Water droplets rolling on a super-hydrophobic surface. The repellency of a super-hydrophobic surface immersed in water causes a depression in the water, while the silvery shine of the surface under water is caused by a microscopic layer of air trapped between the water and the super-hydrophobic surface. Photo: Mika Latikka, Aalto University.In a new perspective article in Science, researchers from Aalto University in Finland call for consistent and standardized testing of super-hydrophobic, i.e. extremely water-repellent, materials.
A unified testing method is needed to allow community-wide comparison between published results, say the researchers. This would significantly advance development of super-hydrophobic materials and their adoption in commercial products for self-cleaning and anti-icing applications.
Currently, research groups around the world use many different kinds of tests to evaluate the durability and wear of super-hydrophobic materials. For example, researchers have used linear abrasion, circular abrasion, sandblasting and water jets to test super-hydrophobic surfaces. However, the results obtained through different methods are not comparable, making it hard to find the best materials for specific applications.
“We therefore propose that researchers should begin using a standardized method when testing the wear and durability of extremely water-repellent materials,” says Robin Ras, assistant professor of experimental condensed matter physics at Aalto University. “We find that linear abrasion of the surface, for example with sandpaper, would be the best general method. It is important to specify the applied pressure, the abrasion distance and the abradant material to make sure the result is reproducible.”
Benefits of the proposed method include, among other things, the easy availability of the testing materials and the simple test setup.
“Nevertheless, the wear test alone is not sufficient to determine the robustness of the super-hydrophobic surface. The second step is to perform tests with water droplets to measure the water-repelling properties of the surface after wear,” adds Tuukka Verho, who is currently preparing his doctoral dissertation.
The super-hydrophobicity of a surface is often measured based on the shape of a water droplet on the surface, i.e. how completely the droplet beads up. However, according to the authors, this so-called static contact angle does not provide enough information about the effects of wear. Water can still form almost spherical droplets on a damaged surface, even if it can no longer easily slide off that surface. Therefore, the authors suggest that droplet mobility should always be measured, by for example using an inclination test.
The Aalto University researchers hope that their perspective article initiates a discussion within the research community on the evaluation of super-hydrophobic surfaces. Even though they propose linear abrasion as a primary test for all super-hydrophobic materials, additional application-specific tests may be needed to, for instance, assess the laundering durability of textiles or the weather-durability of outdoor materials.
This story is adapted from material from Aalto University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.