title: Spatial and temporal coarse graining for dispersion in randomly packed spheres.
abstract: The propagator for molecular displacements P(zeta, t) and its first three cumulants were measured for Stokes flow in monodisperse bead packs with different sphere sizes d and molecular diffusion coefficients D(m). We systematically varied the normalized mean displacement <zeta>/d and diffusion length L(D)=sqrt[2D(m)t]/d. The experimental results map onto each other with this scaling. For L(D)/d<0.2 the propagator remains non-Gaussian, and thus an advection diffusion equation is not obeyed, for mean displacements measured up to <zeta>>10d. A Gaussian shape is approached for large mean displacements when L(D)>0.3d.
authors.affiliation: Schlumberger-Doll Research, Old Quarry Road, Ridgefield, Connecticut 06877, USA.
authors.forename: Ulrich M
authors.initials: UM
authors.lastname: Scheven
authors.forename: Pabitra N
authors.initials: PN
authors.lastname: Sen