Onset of two collective excitations in the transverse dynamics of a simple fluid

TL;DR

This study uses molecular dynamics and exponential expansion theory to analyze transverse current autocorrelation in a simple Lennard-Jones fluid, revealing two collective excitations and their transition from overdamped to underdamped behavior.

Contribution

It provides a detailed account of transverse excitations in a simple fluid, identifying two distinct modes and their dynamical crossover using a novel exponential mode decomposition.

Findings

Identification of the wavevector where shear wave propagation begins.

Observation of two pairs of modes undergoing a smooth transition from overdamped to underdamped.

Demonstration of the continuous nature of the mode transitions in the fluid.

Abstract

A thorough analysis of the transverse current autocorrelation function obtained by molecular dynamics simulations of a dense Lennard-Jones fluid reveals that even such a simple system is characterized by a varied dynamical behavior with changing length scale. By using the exponential expansion theory, we provide a full account of the time correlation at wavevectors $Q$ between the upper boundary of the hydrodynamic region and $Q_p/2$, with $Q_p$ the position of the main peak of the static structure factor. In the $Q$ range studied we identify and accurately locate the wavevector at which shear wave propagation starts to take place, and show clearly how this phenomenon may be represented by a damped harmonic oscillator changing, in a continuous way, from an overdamped to an underdamped condition. The decomposition into exponential modes allows one to convincingly establish not only the crossover related to the onset of transverse waves but, surprisingly, also the existence of a second pair of modes equivalent to another oscillator that undergoes, at higher $Q$ values, a similarly smooth over- to underdamped transition.