Electro-optic effects of aqueous fd-virus suspensions at very low ionic strength

Abstract

The orientation in external electric fields of rod-like fd-virus particles (length ℓ= 895 nm, diameter d = 9 nm) in aqueous suspensions is examined by the electric birefringence method. In aqueous suspensions the negatively charged fd-particles are surrounded by a diffuse Debye cloud of counterions, which is characterized by the Debye-Hückel parameter κ. A special experimental set-up is used to vary the ionic strength of the suspension, i.e. the Debye-Hückel parameter, and therefore the electrostatic interparticle interaction. The birefringence signal Δn is measured as a function of the strength and frequency of the applied electric field in suspensions of very low ionic strength (10-6M-10-4M). At low field strengths Kerr-behaviour is found. From the dependence of the electric anisotropy Δαel on the Debye-Hückel parameter κ it is concluded that the orientation of the fd-particles is correlated to an induced dipole due to a deformation of the diffuse Debye cloud. Saturation electric birefringence values are far from that theoretically expected. This can be interpreted as a destruction of the diffuse Debye cloud at high electric fields. At low field strengths the frequency dispersion below 1 kHz of Δn of the electrostatically interacting fd-virus suspensions shows anomalous behaviour. This negative electro-optic effect is an evidence for the orientation of the particle's long symmetry axis perpendicular to the applied electric field. The dispersion has a positive maximum at about 2 kHz. This maximum could be explained by different frequency dependencies of the electric polarizabilities parallel and perpendicular to the long symmetry axis of the fd-rods

Double-layer polarization induced transitions in suspensions of colloidal rods

“…This complies with the negative birefringence found in Ref. [5]. As can be seen No birefringence is observed, which implies that this state is isotropic.…”

“…A few experimental studies are reported on colloidal dispersions of rods at lower frequencies where interactions between polarized double-layers are important [5], [6]. In Ref.…”

“…These patterns are due to confinement effects that are absent in the bulk experiments in Refs. [5], [6]. The experiments described in this Letter are also concerned with bulk properties.…”

Electric-field-induced polarization of the layer of condensed ions on cylindrical colloids

“…Anomalous alignment of fd-virus particles (very long and thin, stiff rods) perpendicular to the external field has been found in Ref. [1] at low frequencies (below 1 kHz) for fd-concentrations above the overlap concentration. The present authors observed field-induced phase/state transitions, dynamical states and non-equilibrium critical behaviour in concentrated fd-virus suspensions [2,3,4].…”

“…(1,2), the solution of the equation of motion (12) for the surface-charged density must satisfy the condition,…”