Washington: For the first time, researchers have directly calculated the rate at which water crystallises into ice in a realistic computer model of water molecules. The simulations, which were carried out on supercomputers, provide insight into the mechanism by which water transitions from a liquid to a crystalline solid.
Understanding ice formation adds to our knowledge of how cold temperatures affect both living and non-living systems, including how living cells respond to cold and how ice forms in clouds at high altitudes, researchers at the Princeton University said.
A more precise knowledge of the initial steps of freezing could eventually help improve weather forecasts and climate models, as well as inform the development of better materials for seeding clouds to increase rainfall.
The study, conducted by Amir Haji-Akbari, a postdoctoral research associate, and Pablo Debenedetti, a professor of chemical and biological engineering, looked at the process by which, as the temperature drops, water molecules begin to cling to each other to form a blob of solid ice within the surrounding liquid.
These blobs tend to disappear quickly after their formation. Occasionally, a large enough blob, known as a critical nucleus, emerges and is stable enough to grow rather than to melt. The process of forming such a critical nucleus is known as nucleation. To study nucleation, the researchers used a computerised model of water that mimics the two atoms of hydrogen and one atom of oxygen found in real water.
Through the computer simulations, the researchers calculated the average amount of time it takes for the first critical nucleus to form at a temperature of about minus 43 degrees Celsius, which is representative of conditions in high-altitude clouds.
They found that, for a cubic meter of pure water, the amount of time it takes for a critical nucleus to form is about one-millionth of a second.
"The main significance of this work is to show that it is possible to calculate the nucleation rate for relatively accurate models of water," said Haji-Akbari. The study was published in the journal PNAS.
