Structure and dynamics of a layer of sedimented Brownian particles

TL;DR

This study combines experiments, simulations, and theory to analyze the structure and dynamics of sedimented colloidal particles near a surface, revealing layer formation, diffusion behavior, and effects of polydispersity.

Contribution

It introduces a layered quasi-two-dimensional model for sedimented colloids and explores how layering and polydispersity influence particle dynamics.

Findings

Layer formation leads to bimodal height distribution.

Layering weakens the decrease in diffusion coefficient with concentration.

Polydispersity causes size segregation affecting layer properties.

Abstract

We investigate experimentally and theoretically thin layers of colloid particles held adjacent to a solid substrate by gravity. Epifluorescence, confocal, and holographic microscopy, combined with Monte Carlo and hydrodynamic simulations, are applied to infer the height distribution function of particles above the surface, and their diffusion coefficient parallel to it. As the particle area fraction is increased, the height distribution becomes bimodal, indicating the formation of a distinct second layer. In our theory we treat the suspension as a series of weakly coupled quasi-two-dimensional layers in equilibrium with respect to particle exchange. We experimentally, numerically, and theoretically study the changing occupancies of the layers as the area fraction is increased. The decrease of the particle diffusion coefficient with concentration is found to be weakened by the layering. We demonstrate that particle polydispersity strongly affects the properties of the sedimented layer, because of particle size segregation due to gravity.