Identification of time scales of the violation of the Stokes-Einstein relation in Yukawa liquids

Abstract

We investigate the origin of the violation of the Stokes-Einstein (SE) relation in two-dimensional Yukawa liquids. Using comprehensive molecular dynamics simulations, we identify the time scales supporting the violation of the SE relation D ∝ (η / T) - 1, where D is the self-diffusion coefficient and η is the shear viscosity. We first compute the self-intermediate scattering function F s (k, t), the non-Gaussian parameter α2, and the autocorrelation function of the shear stress C η (t). The time scales obtained from these functions include the structural relaxation time τ α, the peak time of the non-Gaussian parameter τ α 2, and the shear stress relaxation time τ η. We find that τ η is coupled with D for all temperatures indicating the SE preservation; however, τ α and τ α 2 are decoupled with D at low temperatures indicating the SE violation. Surprisingly, we find that the origins of this violation are related to the non-exponential behavior of the autocorrelation function of the shear stress and non-Gaussian behavior of the distribution function of particle displacements. These results confirm dynamic heterogeneity that occurs in two-dimensional Yukawa liquids that reflect the presence of regions in which dust particles move faster than the rest when the liquid cools to below the phase transition temperature.