Researchers have also developed a unique micropipette measurement technique to measure the forces acting on water droplets. Photo: Matilda Backholm / Aalto University

Science and technology

Physicists at Aalto University and ESPCI Paris have successfully explained a previously unknown force that inhibits the movement of water droplets on superhydrophobic surfaces, a discovery that promises to revolutionize the design of ultra-slick materials used in industries such as pharmaceuticals and transportation. This finding, recently published in the prestigious journal Proceedings of the National Academy of Sciences, was led by Assistant Professor Matilda Backholm of Aalto University.

The phenomenon under investigation involves water droplets gliding effortlessly over surfaces covered in microscopic cone-like structures, which reduce contact between the surface and the water. While these structures facilitate minimal friction at slow speeds, an unexpected resistance emerges as the speed increases. This resistance, akin to the rolling resistance experienced by tires on soft terrain, arises due to a pressure effect exerted by the moving droplet on the trapped air beneath it, creating a force opposing the droplet's motion.

Professor Robin Ras, who leads the Soft Matter and Wetting team where Backholm worked as a postdoctoral researcher, praised the significance of this work, saying, "It’s rare to have the opportunity to explain the subtleties of microscopic forces related to water dynamics, but this work does exactly that."

The research team discovered that the very mechanism that makes superhydrophobic surfaces exceptionally slippery also contributes to the observed resistance. "It’s a bit of an irony that the smaller we made the cones to increase slipperiness, the greater the effect of air compression became," Backholm explained. This insight has led to a rethinking of the design of ultra-slick surfaces.

To mitigate this resistance, Backholm and her colleagues have proposed a novel approach involving a combination of high pillars and low cones on the surface. This structure allows air to move more freely around the droplet while maintaining the necessary slickness, thereby reducing the compressive effect of the air and increasing the efficiency of the surface.

Furthermore, the research team developed a unique micropipette measurement technique to quantify the forces acting on the water droplets. This technique has also been used by Backholm in her previous work studying the swimming behavior of mesoscopic shrimp swarms and measuring root growth in plants.

With these findings, the research not only provides a deeper understanding of the interaction between liquid droplets and superhydrophobic surfaces but also lays the groundwork for the development of more effective hydrophobic coatings that could have broad applications from improving vehicle aerodynamics to enhancing the cleanliness and efficiency of medical devices.