Electric Birefringence of a Dispersion of Electrically Charged Anisotropic Particles

We have measured the static and dynamic birefringence induced by an electric field on a dispersion of PTFE rods having length of about 0.33 μm and diameter of 0.15 μm. The rods are electrically charged and have a crystalline structure. By using a pulse consisting of a train of square waves of variable frequency, we have been able to separate the contribution of the counterion cloud from that due only to form and intrinsic anisotropy of the rods. We find that the optical anisotropy of the material is n || - n ⊥ = 0.04. At volume fractions above 0.1% we find an important collective effect in the measured Kerr constant. Such an effect disappears upon addition of a few mM NaCl. The dynamics of electric birefringence appears to be controlled by the rotational diffusion constant of the individual rod, irrespective of the volume fraction and the added salt concentration.