Structural disorder, filament growth and self-poisoning in short rods confined onto a flat wall

Confocal microscopy was used to directly observe the structural coarsening of the first layer of short colloidal rods sedimented onto a flat wall. Based on an image analysis algorithm we devised, quantitative information on the location, orientation and length of each particle can be extracted with high precision. At high density the system undergoes structural arrest, and becomes trapped in a disordered state of randomly arranged filaments that are composed of side-by-side aligned rods. The frustration of structural order is signalled by a new peak that emerges in the radial distribution function. Configuration analysis shows that the peak is primarily due to pairs of particles that are arranged in a "T" shape, a configuration that is compatible with neither crystallization nor filament growth. Our results point to a self-poisoning mechanism for the frustration of structural order, and highlight the importance of particle shape in controlling colloidal assembly thus materials properties.