Transient Electrooptics of Soft Matter
Prof. Zoltan A. Schelly (Dept. of Chemistry and Biochemistry Univ. of Texas - Arlington)

The primary effects of application of an electric field E to a colloidal solution are the polarization of the constituents of the system, and reorientation of the resultant μ of the permanent μp and induced μind dipole moments. The ensuing structural anisotropy of the system is exhibited in time (t) dependent optical anisotropy such as electric field-induced birefringence Δn(t) (Kerr ef-fect) and electric field-induced light scattering ΔS(t). However, if the colloidal particles are soft (e.g., reverse micelles, w/o microemulsions, surfactant crystallites and unilamellar bilayer vesicles)—due to their deformability and thermal shape fluctuation prior to perturbation—their instantaneous dipole moment μ_ins contributes to the time evolution of the resultant dipole moment μ(t) of the particles. As a secondary effect, interaction between the induced dipole moments may result in pearl formation in the direction of the force field E, ultimately causing fusion of, or other structural changes in, the particles.

Experimentally, the perturbation is applied as a high-voltage rectangular electric pulse (DC field), a rectangular pulse immediately followed by a second pulse of opposite polarity, or a high intensity linearly polarized laser pulse (AC field, causing the optical Kerr effect,[3]). The field-induced processes are monitored through the transmitted intensity ΔI(t) of a linearly polarized continuous laser. Since field-induced birefringence and light scattering occur concurrently, it is of paramount importance to examine and separate their individual contributions to the ΔI(t) signal.
The relaxation response of soft systems on the nanosecond to millisecond timescale provides information about the structure of amphiphile aggregates and details of their field-induced deformation, rotation and vectorial clustering, the kinetics of phase separation in microemulsions and the mechanism of electroporation of vesicles. The use of electroporation of vesicles for the preparation of ultra-small quantum dots (diameter ≤1 nm) is also discussed.