Solid mechanics in colloidal and bacterial filtration

Microbial and particle transportation and adhesion in porous media in electrolytic solutions is fundamental in many fields related to water treatment, drug delivery and human health. This is a complex multi-physics process which includes, but not limited to, fluid mechanics, microbial properties, surface adhesion and aqueous environments. The conventional, empirically-driven approach is based on a flowthrough sand-packed column test. Although it is widely applied to predict filtration efficiency, the fundamental science and contribution of individual factors in this problem are still missing—for example, flow condition, salt concentration and microbial properties. This dissertation addresses this problem by developing a new microfluidic test method and a theoretical model based on contact mechanics. The new microfluidic test simplifies the physics behind microbial/colloidal filtration, and allows us to focus on the surface attachment/detachment due to inter-surface and hydrodynamic interactions. Filtration efficacy was directly measured using this new microfluidic test. The results of the tests were then compared with the conventional flow-through column test. The two show a good agreement with each other. This implies that surface adsorption is the dominant filtration mechanism in the column test. The coupled effect of flow rate, salt concentration and microbial properties were analysed using the classical DLVO theory, fluid mechanics and contact mechanics. The fundamental physics that control the microbial/colloidal attachment were suggested. We further developed a theoretical model for microbial filtration in a porous medium based on a moment balance method. The fate of an adhered microbial cell after collision with a sand collector was determined by competition between the adhesive moment due to the surface adhesion and the detachment moment due to hydrodynamic interaction. The new model takes into account the effects of flow condition, salt concentration and bacterial micro-properties. The theoretical results agree well with the