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Now showing 1 - 10 of 77
  • Publication
    Kinetics of Gravity-Driven Water Channels Under Steady Rainfall
    (2014-10-21) Cejas, Cesare M; Wei, Yuli; Barrois, Rémi; Frétigny, Christian; Durian, Douglas J; Dreyfus, Rémi
    We investigate the formation of fingered flow in dry granular media under simulated rainfall using a quasi-two-dimensional experimental setup composed of a random close packing of monodisperse glass beads. Using controlled experiments, we analyze the finger instabilities that develop from the wetting front as a function of fundamental granular (particle size) and fluid properties (rainfall, viscosity). These finger instabilities act as precursors for water channels, which serve as outlets for water drainage. We look into the characteristics of the homogeneous wetting front and channel size as well as estimate relevant time scales involved in the instability formation and the velocity of the channel fingertip. We compare our experimental results with that of the well-known prediction developed by Parlange and Hill [D. E. Hill and J. Y. Parlange, Soil Sci. Soc. Am. Proc. 36, 697 (1972)]. This model is based on linear stability analysis of the growth of perturbations arising at the interface between two immiscible fluids. Results show that, in terms of morphology, experiments agree with the proposed model. However, in terms of kinetics we nevertheless account for another term that describes the homogenization of the wetting front. This result shows that the manner we introduce the fluid to a porous medium can also influence the formation of finger instabilities. The results also help us to calculate the ideal flow rate needed for homogeneous distribution of water in the soil and minimization of runoff, given the grain size, fluid density, and fluid viscosity. This could have applications in optimizing use of irrigation water.
  • Publication
    Nonlinear Bubble Dynamics in a Slowly Driven Foam
    (1995-09-25) Gopal, A. D.; Durian, Douglas J
    Sudden topological rearrangement of neighboring bubbles in a foam occur during coarsening, and can also be induced by applied forces. Diffusing-wave spectroscopy measurements are presented of such dynamics before, during, and after an imposed shear strain. The rate of rearrangements is proportional to the strain rate, and the shape of the correlation functions shows that they are spatially and temporally uncorrelated. Macroscopic deformation is thus accomplished by a nonlinear microscopic process reminiscent of dynamics in the propagation of earthquake faults or the flow of granular media.
  • Publication
    Granular Discharge Rate for Submerged Hoppers
    (2014-01-01) Wilson, T. J; Pfeifer, C. R; Meysingier, N.; Durian, Douglas J
    The discharge of spherical grains from a hole in the bottom of a right circular cylinder is measured with the entire system underwater. We find that the discharge rate depends on filling height, in contrast to the well-known case of dry non-cohesive grains. It is further surprising that the rate increases up to about twenty five percent, as the hopper empties and the granular pressure head decreases. For deep filling, where the discharge rate is constant, we measure the behavior as a function of both grain and hole diameters. The discharge rate scale is set by the product of hole area and the terminal falling speed of isolated grains. But there is a small-hole cutoff of about two and half grain diameters, which is larger than the analogous cutoff in the Beverloo equation for dry grains.
  • Publication
    From Avalanches to Fluid Flow: A Continuous Picture of Grain Dynamics Down a Heap
    (2000-11-13) Lemieux, P. A; Durian, Douglas J
    Surface flows are excited by steadily adding spherical glass beads to the top of a heap. To simultaneously characterize the fast single-grain dynamics and the much slower collective intermittency of the flow, we extend photon-correlation spectroscopy via fourth-order temporal correlations in the scattered light intensity. We find that microscopic grain dynamics during an avalanche are similar to those in the continuous flow just above the transition. We also find that there is a minimum jamming time, even arbitrarily close to the transition.
  • Publication
    Divergence of Voronoi Cell Anisotropy Vector: A Threshold-Free Characterization of Local Structure in Amorphous Materials
    (2016-02-26) Rieser, Jennifer M; Goodrich, Carl P; Durian, Douglas J; Liu, Andrea J
    Characterizing structural inhomogeneity is an essential step in understanding the mechanical response of amorphous materials. We introduce a threshold-free measure based on the field of vectors pointing from the center of each particle to the centroid of the Voronoi cell in which the particle resides. These vectors tend to point in toward regions of high free volume and away from regions of low free volume, reminiscent of sinks and sources in a vector field. We compute the local divergence of these vectors, where positive values correspond to overpacked regions and negative values identify underpacked regions within the material. Distributions of this divergence are nearly Gaussian with zero mean, allowing for structural characterization using only the moments of the distribution. We explore how the standard deviation and skewness vary with the packing fraction for simulations of bidisperse systems and find a kink in these moments that coincides with the jamming transition.
  • Publication
    Bubble Kinetics in a Steady-State Column of Aqueous Foam
    (2006-10-13) Feitosa, K.; Kamien, Randall D; Halt, O. L; Durian, Douglas J
    We measure the liquid content, the bubble speeds, and the distribution of bubble sizes, in a vertical column of aqueous foam maintained in steady state by continuous bubbling of gas into a surfactant solution. Nearly round bubbles accumulate at the solution/foam interface, and subsequently rise with constant speed. Upon moving up the column, they become larger due to gas diffusion and more polyhedral due to drainage. The size distribution is monodisperse near the bottom and polydisperse near the top, but there is an unexpected range of intermediate heights where it is bidisperse with small bubbles decorating the junctions between larger bubbles. We explain the evolution in both bidisperse and polydisperse regimes, using Laplace pressure differences and taking the liquid fraction profile as a given.
  • Publication
    Low-Speed Impact Craters in Loose Granular Media
    (2003-05-16) Uehara, J. S; Ambroso, M. A; Ojha, R.; Durian, Douglas J
    We report on craters formed by balls dropped into dry, noncohesive, granular media. By explicit variation of ball density ρb, diameter Db, and drop height H, the crater diameter is confirmed to scale as the 1/4 power of the energy of the ball at impact: Dc∼(ρbD3bH)1/4. Against expectation, a different scaling law is discovered for the crater depth: d ∼ (ρ3/2bD2bH)1/3. The scaling with properties of the medium is also established. The crater depth has significance for granular mechanics in that it relates to the stopping force on the ball.
  • Publication
    Structure and Coarsening at the Surface of a Dry Three-Dimensional Aqueous Foam
    (2013-12-05) Roth, R. E; Chen, B. G; Durian, Douglas J
    We utilize total-internal reflection to isolate the two-dimensional surface foam formed at the planar boundary of a three-dimensional sample. The resulting images of surface Plateau borders are consistent with Plateau's laws for a truly two-dimensional foam. Samples are allowed to coarsen into a self-similar scaling state where statistical distributions appear independent of time, except for an overall scale factor. There we find that statistical measures of side number distributions, size-topology correlations, and bubble shapes are all very similar to those for two-dimensional foams. However, the size number distribution is slightly broader, and the shapes are slightly more elongated. A more obvious difference is that T2 processes now include the creation of surface bubbles, due to rearrangement in the bulk, and von Neumann's law is dramatically violated for individual bubbles. But nevertheless, our most striking finding is that von Neumann's law appears to holds on average, namely, the average rate of area change for surface bubbles appears to be proportional to the number of sides minus six, but with individual bubbles showing a wide distribution of deviations from this average behavior.
  • Publication
    Particle Motions in a Gas-Fluidized Bed of Sand
    (1997-11-03) Menon, Narayanan; Durian, Douglas J
    We report measurements by diffusing-wave spectroscopy of particle motions in a gas-fluidized bed. The homogenous state of the bed known as the uniformly fluidized state is actually a weak solid in which particles are at rest. The only truly fluid state is an inhomogenous mixture of gas bubbles and of liquid regions in which microscopic particle dynamics are shown to be collisional. The motion of macroscopic bubbles is the source of particle motions in the bed. Measurements of mean free paths, collision rates, and velocity fluctuations (the “granular temperature”) show that dissipation occurs nonuniformly in the medium.
  • Publication
    Morphology of Rain Water Channeling in Systematically Varied Model Sandy Soils
    (2014-10-15) Wei, Yuli; Cejas, Cesare M; Barrois, Rémi; Dreyfus, Rémi; Durian, Douglas J
    We visualize the formation of fingered flow in dry model sandy soils under different rain conditions using a quasi-2D experimental setup and systematically determine the impact of the soil grain diameter and surface wetting properties on the water channeling phenomenon. The model sandy soils we use are random closely packed glass beads with varied diameters and surface treatments. For hydrophilic sandy soils, our experiments show that rain water infiltrates a shallow top layer of soil and creates a horizontal water wetting front that grows downward homogeneously until instabilities occur to form fingered flows. For hydrophobic sandy soils, in contrast, we observe that rain water ponds on the top of the soil surface until the hydraulic pressure is strong enough to overcome the capillary repellency of soil and create narrow water channels that penetrate the soil packing. Varying the raindrop impinging speed has little influence on water channel formation. However, varying the rain rate causes significant changes in the water infiltration depth, water channel width, and water channel separation. At a fixed rain condition, we combine the effects of the grain diameter and surface hydrophobicity into a single parameter and determine its influence on the water infiltration depth, water channel width, and water channel separation. We also demonstrate the efficiency of several soil water improvement methods that relate to the rain water channeling phenomenon, including prewetting sandy soils at different levels before rainfall, modifying soil surface flatness, and applying superabsorbent hydrogel particles as soil modifiers.