Black silicon, a material commonly used in solar cell technology, has unique properties that make it superhydrophobic, meaning it repels water. Droplets of water on black silicon move by riding on a thin air-film gap trapped beneath the textured surface. When the droplets move slowly, they glide without any issues, but at faster speeds, a mysterious force tugs on their underbelly. A team of researchers from Aalto University and ESPCI Paris have finally identified this force, which arises from the movement of droplets on super slippery surfaces like black silicon, creating a shearing effect on the air trapped beneath and causing a drag-like force on the droplet itself.
Assistant Professor Matilda Backholm from Aalto University published a paper detailing these findings in the Proceedings of the National Academy of Sciences. She and her team have managed to measure the force, explain how it works, and find a way to eliminate the drag force altogether. By understanding these complex interactions of fluid and soft matter physics, Backholm has opened up the possibilities of creating better superhydrophobic surfaces that could enhance the slipperiness of transportation systems, make medical devices more sterile, and improve various applications requiring liquid-repellent surfaces.
Black silicon maximizes the surface tension of water to minimize contact between droplets and the surface. Cones etched onto the material create a plastron, an air-film gap that allows droplets to glide smoothly. However, this mechanism also leads to the shearing effect that Backholm has explained in her research. While researchers have previously observed this force, Backholm’s work is the first to provide a comprehensive explanation. She suggests modifying the surface by adding taller cones with textured caps to reduce the total contact area of the droplets and minimize the air-shearing effect, thus improving the efficiency of superhydrophobic surfaces.
Backholm’s research showcases the effectiveness of a specialized micropipette measurement technique in quantifying the forces acting against water droplets on black silicon. By oscillating the droplets and probe, she was able to identify the subtle forces at work, ruling out other potential factors by testing carbonated droplets. This force acts independently of the contact between droplets and the surface, demonstrating the unique nature of the shearing effect. Backholm’s discoveries using this technique provide valuable insights for physicists and engineers looking to develop improved hydrophobic surfaces for various applications in science and technology.
A key takeaway from Backholm’s research is the importance of considering the microscopic forces at play in wetting dynamics when designing ultrahydrophobic surfaces. By understanding the shearing effect and its implications for droplet movement on textured surfaces like black silicon, researchers can tailor their designs for better performance. Backholm’s work builds upon the expertise of the Soft Matter and Wetting research group at Aalto University and offers a new perspective on how to enhance the slipperiness of surfaces. Her findings are expected to drive further advancements in the field of superhydrophobic materials and could lead to a range of innovative applications in the future.
Now leading the Living Matter research group at the Department of Applied Physics, Backholm continues her work on understanding the intricacies of fluid and soft matter physics. Her discoveries have shed light on the previously unknown shearing effect and its impact on droplet behavior, paving the way for new developments in the field of superhydrophobic surfaces. By combining specialized techniques with a deep understanding of the forces at work, Backholm has made significant contributions to the study of wetting dynamics and opened up exciting possibilities for future research and applications in material science.
