Superhydrophobic surfaces to be tested for real-world applications
In their perspective article in the journal Science, researchers from Aalto University call for consistent and standardised testing of superhydrophobic, i.e. extremely water-repellent, materials. Agreeing on a unified testing method is needed to allow community-wide comparison between published results. This would significantly progress development of superhydrophobic materials and their transfer to commercial products in, for instance, 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 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.
Benefits of the proposed method include, among other things, the easy availability of the testing materials and the simple test setup.
The superhydrophobicity 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 in the surface. However, according to the authors, 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, but no longer easily slides off the surface. Therefore, the authors encourage to always measure droplet mobility, for example using an inclination test.
The Aalto University researchers wish that their perspective article initiates in the research community a discussion 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.