The unusual amorphous ice has a density close to that of liquid water.
Ice cubes float in water because they are less dense than liquid. But the density of the new type of ice is almost equal to the density of your glass of water, researchers report in Science. If you could drop that ice into your cup without it melting immediately, it would wobble without floating or sinking.
The new ice is a special type called amorphous. This means that the water molecules inside it are not arranged in a clear order, as in ordinary crystalline ice. Other types of amorphous ice are already known, but they have a density either lower or higher than the density of water under standard conditions. Some scientists hope that this newly formed amorphous ice can help solve the scientific mysteries that rage around water.
To create new ice, scientists used a surprisingly simple technique. It’s called ball milling and involves shaking a container of ice and stainless steel balls cooled to 77 Kelvin (almost -200° Celsius). The researchers were motivated by curiosity; they did not expect the technique to create new amorphous ice. “It was kind of an idea on a Friday afternoon, just to try it and see what happens,” says physical chemist Christoph Salzmann of University College London.
Analysis of how X-rays were scattered by the frosty material suggested they created amorphous ice. And computer simulations that simulated the effects of crushing the balls showed that the disordered structure could be created by layers of ice sliding past each other in random directions in response to the forces exerted by the balls.
“You have to be open as a scientist to the unexpected,” says physicist Anders Nilsson of Stockholm University, who was not involved in the study. According to him, the technique of the ball mill “was quite innovative.”
Since the material was made by grinding regular ice, its relationship to liquid water is unknown. It is unclear whether it can be produced directly by cooling liquid water. Not all amorphous ices have such a connection with their liquid state.
If the new ice does have such a relationship with the liquid, the ice could help scientists better understand the properties of water. Water is surprising because it violates the norms for liquids. For example, while most liquids become denser as they cool, water becomes denser as it approaches 4 °C, but becomes less dense as it cools.
Many scientists suspect that the strangeness of water is related to its behavior as a supercooled liquid. Pure water can remain a liquid at temperatures well below the freezing point. Under these conditions, liquid water is believed to exist in two distinct phases, a high-density liquid and a low-density liquid, and this dual nature may explain the behavior of water under more typical conditions. But much remains unclear about this idea.
Salzmann and his colleagues suggest that the new ice may be a special form of water called glass. Glasses can be made by cooling a liquid fast enough that the molecules cannot rearrange into a crystalline structure. The glass in window glass is an example of such a material made by cooling molten quartz sand, but other substances can also form glass.
If the new ice is the glassy state of water, scientists will need to determine how it fits into the dual-liquid picture. And it could help scientists understand what’s really going on in hard-to-probe subcooling conditions.
But some researchers are skeptical that the new material has anything to do with the strange physics of liquid water. Physical chemist Thomas Loerting of the University of Innsbruck in Austria believes that ice is “closely associated with very small distorted ice crystals” rather than a liquid form of water.
However, earlier computer simulations suggested that water could form glasses with a density range close to that of liquid water, says computational physicist Nicholas Giovambattista of the City University of New York’s Brooklyn College. According to Jovambattista, who was not involved in the new study, this simulation produced structures similar to those seen in computer simulations of ball mill ice.
“It opens the door to new questions. This is new, so what is it?”
