A hydrophobic metallic structure that can’t be sunk has been developed by a team based at the laboratory of Chunlei Guo at the University of Rochester in the US. The structure, which is inspired by the natural characteristics of diving bell spiders and fire ants, is so water repellent that it won’t sink regardless of how much damage it receives or water it contains, a breakthrough that could lead to wearable flotation devices able to remain operational when punctured, electronic monitoring devices that can survive long immersion in the ocean and even unsinkable ships.

Spiders and fire ants are able to survive for a long time under water by trapping air in an enclosed area such as a dome-shaped web or raft, which they fill with air from the surface and then transport it between their superhydrophobic legs or in their bodies. As described in the journal ACS Applied Materials and Interfaces [Zhan et al. ACS Appl. Mater. Interfaces (2019) DOI: 10.1021/acsami.9b15540], here the researchers developed a structure where the treated surfaces of two parallel inward-facing aluminum plates produce a watertight compartment that suffers no external wear or abrasion.

The surfaces of the structure, separated by a specific distance to trap and hold sufficient air to keep the structure afloat, are based on a technique that uses femtosecond bursts of lasers to “etch” the surface of metals with detailed micro- and nanoscale patterns that trap air and make the surfaces water repellent. These multifaceted superhydrophobic surfaces can potentially trap a large volume of air, making them suitable for buoyancy devices.

Despite the structures being tested by being them submerged for up to two months, they immediately returned to the surface once released. They also retained this ability after being punctured many times since air remained trapped in parts of the compartment or in adjoining structures. While aluminum was used, the etching process can work for other metals or materials, and although metals don’t normally float, their superhydrophobic surface made the structure completely waterproof and able to stay afloat even when damaged.

The team are now working on developing other shapes and geometries for the metallic assembly based on their superhydrophobic surface, as well as exploring many other potential applications. The most obvious areas for the technology are ships, flotation devices and devices that need to be deployed at sea, while another idea that has been “floated” is their potential use in water-based cities.

Floating aluminum structure that can't be sunk (Credit: J. Adam Fenster/University of Rochester)
Floating aluminum structure that can't be sunk (Credit: J. Adam Fenster/University of Rochester)