Static light scattering measurements are presented for rod-like fd-virus particles (L=880 nm, D= 9 nm) subjected to a pulsed alternating electric field in aqueous suspensions at very low ionic strength. In aqueous suspensions the dispersed fd-particles are negatively charged and surrounded by a diffuse Debye counterion cloud. In an external electric field an induced dipole originating from a deformation of the diffuse counterion cloud causes the alignment of the macromolecules. The anisotropic orientation distribution of the particles in the presence of the electric field results in a change of the angular distribution of the scattered light intensity with regard to the isotropic case. The steady-state electric field light scattering effect ΔI/I0 is measured as a function of the electric field strength and its frequency at a fixed scattering angle. The determination of the anisotropy of the electric polarizability Δαel of a fd-virus particle at higher electric field strengths, above the Kerr regime, shows a decrease of Δαel with increasing field. This is interpreted as a destruction of the diffuse Debye cloud in high electric fields. The orientational order parameter has been found to be as large as 0.93 indicating an almost complete particle orientation along the external field at the highest fields. It is also shown that in the frequency regime below 1 kHz electrostatically interacting rods can align perpendicular to the external electric field, whereas at higher frequencies this anomalous behaviour disappears. From the scattered intensity the form-factor and the static structure factor of interacting fd-virus particles have been determined. With increasing fields a substantial increase in the peak height of the static structure factor is found. The data is in good agreement with Monte Carlo simulations using a simple interaction model for the system. The orientation of the macromolecules in the presence of an electric field is affected by the intermolecular electrostatic repulsion.
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