ABSTRACT
Inelastic x-ray scattering data have been collected for liquid sodium at T=390 K, i.e., slightly above the melting point. Owing to the very high instrumental resolution, pushed up to 1.5 meV, it has been possible to determine accurately the dynamic structure factor S(Q,omega) in a wide wave-vector range, 1.5-15 nm(-1), and to investigate on the dynamical processes underlying the collective dynamics. A detailed analysis of the line shape of S(Q,omega), similarly to other liquid metals, reveals the coexistence of two different relaxation processes with slow and fast characteristic time scales. The present data lead to the conclusion that (i) the picture of the relaxation mechanism based on a simple viscoelastic model fails and (ii) although the comparison with other liquid metals reveals similar behavior, the data do not exhibit an exact scaling law as the principle of the corresponding state would predict.
ABSTRACT
The longitudinal and shear viscosity of water are calculated by molecular dynamics simulation with a polarizable potential model at room temperature. To overcome the difficulty of evaluating directly the stress autocorrelation function of a system with intrinsically many-body forces, we have resorted to the analysis of the wave-vector-dependent longitudinal and transverse-current correlation functions. In a memory function formalism, the generalized viscosity can be evaluated as a function of the wave vector k. By extrapolating to k=0, we find longitudinal and shear viscosity values in better agreement with the experimental value than the corresponding quantities evaluated by making use of a nonpolarizable potential model. This result points out that for a realistic reproduction of transport quantities, it is crucial to take into account many-body contributions to the interaction potential.
ABSTRACT
New inelastic x-ray scattering experiments have been performed on liquid lithium in a wide wave vector range. With respect to the previous measurements, the instrumental resolution, improved up to 1. 5 meV, allows one to accurately investigate the dynamical processes determining the observed shape of the dynamic structure factor S(Q, omega). A detailed analysis of the line shapes shows the coexistence of relaxation processes with both slow and fast characteristic time scales, and therefore shows that pictures of the relaxation mechanisms based on a simple viscoelastic model must be abandoned.
