Direct microscopic observation of particle deposition in porous media: Role of the secondary energy minimum

A flow-cell has been constructed where micrometer-sized particles flow horizontally through a single layer of packed glass beads. Deposition is viewed through a fluorescent microscope with phase contrast capabilities. This allows the visualization of particle deposition in porous media in real time and provides insight into phenomena such as secondary minimum deposition of particles and microbial pathogens during physicochemical filtration. At low ionic strengths (1–3 mM), no deposition was observed on the collector beads. As the monovalent salt concentration was raised to moderate ionic strengths (10–30 mM), the particles became trapped in the secondary minimum and translated along the glass bead surface, accumulating near the rear of the spherical collectors. At high ionic strength (100 mM), no calculated electrostatic energy barrier to deposition exists and particles deposited over the entire surface of the spherical collector. It is concluded that deposition in the secondary energy minimum is an important removal mechanism for micrometer-sized particles in flow through granular porous media.