Are strongly confined colloids good models for two dimensional liquids?

TL;DR

This study uses simulations to compare the dynamics of quasi-2D colloidal suspensions with true 2D systems, revealing that vertical space in quasi-2D samples affects diffusion and relaxation, but the underlying physics remains similar when properly characterized.

Contribution

The paper demonstrates that quasi-2D colloids differ in dynamics from true 2D systems due to vertical motion, but their glass-formation physics are fundamentally comparable when using appropriate structural measures.

Findings

Quasi-2D colloids exhibit faster diffusion than 2D systems at the same area fraction.

Vertical space allows particle overlap in quasi-2D, affecting dynamics.

Structural quantities related to the radial distribution function unify the description of both geometries.

Abstract

Quasi-two-dimensional (quasi-2D) colloidal hard-sphere suspensions confined in a slit geometry are widely used as two dimensional (2D) model systems in experiments that probe the glassy relaxation dynamics of 2D systems. However, the question to what extent these quasi-2D systems indeed represent 2D systems is rarely brought up. Here, we use computer simulations that take into account hydrodynamic interactions to show that dense quasi-2D colloidal bi-disperse hard-sphere suspensions exhibit much more rapid diffusion and relaxation than their 2D counterparts at the same area fraction. This difference is induced by the additional vertical space in the quasi-2D samples in which the small colloids can move out of the 2D plane, therefore allowing overlap between particles in the projected trajectories. Surprisingly, this difference in the dynamics can be accounted for if, instead of using the surface density, one characterizes the systems by means of a suitable structural quantity related to the radial distribution function. This implies that in the two geometries the relevant physics for glass-formation is essentially identical. Our results provide not only practical implications on 2D colloidal experiments but also interesting insights into the 3D-to-2D crossover in glass-forming systems.