured how water droplets rolled off each surface as they were tilted from vertical to flat positions and compared the behavior with that of untextured solids.

The other important finding was that the water-repelling ability of cone-shaped nanotexturing held up even when water droplets were sprayed onto the surface with a pressurizing syringe. Such pressure could potentially force water into the nanosized pockmarks between the conical or cylindrical pillars, displacing the air bubbles and destroying the water-repelling effect.

The team is working on extending this technique to other materials, including glass and plastics, and on fabricating surfaces that are also oil-repellent by further tweaking the feature shape.

The nanopatterning technique used in this study also enables the design of a wide variety of materials with different texturing—and therefore different water-repelling properties—on different parts of a single surface. This approach could be used, for example, to fabricate nanoscale channels with self-cleaning and low fluid friction properties for diagnostic applications such sensing the presence of DNA, proteins, or biotoxins.

This story is reprinted from material from Brookhaven National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.