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Physicists working in the weird field of quantum mechanics have managed to find a way to make the thinnest liquid films ever with help from graphene.
Many of us will be familiar with water droplets forming on our smartphones that are easy to brush off, but there is also the problem whereby after a shower our mirrors fog up and it’s a lot more difficult to remove the water.
Both of these realities are down to well-understood physics, but there is a third, mysterious possibility for how water interacts with a surface. Called ‘critical wetting’, it has been theorised since the 1950s, but no one had been able to determine whether it was actually real.
In this process, atoms of liquid would start to form a film on a surface, but then would stop building up when they were just a few atoms thick.
As it turns out, the discovery of the atom-thin wonder material graphene in 2010 was the perfect candidate to test critical wetting in the real world, and now it has contributed to a breakthrough scientific paper showcasing a newfound ability to create the thinnest liquid films ever.
In doing so, the breakthrough could allow for the engineering of a new class of surface coatings and lubricants just a few atoms thick.
In a vacuum, a suspended sheet of one-atom-thick graphene (brown lattice) could be manipulated to create a liquid film (atoms in dark blue) that stops growing at a thickness between three and 50 nanometres. Image: Adrian Del Maestro et al
Controllable by a ‘quantum-sized knob’
Publishing its findings in the journal Physical Review Letters, the team of physicists from the University of Vermont in the US detailed how it explored how three light gases – hydrogen, helium and nitrogen – would behave near graphene.
In a vacuum and other conditions, they calculated that a liquid layer of these gases will start to form on the one-atom-thick sheet of graphene.
But this suddenly stops when it reaches a thickness of about 10 or 20 atoms, due to a law of quantum mechanics known as Van der Waals force, which can only allow a surface to hold on to the liquid for so long because of graphene’s lack of electrostatic charge or chemical bond to hold the liquid.
By comparison, water on a bathroom mirror – where the Van der Waals force is not the only thing at play – is around 109 atoms thick, so engineering a surface where this kind of weak force can be observed has proven very challenging.
Yet a suspended sheet of graphene in a vacuum has now been shown to allow for the creation of a liquid film that stops growing at a thickness of as much as 50 nanometres, down to a thickness of just three nanometres.
Most importantly, this thickness is tuneable, allowing for the control of an ultra-thin film, equating to a ‘quantum-sized knob’ by the team.
While the researchers are left wondering about its industrial potential, they believe that with the control of critical wetting, engineers might be able to customise nanoscale coatings that wouldn’t blot out the desired properties of graphene, making it ideal for next-generation wearable electronics and displays.
