GRANULAR MATERIALS UNDER VIBRATION AND THERMAL CYCLES

Open Access
Author:
Chen, Ke
Graduate Program:
Physics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 12, 2008
Committee Members:
  • Peter E Schiffer, Committee Chair
  • Julian Decatur Maynard Jr., Committee Member
  • Reka Z Albert, Committee Member
Keywords:
  • thermal cycles
  • Granular flow
  • vibration
Abstract:
We report flow rate measurement of granular materials from a lab size silo with and without sinusoidal vibration, and the flows from a jammed container under mechanical shocks. We also report the investigation of fragility in granular materials using controlled cyclic temperature variation, or thermal cycling that induces microscopic changes in the size of the grains and the container. When placed under sinusoidal vibration, the flow rate or flux from an unjammed container decreases with the peak velocity of the vibration, and becomes a constant at the highest peak velocities. The flux under vibration follows a 5/2 power scaling rule to corrected orifice diameter, the same scaling rule that is also observed in the absence of vibration. Under vibration, granular flux is no greater than the flux without vibration. Density dilution of granular packs under vibration is likely the cause for such reduced flux, and can be described by a model based on energy balance at the vibrating boundary. The eventual saturation of flux at the highest peak velocities signifies a possible transition from granular fluid to granular gas, as the density decreases and inter-grain interaction changes. Brief flows can be initiated from a jammed container using mechanical impacts. The number of grains flowing out of the container as well as the duration of these flows follows an almost exponential decay distribution. The probability that a flow can be initiated by an impact increases with impact intensity and ratio the diameters of the orifice and the grain. The possible container size and filling depth dependence are also discussed. For the thermal cycling measurement, data show that the packing fraction of granular samples increases under thermal cycles regardless of the relative thermal expansions of the grains or the container. A heavy intruder, when passing a density threshold, sinks in a granular pile under thermal cycles. The results show that the bulk property of granular materials can be impacted by microscopic changes orders or magnitudes less, and without the input of mechanical energy. We believe both the packing fraction relaxation and the intruder displacement in thermally cycled granular systems demonstrate the fragility of disordered granular media, which can be defined as the “inability to elastically support some infinitesimal loads”.