AsianScientist (Nov. 6, 2017) – Scientists in Japan have simulated supercooled water in unprecedented detail to explain the anomalous behavior of water at low temperatures. They published their findings in Science Advances.
When Einstein was working toward his PhD, he was among the first to explain how particles exhibit random motions in fluids. Diffusion is an important physical process and the Stokes-Einstein relationship describes how particles diffuse through a fluid based on hydrodynamic theory. However, supercooled liquids—liquids at temperatures below their melting temperature—are viscous and glassy and their behavior can no longer be explained by the simple Stokes-Einstein relationship.
In this study, researchers from Osaka University and Nagoya University have used simulations to show that different regions of water become differentially affected by supercooling, forming hydrogen bonds heterogeneously, with partial solidification. Their simulations allowed them to examine how the supercooled water hydrogen bonding network changed over time.
“Most liquids obey the Stokes-Einstein equation over a wide range of temperatures, but some unexpected changes in behavior are found in supercooled water and other glassy materials,” said Associate Professor Kim Kang of Osaka University. “Breakdown of Stokes-Einstein behavior suggests some kind of anomalous molecular motions even in a liquid state, but it’s not clear what those behaviors are.”
The researchers demonstrated that the movement of water molecules in the viscous supercooled state was reflective of jumps related to hydrogen bond breaking. The erratic timing of this kind of movement is not accounted for by the Stokes-Einstein equation.
“There are interesting physical implications of these findings as the Stokes-Einstein violation is regarded as a hydrodynamic anomaly of many glassy materials systems,” said Kim. “Our simulations help to answer questions about what happens in pure supercooled water and could also help to explain other dynamic behaviors in other technologically important glassy materials.”
The article can be found at: Kawasaki & Kim. (2017) Identifying Time Scales for Violation/preservation of Stokes-Einstein Relation in Supercooled Water.
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Source: Osaka University.
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