Stress response function of a granular layer: quantitative comparison between experiments and isotropic elasticity

We measured the vertical pressure response function of a layer of sand submitted to a localized normal force at its surface. We found that this response profile depends on the way the layer has been prepared: all profiles show a single centered peak whose width scales with the thickness of the layer, but a dense packing gives a wider peak than a loose one. We calculate the prediction of isotropic elastic theory in presence of a bottom boundary and compare it to the data. We found that the theory gives the right scaling and the correct qualitative shape, but fails to really fit the data.Comment: 22 pages, 9 figures, submitted to Euro. Phys. J.

Larvicidal, pupicidal and insecticidal activities of Cosmos bipinnatus, Foeniculum vulgare and Tagetes minuta against Culex quinquefasciatus mosquitoes

Purpose: To evaluate the larvicidal, pupicidal and insecticidal activities of Cosmos bipinnatus, Foenuculum vulgare and Tagetes minuta leaf extracts against Culex quinquefasciatus mosquitoes.Methods: The leaves of the plants were extracted with distilled water, ethanol (95 %), and hexane and the extracts screened for their phytochemical profile. While larvicidal and pupicidal activities were assayed at concentrations ranging from 0.1 - 10 mg/mL, insecticidal property was tested at varying amounts (0.25 - 2 g) of the plant sample. The respective larval mortality was thereafter evaluated using Probit analysis.Results: Saponins, terpenoids, flavonoids and steroids were detected in the plant extracts. The ethanol extracts of F. vulgare, T. minuta and C. bipinnatus exhibited larvicidal activity half-maximal lethal concentration (LC50) of 0.10, 1.17 and 1.18 mg/mL, followed by hexane extracts with LC50 value of 1.03, 1.01 and 1.27 mg/mL, respectively, against the larvae of C. quinquefasciatus mosquito. Hexane extracts displayed pupicidal activity with LC50 of 1.07, 1.12 and 1.16 mg/mL against F. vulgare, T. minuta and C. bipinnatus, respectively, while the ethanol extracts of T. minuta, C. bipinnatus and F. vulgare displayed pupicidal activity at LC50 of 1.11, 1.14 and 1.31 mg/mL respectively, against pupa of C. quinquefasciatus mosquito. The aqueous extracts had no (p > 0.05) lethal effects on both larvae and pupa of C. quinquefasciatus at all evaluated concentrations. F. vulgare had the highest (p < 0.05) half-maximal knock-down effect (KD50 = 7.52 min-1), followed by T. minuta (KD50 = 8.64 min-1) on adult C. quinquefasciatus mosquitoes after 6 h of exposure. F. vulgare and T. minuta killed all evaluated mosquito adults within 12 h with LD99 = 0.25 g/air, while the leaves of C. bipinnatus had no (p > 0.05) knock-down or lethal effects on the adult mosquito.Conclusion: C. bipinnatus, F. vulgare and T. minuta possess larvicidal and pupicidal properties against C. quinquefasciatus, whereas only F. vulgare and T. minuta displayed insecticidal properties. Consequent upon these findings, all the plants can be considered naturally potent larvicidal and pupicidal agents against C. quinquefasciatus.Keywords: Cosmos bipinnatus, Culex quinquefasciatus, Botanical insecticides, Knock-down effect, Larvicidal, Pupicidal, Insecticidal, Foeniculum vulgare, Tagetes minut

Force Chains, Microelasticity and Macroelasticity

It has been claimed that quasistatic granular materials, as well as nanoscale materials, exhibit departures from elasticity even at small loadings. It is demonstrated, using 2D and 3D models with interparticle harmonic interactions, that such departures are expected at small scales [below O(100) particle diameters], at which continuum elasticity is invalid, and vanish at large scales. The models exhibit force chains on small scales, and force and stress distributions which agree with experimental findings. Effects of anisotropy, disorder and boundary conditions are discussed as well.Comment: 4 pages, 11 figures, RevTeX 4, revised and resubmitted to Phys. Rev. Let

Effective application of economic principles to border post planning and design : the Maseru bridge example

Decisions about border post location and design are often based on operational, design and implementation cost considerations, without due consideration of the economic impact of these choices. Although the resultant changes in regional and local travel patterns, local impacts on border post users, and the economic opportunities presented to local communities could have a fundamental impact on implementation choices, there is, however, no set uniform procedure for considering these impacts in a consistent manner provided in the available literature.Paper presented at the 34th Annual Southern African Transport Conference 6-9 July 2015 "Working Together to Deliver - Sakha Sonke", CSIR International Convention Centre, Pretoria, South Africa.The Minister of Transport, South AfricaTransportation Research Board of the US

Structure and rheological properties of model microemulsion networks filled with nanoparticles

Model microemulsion networks of oil droplets stabilized by non ionic surfactant and telechelic polymer C18-PEO(10k)-C18 have been studied for two droplet-to-polymer size ratios. The rheological properties of the networks have been measured as a function of network connectivity and can be described in terms of simple percolation laws. The network structure has been characterised by Small Angle Neutron Scattering. A Reverse Monte Carlo approach is used to demonstrate the interplay of attraction and repulsion induced by the copolymer. These model networks are then used as matrix for the incorporation of silica nanoparticles (R=10nm), individual dispersion being checked by scattering. A strong impact on the rheological properties is found for silica volume fractions up to 9%

Dielectric losses in hydrogenated VT1-0 titanium in distribution in eddy current

For the first time introduced the concept of the value of the dielectric loss in the hydrogenated titanium. Measurement of this value allowing more sensitive measurement of the distribution of hydrogen in the titanium sample according to the depth. This concept is widely used in the analysis of the plasma properties of semiconductors and other materials

Particle displacements in the elastic deformation of amorphous materials: local fluctuations vs. non-affine field

We study the local disorder in the deformation of amorphous materials by decomposing the particle displacements into a continuous, inhomogeneous field and the corresponding fluctuations. We compare these fields to the commonly used non-affine displacements in an elastically deformed 2D Lennard-Jones glass. Unlike the non-affine field, the fluctuations are very localized, and exhibit a much smaller (and system size independent) correlation length, on the order of a particle diameter, supporting the applicability of the notion of local "defects" to such materials. We propose a scalar "noise" field to characterize the fluctuations, as an additional field for extended continuum models, e.g., to describe the localized irreversible events observed during plastic deformation.Comment: Minor corrections to match the published versio

Numerical Study of the Stress Response of Two-Dimensional Dense Granular Packings

We investigate the Green function of two-dimensional dense random packings of grains in order to discriminate between the different theories of stress transmission in granular materials. Our computer simulations allow for a detailed quantitative investigation of the dynamics which is difficult to obtain experimentally. We show that both hyperbolic and parabolic models of stress transmission fail to predict the correct stress distribution in the studied region of the parameters space. We demonstrate that the compressional and shear components of the stress compare very well with the predictions of isotropic elasticity for a wide range of pressures and porosities and for both frictional and frictionless packings. However, the states used in this study do not include the critical isostatic point for frictional particles, so that our results do not preclude the fact that corrections to elasticity may appear at the critical point of jamming, or for other sample preparation protocols, as discussed in the main text. We show that the agreement holds in the bulk of the packings as well as at the boundaries and we validate the linear dependence of the stress profile width with depth.Comment: 7 pages, 5 figure