### Session MM: Granular Segregation

Chair: Karen E. Daniels, North Carolina State University
Room: 103B

 Tuesday, November 25, 2008 8:00AM - 8:13AM MM.00001: Phase Transitions of Bidisperse Granular Mixtures in Rotating Tumblers Gabriel Juarez , Richard M. Lueptow , Julio M. Ottino Various aspects of axial banding of size-varying bidisperse granular mixtures in cylindrical tumblers has been documented repeatedly over a decade or so, but the dependence of band formation on the relative concentration of particles or rotation rate has not been thoroughly examined. Phase transitions analogous to nucleation and spinodal decomposition appear to occur as the relative concentration of small and large particles and the rotation rate (granular temperature) of the tumbler are varied. Phase separation similar to nucleation occurs near the endwalls for very low or high concentrations of small particles in which relatively few axial bands form and grow but do not coarsen. Phase separation similar to spinodal decomposition occurs for moderate relative particle concentrations in which bands form along the entire length of the tumbler and eventually merge, leading to coarsening of the segregation pattern. A phase diagram with a miscibility gap can be constructed from the space-time plots. Tuesday, November 25, 2008 8:13AM - 8:26AM MM.00002: Translational and Rotational Velocity Statistics in a Rotating Granular Tumbler Jacob Jantzi , Jeffrey Olafsen Several hundred stainless steel cylinders in a rotating tumbler were used to examine translational and rotational velocity statistics within a granular flow of only a few layers. The particles at the boundary are strongly influenced by the shear of the wall and act as a lubrication layer between the boundary and the bulk flow. The particles in the bulk flow do not appear to have any mean rotational velocity about their axes, and instead chatter,'' fluctuating back and forth without bias. Inertial effects due to the particle layers were observed as well, with the rotational velocities of the boundary layer dependent on the height of the bulk above it. Both the translational and rotational velocity distributions in the flow were examined for deviations from Gaussian. This analysis was accomplished using a newly developed stereoscopic CCD camera array. Tuesday, November 25, 2008 8:26AM - 8:39AM MM.00003: Axial Band Switching in Spherical Tumblers Bryan Lochman , Pengfei Chen , Julio M. Ottino , Richard M. Lueptow Axial band formation of bidisperse granular mixtures is typically studied in long cylindrical tumblers. Similar bands occur in spherical tumblers where there are no flat endwalls to initiate the bands. For a 50:50 mixture of large and small particles, three bands form: one at each pole and one at the equator. It is curious, though, that which particles appear in the equatorial band depends on the fill level of the tumbler. For low fill levels, large particles are at the equator; for high fill levels, the opposite occurs. The result is robust, occurring for several combinations of 1, 2, and 4 mm particles in a 140 mm diameter tumbler, though the fill level at which the transition occurs varies depending on particle size and rotational speed. Discrete element method simulations produce identical results. Particle tracking indicates that small particles flow further toward the poles than large particles in the upstream portion of the flowing layer for low fill levels leading to a band of small particles at each pole. The opposite is true for high fill levels, though the deviation between the paths for the small and large particles is smaller, resulting in slower segregation. Tuesday, November 25, 2008 8:39AM - 8:52AM MM.00004: Experimental and computational studies of segregation mechanisms using a split-bottom cell Yi Fan , Kimberly Hill We investigate the relative importance of certain microscopic'' (that is, particlescale) segregation mechanisms for densely sheared granular mixtures using a split-bottom cell. Unlike other experiments more commonly used to study segregation in dense flows, the split bottom cell induces a shear primarily in the horizontal direction and thus isolates the shear-related segregation mechanisms from the effect of gravity Experimentally, we find that (unlike in gravity driven shear flow) in the vertical direction, segregation associated with particle density is much faster than segregation associated with particle size. Further, investigations of the segregation structure in the bulk show that for particles differing only in size, the vertical segregation structure is somewhat restricted to the region close to free surface likely associated with a surficial porosity gradient We also perform Distinct Element Method (DEM) simulations to access the bulk kinematics, and find stratification associated with relative particle sizes can strongly influence the segregation dynamics. Tuesday, November 25, 2008 8:52AM - 9:05AM MM.00005: Granular segregation in spinning pipes and circular Hele-Shaw cells Maximo Pliego , Abraham Medina , Francisco Higuera Spinning vertical pipes and spinning horizontal Hele-Shaw cells, both filled with dry, polydisperse, non cohesive granular matter, yield complex flows whose expanding shapes depend on the reached angular velocity $\Omega$, the grain sizes and the inner radii of the pipes and the circular cells, respectively. In this work, we present a simple theoretical treatment and a series of experiments related to how the grain's trajectories for grains of several sizes produce segregation Tuesday, November 25, 2008 9:05AM - 9:18AM MM.00006: Filtration of Rod-like Granular Materials and the Buffon-Laplace Needle Problem Scott Franklin , Zack Dell , Maddie Pelz We investigate the efficacy of filtering rod-like granular materials from solution by a square-grid mesh. Because rods of large aspect ratio are much longer than they are wide, the probability of getting caught by a sieve depend sensitively on the particle orientation. We have measured the probability for a needle to be filtered as a function of mesh size, particle length, and aspect ratio. Results are compared with a theoretical calculation based on the Buffon-Laplace Needle problem. In 1770, Buffon solved for the probability that a needle dropped on a surface covered by parallel lines will intersect a line., Laplace correctly generalized this to a grid in 1812, hence the name Buffon-Laplace Needle Problem'' (BLNP). We have extended the BLNP to account for sphero-cylinders of finite width, and thus well-defined aspect ratio and to include an isotropic angular distribution in the zenith angle $\phi$. The solution is the probability that a sphero-cylinder in three dimensions will make contact with a 2D sieve-like mesh, which we then compare with our experimental data. Tuesday, November 25, 2008 9:18AM - 9:31AM MM.00007: Force distribution functions within thick layers of gravity-driven granular mixtures K.M. Hill , J. Zhang , B. Yohannes , J.B. Freund We investigate computationally the probability distribution of the magnitudes of interparticle forces P(F) in thick gravity-driven layers of granular mixtures in half-filled rotated drums. In the majority of the layer where the particles move in solid-like rotation with the drum, P(F) decays exponentially above the average force, as reported in other jammed granular systems. However, from the bottom of the flowing layer to the top of the free surface, the shape of P(F) above the average force undergoes a gradual transition from one that decays exponentially to one that decays as a power-law. Over the same distance, the coordinate number decreases from near 4 at the bottom of the flowing layer to far less than 1 near the free surface. We show how these signatures of the changes in the flow structure modify the dominant segregation mechanisms for the different regions of the flowing layer. Tuesday, November 25, 2008 9:31AM - 9:44AM MM.00008: Shear segregation of granular materials as a function of particle size and confining pressure Laura Golick , Karen Daniels We experimentally investigate the dependence of granular shear segregation rates on particle size ratio and confining pressure. Within a cylindrical annulus, we shear two monodisperse layers of spherical glass beads, with an equal volume of small beads initially placed in a layer above the large. From changes in the height of the sample, we compute the mixing and segregation rates as a function of particle size ratio. We observe that contrasting as well as similar particle size ratios segregate and mix more slowly than intermediate particle sizes, in disagreement with kinetic sieving theory. Increasing pressure reduces the segregation and mixing rates. Tuesday, November 25, 2008 9:44AM - 9:57AM MM.00009: The Underlying Physics in Wetted Particle Collisions Carly Donahue , Christine Hrenya , Robert Davis Wetted granular particles are relevant in many industries including the pharmaceutical and chemical industries and has applications to granulation, filtration, coagulation, spray coating, drying and pneumatic transport. In our current focus, we investigate the dynamics of a three-body normal wetted particle collision. In order to conduct collisions we use an apparatus called a Stokes Cradle,'' similar to the Newton's Cradle (desktop toy) except that the target particles are covered with oil. Here, we are able to vary the oil thickness, oil viscosity, and material properties. With a three particle collision there are four possible outcomes: fully agglomerated (FA); Newton's Cradle (NC), the striker and the first target ball are agglomerated and the last target ball is separated; Reverse Newton's Cradle (RNC), the striker is separated and the two targets are agglomerated; and fully separated (FS). Varying the properties of the collisions, we have observed all four outcomes. We use elastohydrodynamics as a theoretical basis for modeling the system. We also have considered the glass transition of the oil as the pressure increases upon impact and the cavitation of the oil as the pressure drops below the vapor pressure upon rebound. A toy model has been developed where the collision is modeled as a series of two-body collisions. A qualitative agreement between the toy model and experiments gives insight into the underlying physics. Tuesday, November 25, 2008 9:57AM - 10:10AM MM.00010: Computational and experimental studies of the flow, mixing, and size segregation phenomena of heterogeneous granular materials Masato R. Nakamura , Marco J. Castaldi , Nickolas J. Themelis Flow, mixing, and size segregation of heterogeneous granular particles are intriguing phenomena. In order to characterize the behavior of heterogeneous particle, a two-dimensional stochastic model of particle flow and mixing within the packed bed on a traveling grate was developed. The model was calibrated and validated by means of a physical model of the reverse acting grate, using tracer particles ranging from 6 -- 22 cm in diameter. It was found that the motion of the traveling grate, whose speed ranged from 15 to 90 reciprocations/hr, increases the mean residence time of small and medium particles by 69{\%} and 8{\%}, respectively, while decreasing the mean residence time of large particles by 19{\%}. This is because of size segregation known as the Brazil Nut Effect. When the ratio of particle diameter to the height of moving bar, d/h, increases from 0.46 to 1.69, the mixing diffusion coefficient, De at 60/hr., decreases from 96 to 38.4. This indicates that the height of the moving bars should be greater than the diameter of targeted particles.