Charged fibrous viruses (fd) in external electric fields: dynamics and orientational order

We recently found a number of phases and dynamical states that are induced in a concentrated suspension of charged, colloidal rods (fd-viruses) by an alternating external electric field (Kang and Dhont 2008 Eur. Phys. Lett. 84 14005; 2010 Soft Matter 6 273). The various phases and dynamical states are the result of interactions between the charged rods through their polarized electric double layers, polarized layers of condensed ions and/or electro-osmotic flow. At a relatively high frequency, a homogeneous, homeotropically aligned phase is induced (the H-phase). We present a dynamic light-scattering study of the microscopic dynamics of the rods, varying the frequency and field amplitudes along different pathways within this phase. Scattering experiments are performed at very small scattering angles with a home-made vertically mounted dynamic light-scattering setup, where Brownian motion perpendicular to the direction of alignment is probed. The orientational order is measured by means of birefringence experiments. The remarkable finding is that relaxation times and the degree of alignment are independent of the frequency and the amplitude of the applied electric field throughout the entire H-phase. Only within a small region in the neighborhood of the transition line, where the H-phase transforms to an inhomogeneous chiral-nematic phase, is there a frequency and amplitude dependence of relaxation times, which are shown to be the result of the appearance of transient, pre-transitional domains. We also recently identified a non-equilibrium critical point, where a time-and length-scale connected to a dynamical state are shown to diverge (Kang and Dhont 2009 Eur. Phys. J. E 30 333). Approaching this critical point from the side of the H-phase, we find that the light-scattering correlation functions develop a very slowly decaying mode, the origin of which requires further investigation.