That’s unsinkable behaviour


Lasers, spiders, ants and air help create a metallic structure that keeps coming back to the surface.


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

By Nick Carne

US scientists have created a metallic structure they say is so water repellent it refuses to sink – even if damaged. When forced underwater, it just keeps bobbing back up.

The secrets to success are incredibly short bursts of lasers (we’re talking femtoseconds, or millionths of a billionth of a second) and the smarts of certain spiders and ants.

The lasers are used to "etch" the surfaces of metals with intricate micro- and nanoscale patterns that trap air and make the surfaces super-hydrophobic, or water repellent.

However, Chunlei Guo and colleagues at the University of Rochester found this only works up to a point. After being immersed in water for long periods of time, the surfaces can start to lose their hydrophobic properties.

So they looked to nature for inspiration.

The diving bell spider (Argyroneta Aquatica), for example, can survive long periods below the surface by trapping air in a dome-shaped web. Similarly, fire ants (genus Solenopsis) can form a raft by trapping air among their superhydrophobic bodies.

"The key insight,” the researchers write in a paper in the journal ACS Applied Materials and Interfaces, “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."

They are not, it must be said, the only scientists to notice this. Earlier this year, Cosmos reported on French research inspired by A. Aquatica, and its potential to help industry control toxic or inflammable gases in fluid lines.

What Guo's team did was to create a structure in which the treated surfaces on two parallel aluminium 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.

Even after being forced to submerge for two months, the structures immediately bounced back to the surface after the load was released, Guo says.

They also retained this ability even after being punctured multiple times, because air remains trapped in remaining parts of the compartment or adjoining structures.

The process "could be used for literally any metals, or other materials," Guo adds. Now they are working to speed up the process to make it feasible for scaling up for commercial applications.

“The potential use of the SH floating metallic assembly ranges from floating devices and electronic equipment protection, to highly floatable ships and vessels,” the authors write.

Explore #lasers #spiders
  1. https://pubs.acs.org/doi/10.1021/acsami.9b15540
  2. https://cosmosmagazine.com/physics/underwater-spider-may-help-make-industry-safer
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