The droplet probe of the SDAM above the superhydrophobic wing of a butterfly known as a golden birdwing (Troides aeacus). Photo: Matti Hokkanen/Aalto University.
The droplet probe of the SDAM above the superhydrophobic wing of a butterfly known as a golden birdwing (Troides aeacus). Photo: Matti Hokkanen/Aalto University.

Wetting is an everyday phenomenon that relates to how well liquid spreads on a surface. When water comes into contact with an extremely water-repellent, or 'superhydrophobic', surface, droplets bead up and roll off easily. Researchers at Aalto University in Finland have now developed a measurement technique called Scanning Droplet Adhesion Microscopy (SDAM) to understand and characterize the wetting properties of such superhydrophobic materials. They describe this new technique in a paper in Nature Communications.

"Our novel microscope will promote the understanding of how wetting emerges from surface microstructures," explains Robin Ras, a professor in Aalto University School of Science. "The measuring instrument can also detect microscopic defects of the surface, which could allow coating manufacturers to control the quality of materials. Defects in self-cleaning, anti-icing, anti-fogging, anti-corrosion or anti-biofouling products can impeach the functional integrity of the whole surface."

SDAM is extremely sensitive and 1000 times more precise than current state-of-the-art wetting characterization methods, able to measure minuscule features and inconsistencies of surfaces with microscale resolution. Existing instruments for measuring droplet adhesion forces only detect forces down to the micronewton level – not sensitive enough for superhydrophobic surfaces.

"We have used a droplet of water to measure the water-repellent properties of a surface by recording the very tiny nanonewton force when the droplet touches the surface and when it separates from the surface," says Quan Zhou, a professor in Aalto University School of Electrical Engineering. "By measuring on many locations with micrometer spacing between the measurement points, we can construct a two-dimensional image of the surface's repellency, called a wetting map."

Wetting maps are a new concept for hydrophobic surface characterization and open a window on investigating structure-property relationships in surface wetting.

Up to now, 'contact angle measurement' has been the typical method for measuring the wetting properties of surfaces, but it is prone to inaccuracies for surfaces that are highly repellent to liquid. Unlike contact angle measurement, SDAM does not require a direct line of sight, which allows it to measure uneven surfaces such as fabrics or biological surfaces. SDAM can also detect wetting properties of microscopic functional features that were previously very hard to measure. Such microscopic features are important in many biochips, chemical sensors and microelectromechanical components and systems.

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.