Viscosity and self-diffusion of supercooled and stretched water from molecular dynamics simulations



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Montero de Hijes, Pablo and Sanz García, Eduardo and Joly, Laurent and Valeriani, Chantal and Caupin, Frederic (2018) Viscosity and self-diffusion of supercooled and stretched water from molecular dynamics simulations. Journal of chemical physics, 149 (9). ISSN 0021-9606

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Among the numerous anomalies of water, the acceleration of dynamics under pressure is particularly puzzling. Whereas the diffusivity anomaly observed in experiments has been reproduced in several computer studies, the parallel viscosity anomaly has received less attention. Here we simulate viscosity and the self-diffusion coefficient of the TIP4P/2005 water model over a broad temperature and pressure range. We reproduce the experimental behavior and find additional anomalies at negative pressure. The anomalous effect of pressure on dynamic properties becomes more pronounced upon cooling, reaching two orders of magnitude for viscosity at 220 K. We analyze our results with a dynamic extension of a thermodynamic two-state model, an approach which has proved successful in describing experimental data. Water is regarded as a mixture of interconverting species with contrasting dynamic behaviors, one being strong (Arrhenius) and the other fragile (non-Arrhenius). The dynamic parameters of the two-state models are remarkably close between experiment and simulations. The larger pressure range accessible to simulations suggests a modification of the dynamic two-state model, which in turn also improves the agreement with experimental data. Furthermore, our simulations demonstrate the decoupling between viscosity eta and self-diffusion coefficient D as a function of temperature T. The Stokes-Einstein relation, which predicts a constant D eta/T, is violated when T is lowered, in connection with the Widom line defined by an equal fraction of the two interconverting species. These results provide a unifying picture of thermodynamics and dynamics in water and call for experiments at negative pressure. Published by AIP Publishing.

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© 2018 Author(s). P.M.H., E.S., and C.V. have been funded by Grant Nos. FIS2013/43209-P, FIS2016-78117-P, and FIS2016-78847-P of the MEC and the UCM/Santander Nos. 910570 and PR26/16-10B-2. P.M.H. acknowledges financial support from a FPI Ph.D. fellowship. L.J. acknowledges support from Institut Universitaire de France. This work was partially supported by CNRS (France) through a PICS program.

Uncontrolled Keywords:Stokes-Einstein relation; Liquid water; Pressure-dependence; Density anomalies; Glass-transition; Relaxation; Model; Mobility; Temperature; Breakdown
Subjects:Sciences > Physics > Nuclear physics
ID Code:51148
Deposited On:13 Feb 2019 10:11
Last Modified:13 Feb 2019 11:50

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