Tokyo, Japan – Water is everywhere and comes in many forms: snow, sleet, hail, hoarfrost… However, despite water being so commonplace, scientists still do not fully understand the predominant physical process that occurs when water transforms from liquid to solid.
Now, in an article recently published in the Journal of Colloid and Interface Science, researchers from the Institute of Industrial Science, The University of Tokyo, have carried out a series of molecular-scale simulations to uncover why ice forms more easily on surfaces than in bodies of water.
While it is common knowledge that water freezes at 0°C (32°F), water does not instantly turn into ice the moment this temperature is reached. Instead, ice crystals begin forming at tiny 'nuclei' and spread throughout the body of water in a process called nucleation. Lower temperatures promote nucleation events and hence speed up the freezing process. Although, at the microscopic level, other factors can also play a role.
"If you watch a glass of water freezing, you will notice that the ice first forms at the water–glass interface and gradually moves inward," says Gang Sun, lead author of the study. "So, it is clear that the way in which water molecules interact with surfaces is important to the nucleation process."
To understand the microscopic effects responsible for ice formation, the team employed sophisticated, state-of-the-art molecular dynamics simulations. The simulations considered many physical parameters, such as temperature and intermolecular interaction strength, but one stood out as particularly surprising.
"Most people would assume that a surface's affinity for ice dictates the nucleation pathway," explains Hajime Tanaka, senior author. "However, our simulations show that the arrangement of water molecules in the two layers closest to the surface is even more important. This structured layering promotes the formation of a low-dimensional hexagonal crystal lattice at the surface, which then propagates into the bulk."
That said, the surface's hydrophilicity (the strength with which it attracts water) remains a key factor. Excessive hydrophilicity disrupts the bilayered hexagonal ordering of water molecules, hindering nucleation. However, there is a 'Goldilocks zone' for ice formation: where the surface interaction is neither too strong nor too weak to interfere with crystallization.
This knowledge lends researchers a tool with which they could potentially control ice formation, which would be useful for anti-ice coatings and other materials. The proposed mechanism may also apply to other liquids with other tetrahedrally bonding liquids, such as silicon and carbon. This could then inform crucial fields such as climate science and semiconductor manufacturing, where the control of nucleation events has societal and economic applications.
