James B. Knight

Granular convection observed by magnetic resonance imaging

Vibrations in a granular material can spontaneously produce convection rolls reminiscent of those seen in fluids. Magnetic resonance imaging provides a sensitive and noninvasive probe for the detection of these convection currents, which have otherwise been difficult to observe. A magnetic resonance imaging study of convection in a column of poppy seeds yielded data about the detailed shape of the convection rolls and the depth dependence of the convection velocity. The velocity was found to decrease exponentially with depth; a simple model for this behavior is presented here.

Reversibility and irreversibility in the packing of vibrated granular material

Abstract We report on the settling of loosely packed, cohesionless granular material under mechanical vibrations. Monodisperse spherical beads were confined to a long vertical cylinder that was driven by an electromagnetic vibration exciter. Under vibrations the bead packing evolves from an initial, low-density configuration towards higher density. Ramping the vibration intensity repeatedly up and back down again reveals the existence of both an irreversible and a reversible branch in the response. The reversible branch represents a steady state in which the packing density depends monotinically on the vibration intensity. We have investigated the bead size, depth, and ramp rate dependence of the compaction process. Our results indicate how the occupied volume fraction can be optimized by slowly reducing the vibration intensity along the reversible branch.

Standing wave patterns in shallow beds of vibrated granular material

Conventional and high-speed video imaging was used to observe surface waves in vertically oscillated, thin layers of dry, noncohesive granular material. The onset of wave formation is pressure dependent and sensitive to the preparation of the particle surface. The dispersion relation of these waves is different at high and low frequencies, and the transition can be explained within the context of a hydrodynamical model by the introduction of a frequency dependent viscous cut-off that dominates at high frequency. High-speed imaging also reveals a qualitative change in individual particle movement at high and low frequencies, and the compressibility of the granular layer.

What Is Shaking in the Sandbox

In 1831, Faraday reported to the Royal Society of London that granular material inside a container, when vibrated, would spontaneously begin to exhibit convection rolls, similar to what is observed in normal fluids when heated from below. This observation indicated that not only can a granular material act like a fluid, but also that vibrations can affect the properties of these materials in important ways. Such phenomena are of immediate practical importance because granular materials exist all around us. We use sand and gravel to build the roads we drive on; we process grain to provide our food supply; we mine ore to provide coal, minerals, and precious commodities; we take powders and pills to cure what ails us. Many of the phenomena observed in granular media are prototypical examples of complex, nonequilibrium behavior that is also found in an increasing number of other systems. As a result, sandpiles have served as a macroscopic and visually appealing metaphor for thinking about a number of microscopic systems that are not directly accessible to our senses. Despite the common occurrence of these materials, their properties are not at all well understood and most of our knowledge centers on the subset of static, equilibrium properties of granular matter. Only over the last few years have physicists and engineers begun to unravel some of the exceptional time-dependent, nonequilibrium properties that these seemingly simple materials exhibit. This review focuses on recent developments in the newly emerging field of granular dynamics and, in particular, addresses the role of vibration in determining the phenomena observed in such media.