Microscopic Origin of Shear Relaxation in a Model Viscoelastic Liquid

An atomistic description of shear stress relaxation in a viscoelastic liquid is developed from first principles through accurate molecular dynamic simulations in a model Yukawa system. It is shown that the relaxation time tex M of the excess part of the shear stress autocorrelation function provides a correct measure of the relaxation process. Below a certain critical value Gamma(c) of the Coulomb coupling strength, the lifetime of local atomic connectivity tau(LC) converges to tau(ex)(M) and is the microscopic origin of the relaxation. At Gamma >> Gamma(c), i.e., in the potential energy dominated regime, tau(ex)(M) -> tau(M) (the Maxwell relaxation time) and can, therefore, fully account for the elastic or solidlike behavior. Our results can help provide a better fundamental understanding of viscoelastic behavior in a variety of strongly coupled systems such as dusty plasmas, colloids, and non-Newtonian fluids.