Electric-field induced transitions in suspensions of charged colloidal rods

Abstract

We explore transitions in suspensions of fd virus at a low ionic strength, induced by external electric fields at frequencies where double layers are polarized. On the basis of the different optical morphologies, phase/state diagrams are constructed in the field-amplitude versus frequency plane and the field-amplitude versus concentration plane. Due to interactions between polarized double layers, for low frequencies, various phases and dynamical states are found: a nematic phase, a striped phase and a dynamical state where nematic domains melt and reform. At relatively high frequencies of a few kHz, a uniform homeotropic phase is induced. The various phases and states are characterized by means of polarization microscopy, birefringence, dynamic light scattering and video-correlation spectroscopy. An expression is derived for the attenuation of the electric field due to electrode polarization, which is tested experimentally. This theory is used to correct phase/state diagrams for electrode polarization.

Image time-correlation, dynamic light scattering, and birefringence for the study of the response of anisometric colloids to external fields

“…In order to estimate the accuracy with which diffusion coefficients can be obtained with our setup, we measured the diffusion coefficient of silica spheres as a function of the scattering angle. 10 The scatter in measured diffusion coefficients is about 10%, independent of the scattering angle within the range of 5 o -50 o . The same system was also probed with a standard DLS apparatus, where the measurement error was found to be about 2%.…”

“…The corresponding dynamical state is found at relatively low frequencies and somewhat higher field amplitudes. 9,10 Figure 3(a) shows a few depolarized optical morphologies of small nematic domains that melt and reform, resulting in time-changes of the images, where the white dotted circles mark a region where time-changes are particularly clear.…”

“…10,29 In order to measure the orientational order in this homeotropically aligned phase, we have to able to rotate the entire optical train. This is depicted in Fig.…”

“…More details on the determination of the direction of alignment of the rods in the Hphase and the interpretation of birefringence measurements can be found in Ref. 10.…”

“…In order to gain in the understanding of these complicated phenomena, we developed an in situ set of instruments to probe the bulk phase behavior of charged colloidal rods and their interactions under an externally applied electric field. We recently reported on the response of charged colloidal rods to an alternating electric field, 9,10 where it was found that different stationary phases/states and dynamical states can be induced, depending on the field amplitude and frequency. These phases/states and dynamical states are the result of inter-particle interactions that are due to polarization of the particle's double layers as well as electro-osmotic flow.…”