Flow-induced retention of stable colloidal particles during low Reynolds number flow of dilute suspensions through porous media.

Abstract

The flow behavior of colloidal particles, suspended in a fluid, within porous media is of fundamental importance to many engineering and natural systems. Flow behavior refers to the nature of retention or capture of the suspended particles within the porous medium and its effect on the macroscopic, measurable quantities such as the effluent concentration and the hydraulic conductivity (permeability) of the porous medium. For reliable prediction of the particle flow behavior, it is imperative that all relevant mechanisms by which particles are retained or captured within the porous medium be clearly understood. Straining and depth filtration are the only mechanisms accounted for in most of the models describing plugging by particles. The objective of this thesis is to obtain a fundamental understanding of two flow-induced particle retention mechanisms during the low Reynolds number flow of dilute, aqueous suspension through porous media. The two mechanisms are hydrodynamic bridging and flow-induced deposition on previously deposited particles. These mechanisms can cause particle retention in the absence of straining and in the presence of strong interparticle electrostatic repulsion. Hydrodynamic bridging is the phenomenon of blocking of pores by simultaneously arriving (stable) particles whose size is smaller than the pore size. Flow-induced deposition on previously deposited particles will lead to multilayer deposition of stable particles within pores. These mechanisms can therefore cause unexpected particle retention and are governed by the competition between hydrodynamic and colloidal forces acting on particles in the vicinity of a pore. Because of the lack of previous studies of these mechanisms, a model experimental system consisting of spherical, nearly monodisperse latex colloidal particles, and track-etched membranes having cylindrical pores was used to understand the mechanisms. The experiments unambiguously demonstrate both the phenomena and elucidate the dependence of plugging by these mechanisms on the relevant parameters. Theoretical calculations of the particle(s) trajectories were also performed to prove that the phenomena are indeed flow-induced for the experimental conditions considered. These calculations provide clear physical insight into the phenomena and explain the dependence of the particle flow behavior near the pore entrance on the relevant parameters.