Superhydrophobic metal won't sink -- even when damaged
A metallic structure etched by lasers, right, floats to the top on the water's surface in professor Chunlei Guo's lab. [University of Rochester photo/J. Adam Fenster]
University of Rochester researchers, inspired by diving bell spiders and rafts of fire ants, have created a metallic structure that is so water repellent, it refuses to sink -- no matter how often it is forced into water or how much it is damaged or punctured.
Could this lead to an unsinkable ship? A wearable flotation device that will still float after being punctured? Electronic monitoring devices that can survive for long periods in the ocean?
A superhydrophobic metallic structure is weighed down to keep it submerged in water in the Guo lab. Even after two months, it will bounce back to the surface.
The structure features a groundbreaking technique the lab developed for using femtosecond bursts of lasers to "etch" the surfaces of metals with intricate micro- and nanoscale patterns that trap air and make the surfaces superhydrophobic, or water repellent.
Initially, the researchers found that after being immersed in water for long periods of time, single-layer surfaces may start to lose their hydrophobic properties.
Enter the spiders and fire ants, which can survive long periods under or on the surface of water. How? By trapping air in an enclosed area. Argyroneta aquatic spiders, for example, create an underwater dome-shaped web -- a so-called diving bell -- that they fill with air carried from the surface between their super-hydrophobic legs and abdomens. Similarly, fire ants can form a raft by trapping air among their superhydrophobic bodies.
The superhydrophobic structure remains afloat even after significant structural damage -- punctured with six 3-mm-diameter holes and one 6-mm hole.
"That was a very interesting inspiration," Guo says. As the researchers note in the paper: "The key insight is that multifaceted superhydrophobic (SH) surfaces can trap a large air volume, which points towards the possibility of using SH surfaces to create buoyant devices."
Guo's lab created a structure in which the treated surfaces on two parallel aluminum plates face inward, not outward, so they are enclosed and free from external wear and abrasion. The surfaces are separated by just the right distance to trap and hold enough air to keep the structure floating -- in essence creating a waterproof compartment. The superhydrophobic surfaces will keep water from entering the compartment even when the structure is forced to submerge in water.
Even after being forced to submerge for two months, the structures immediately bounced back to the surface after the load was released, Guo says. The structures also retained this ability even after being punctured multiple times, because air remains trapped in remaining parts of the compartment or adjoining structures.
VIDEO: University of Rochester researchers, inspired by diving bell spiders and rafts of fire ants, have created a metallic structure that is so water repellant, it refuses to sink -- no matter how often it is punctured.
Though the team used aluminum for this project, the special etching process could be used for literally any metals," Guo says.
When the Guo lab first demonstrated the etching technique, it took an hour to pattern a 1-in. x 1-in. area of surface. Now, by using faster-scanning lasers that are also seven times as powerful, the lab has sped up the process dramatically, making it more feasible for scaling for commercial applications.
Coauthors include lead author Zhibing Zhan, Mohamed ElKabbash, Jihua Zhang, and Subhash Singh, all PhD candidates or postdoctoral fellows in Guo's lab, and Jinluo Cheng, associate professor at the Changchun Institute of Optics, Fine Mechanics, and Physics in China.
The project was supported by funding from the Bill & Melinda Gates Foundation, the US Army Research Office, and National Science Foundation.