The stress transmission universality classes of periodic granular arrays

The transmission of stress is analysed for static periodic arrays of rigid grains, with perfect and zero friction. For minimal coordination number (which is sensitive to friction, sphericity and dimensionality), the stress distribution is soluble without reference to the corresponding displacement fields. In non-degenerate cases, the constitutive equations are found to be simple linear in the stress components. The corresponding coefficients depend crucially upon geometrical disorder of the grain contacts

The missing stress-geometry equation in granular media

The simplest solvable problem of stress transmission through a static granular material is when the grains are perfectly rigid and have an average coordination number of $\bar{z}=d+1$. Under these conditions there exists an analysis of stress which is independent of the analysis of strain and the $d$ equations of force balance $\nabla_{j} \sigma_{ij}({\vec r}) = g_{i}({\vec r})$ have to be supported by $\frac{d(d-1)}{2}$ equations. These equations are of purely geometric origin. A method of deriving them has been proposed in an earlier paper. In this paper alternative derivations are discussed and the problem of the "missing equations" is posed as a geometrical puzzle which has yet to find a systematic solution as against sensible but fundamentally arbitrary approaches.Comment: 10 pages, 4 figures, accepted by Physica

Interconnectivity of habitats in soil:combining X-ray micro tomography and thin sectioning to reveal fungal-soil structure interactions

The extreme heterogeneity and interconnectivity of the 3-dimensional pore space within soil makes it a unique habitat for the diverse microbial population and has a pivotal role in microbial interactions. Manipulation and quantification of the 3-dimensional pore space and the spatial distribution of micro-organisms is therefore essential if we are to fully understand microbial interactions. Here we pack soil microcosms at different bulk-densities to manipulate soil structure and use x-ray micro tomography and soil thin sections to analyse the effect on the connectivity of the pore volume and on fungal exploration

Overlapping Resonances Interference-induced Transparency: The S0→S2/S1S_0 \to S_2/S_1 Photoexcitation Spectrum of Pyrazine

The phenomenon of "overlapping resonances interference-induced transparency" (ORIT) is introduced and studied in detail for the $S_0 \to S_2/S_1$ photoexcitation of cold pyrazine (C$_4$H$_4$N$_2$). In ORIT a molecule becomes transparent at specific wavelengths due to interferences between envelopes of spectral lines displaying overlapping resonances. An example is the $S_2\leftrightarrow S_1$ internal conversion in pyrazine where destructive interference between overlapping resonances causes the $S_0 \to S_2/S_1$ light absorption to disappear at certain wavelengths. ORIT may be of practical importance in multi-component mixtures where it would allow for the selective excitation of some molecules in preference to others. Interference induced cross section enhancement is also shown.Comment: 13 pages, 7 figure

Comparative Study of Hydrogen Adsorption in Slit-like Pores of Carbon Adsorbents and on Fullerene Molecules

Adsorption of hydrogen in slit-like pores of carbon adsorbents and on fullerene molecules was investigated using the classical density functional theory. Hydrogen adsorption in a gap between two graphene walls was calculated at different temperatures and pressures. The obtained results agree with the data found using the Dubinin theory of the volume pore filling and with the available molecular dynamics results. It has been shown that conventional carbon adsorbents corresponding to the slit-like model and fullerene materials should have approximately equal storage capacities. However, such a capacity is sufficient for practical storage and use of hydrogen at low temperatures only (at about 20 K), and at room temperatures some special active sites of adsorption should be used to solve the problem under consideration. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3517

The Stress Transmission Universality Classes of Periodic Granular Arrays

The transmission of stress is analysed for static periodic arrays of rigid grains, with perfect and zero friction. For minimal coordination number (which is sensitive to friction, sphericity and dimensionality), the stress distribution is soluble without reference to the corresponding displacement fields. In non-degenerate cases, the constitutive equations are found to be simple linear in the stress components. The corresponding coefficients depend crucially upon geometrical disorder of the grain contacts.Comment: 7 pages, 1 figur

Statistical Mechanics of Vibration-Induced Compaction of Powders

We propose a theory which describes the density relaxation of loosely packed, cohesionless granular material under mechanical tapping. Using the compactivity concept we develope a formalism of statistical mechanics which allows us to calculate the density of a powder as a function of time and compactivity. A simple fluctuation-dissipation relation which relates compactivity to the amplitude and frequency of a tapping is proposed. Experimental data of E.R.Nowak et al. [{\it Powder Technology} 94, 79 (1997) ] show how density of initially deposited in a fluffy state powder evolves under carefully controlled tapping towards a random close packing (RCP) density. Ramping the vibration amplitude repeatedly up and back down again reveals the existence of reversible and irreversible branches in the response. In the framework of our approach the reversible branch (along which the RCP density is obtained) corresponds to the steady state solution of the Fokker-Planck equation whereas the irreversible one is represented by a superposition of "excited states" eigenfunctions. These two regimes of response are analyzed theoretically and a qualitative explanation of the hysteresis curve is offered.Comment: 11 pages, 2 figures, Latex. Revised tex

Soil water percolation at different bulk densities

Soil structure, and specifically bulk density, porosity and connectivity have strong influence on water transport in the soil. In this work we describe the percolation of a fluid particle through a soil simulating its movement through voxel-thick images of the soil, imposing a downwards movement as a fluid particle randomly delivered from the top of a soil image. From the simulation, porosity, frequency distribution of random walk time (expressed as number of simulation steps), and depth reached by random walks was obtained. This work extended the analysis presented in Ruiz-Ramos et al. (2009). An arable sandy loam soil was packed into polypropylene cylinders of 6 cm diameter and 5 cm high at five different bulk densities: 1.2, 1.3, 1.4, 1.5 and 1.6 Mgm3. The image stacks of 260x260x260 with voxel-thick slices were generated from the 3D volumes by using VGStudioMax v.1.2.1. Simulation of the percolation was done applying a set of 5 to 7 threshold values based on the analysis of the histogram region corresponding to 5 voxels. From each image, corresponding to a bulk density, percolation speed distribution in depth was estimated from the simulation outputs. Consequences and relationships among density, grey threshold, porosity and connectivity were discussed. Obtained distributions did not fit to a normal equation, preventing from applying the Darcy’s Laws for describing water movement on these soils