Prof. Huiling Duan’s group in the Department of Mechanics and Engineering Science and the Center for Applied Physics and Technology, has made a major breakthrough in maintaining the ultimate stable underwater superhydrophobic state. The work was published online in Physical Review Letters on September 27 (Phys. Rev. Lett.119, 134501, (2017)).The Physics Synopsis column of the APS website highlighted this discovery by publishing an article entitled “How to Make Superhydrophobicity Last”, stating that “researchers find tricks to prolong the typically short-lived water repellency of a superhydrophobic surface”.
In nature, when many aquatic plants and insects submerge into the water, due to the need for survival, the microstructures on the surface of their bodies will contain large amounts of gas to ensure that they can maintain life activities underwater such as breathing. In addition, the existence of gas layers also makes the surface of plants and animals possess such characteristics as self-cleaning, anti-adhesion, etc. Inspired by nature, people can prepare a bionic functional surface that can seal gas in the microstructures, which has a brilliant application prospect in the drag reduction of ships and underwater vehicles. However, influenced by factors such as water pressure and water flowing, the gas layer may gradually lose stability or even disappear, which result in the loss of original performance of the functional surface. Therefore, making the gas layer keep stable underwater, especially in deep water and under high pressure, is one of the biggest challenges that the application of underwater functional surface may face.
Prof. Duan’s group in the College of Engineering, by considering the saturation of the dissolved gas in the water and the dissolution and diffusion equilibrium within the cavitation, makes it possible to keep the superhydrophobic state stable for a long time underwater. The researchers have drawn phase diagrams to reflect whether the gas layer can remain stable for long under different conditions by establishing a generalized thermodynamic theoretical framework. With the help of confocal microscopes, they have also respectively observed the evolution processes of gas layers on the artificial bionic functional surfaces and on the fresh lotus leaves under different experimental conditions, finding that the stability of the superhydrophobic state can become a reality through regulating the saturation of the dissolved gas underwater. Their study on lotus leaves proves that the long-hour stable state of the gas layer can be implemented on any rough surface. All in all, this research reveals the mechanism of how gas layers in underwater microstructures keep stable, and plays an important guiding role in the application of hydrophobic bionic functional surfaces in the water.
Figure 1. (a), (b) show that at 1.5 atmospheric pressure and in the saturated water, the gas layers on the surface of lotus leaves can stably exist for a long time; (c), (d) manifest that in unsaturated water, the gas layers disappear soon.
In the past few years, Prof. Duan’s group has always been devoted to studying the stability and drag reduction of the superhydrophobic surface underwater, and has published a series of papers on important international journals (Phys. Rev. Lett. 112, 196101, 2014 ; Physics of Fluids 27, 092003, 2015 ; Physics of Fluids 29, 032001, 2017).
The first author of this paper is Mr. Yaolei Xiang, a postgraduate in the Department of Mechanics and Engineering Science, Peking University. The research is financially supported by the National Natural Science Foundation of China.