The properties of graphene seem to amaze every day, but the latest finding of the researchers at Rensselaer Polytechnic Institute and Rice University may be the most surprising of recent months. The team, led by Professor Nikhil Koratkar and charged with determining the wetting transparency of the nanomaterial, have discovered that graphene is practically invisible to water; a unique characteristic of the substance that means water acts exactly as if it wasn’t there.
By laying a single sheet of graphene on pieces of gold, copper, and silicon, and then placing a drop of water on the coated surfaces the team were able to show that the graphene had virtually no effect on the way the water spread across the surface. This finding doesn’t seem at all remarkable until the fact of graphene’s impermeability is taken into account. Graphene’s structure is such that water molecules cannot pass through the lattice of carbon atoms, they are simply too large, yet the latest findings suggest that despite the graphene barrier the water continues to act as though it were in direct contact with the other elements.
“The graphene was completely transparent to the water,” said Koratkar, a faculty member in the Department of Mechanical, Aerospace, and Nuclear Engineering and the Department of Materials Science and Engineering at Rensselaer. “The single layer of graphene was so thin that it did not significantly disrupt the non-bonding van der Waals forces that control the interaction of water with the solid surface. It’s an exciting discovery, and is another example of the unique and extraordinary characteristics of graphene.”
The wetting of a substance is calculated by measuring the angle between the droplet and the surface on which it sits; a hydrophobic surface will produce a high angle and a hydrophilic substance a low angle. In the case of a single sheet of graphene the contact angle for each of the substances was hardly affected by the presence of graphene, so for gold the angle was 78 degrees as opposed to 77 degrees, silicon 33 degrees instead of 32, and copper 86 degrees instead of 85 degrees. As can be seen the contact angles suggest that the water was sitting on different substances rather than a single substance as was the case.
As the number sheets was gradually increased from one up to six the effect was seen to lessen. After six layers the contact angle became the same as if the water were sat atop of graphite.
“This effect is an artifact of the extreme thinness of graphene—which is only about 0.3 nanometers thick. Nothing can rival the thinness of graphene. Because of this, graphene is the ideal material for wetting angle transparency… Moreover, graphene is strong and flexible, and it does not easily crack or break apart. Additionally, it is easy to coat a surface with graphene using chemical vapor deposition, and it is relatively uncomplicated to deposit uniform and homogeneous graphene coatings over large areas. Finally, graphene is chemically inert, which means a graphene coating will not oxidize away. No single material system can provide all of the above attributes that graphene is able to offer.”Professor Koratkar has added these findings to the other notable properties of graphene,
But what use can be found for the new discovery? Well, one idea put forward is that graphene can be used to coat the copper found in dehumidifiers in order to prevent oxidation from contact with water. The graphene would essentially provide a barrier whilst enabling the machine to function without any decreases in efficiency. The graphene has been shown to have little or no effect on the wettability of copper, yet it would simultaneously enrobe the copper surface and prevents it from oxidizing.
The study can be read in the Nature Materials paper “Wetting transparency of graphene.” Or online at: http://dx.doi.org/10.1038/NMAT3228