Theses and Dissertations from UMD
Permanent URI for this communityhttp://hdl.handle.net/1903/2
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item Mechanisms for Axial Band Formation in a Rotating Drum of Granular Material(2006-12-01) Newey, Michael Kent; Losert, Wolfgang; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We study granular particles, like sand or glass beads, that are mixed in a partially filled, horizontal, rotating, cylindrical drum. When particles of different sizes are placed in the drum spatial segregation of the particles by size is observed. This segregation occurs in two phases. During the first phase, called radial segregation, the smaller particles form a radial core. In the second, called axial segregation, particles segregate into alternating bands along the axis of the drum. We perform a detailed study of the characteristics of the flow to determine the physical mechanisms driving axial segregation. We characterize the top surface of the flowing layer by tracking particles using a high-speed camera. We then extract average quantities such as velocity and diffusion. The average velocities show surprising behavior: Particles in small particle bands have a higher downhill flow velocity than particles in large particle bands. We also observe that there is a pattern of sideways velocity as a function of position down the flow. Particles flow into small particle bands in the middle of the flow but flow out of small particle bands at the bottom. We present the framework for a new model based on our experimentally observed results. We explain the axial band formation in terms of the observed surface flow patterns. Our framework connects differences in flow velocities on the surface of the drum with the radial segregation in the bulk of the drum. We compare these results to current models. We test the general model assumption that the particles always flow in the direction of steepest descent by measuring the surface height of the banded state with a laser line. We find that although there is some indication that particles flow in the direction of steepest descent, there is strong flow that is not in the direction of steepest descent, contrary to the given models. Finally, we study oscillating patterns in mixtures of three sizes of glass beads. Ternary mixtures of particles form bands within bands of the different particle sizes. For certain experimental conditions we observe traveling and oscillating patterns.Item Onset of Granular Flows by Local and Global Forcing(2006-08-04) Toiya, Masahiro; Losert, Wolfgang; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis focuses on the onset of granular flows and memory effects in granular materials under local and global forcing conditions. Global flows are induced in a shear cell of Taylor-Couette type by moving a boundary wall. We find that how a granular shear flow starts depends strongly on the prior shear direction. We observe that when the shear direction is reversed, the material goes through a transient period during which the material compacts, the shear force is small, and the shear band is wide. Three dimensional confocal imaging of particle rearrangements during shear reversal shows that bulk and surface flows are comparable. Local flows are induced by forcing a rod into a fluid immersed granular bed with various preparation methods. Particle rearrangements are observed in 3D by confocal microscopy and by moving a laser sheet through the sample. Image analysis indicates that rearrangements spread farthest not directly under the penetrometer but in a ring around the penetrometer. In addition, the direction of preformed stress chains in the material influences the particle rearrangements. Material compressed from one side exhibits anisotropic particle rearrangements under penetrometer testing.Item The Self-Assembly of Particles with Multipolar Interactions(2004-12-17) Stambaugh, Justin John; Losert, Wolfgang; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis, we describe results from investigations of the self-assembly of anisotropically interacting particles. In particular, we focus upon the roles of dipolar and higher order multipolar interactions on the patterns of self-organization. Using an experimental model system of vertically vibrated magnetic spheres, we investigate the e®ects of octopolar and higher order interactions on the pattern of self-assembly. We show that simple theoretical point charge models can be used to provide insight into the underlying causes of the observed phenomena. We also show that such models can be used to better understand the pattern formation in several related physical systems, including biological macromolecular self-assembly and cohesive granular materials.