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  Connection of translational and rotational dynamical heterogeneities with the breakdown of the Stokes-Einstein and Stokes-Einstein-Debye relations in water

Mazza, M. G., Giovambattista, N., Stanley, H. E., & Starr, F. W. (2007). Connection of translational and rotational dynamical heterogeneities with the breakdown of the Stokes-Einstein and Stokes-Einstein-Debye relations in water. Physical Review E, 76(3): 031203. doi:10.1103/PhysRevE.76.031203.

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Item Permalink: http://hdl.handle.net/11858/00-001M-0000-002B-2FA8-D Version Permalink: http://hdl.handle.net/11858/00-001M-0000-002B-2FA9-B
Genre: Journal Article

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 Creators:
Mazza, Marco G.1, Author              
Giovambattista, Nicolas, Author
Stanley, H. Eugene, Author
Starr, Francis W., Author
Affiliations:
1Group Non-equilibrium soft matter, Department of Dynamics of Complex Fluids, Max Planck Institute for Dynamics and Self-Organization, Max Planck Society, ou_2063308              

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 Abstract: We study the Stokes-Einstein SE and the Stokes-Einstein-Debye SED relations, Dt=kBT/6R and Dr=kBT/8R3, where Dt and Dr are the translational and rotational diffusivity, respectively, T is the temperature, the viscosity, kB the Boltzmann constant, and R the “molecular” radius. Our results are based on molecular dynamics simulations of the extended simple point charge model of water. We find that both the SE and SED relations break down at low temperature. To explore the relationship between these breakdowns and dynamical heterogeneities DHs, we also calculate the SE and SED relations for subsets of the 7% “fastest” and 7% “slowest” molecules. We find that the SE and SED relations break down in both subsets, and that the breakdowns occur on all scales of mobility. Thus these breakdowns appear to be generalized phenomena, in contrast with a view where only the most mobile molecules are the origin of the breakdown of the SE and SED relations, embedded in an inactive background where these relations hold. At low temperature, the SE and SED relations in both subsets of molecules are replaced with “fractional” SE and SED relations, Dt /T −t and Dr /T −r, where t0.841 and r0.751. We also find that there is a decoupling between rotational and translational motion, and that this decoupling occurs in both the fastest and slowest subsets of molecules. Further, we find that, the decoupling increases upon cooling, but that the probability of a molecule being classified as both translationally and rotationally fastest also increases. To study the effect of time scale for SE and SED breakdown and decoupling, we introduce a time-dependent version of the SE and SED relations, and a time-dependent function that measures the extent of decoupling. Our results suggest that both the decoupling and SE and SED breakdowns originate at a time scale corresponding to the end of the cage regime, when diffusion starts. This is also the time scale when the DHs are more relevant. Our work also demonstrates that selecting DHs on the basis of translational or rotational motion more strongly biases the calculation of diffusion constants than other dynamical properties such as relaxation times.

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Language(s): eng - English
 Dates: 2007-09-26
 Publication Status: Published in print
 Pages: -
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 Table of Contents: -
 Rev. Method: -
 Identifiers: DOI: 10.1103/PhysRevE.76.031203
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Title: Physical Review E
Source Genre: Journal
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Publ. Info: Melville, NY : American Physical Society
Pages: 12 Volume / Issue: 76 (3) Sequence Number: 031203 Start / End Page: - Identifier: ISSN: 1539-3755
CoNE: /journals/resource/954925225012