By flattening a droplet of water between two sheets of graphene, researchers have created a new form of ice. Just a few molecules thick, its atoms are locked in a square grid pattern1. The discovery of ‘square ice’ highlights another remarkable property of graphene, which consists of flat, atom-thick sheets of carbon. Not only are graphene sheets remarkably stiff, strong and conductive, but they can also exert immense pressure on molecules trapped between them. This could explain why water seeps through stacks of graphene very quickly — a property that suggests the material could be used in desalination membranes to purify water.

Back in 2012, a team led by Andre Geim at the University of Manchester, UK — who shared the 2010 Nobel Prize in Physics for isolating and studying graphene — found that water vapour could pass through laminated sheets of graphene oxide, something that not even helium gas could manage2. Two years later, they showed that liquid water performed the same trick through stacks of graphene oxide, even though those stacks filtered out other molecules3.

Computer simulations suggested that water was forming layers of square ice between the graphene sheets. Pushing the ice from one end shunted all the molecules forward in concert, like carriages in a high-speed train. “But you never trust molecular-dynamics simulations,” says Geim. Hence the latest experiment.

Ice to meet you

Geim’s team dropped one microlitre of water on to a sheet of graphene, and then placed a second graphene wafer on top, all at room temperature. As the water slowly evaporated, the graphene sheets were squeezed together until they were less than one nanometre apart, trapping pockets of water in the sandwich.

Transmission electron microscopy revealed that these pockets contained square ice. “It’s not totally unexpected,” says Alan Soper, a physicist at the Rutherford Appleton Laboratory in Harwell, UK, who wrote a News & Views article4 that accompanies the report of the discovery, which is published inNature1. When water gathers into small clusters of just eight molecules, for example, it forms a cubic structure. “But it’s never been observed in such an extended layer,” he says.

Soper reckons that square ice qualifies as a new crystalline phase of ice, joining 17 others that have already been observed.

Flat hunting

Square ice is strikingly different from normal ice. In a single, V-shaped water molecule (H2O), an oxygen atom is connected to two hydrogen atoms by strong bonds. But it also forms weaker attractions to hydrogen atoms in two neighbouring water molecules. In ice, these four bonds are usually arranged in a tetrahedral (pyramid) shape.

But in a layer of square ice, all the atoms lie in a flat plane with a right angle between each oxygen–hydrogen bond. Geim’s patches of square ice contained one, two or three of these layers, with oxygen atoms in adjacent layers sitting directly on top of one another.

The team calculated that the graphene sheets must be exerting more than 10,000 times atmospheric pressure to flatten water in this way. “It was a surprise the pressure was so high,” says Geim. That pressure is generated when the graphene’s carbon atoms get close enough to distort each other’s electron clouds. This causes a mutual attraction, known as the van der Waals force, between carbon atoms in adjacent graphene layers. “It’s like having millions of little springs holding them together,” says Soper.

Geim thinks that square ice could turn up in other tight spaces, such as the interiors of nanotubes. And pinning down its properties should help the development of improved desalination filters based on graphene, he adds. “Finding out how the water behaves in a capillary is a big part of what we need to do to make a good filter,” says Geim. “This is a very important step.”