Packing and voids in electro-rheological structures of polarized clay particles

Oil suspensions of fluorohectorite clay particles exhibit a dramatic meso-structural ordering when submitted to a strong electric field. This is due to dipolar interaction between polarized fluorohectorite particles, which orientate and aggregate to form chains and/or bundle-like structures along the direction of the applied electric field. We have used synchrotron small angle X-ray scattering to get insight into the nature of the porous medium in the bundles. Three types of fluorohectorite clay samples corresponding to three different intercalated cations Na+, Ni2+ and Fe3+ were studied. The two-dimensional SAXS images from bundles of fluorohectorites exhibit a marked anisotropy which is analyzed by fitting ellipses to iso-intensity lines of SAXS patterns. This also provides principal directions along which one-dimensional spectra are computed. They display a power law behavior typical of porous media, separated by crossovers. The crossovers are interpreted in terms of typical length scales for the clay particle bundles, providing for the first time a quantitative image of the 3D geometry inside such bundles of polarized clay particles. The exponents of the power laws indicate either predominant surface- (for 2 types of samples) or bulk- (for the last type) scattering, at all length scales investigated.