Granular origins of cohesion: Fluid-driven assembly of erosion-resistant aggregates
Abstract
Erosion of natural deposits (soils and muds) depends on the ability of their aggregates to resist disruptive forces during fluvial or aeolian processes. Understanding cohesion of natural aggregates is a key element to predict the onset of sediment transport, evolution of dune fields, the frictional behavior of hillslope creep, and the collapse of natural deposits upon re-wetting. Geological materials are exposed to periodic wetting and drying; we posit that the process of drying forms aggregates by driving suspended particles together via hydrodynamic forces. In the absence of attractive inter-particle forces, however, aggregates should be unstable when subject to wetting or fluid shear. The origin of interparticle cohesion in stabilizing natural aggregates is not well-understood. Here we present results from a novel microfluidic experiment that examined the particle-by-particle assembly of aggregates forced by evaporation of a colloidal suspension, and probes the stability of these aggregates subject to controlled re-wetting. Aggregates are created from natural and artificial silicate-based particles, including silica microspheres and natural clays. We observe that capillary forces during evaporation can overcome interparticle repulsion, condensing submicron-sized particles into capillary bridges. Further evaporation results in the formation of "solid bridges" composed of submicron-sized particles (if present), that bind larger particles together and form aggregates. We show that solid bridges contribute to stabilizing the entire aggregate during re-wetting, by imposing a cohesive force that likely arises from van der Waals interactions. In the absence of submicron-sized particles and their associated solid bridges, the disruptive hydrodynamic forces disintegrate aggregates and easily transport them. These experiments suggest that the cohesion in natural soil and sediment aggregates can be induced by: (1) the presence of submicron/nano-sized particles, whose small mass and large surface area makes them surface reactive; and (2) mechanical stresses that are large enough to overcome electrostatic repulsion, and push particles into the short-range attraction (e.g., van der Waals) domain where solid bridges form.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMEP41B2662S
- Keywords:
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- 1810 Debris flow and landslides;
- HYDROLOGYDE: 1824 Geomorphology: general;
- HYDROLOGYDE: 1862 Sediment transport;
- HYDROLOGYDE: 3265 Stochastic processes;
- MATHEMATICAL GEOPHYSICS