Shear thinning effect in pair-ion plasmas

Abstract This work investigates the non-Newtonian shear thinning behavior in a two-dimensional (2D) pair-ion plasma (PIP) system using Molecular Dynamics (MD) simulations. The PIP system consists of oppositely charged ions of equal masses are interacting via a repulsive Yukawa-like interaction potential. The system is subjected to shear flow using two distinct simulation techniques: the Artificial Momentum Exchange algorithm (AMEA) and the Homogeneous Shear algorithm (HSA). While measuring the shear viscosity of the system using the AMEA technique, the general idea of the cause-and-effect is reversed. The simulation imposes the momentum flux as the effect and measures the velocity gradient (shear rate) as the cause. In HSA, the shear flow is modelled using Lees-Edwards boundary conditions (LEBC) giving rise to a uniform flowing field in the simulation box. The system is characterized by the thermostatted SLLOD equations of motion, which are solved using the operator splitting technique. While AMEA is found to be effective for low shear rates, it fails to capture high-shear-rate effects due to system heating and stability constraints. In contrast, HSA successfully induces homogeneous shear flow, enabling a comprehensive analysis of the system’s viscosity over a wide range of shear rates and Coulomb coupling parameters. The results reveal a well-defined shear thinning regime, wherein shear viscosity decreases with increasing shear rate, exhibiting a square-root dependence on the applied shear rate. Additionally, a minimum in shear viscosity is observed at a critical coupling parameter, attributed to a balance between kinetic and potential contributions to momentum transport. This behavior aligns with trends reported in Yukawa and one-component plasma systems. The study provides new insights into the transport properties of PIP systems and highlights their relevance in understanding non-Newtonian plasma flows, with potential implications for astrophysical and laboratory plasma environments.