Structural arrest and texture dynamics in suspensions of charged colloidal rods

Kyongok Kang,Jan K. G. Dhont

doi:10.1039/c3sm27754b

Abstract

There is an abundance of experiments and theories on the glass transition of colloidal systems consisting of spherical particles. Much less is known about possible glass transitions in suspensions of rod-like colloids. In this study we present observations of a glass transition in suspensions of very long and thin rod-like, highly charged colloids. We use as a model system fd-virus particles (a DNA strand covered with coat proteins) at low ionic strength, where thick electric double layers are present. Structural arrest as a result of particle-caging is observed by means of dynamic light scattering. The glass-transition concentration is found to be far above the isotropic–nematic coexistence region. The morphology of the system thus consists of nematic domains with different orientations. Below the glass-transition concentration the initial morphology with large shear-aligned domains breaks up into smaller domains, and equilibrates after typically 50–100 hours. We quantify the dynamics of the transitional and the equilibrated texture by means of image time-correlation. A sharp increase of relaxation times of image time-correlation functions is found at the glass-transition concentration. The texture dynamics thus freezes at the same concentration where structural arrest occurs. We also observe a flow instability, which sets in after very long waiting times (typically 200–300 hours), depending on the rod concentration, which affects the texture morphology.

Highlights

Diffusion of a colloidal particle at high packing fractions can be visualized in terms of a cage of neighboring particles, where at short-times the particle diffuses within the cage, while long-time diffusion relates to rare escapes from the cage
For spherical colloids the dynamics is characterized by the density autocorrelation function, which can be measured by dynamic light scattering
In the present paper we report on a glass transition in dispersions of very long and thin, stiff, and highly charged colloidal rods at low ionic strengths

Summary

Introduction

Paper studied with computer simulations, where it is for example found that assemblies of particles move coherently in strings and loops.[23]. Much less experimental work exists on the glass transition of purely repulsive rod-like colloids. There is a single experimental study where a glass transition due to repulsive interactions in suspensions of very thin and long colloidal rods is reported by Wierenga et al.[28] ( the effect of attractions cannot be completely ruled out), while a brief account of the glassy behaviour of highly charged, very thin and long rods (fd-virus particles) by the present authors can be found in ref. We nd in the present study a different scenario, where the texture dynamics freezes at the same concentration where the translational dynamics of rods within the domains freeze.

The system and technical details

Texture dynamics: decay rates of image time-correlation functions

Particle dynamics: dynamic light scattering

The onset of flow

Summary and discussion