Superhydrophobic materials are a hot topic in science right now, but they’re hardly new; not only did evolution create such water-repelling surfaces to protect the leaves of many plant species, mankind has even made hydrophobic surfaces with devices as simple as sandpaper. It’s not actually all that hard to make something hydrophobic, it’s just hard to make it really hydrophobic without sacrificing some other crucial element of its design.
Most recently, much-ballyhooed super-hydrophobic sprays released not with a bang but a whimper, as they have tended to flake off soon after application. Now, an advanced technique using high-powered lasers could finally help us have durable superhydrophobic surfaces in the real world.
The innovation comes by way of advanced laser technology, and the ability to deliver lots of power to extremely small areas with high precision. Using these lasers, the team from the University of Rochester etched nested details into the surfaces of various metal plates (made of platinum, brass, or titanium), nano-scale divets carved into the surface of the micro-scale divets it just carved. These kinds of detail-within-detail structures are called “monolithic” when they’re carved directly out of a single piece of material. The result is a superhydrophobic “coating” that won’t peel off, and which will be much more resistant to being flattened by physical contact.
The reason these etchings repel water is colloquially known as the Lotus Effect: the leaves of the lotus flower have long been known to be extremely slippery and water repulsive. Upon investigation into this so-called “self cleaning” mechanism, researchers found that a system of surface details much like those carved out in this experiment was likely responsible.
These nano-structures both minimize contact between the surface and the droplet of water (imagine lying on a bed vs. a bed of nails), and create spaces in which air will be trapped beneath a descending droplet. With low adhesion to the surface and upward-pushing pockets of air caught in surface of the metal itself, water tends to shoot off of the surface in the classical hydrophobic fashion. The “self cleaning” aspect comes from the tendency of most contaminants to, a) have lots of water in them and, b) jump aboard and ride away on any water droplets that do transiently hit the leaf.
This obviously has applications for smartphones, which still get smudged and covered with finger-grease. Not only would a super-hydrophobic screen self-clean its way out of this problem, it could reduce glare by scattering incoming light rays in all directions. The metals etched in this experiment became dark, dark black for this very reason, endlessly bouncing and eventually absorbing almost all incoming photons. Applied to glass, this etching might be able to be made to reduce reflections and glare — it’s certainly not unheard of.
The researchers envision being able to etch superhydrophobicity on to the interior of a toilet, dramatically reducing the amount of water it would need to use. Not to get too into this point, but the principle wouldn’t just apply to urine, thanks to the “self cleaning” properties mentioned above. Maybe the most exciting possible application, though, is in solar power, as light-absorbing, moisture-repelling metals could help increase efficiency while also extending the life of parts by resisting rust and dirt accumulation.
This is true superhydrophobicity, and it can be tailored down to the nanometer scale. Obviously we don’t yet know the specifics of exactly how durable these tiny details will be in the long term, but at the very least this could have meaningful implications for industry. Depending on how cheap a metal can support this kind of etching (brass is pretty cheap…) and how cheap the etching lasers can be made, this could end up making its way into a dizzying array of devices.