Permeability of mixed soft and hard granular material: hydrogels as drainage modifiers

We measure the flow of water through mixed packings of glass spheres and soft swellable hydrogel grains, at constant sample volume. Permeability values are obtained at constant sample volume and at porosities smaller than random close packing, for different glass bead diameters $D$ and for variable gel grain diameter $d$, as controlled by the salinity of the water. The gel content is also varied. We find that the permeability decays exponentially in $n(D/d)^b$ where $n=N_{gel}/N_{glass}$ is the gel to glass bead number ratio and $b$ is approximately 3. Therefore, flow properties are determined by the volume fraction of gel beads. A simple model based on the porosity of overlapping spheres is used to account for these observations

Importance of Boundary Reflections in the Theory of Diffusive Light Scattering

This PDF file contains the letter “Letter: Importance of boundary reflections in the theory of diffusive light scattering [see 33(12)3849-3852(Dec1994)]” for OE Vol. 34 Issue 1

Effective exponents for the diffusive coarsening of wet foams and analogous materials

Empirical exponents for the growth of average domain size commonly lie between the known limits of 1/2 and 1/3. Here, a framework is developed in the context of foams to quantitatively explain such intermediate exponents, based on a model of gas diffusion and microstructure in which bubbles are approximately spherical and meet at approximately circular films. This predicts growth that is only logarithmically different from a 1/3 power law for nearly-kissing spheres at jamming. It also predicts how the growth law and effective exponents vary with liquid content, as set by the film radius. These predictions compare well with experimental data for two different foaming systems

Characterization of the Drag Force in an Air-Moderated Granular Bed

We measure the torque acting on a rod rotated perpendicular to its axis in a granular bed, through which an upflow of gas is utilized to tune the hydrostatic loading between grains. At low rotation rates the torque is independent of speed, but scales quadratically with rod-length and linearly with depth; the proportionality approaches zero linearly as the upflow of gas is increased towards a critical value above which the grains are fluidized. At high rotation rates the torque exhibits quadratic rate- dependence and scales as the rod's length to the 4th power. The torque has no dependence on either depth or airflow at these higher rates. A model used to describe the stopping force experienced by a projectile impacting a granular bed can be shown to predict these behaviors for our system's geometry, indicating that the same mechanics dictate both steady-state and transient drag forces in granular systems, regardless of geometry or material properties of the grains.Comment: 14 pages, 5 figure

Bubble-Scale Model of Foam Mechanics: Melting, Nonlinear Behavior, and Avalanches

By focusing on entire gas bubbles, rather than soap films or vertices, a microscopic model was recently developed for the macroscopic deformation and flow of foam in which dimensionality, energy storage, and dissipation mechanisms, polydispersity, and the gas-liquid ratio all can be varied easily [D. J. Durian, Phys. Rev. Lett. 75, 4780 (1995)]. Here, a more complete account of the model is presented, along with results for linear rheological properties as a function of the latter two important physical parameters. It is shown that the elastic character vanishes with increasing liquid content in a manner that is consistent with rigidity percolation and that is almost independent of polydispersity. As the melting transition is approached, the bubble motion becomes increasingly nonaffine and the relaxation time scale appears to diverge. Results are also presented for nonlinear behavior at large applied stress, and for the sudden avalanchelike rearrangements of bubbles from one tightly packed configuration to another at small applied strain rates. The distribution of released energy is a power law for small events, but exhibits an exponential cutoff independent of system size. This is in accord with multiple light scattering experiments, but not with other simulations predicting self-organized criticality

Accuracy of Diffusing-Wave Spectroscopy Theories

Random walk computer simulations are reported for the electric field autocorrelation of photons transmitted through multiple-scattering slabs. The results are used as a benchmark for judging the accuracy of competing theories of diffusing-wave spectroscopy (DWS), and also for distinguishing between errors introduced from the approximation of diffusive photon transport and from the continuum approximation that the total square wave-vector transfer of a transmitted photon is proportional to its path length in the material. An important conclusion is that these errors partially cancel, giving accuracies on the order of a few percent for typical experimental situations. Detailed comparisons are made as a function of optical thickness, boundary reflectivity, as well as scattering anisotropy; guidelines are generated for optimizing the analysis of actual DWS data in terms of the dynamics of individual scattering sites

Foam Mechanics at the Bubble Scale

By focusing on entire bubbles rather than films or vertices, a simple model is proposed for the deformation and flow of foam in which dimensionality, polydispersity, and liquid content can easily be varied. Simulation results are presented for the linear elastic properties as a function of bubble volume fraction, showing a melting transition where the static shear modulus vanishes and the relaxation time scale peaks. Results are also presented for shear stress versus strain rate, showing intermittent flow via avalanchelike topological rearrangements and Bingham-plastic behavior