Particle swarms in confining geometries

The transport of micro- and nano-particles in subsurface fluid deposits is an area of increasing interest due to the rising use of these particles for consumer and industrial purposes. Subsurface particle transport is complicated by the presence of fractures and fracture networks which govern the paths that particles will be able to take. In this thesis, subsurface particle transport will be investigated using particle swarms; collections of hydro-dynamically interacting particles which exhibit group behavior. The effects of fluid viscosity, particle properties, fracture geometry, and fracture aperture on swarm behavior were experimentally investigated. Swarm parameters were examined in time with an emphasis on geometry (height, width) and speed. Fracture geometry and aperture strongly affected these parameters. As a result, swarms in artificial fluid filled fractures displayed behavior that was not obvious or expected based on current theory. The most significant of these is what we have termed the "Enhanced Transport Regime." In uniform aperture fractures (two finite parallel plates), a range of apertures exists in which swarms travel more quickly than swarms in larger apertures. This behavior was observed in 3 separate experimental sets using different combinations of bulk fluid and particles. In fractures with variable apertures, swarms changed shape and speed in response to fracture features: accelerating/elongating when apertures increased and decelerating/expanding when apertures decreased. Experiments and numerical models were also undertaken to investigate the importance of finite fractures on particle swarms. Closing the open boundaries on the sides and bottom of the uniform aperture fracture had a dramatic effect on the behavior of particle swarms, either eliminating or enhancing the enhanced transport regime. This was investigated with a numerical model which determined that a finite fracture allows fluid to exit the confined space and requires that fluid re-enter at a different location. This creates global scale fluid flow that interacts with particle swarms in ways that are impossible if the fracture has infinite length. The experimental results demonstrate the critical importance of the collective nature of particle swarms. As collections of particles that are free to move relative to each other, swarms are able to respond to fractures in ways that a single spherical object cannot (i.e. expanding, contracting, elongating, etc.). Additionally, the finite sizes of the fractures used in these experiments play a significant role in governing the behavior of particle swarms.