From left to right: Dr Isaac Gresham, Professor Chiara Neto, Mr Seamus Lilley [Credit: Isaac Gresham]
The slippery nano-thin layers, between two and five billionths of a metre thick or 10,000 times thinner than a human hair, are made up of oil molecules that are only a hundred atoms long.
“A water droplet glides with no friction over a thick oil film, but if you completely remove the oil film, say by using soap, most water droplets will stick to solid surfaces,” Professor Neto said.
“How thin can the oil layer be on a solid surface before it is no longer ‘liquid-like’? At the nanoscale, the definition of a liquid becomes somewhat slippery.”
To unravel the secrets of their ultra-thin liquid coatings, the team used two techniques to ‘see’ the surface layers.
The first technique is single-molecule force spectroscopy, which measures the length of individual molecules and the force required to stretch or compress them.
The second is neutron reflectometry, which allows scientists to measure the length and grafting density of molecules.
“We found that if the liquid molecules were too short and sparsely grafted on the solid surface, they did not adequately cover the underlying solid surface and remained sticky,” Professor Neto said.
“On the other hand, if molecules were too long or grafted too densely, they did not have enough flexibility to act like a liquid.
“For SCALS to be effective, they needed to be in a Goldilocks zone, where they are neither too short nor too long, nor packed too loose or too tight.”
To show definitively that the exceptional properties of these layers are due to their ‘liquid-like’ state, the team measured the speed that a small probe molecule diffused inside the layer.
Molecules can diffuse through liquids, but not through solids. Professor Neto said the fastest molecular diffusion was observed in the Goldilocks zone, where the oil molecules are just the right length and grafted with moderate density.
