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Mechanics of supercooled liquids

Zhigang Suo's picture

In a pure liquid, molecules touch one another but change neighbors frequently.  External forces cause the liquid to change shape by viscous flow.  Thermal agitation causes  molecules to undergo self-diffusion.  The two phenomena--viscous flow and self-diffusion--often result from a single rate-limiting process:  molecules change neighbors.  This simple picture is amply confirmed by the Stokes-Einstein relation, which links the viscosity and self-diffusivity for many liquids over wide ranges of temperature.

In recent decades, however, evidence has accumulated that this simple picture is wrong for many supercooled liquids, substances that remain in the liquid state and do not crystallize when the temperature drops below their melting points.  In many cases a supercooled liquid forms a dynamic structure that consists of regions larger than individual molecules.  Molecular rearrangement is much slower in some regions than others. Viscous flow proceeds by disrupting the dynamic structure, but self-diffusion proceeds by the migration of individual molecules through the fast regions. As the temperature drops, the dynamic structure increasingly jams viscous flow, but does not retard self-diffusion as much.

We have just formulated a continuum theory of supercooled liquids by regarding viscous flow and self-diffusion as distinct, concurrent processes.  We show that the relative rate of viscous flow and self-diffusion defines a length, which, for some supercooled liquids, is much larger than the molecular dimension. Large objects evolve by viscous flow, and small objects evolve by self-diffusion.

We hope that this theoretical paper will stimulate people to identify suitable experimental systems to demonstrate this transition.  Concurrent diffusion and viscous flow should also be important in other partially jammed systems, such as gels consisting of macromolecules and solvents, and glasses consisting of dissimilar atoms.

The paper, entitled Mechanics of Supercooled Liquids, is co-authored by Jianguo Li, Qihan Liu, Laurence Brassart, and Zhigang Suo.  The paper has just appeared in Journal of Applied Mechanics.  

We would be grateful if you can comment.

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Zhigang Suo's picture

I have just given a talk on supercolled liquids at the Prager Medal Symposium in honor of Bob McMeeking.  I attach the slides of the talk.  The paper has just appeared in JAM.  

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