Viscoelasticity of liquid water investigated using molecular dynamics simulations

Abstract

Real liquids exhibit a viscoelastic response when excited mechanically to deform at sufficiently high frequency. We use classical nonequilibrium molecular dynamics simulations to calculate the linear viscoelastic response of extended simple point charge (SPC/E) water under both shear and elongation, for frequencies between 50 GHz and 10 THz and temperatures spanning the liquid phase of water at atmospheric pressure. These simulations are validated using equilibrium simulations that make use of Green-Kubo relations. Data up to and including 2 THz is fit to a single relaxation time linear Maxwell model, to facilitate comparison with reported experiments. We find that the resulting elastic moduli agree well with measurement, but this is not the case for the viscous moduli. This data also obeys a generalized Cauchy relation, implying that the elastic response of SPC/E water is dominated by central forces. This opens a pathway toward development of a simplified, molecular elastic water model for viscoelastic flows at high frequency. Furthermore, both elastic and loss moduli obey the time temperature superposition principle for frequencies up to 2 THz; an anomaly is observed above 2 THz, pointing to different physics. This behavior remains to be observed experimentally.