Plugging by hydrodynamic bridging during flow of stable colloidal particles within cylindrical pores

Abstract

This paper describes the flow-induced retention of charge stabilized colloidal particles during flow through cylindrical pores. Current models describing the low-Reynolds-number flow behaviour of particulate suspensions through porous media do not predict retention of stable colloidal particles if the particles are smaller in size than the pores, and the particles and the pores have like surface charges. Retention is not expected under these conditions because the small particle size relative to the pore constriction size precludes straining (physical capture of particles larger than the pore constriction) while particle–pore surface electrostatic repulsion prevents deposition. However, the experiments show that substantial particle retention can occur under these conditions. The mechanism causing particle retention under these conditions, hydrodynamic bridging, is flow-induced. In this mechanism, hydrodynamic forces acting on particles arriving at a pore entrance do not allow their simultaneous passage through the pore, resulting in the formation of a particle bridge across the pore constriction. This paper reports experiments elucidating the effects of velocity, particle concentration, and the ratio of pore size to particle size on retention by hydrodynamic bridging. For flow through cylindrical pores, the effect of velocity on retention by bridging is opposite to that of retention by deposition. Furthermore, observations indicate the existence of a critical flow velocity necessary for particle bridging to occur. This critical velocity is a measure of the net colloidal interparticle and particle–porous medium repulsion that must be overcome by the hydrodynamic forces for bridging to occur. Approximate theoretical calculations of the trajectories of two particles approaching an isolated cylindrical pore are also presented. These calculations show that bridging is indeed possible in the Stokes flow regime for the experimental conditions considered.