Molecular velocity auto-correlation of simple liquids observed by NMR MGSE method

The velocity auto-correlation spectra of simple liquids obtained by the NMR method of modulated gradient spin echo show features in the low frequency range up to a few kHz, which can be explained reasonably well by a $t^{-3/2}$ long time tail decay only for non-polar liquid toluene, while the spectra of polar liquids, such as ethanol, water and glycerol, are more congruent with the model of diffusion of particles temporarily trapped in potential wells created by their neighbors. As the method provides the spectrum averaged over ensemble of particle trajectories, the initial non-exponential decay of spin echoes is attributed to a spatial heterogeneity of molecular motion in a bulk of liquid, reflected in distribution of the echo decays for short trajectories. While at longer time intervals, and thus with longer trajectories, heterogeneity is averaged out, giving rise to a spectrum which is explained as a combination of molecular self-diffusion and eddy diffusion within the vortexes of hydrodynamic fluctuations.Comment: 8 pages, 6 figur

Long time behaviour of the velocity autocorrelation function at low density and near critical point of simple fluids

15 pagesInternational audienceNumerous theoretical and numerical works have been devoted to the study of the algebraic decrease at large times of the velocity autocorrelation function of simple fluid particles. The derivation of this behaviour, so-called the long-time tail, generally based on linearized hydrodynamic makes no reference to any specific characteristic of the particle interactions. However, in the literature, doubts have been expressed on the possibility that by numerical simulations the long-time tail can be observed in all the fluid phase domain of systems where the particles interact by soft-core and attractive pair potentials. In this work, extensive and accurate molecular dynamics simulations establish that the predicted long-time tail of the velocity autocorrelation function exists in low density fluid of particles interacting by a soft-repulsive potential and near the liquid-gas critical point of a Lennard-Jones system. These results contribute to confirm that the algebraic decay of the velocity autocorrelation function is universal in the fluid systems