Self-diffusion in dense granular shear flows.

摘要: Diffusivity is a key quantity in describing velocity fluctuations granular materials. These are the basis of many thermodynamic and hydrodynamic models which aim to provide statistical description systems. We present experimental results on diffusivity dense, shear flows two-dimensional Couette geometry. find that self-diffusivities D proportional local rate gamma; with diffusivities along direction mean flow approximately twice as large those perpendicular direction. The magnitude gamma;a(2), where particle radius. However, gradient rate, coupling flow, strong drag at moving boundary lead displacements can appear subdiffusive or superdiffusive. In particular, diffusion appears be superdiffusive due Taylor dispersion effects rate. anisotropic force network leads an additional anisotropy property dense systems has no obvious analog rapid flows. Specifically, suppressed network. A simple random walk simulation reproduces features data, such apparent behavior arising from field, confirming underlying diffusive motion. not observed since included. Examples correlated motion, transient vortices, Levy flights also observed. Although motion creates fields qualitatively different collisional Brownian introduce nondiffusive effects, average system simply diffusive.