Black as a moth eye

A femtosecond laser created detailed hierarchical structures in the metals, as shown in this SEM image of the platinum surface

Sometimes I run into a scientific/research news that hooks me up not for the result obtained, although they are usually noteworthy, but for the “tools” and methods used to achieve those results. And this is a point in case.

Researchers at the University of Rochester, NY, have used a laser beam to etch the surface of a metal plate in such a way that the resulting “scars” render the surface completely black (that is the surface absorb all lights visible wavelengths) and additionally makes the surface hydrophobic (repellent to water).

Now I learnt that the laser was shooting bullets of photons in burst each a millionth of a billionth of a second. Just to put this into perspective a beam of light in such time window will travel for 0.3µm.  And that’s not all: the intensity of the photons burst is equivalent to the power of the whole US electrical grid! Given the infinitesimal length of time the actual power is very very low but its effect on a metal surface is amazing. It can create, in a controllable way (and here’s the trick) nanostructure on the metal surface that can result in complete absorption of light wavelengths and in amazing hydrophobic characteristics.  The researchers have pointed out that a material like Teflon considered highly hydrophobic will let water “drop” away once the surface is inclined by 70°. Surfaces produced by their method of laser etching will cause water to drop away as the surface is inclined by 2°! Take a look at the video to see the amazing behaviour of water poured on this kind of surface.

Interestingly, this laser etching method works well on several metallic materials: they have experimented with platinum, titanium and brass but it can be applied to other metals as well.

What amazes me is the kind of control that we can have on Nature at the nano-level and how a laser can be used in such a precise and powerful manner to create nano-patterns on a surface.

The results have very practical application, or so the researchers say. 
 You can use this fully absorbing light surface to increase the yield of photovoltaic panels, you can create special sensors to detect specific molecules, you can even exploit the hydrophobic properties to have a surface that is “rust” resistant and remains so since its resistant is not dependent on a protective varnish but is a physical characteristic of the surface.

Also, the researchers pointed out that such an amazing hydrophobic characteristics would make possibile to have toilets where you need very little water to flush (or possibly none at all) and still the toilet surface will remain completely clean. A bonus in many Countries were water availability is scarce and dirt creates the perfect environment for germs and diseases.

I forgot. Why Black as a moth eye?

Well, it turns out that moth’s eyes have a nanostructure surface that is not reflective at all (hence they are as black as possibile) making them resistant to glare (which is good if you plan to fly nighttime…) and also invisible to potential predator. Scientists noticed this characteristics and the researchers have been able to duplicate this nano-pattern on metal surfaces with their laser etching making them as black as a moth eye|

Author - Roberto Saracco

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