Effect of sample height and particle elongation in the sedimentation of colloidal rods

TL;DR

This study theoretically explores how gravity and particle elongation influence the layering and phase behavior of colloidal rod suspensions, revealing complex stacking sequences and effects of sample height.

Contribution

It introduces a theoretical framework combining equations of state and sedimentation path theory to analyze gravitational effects on colloidal rods with varying aspect ratios.

Findings

Gravity stabilizes multiple stacking sequences in rod suspensions.

Sample height non-trivially affects the stable phase stacking.

Polydispersity in buoyant mass influences sedimentation behavior.

Abstract

We study theoretically the effect of a gravitational field on the equilibrium behaviour of a colloidal suspension of rods with different length-to-width aspect ratios. The bulk phases of the system are described with analytical equations of state. The gravitational field is then incorporated via sedimentation path theory, which assumes a local equilibrium condition at each altitude of the sample. The bulk phenomenology is significantly enriched by the presence of the gravitational field. In a suspension of elongated rods with five stable phases in bulk, the gravitational field stabilizes up to fifteen different stacking sequences. The sample height has a non-trivial effect on the stable stacking sequence. New layers of distinct bulk phases appear either at the top, at the bottom, or simultaneously at the top and the bottom when increasing the sample height at constant colloidal concentration. We also study sedimentation in a mass-polydisperse suspension in which all rods have the same shape but different buoyant masses.