Experimental study of random close packed colloidal particles

TL;DR

This study uses confocal microscopy to analyze the structure of experimentally realized random close packed colloidal particles, revealing small crystallites and long wavelength density fluctuations that differ from simulated models.

Contribution

It provides the first detailed experimental 3D characterization of colloidal RCP, highlighting the presence of tiny crystallites and long-range density fluctuations.

Findings

Less than 3% crystallites observed

Long wavelength density fluctuations present (S(0) ≈ 0.049)

Experimental RCP differs from simulated models

Abstract

A collection of spherical particles can be packed tightly together into an amorphous packing known as "random close packing" (RCP). This structure is of interest as a model for the arrangement of molecules in simple liquids and glasses, as well as the arrangement of particles in sand piles. We use confocal microscopy to study the arrangement of colloidal particles in an experimentally realized RCP state. We image a large volume containing more than 450,000 particles with a resolution of each particle position to better than 0.02 particle diameters. While the arrangement of the particles satisfies multiple criteria for being random, we also observe a small fraction (less than 3%) of tiny crystallites (4 particles or fewer). These regions pack slightly better and are thus associated with locally higher densities. The structure factor of our sample at long length scales is non-zero, $S(0) \approx 0.049$, suggesting that there are long wavelength density fluctuations in our sample, perhaps due to the tiny crystallites. Our results suggest that experimentally realizable RCP systems may be different from simulated RCP systems, in particular, with the presence of these long wavelength density fluctuations.