Understanding particle transport in porous media is important to a number of problems involving subsurface flow and transport, water and waste-water treatment, and soil pedology. The migration behavior of particles in porous media is complex. Factors influencing this behavior include the particle density, size and surface chemistry, the water chemistry, the interstitial velocity, and the characteristics of the porous medium. In the case of biological particles, motility, chemotaxis, growth and decay are also influential. Considerable insight into particle transport in porous media has been gained from experimental programs based on batch and column testing. Nonetheless, work of this nature cannot resolve, in real-time, the processes governing particle transport in the interior of a porous medium. This limits the scale of understanding that can be gained from such experimental approaches. In order to further understanding of the fundamental processes governing particle fate and transport in porous media, alternative methods that involve visualization studies of particle behavior within a porous medium are needed. In prior work, the Investigators of this proposal developed a visualization technique to study particle behavior in the interior of a porous medium. Use of this technique enabled the proposition of new hypotheses regarding particle transport behavior. The goals of this exploratory research project are to further the new visualization technique, and to provide further support for the emerging hypotheses on particle transport behavior. Specifically, the project will: 1. Extend the new visualization technique to enable better resolution of particle behavior, including resolution of individual particle tracks at the micro-scale and distinction between aqueous and solid phase particles at the macro-scale. 2. Prove, irrefutably, that, during non-Brownian particle transport under unfavorable electrostatic conditions, observations of decreasing particle attachment rate with transport distance are attributable to the early filtration of heavier particles as a result of gravitational sedimentation, and 3. Establish the impact of flow direction on particle transport for micron-sized particles whose behavior is, theoretically, dominated by Brownian motion. Broader Impacts The proposed research will provide a visualization tool that can spatially and temporally resolve microscopic and macroscopic particle concentrations in the interior of a porous medium. The research will also improve understanding of attachment and detachments mechanisms that impact particle interaction with a medium's solid phase. In particular, it will provide an explanation for why irreversible particle attachment rates decrease with transport distance under unfavorable electrostatic conditions. Research training will be provided for one graduate student. Undergraduate students will also be engaged in the research through MIT's Undergraduate Research Opportunities Program.
