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Stability of Natural Aggregates in the Environment: The Role of a Solid Bridging Mechanism

Abstract:

Stability of natural aggregates depends on their ability to resist disruptive forces during fluvial or aeolian processes. Diffusion-and reaction-limited colloid aggregation are the two distinct limiting mechanisms that form aggregates. In addition, the process of drying colloidal suspensions can drive particles together and form aggregates. In

the absence of attractive inter-particle forces, however, aggregates are unstable when subject to wetting or fluid shear. Here we directly observe the formation of aggregates under evaporation, using a wide range of particle sizes and materials. Experiments show that, when present, small particles ( 1m) condense within shrinking capillary

bridges to form “solid bridges” that bind large particles (≫ 1m) together. These aggregates are stable to rewetting, whereas aggregates made only from large particles disintegrate when wetted. For polydisperse mixtures, particles sizesegregate to form remarkable hierarchical (fractal), stable clusters. Using Atomic Force Microscopy we directly measure the strength of solid bridging bonds between large and small particles, and find that it is orders of magnitude larger than other relevant hydrodynamic and adhesion forces. A direct macroscopic consequence of

solid bridges is thus an effective cohesion, that is facilitated by polydispersity and transient hydrodynamic forcing. This effect must be relevant for the strength and erodibility of natural soils, and other polydisperse particulates that experience cycles of wetting and drying.

Bio:

Dr. Ali Seiphoori is a visiting scientist at the Rock Mechanics Laboratory, the Department of Earth, Atmospheric and Planetary Sciences (EAPS), MIT. Prior to EAPS, he held a postdoctoral associate appointment at the Department of Earth and Environmental Sciences, the University of Pennsylvania (UPenn), and a postdoctoral fellow appointment at the MIT Department of Civil and Environmental Engineering. He obtained his Ph.D. in Civil and Environmental Engineering from the Swiss Federal Institute of Technology at Lausanne (EPFL), where he focused on the characterization

and modeling of the coupled THM processes in bentonite barriers. His current research activities are focused on the experimental analysis and modeling of the flow and transport in porous and fractured media.

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