Experiments were performed to determine the effect of a rough walled fracture on the evolution and maintenance of particle swarms, i.e. coherent collections of colloidal size particles. Of particular interest is the response of a swarm to sudden changes in confinement and drag caused by variations in fracture aperture. Two different rough fracture geometries were used and are compared to smooth-walled converging and diverging synthetic fractures. The variation in aperture along a swarm’s transport path exerted a strong influence swarm evolution and, in many cases, enhanced swarm stability and velocity. Converging apertures tend to decelerate a swarm because of increasing confinement and drag from the fracture surfaces, while diverging apertures resulted in the acceleration of a swarm. As long as an unrestricted flow path exists, swarms will maintain their cohesion, even if a fracture contains obstacles to swarm flow (asperities). However, if the aperture of the flow path is too small, swarms will bifurcate around an obstacle and produce smaller sub-swarms that will continue on separate paths.

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