Calculation of displacement correlation tensor indicating vortical cooperative motion in two-dimensional colloidal liquids

TL;DR

This paper analytically and numerically investigates the displacement correlation tensor in two-dimensional colloidal liquids, revealing vortical cooperative motion, cage effects, and self-similar cage structures through tensorial analysis.

Contribution

It introduces an analytical framework relating displacement correlation to deformation gradient tensors, capturing vortical motion and cage effects in colloidal liquids.

Findings

Displacement correlation tensor exhibits vortical structures.

Negative long-time tail in velocity autocorrelation due to cage effects.

Cages form a self-similar nested structure in space-time.

Abstract

As an indicator of cooperative motion in a system of Brownian particles that models two-dimensional colloidal liquids, displacement correlation tensor is calculated analytically and compared with numerical results. The key idea for the analytical calculation is to relate the displacement correlation tensor, which is a kind of four-point space-time correlation, to the Lagrangian two-time correlation of the deformation gradient tensor. Tensorial treatment of the statistical quantities, including the displacement correlation itself, allows capturing the vortical structure of the cooperative motion. The calculated displacement correlation also implies a negative longtime tail in the velocity autocorrelation, which is a manifestation of the cage effect. Both the longitudinal and transverse components of the displacement correlation are found to be expressible in terms of a similarity variable, suggesting that the cages are nested to form a self-similar structure in the space-time.