Glass Transition in Suspensions of Charged Rods: Structural Arrest and Texture Dynamics

We report on the observation of a glass transition in suspensions of very long and thin, highly charged colloidal rods (fd-virus particles). Structural particle arrest is found to occur at a low ionic strength due to caging of the charged rods in the potential setup by their neighbors through long-ranged electrostatic interactions. The relaxation time of density fluctuations as probed by dynamic light scattering is found to diverge within a small concentration range. The rod concentration where structural particle arrest occurs is well within the full chiral-nematic state, far beyond the two-phase isotropic-nematic coexistence region. The morphology of the suspensions thus consists of nematic domains with various orientations. We quantify the dynamics of the resulting texture with image-time correlation spectroscopy. Interestingly, the decay times of image correlation functions are found to diverge in a discontinuous fashion at the same concentration of charged rods where structural particle arrest is observed. At the glass-transition concentration, we thus find both structural arrest and freezing of the texture dynamics.

Figure 1

(a) Depolarized CCD images for a fd concentration of $4.0\mathrm{mg}/\mathrm{ml}$. The top row shows images of the entire cuvette, with a diameter of 20 mm, at two waiting times $t_w$ as indicated. The three images below cover $7\times 7{\mathrm{mm}}^2$ areas (see the yellow indicated square in the top-right image), for longer waiting times. These types of images are used to construct image time-correlation functions. (b) Image time-correlation functions for different waiting times (given in hours), as indicated in the figure. (c) The initial decay rate $\Gamma$ as a function of the inverse waiting time.

Figure 2

Same as in Fig. 1, but now for a fd concentration of $13.6\mathrm{mg}/\mathrm{ml}$. In (a) images of the entire cuvette are given, (b) the image time-correlation functions for different waiting times (given in hours), and (c) plot of the initial relaxation rate as a function of the inverse waiting time.

Figure 3

Initial image time-correlation decay rates as a function of fd concentration. The top two images on the right are for the fd concentration of $11.1\mathrm{mg}/\mathrm{ml}$ just below the texture-freezing concentration, and the bottom two images for a concentration of $12.3\mathrm{mg}/\mathrm{ml}$ just above the glass transition. The left-hand images are taken right after filling the cuvette, the right-hand images after a waiting time of 40 h.

Figure 4

(a) Ensemble averaged DLS correlation functions for various fd concentrations. Solid lines are fits to a stretched exponential function. (b) The decay rate ${\Gamma }_{\mathrm{DLS}}=1/\tau$ as a function of fd concentration. The gray region indicates the region where structural arrest occurs. The inset shows the stretching exponential $\beta$ as a function of fd concentration.