(180i) Nanorheological Behaviors and Flow Dynamics of Isotropic/Anisotropic Geocolloids in Aqueous Medium

Geocolloids are known to play a major role on facilitating the transport and distribution of energy-related contaminants (e.g. naphthalene and cesium etc.) in environment. However, there are many fundamental questions to be answered pertaining to their transport processes and molecular interactions at the nano- and micro-length scales, that are closely related to corresponding relaxation dynamics of colloidal suspension.

In this work, highly monodisperse silica particles of diameter ranging from 50 to 400nm, synthesized using a modified version of the Stöber process, were selected as model geocolloids. Trichlorosilane was deposited onto geocolloid surfaces by chemical vapor deposition with controlled degrees of surface coverage to serve as adsorbed chemical contaminants. The viscosity and flow characteristics of geocolloidal suspensions containing bare as well as modified silica nanoparticles were evaluated utilizing the Surface Forces Apparatus (SFA) under different degrees of nanoconfinment. The viscosity of the geocolloidal suspension demonstrated highly discontinuous behaviors below a critical nanoconfinement level of 2000 nm. The increase in the viscosity was drastic, suggesting that these colloids form jammed aggregates under these conditions. Furthermore, the geocolloidal suspension displayed a shear-thinning behavior under nanoscale confinement contrasting to the case in bulk regime where the Newtonian behavior was observed. When salts were introduced into the dispersion, the increase in measured viscosity appears to be much more dramatic at small separation distances. This phenomena can be attributed to salt ions which can effectively screen electrostatic repulsions arising among geocolloids, thereby giving rise to promoted aggregation, especially in small confining zones.

From a physical sciences perspective, we anticipate this project to provide a better understanding on the nanorheology of isotropic/anisotropic geocolloids as a function of confinement. This can have significant impacts on the geophysics, fluid dynamics, and environmental science communities at large, ultimately bringing new perspectives to the fields of geophysics and geochemistry.