Origin of viscosity at individual particle level in Yukawa liquids

Abstract

The transition of the viscosity $\ensuremath{\eta}$ from a collisional gas through a minimum value to a correlated liquid is investigated using computer simulations with the Green-Kubo relation. It is discovered that, as the temperature varies, the transition of $\ensuremath{\eta}$ is well described by the unity ratio of the instantaneous transverse sound speed ${C}_{T}$ to the average particle speed ${\overline{v}}_{p}$. While ${C}_{T}/{\overline{v}}_{p}<1, \ensuremath{\eta}$ increases with the temperature, since in this regime the viscosity is dominated by the gaslike individual dynamics. However, when ${C}_{T}/{\overline{v}}_{p}>1$ where the cooperative dynamics dominates, the fundamental origin of viscosity of liquids is found to be just losing or gaining neighbors for individual particles, so that the viscosity of a typical liquid reasonably decreases with the temperature. Our results reveal that the viscosity transition point of ${C}_{T}/{\overline{v}}_{p}=1$ is just $\ensuremath{\approx}20$ times the corresponding melting point for both two-dimensional and three-dimensional Yukawa liquids with various screening parameters, which probably can be used as a new criterion to distinguish the strong and weak couplings in plasma physics.