Electric field-induced structure in dense colloidal suspensions

The application of electric and shear fields to suspensions of dielectric particles in a dielectric liquid, results in a drastic change in the fluids microstructure, and hence, its rheological behavior. In this work, we focus on the effects of varying the particle size and concentration and how these parameters effect the field induced structure for different electric and shear field strengths.; The fluids used in this work were composed of sterically stabilized silicon dioxide particles suspended in index matching solvents. This index matching minimized multiple scattering and gave transparent solutions for particle volume fractions as high as 40%. These particles are monodisperse and were made in sizes from 90 to 700 nm. The dielectric constants of these particles in suspension, were measured using a dielectrophoresis experiment.; Calculations of scattering dichroism for perfect particle alignment in the electric field direction, using the Rayleigh-Gans-Debye approximation, provided evidence that dichroism is sensitive to the ratio of the particle and aggregate size relative to the wavelength of light used in the measurement. A variable wavelength polarimetry apparatus was constructed to measure electric field induced dichroism over a range of wavelengths from 350 to 750 nm. These measurements provide a mapping of the pair distribution function in these systems, and show preferential particle alignment in the direction of the applied electric field. The results compare favorably with theoretical calculations.; A second polarimetry experiment was used to measure the magnitudes of birefringence and dichroism, and the principal axis of the field induced structure under combined shear and electric fields. The results show that the angle is a function of the ratio of the applied fields, whereas the magnitudes are functions of the ratio and the electric field alone. The angle results also compare favorably with theoretical calculations.; Time scales of structure formation and decay were considered. The build up scales with the square of the applied field strength as it is controlled by the dipole strength of the particles, and is dependent on the volume fraction as well as the particle size. The decay is diffusion controlled and scales with the particle diffusion constant.