Call to standardise the testing of superhydrophobic materials

Researchers from Aalto University have called for consistent and standardised testing of superhydrophobic materials. They argue that agreeing on a unified testing method is needed to allow community-wide comparison between published results and that this would progress the development of superhydrophobic materials to commercial products.

Currently, research groups around the world use many tests to evaluate the durability and wear of superhydrophobic materials. For example, researchers have used linear abrasion, circular abrasion, sandblasting and water jets in testing the surfaces. However, the results obtained through different methods are not comparable, which makes it hard to find the best materials for applications.

Professor Robin Ras, Department of Applied Physics, Aalto University, explained: “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 availability of the testing materials and the simple test setup.

However, Tuukka Verho, doctoral candidate in Prof Ras’ team, added: “Nevertheless, the wear test alone is not sufficient to determine the robustness of the superhydrophobic surface. The second step is to perform tests with water droplets to measure the water-repelling properties of the surface after wear.”

The superhydrophobicity of a surface is often measured based on how completely the water droplet beads up in the surface. However, according to the scientists, this so-called static contact angle does not provide enough information about the effects of wear, as water often forms almost spherical droplets even on a damaged surface. Their solution is to measure droplet mobility, for example using an inclination test.

Verho and Prof Ras say they want their suggestions to initiate a discussion in the research community on the evaluation of superhydrophobic surfaces. Even though they propose linear abrasion as a primary test for all superhydrophobic materials, additional application-specific tests may be needed, for instance, to assess the laundering durability of textiles or the weather-durability of outdoor materials.

Author
Tom Austin-Morgan

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