Confined ionic liquids (ILs) tend to form near-wall layers. Quantifying and visualizing the formation of these near-wall IL layers and their temporal evolution is essential for understanding and controlling the tribo-rheological properties of ILs when used as lubricants.This paper aims to develop and demonstrate a numerical scheme for quantifying and visualizing the formation of near-wall ionic liquid layers and their evolution over time.
The implemented numerical scheme used to quantify and visualize any tendency in ordering is based on the concept of rotational invariant bond-orientational order parameters (BOOPs). These quantities were introduced to identify the crystallographic type of the local symmetry around a given position within a system as a function of the distance relative to the nearest neighbors.
Various rotational invariant BOOPs are calculated for neighbors up to the fifth coordination shell. The pairings of so resulting BOOPs are shown to unambiguously identify the local symmetry even when the confinement destroys the overall translational symmetry. Furthermore, these pairings of BOOPs are found to be characteristic of 2D/3D Bravais lattices.
Analyzing the coarse-grained molecular dynamics data of confined ILs by using typical pairings of BOOPs, one directly visualizes the tendency to near-wall layering. Thus, the circumstances in which this layering starts and how it evolves in time can be also investigated. Accordingly, this local symmetry analysis can be then directly considered when determining the impact of ordering on the viscosity of confined ILs, for example.
