Influence of pore-scale disorder on viscous fingering during drainage

We study viscous fingering during drainage experiments in linear Hele-Shaw cells filled with a random porous medium. The central zone of the cell is found to be statistically more occupied than the average, and to have a lateral width of 40% of the system width, irrespectively of the capillary number $Ca$. A crossover length $w_f \propto Ca^{-1}$ separates lower scales where the invader's fractal dimension $D\simeq1.83$ is identical to capillary fingering, and larger scales where the dimension is found to be $D\simeq1.53$. The lateral width and the large scale dimension are lower than the results for Diffusion Limited Aggregation, but can be explained in terms of Dielectric Breakdown Model. Indeed, we show that when averaging over the quenched disorder in capillary thresholds, an effective law $v\propto (\nabla P)^2$ relates the average interface growth rate and the local pressure gradient.Comment: 4 pages, 4 figures, submitted to Phys Rev Letter

Stress Transmission through Three-Dimensional Ordered Granular Arrays

We measure the local contact forces at both the top and bottom boundaries of three-dimensional face-centered-cubic and hexagonal-close-packed granular crystals in response to an external force applied to a small area at the top surface. Depending on the crystal structure, we find markedly different results which can be understood in terms of force balance considerations in the specific geometry of the crystal. Small amounts of disorder are found to create additional structure at both the top and bottom surfaces.Comment: 9 pages including 9 figures (many in color) submitted to PR

Effect of boundaries on the force distributions in granular media

The effect of boundaries on the force distributions in granular media is illustrated by simulations of 2D packings of frictionless, Hertzian spheres. To elucidate discrepancies between experimental observations and theoretical predictions, we distinguish between the weight distribution {\cal P} (w) measured in experiments and analyzed in the q-model, and the distribution of interparticle forces P(f). The latter one is robust, while {\cal P}(w) can be obtained once the local packing geometry and P(f) are known. By manipulating the (boundary) geometry, we show that {\cal P}(w) can be varied drastically.Comment: 4 pages, 4 figure

Force distributions near the jamming and glass transitions

We calculate the distribution of interparticle normal forces $P(F)$ near the glass and jamming transitions in model supercooled liquids and foams, respectively. $P(F)$ develops a peak that appears near the glass or jamming transitions, whose height increases with decreasing temperature, decreasing shear stress and increasing packing density. A similar shape of $P(F)$ was observed in experiments on static granular packings. We propose that the appearance of this peak signals the development of a yield stress. The sensitivity of the peak to temperature, shear stress and density lends credence to the recently proposed generalized jamming phase diagram.Comment: 4 pages, 3 postscript figures;Version 3 replaces figure 1 and removes figure 2 from version 1. Significant rewording of version 1 to emphasize the formation of peak in P(F) when these systems jam along five different routes of the recently proposed jamming phase diagram. Version 2 displayed the incorrect abstrac

Statistics of the contact network in frictional and frictionless granular packings

Simulated granular packings with different particle friction coefficient mu are examined. The distribution of the particle-particle and particle-wall normal and tangential contact forces P(f) are computed and compared with existing experimental data. Here f equivalent to F/F-bar is the contact force F normalized by the average value F-bar. P(f) exhibits exponential-like decay at large forces, a plateau/peak near f = 1, with additional features at forces smaller than the average that depend on mu. Computations of the force-force spatial distribution function and the contact point radial distribution function indicate that correlations between forces are only weakly dependent on friction and decay rapidly beyond approximately three particle diameters. Distributions of the particle-particle contact angles show that the contact network is not isotropic and only weakly dependent on friction. High force-bearing structures, or force chains, do not play a dominant role in these three dimensional, unloaded packings.Comment: 11 pages, 13 figures, submitted to PR

Force distributions in 3D granular assemblies: Effects of packing order and inter-particle friction

We present a systematic investigation of the distribution of normal forces at the boundaries of static packings of spheres. A new method for the efficient construction of large hexagonal-close-packed crystals is introduced and used to study the effect of spatial ordering on the distribution of forces. Under uniaxial compression we find that the form for the probability distribution of normal forces between particles does not depend strongly on crystallinity or inter-particle friction. In all cases the distribution decays exponentially at large forces and shows a plateau or possibly a small peak near the average force but does not tend to zero at small forces.Comment: 9 pages including 8 figure

Influence of Viscous Fingering on Dynamic Saturation–Pressure Curves in Porous Media

We report on results from primary drainage experiments on quasi-two-dimensional porous models. The models are transparent, allowing the displacement process and structure to be monitored in space and time during primary drainage experiments carried out at various speeds. By combining detailed information on the displacement structure with global measurements of pressure, saturation and the capillary number Ca, we obtain a scaling relation relating pressure, saturation, system size and capillary number. This scaling relation allows pressure-saturation curves for a wide range of capillary numbers to be collapsed on the same master curve. We also show that in the case of primary drainage, the dynamic effect in the capillary pressure-saturation relationship observed on partially water saturated soil samples might be explained by the combined effect of capillary pressure along the invasion front of the gaseous phase, and pressure changes caused by viscous effects in the wetting fluid phase. © 2010 The Author(s)

Complement Activity in the Egg Cytosol of Zebrafish Danio rerio: Evidence for the Defense Role of Maternal Complement Components

Most fish embryos that develop externally are exposed to an environment full of microbes. How they survive microbial attacks are not understood to date. Here we demonstrated that the egg cytosol prepared from the newly fertilized eggs of zebrafish Danio rerio is capable of killing the Gram-negative bacterium Escherichia coli, via in vitro assay system of the complement activity established. All findings indicate that it is the complement system operating via the alternative pathway that is attributable to the bacteriolytic activity. This is the first report providing the evidence for the functional role of the maternal complement components in fish eggs, paving the way for study of maternal immunity in other organisms whose eggs are fertilized in vitro

Effects of Pressure Oscillations on Drainage in an Elastic Porous Medium

The effects of seismic stimulation on the flow of two immiscible fluids in an elastic synthetic porous medium is experimentally investigated. A wetting fluid is slowly evacuated from the medium, while a pressure oscillation is applied on the injected non- wetting fluid. The amplitude and frequency of the pressure oscillations as well as the evacuation speed are kept constant throughout an experiment. The resulting morphology of the invading structure is found to be strongly dependent on the interplay between the amplitude and the frequency of the applied pressure oscillations and the elasticity of the porous medium. Different combinations of these properties yield morphologically similar structures, allowing a classification of structures that is found to depend on a proposed dimensionless number. © 2010 Springer Science+Business Media B.V

Steady-state two-phase flow in porous media: statistics and transport properties.

We study experimentally the case of steady-state simultaneous two-phase flow in a quasi-two-dimensional porous media. The dynamics is dominated by the interplay between a viscous pressure field from the wetting fluid and bubble transport of a less viscous, nonwetting phase. In contrast with more studied displacement front systems, steady-state flow is in equilibrium, statistically speaking. The corresponding theoretical simplicity allows us to explain a data collapse in the cluster size distribution as well as the relation |nablaP| proportional, sqrt[Ca] between the pressure gradient in the system and the capillary number