Visualizing water molecules

Model of graphene

If you look extremely closely, there’s a coating of water a few molecules thick on most surfaces. Although scientists know the water is there, it has been extremely difficult to see the water, not only because it’s in microscopically thin layers, but also because the water tends to slide around. Ke Xu, Peigen Cao and James Heath of Caltech have devised a novel way of trapping and examining the surface water.

Originally, the team was simply studying graphene (a one-atom-thick carbon lattice) layered onto atomically flat surfaces (mica). When they noticed nanoscale structures that grew in prevalence under high humidity and disappeared under completely dry conditions, they realized they’d found a way to visualize the thinnest layers of water.

The graphene lies down so tightly over the flat surface of the mica that any anomalies, such as water molecules, are essentially shrink-wrapped and become visible under an atomic force microscope.

To their surprise, the researchers found that as the humidity increased, the first layer of water deposited was a two-molecule thick layer of ice. This was true even at room temperature. On top of that, a second two-molecule thick layer of ice was deposited. Only on top of that would you finally get liquid droplets of water, if the humidity were high enough.

The scientists hope to use their method to look at proteins and other larger molecules.

Atomic force micrograph of ~1 micrometer wide x 1.5 micrometers (millionths of a meter) tall area. The ice crystals (lightest blue) are 0.37 nanometers (billionths of a meter) high, which is the height of a 2-water molecule thick ice crystal. Detailed analysis of such images reveals that this first layer of water is ice, even at room temperature. At high humidity levels, a second layer of water will coat the first layer, also as ice. At very high humidity levels, additional layers of water will coat the surface as droplets.

Credit: Heath group/Caltech