Microscopic Origin of Shear Relaxation in Strongly Coupled Yukawa Liquids

We report accurate molecular dynamics calculations of the shear stress relaxation in a two-dimensional strongly coupled Yukawa liquid over a wide range of the Coulomb coupling strength $Γ$ and the Debye screening parameter $κ$. Our data on the relaxation times of the ideal- , excess- and total shear stress auto-correlation ($τ^{id}_M, τ^{ex}_M, τ_M$ respectively) along with the lifetime of local atomic connectivity $τ_{LC}$ leads us to the following important observation. Below a certain crossover $Γ_c(κ)$, $τ_{LC} \rightarrow τ^{ex}_M$, directly implying that here $τ_{LC}$ is the microscopic origin of the relaxation of excess shear stress unlike the case for ordinary liquids where it is the origin of the relaxation of the total shear stress. At $Γ>> Γ_c(κ)$ i.e. in the potential energy dominated regime, $τ^{ex}_M\rightarrow τ_M$ meaning that $τ^{ex}_M$ can fully account for the elastic or "solid like" behavior.

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