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[16:00-17:00] MST colloquium
- Description:
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Thursday September 28, 2017
Speaker: Dr. Jack F. Douglas
Affiliation: National Institute of Standards and Technology, Materials Science and Engineering Division, Gaithersburg, MD 20899 USA
Title: “Cooperative Motion and Structural Relaxation in Glass-Forming Materials”
Location: Computer Science Department, room A115 (ground floor), Voutes
Time:16:00
Language:English
Abstract
We investigate collective motion and relaxation in model polymeric glass-forming liquids by molecular dynamics simulation to quantify the
nature of cooperative motion in these liquids and to understand the significance of cooperative particle exchange motion for understanding relaxation properties of glass-forming liquids generally. We find that
relaxation occurs as multi-stage process involving cooperative molecular motion. First, there is a ‘fast’ or b-relaxation process dominated by the inertial motion of the molecules, followed by a longer time
a-relaxation process involving large scale diffusive motion. Our molecular dynamics simulations indicate that as the collective motion of the b-relaxation regime becomes progressively suppressed upon approaching Tg, material relaxation requires larger scale collective motion involving molecular diffusion, explaining the emergence of the a-relaxation regime. In both relaxation regimes, relaxation occurs through a string-like collective particle exchange motion having a common string-like geometrical form and quantitative relationships are derived relating the length of the ‘stringlets’ found in the b-relaxation regime to the ‘strings’ characterizing collective motion at long times associated with the thermally activated diffusive motion. Based on this physical picture, we find that the a-relaxation time data for a wide range of simulated glass-forming liquids can be described by an extension of the Adam-Gibbs model in which the free energy of activation (including the entropy of activation neglected by AG) is proportional to the average length L of string-like particle exchange clusters. This same type of collective motion arises in the grain boundaries of polycrystalline materials, the interfaces of
nanoparticles, superheated crystals, and in biological materials such lipid membranes and proteins so that string-like collective motion is evidently a common feature of strongly-interacting condensed matter.
Host: Dimitris Vlassopoulos
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September 2017
