The research team of Okayama University has elucidated the molecular mechanism of the initial stage of ice melting !
June 20, 2013
The research team of Okayama University has revealed the molecular mechanism of melting ice from the inside by computer simulation. The mechanism by which solid matter melts is a challenging, basic scientific research theme that also provides important suggestions for understanding mechanisms of phase transition and other research fields.
In general, the ice surface helps to initiate melting and therefore ice melts starting from its surface at 0 degrees Celsius. This is termed heterogeneous melting. On the other hand, an ideal melting process occurs from inside the ice if there is no surface. In this case, where ice spontaneously loses its order by thermal agitations, the process is termed homogeneous melting. A familiar example is the Tyndall figure that appears inside the surface ice of a pond when it is exposed to sunlight. (Figure 1)
Homogeneous melting is one of the most important processes in physics and chemistry and is termed the first order phase transition. In ice, each water molecule has four hydrogen bonds and four neighboring water molecules make an ordered structure with a beautiful six-fold symmetry. (Figure 2) In liquid water, most of the hydrogen bonds are preserved but its order is completely lost. (Figure 3) Probing the mechanism by which water molecules introduce disorder spontaneously to end up melting is a major challenge.
Using the computer simulations, Prof. Masakazu Matsumoto of Okayama University and his collaborators, Mr. Kenji Mochizuki and Prof. Iwao Ohmine of the Institute for Molecular Science, succeeded in probing the initial process of homogeneous ice melting in full detail for the first time. Their paper, “Defect pair separation as the controlling step in homogeneous ice melting.” was published on June 20th in the journal Nature as the cover article.
“It was really challenging,” said Mr. Mochizuki, the 4th grade Ph.D. student of the Graduate University for Advanced Studies, “because the melting process at a molecular scale looks quite different every time at a first glance. It took almost 10 years to obtain the generalized view of the melting process.”
The team simulated the melting process of ice using a computational technique called molecular dynamics. They applied many analytical techniques of physical chemistry, and examined the full mechanisms of melting from the emergence of disorder to the growth of a molten droplet toward the total collapse of ice.
“Even the ice structure looks quite ordered, it might have lost its way back to the perfect ice structure,” said Dr. Matsumoto, an associate professor of Okayama University. “Our new method estimates how difficult it is for partially disordered ice to recover the perfect ice order, which we call entanglement.”
This entanglement emerges from the separation of pair defects. Hydrogen bonds strongly bind water molecules in ice. Therefore, even if the ice structure is partially broken by thermal fluctuation, it recovers its order in a short space of time. However, once a pair defect is generated and separated, random motion of the defects prevents it from reuniting by going back to the correct return paths. (Figure 4) Pair defects also work as a catalyst to accelerate structural changes in ice, leading ice to melt. “Melting of ice is amazingly complicated,” said Mr. Mochizuki.
“Such fundamental research may not be directly linked to applications,” said Dr. Ohmine, a professor of the Institute for Molecular Science. “But it gives us insights about the structural changes in many other materials including biomolecules, and might bring us to unravel the ‘mechanism of life’ in the future.”
Paper Reference
"Defect separation as the controlling step in homogeneous ice melting" ,
Kenji Mochizuki, Masakazu Matsumoto, and Iwao Ohmine,
Nature, 20 June 2013 (Cover Article)
DOI: 10.1038/nature12190 (http://dx.doi.org/10.1038/nature12190)
Contact and Authors Information
Kenji Mochizuki
School of Physical Sciences,
The Graduate University for Advanced Studies (SOKENDAI)
Myodaiji, Okazaki 444-8585, Japan
+81-564-55-7394
Masakazu Matsumoto
Graduate School of Natural Science and Technology, Okayama University
3-1-1 Tsushima, Okayama 700-8530, Japan
+81-86-251-7846
Iwao Ohmine
Institute for Molecular Science
Myodaiji, Okazaki 444-8585, Japan
