Model for dynamics in supercooled water

Journal Article · Wed Dec 01 04:00:00 EST 1999 · Physical Review. E, Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics

DOI:https://doi.org/10.1103/PhysRevE.60.6776· OSTI ID:20217799

Liao, C Y ^[1]; Sciortino, F ^[2]; Chen, S H ^[1]

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1. Department of Nuclear Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (United States)
2. Dipartimento di Fisica, Universita di Roma La Sapienza, Piazalle Aldo Moro 2, I-00185 Roma, (Italy)

We propose a phenomenological model for the intermediate scattering function (ISF) associated with density fluctuation in low temperature water. The motivation is twofold: to extract various physical parameters associated with the ISF computed from extended simple-point-charge model water at supercooled temperatures, and to apply this model to analyze high resolution inelastic x-ray scattering data of water in the future. The ISF of the center of mass of low temperature water computed from 10 M-step molecular dynamics (MD) data shows clearly time-separated two-step relaxation with a well-defined plateau in-between. We interpret this result as due to the formation of a stable hydrogen-bonded, tetrahedrally coordinated cage around a typical molecule in low temperature water. We thus model the long-time cage relaxation by the well-known Kohlrausch form exp[-(t/{tau}){sup {beta}}] with an amplitude factor which is a k-dependent Debye-Waller factor A(k), and treat the short-time relaxation as due to molecular collisional motions within the cage. The latter motions can be described by the generalized Enskog equation, taking into account the confinement effect of the cage. We shall show that the effect of the confinement changes the collisional dynamics by modifying certain input parameters in the kinetic theory by a factor [1-A(k)]{sup 1/2}. We solve the generalized Enskog equation approximately but analytically by a Q-dependent triple relaxation time kinetic model. This kinetic model was previously shown to account for the large k behavior of Rayleigh-Brillouin scattering from moderately dense, simple fluids. We find that our model fits well with the MD generated collective as well as single-particle ISFs. For the short-time collisional dynamics, we fix values of the hard sphere diameter {sigma} and pair correlation function at contact g({sigma}), without introducting any adjustable parameters. The calculated ISFs reproduce the correct Brillouin peak frequencies at low k values. From the long-time dynamics, we deduce values of the Debye-Waller factor A(k), the Kohlrausch exponent {beta}(k), and the cage relaxation time {tau}(k). (c) 1999 The American Physical Society.

• OSTI ID: 20217799
• Journal Information: Physical Review. E, Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, Journal Name: Physical Review. E, Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics Journal Issue: 6 Vol. 60; ISSN PLEEE8; ISSN 1063-651X
• Country of Publication: United States
• Language: English

Related Subjects

71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
DEBYE-WALLER FACTOR
DENSITY
FLUCTUATIONS
KINETIC EQUATIONS
MOLECULAR DYNAMICS METHOD
RELAXATION
THEORETICAL DATA
WATER
X-RAY SPECTRA