Onset of Granular Flows by Local and Global Forcing

Title of dissertation: ONSET OF GRANULAR FLOWS BY LOCAL AND GLOBAL FORCING Masahiro Toiya, Doctor of Philosophy, 2006 Dissertation directed by: Professor Wolfgang Losert Department of Physics, Institute for Research in Electronics and Applied Physics and Institute for Physical Sciences and Technology 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. Particle rearrangements that do not lead to steady flows but overall increase in the packing fracion have also been investigated. Vibration-induced compaction of spherical grains indicates that in addition to the cage motion there are rare ”jump” events in which a single particle moves significantly more compared to its neighbors. Although rare, such ”jumps” play a significant role in the compaction of the material as a whole. Temperature cycling experiments show similar results. Compaction of material has also been observed in the the Couette cell. Repeated reversals or oscillations of the shear direction lead to additional compaction, which can be described by a stretched exponential, similar to compaction induced by tapping. For 3D imaging of dry and non-transparent materials, X-ray microtomography is used. Images taken during the ongoing penetrometer studies of spherical particles and the lunar simulant JSC-1, along with an outline for future experiments are presented. ONSET OF GRANULAR FLOWS BY LOCAL AND GLOBAL FORCING