Oil dispersions of nano-layered silicates in an external electric field: An experimental study

Abstract

The work in this thesis concerned with the experimental study of structural morphologies and rheological properties of chains and/or- column like-structures of electrorheological (ER) fluids of 2:1 synthetic nano-layered silicate clay minerals, whose particles are polydisperse (lateral size~ few nm to a few μm and longitudinal size~ 1 nm to 0.1 μm). The structural morphology of chains and/-or column like structures were mapped out by two different types of Synchrotron Xray scattering experiments namely, Wide Angle X-ray Scattering (WAXS) and Small Angle X-ray scattering (SAXS). The rheological properties of ER fluids are measured using a Physica MCR 300 rotational rheometer equipped with co-axial cylindrical geometry.

Novel synchrotron wide angle X-ray scattering (WAXS) experiments presented here, show that individual clay particle (stack of nano-layered clay platelets) polarize along its lateral direction indicating that intercalated or attached ions or water molecules play a role in its electric polarization. The preferred orientational distribution of polarized clay particles along the direction of applied electric field and characteristic shape of their orientational distribution functions is found to depend on the nature of intercalated cations and are well described by a Gaussian function and more accurately by means of a Mayer-Saupe function of the type that is generally used to described nematic liquid crystalline ordering. Further, a small increases (~0.05 Å) in distance between the nano-layered clay platelets were observed in an applied electric field (~1 kV/mm).

Small Angle X-Ray scattering (SAXS) experiments show the porous nature of the clay particles assembly inside the chains and/-or column like-structures. The anisotropic SAXS patterns were analyzed in terms of their iso-intensity lines and one dimensional profiles have been computed along their principal directions. Along the principal directions, power law behaviors are observed with crossovers corresponding to the average particle thickness and to typical pore sizes. The exponents of power laws indicated the dominating role of surface scattering of clay particles and provides pictures of their assembly inside the chains and/-or columns like-structures, consistent with configurations inferred from WAXS experiments.

Rheological measurements on ER fluids of dehydrated polydisperse smectites particles show that the induced polarizations of smectites clay particles in applied electric fields (dc) are controlled by their conductivity mismatch to that of suspending liquid (silicone oil), thus confirming that migration of surface charge on the clay particles surface and possible movement of intercalated cations play a major role in their polarizations in external electric fields, consistent with the results of the WAXS experiments. In absence of external electric fields, the steady state shear behaviors of ER fluids are Newtonian-fluid and in presence of external electric fields (~kV/mm) the behavior becomes Bingham-like solid. The dynamic yield stresses of these ER fluids (Bingham-like solids) and static yield stress of undisrupted ER fluids are drastically affected by the magnitude of applied electric field and the volume fraction of the investigated clay particles.