Molecular dynamics simulations of structure and dynamics of ionic solutions under Couette shear flow

Comprehending the behaviors of ionic solutions under shear flow is essential for the development of lubrication, electroplating, electrochemical sensing, energy conversions, and many other research fields. In this study, we propose a novel coarse-grained simulation method to investigate the physicochemical properties of ionic solutions under Couette shear flow, in which the ions are modeled as charged beads, the solvent molecules are coarse-grained as dipolar beads using the Stockmayer fluid model, and the SLLOD algorithm is employed to account for the movements of ions and solvent under shear. This method can effectively capture the interplay between ion-ion, ion-dipole, and dipole-dipole electrostatic interactions, as well as their coupling with the imposed flow field. We systematically investigate the impacts of ionic concentrations, dipole moments, and shear rates on the microscopic structure, steady-state viscosity, and ionic diffusivity of ionic solutions. Our results reveal that the formation of ionic clusters via cation-anion electrostatic interactions leads to increased viscosity and suppressed ionic diffusion. Increasing dipolar interactions or applying stronger shear fields can both lead to the disassembly of ionic clusters; however, these two effects show distinct impacts on solution viscosity and ionic diffusion. In particular, enhancing the solvent dipole moment strengthens ion solvation, resulting in increased viscosity and diffusion, whereas elevating the shear rate weakens solvation by displacing bound solvent molecules from the ions, thereby reducing viscosity and enhancing ionic diffusivity. Our study confers insights into the fundamental understanding of the physicochemical properties of ionic solutions and provides guidance for the design and optimization of the functional ion-containing liquid materials.