Some aspects of electrical conduction in granular systems of various dimensions

We report on measurements of the electrical conductivity in both a 2D triangular lattice of metallic beads and in a chain of beads. The voltage/current characteristics are qualitatively similar in both experiments. At low applied current, the voltage is found to increase logarithmically in a good agreement with a model of widely distributed resistances in series. At high enough current, the voltage saturates due to the local welding of microcontacts between beads. The frequency dependence of the saturation voltage gives an estimate of the size of these welded microcontacts. The DC value of the saturation voltage (~ 0.4 V per contact) gives an indirect measure of the number of welded contact carrying the current within the 2D lattice. Also, a new measurement technique provides a map of the current paths within the 2D lattice of beads. For an isotropic compression of the 2D granular medium, the current paths are localized in few discrete linear paths. This quasi-onedimensional nature of the electrical conductivity thus explains the similarity between the characteristics in the 1D and 2D systems.Comment: To be published in The European Physical Journal

Effects of electromagnetic waves on the electrical properties of contacts between grains

A DC electrical current is injected through a chain of metallic beads. The electrical resistances of each bead-bead contacts are measured. At low current, the distribution of these resistances is large and log-normal. At high enough current, the resistance distribution becomes sharp and Gaussian due to the creation of microweldings between some beads. The action of nearby electromagnetic waves (sparks) on the electrical conductivity of the chain is also studied. The spark effect is to lower the resistance values of the more resistive contacts, the best conductive ones remaining unaffected by the spark production. The spark is able to induce through the chain a current enough to create microweldings between some beads. This explains why the electrical resistance of a granular medium is so sensitive to the electromagnetic waves produced in its vicinity.Comment: 4 pages, 5 figure

MILES extended: Stellar population synthesis models from the optical to the infrared

We present the first single-burst stellar population models which covers the optical and the infrared wavelength range between 3500 and 50000 Angstrom and which are exclusively based on empirical stellar spectra. To obtain these joint models, we combined the extended MILES models in the optical with our new infrared models that are based on the IRTF (Infrared Telescope Facility) library. The latter are available only for a limited range in terms of both age and metallicity. Our combined single-burst stellar population models were calculated for ages larger than 1 Gyr, for metallicities between [Fe/H] = -0.40 and 0.26, for initial mass functions of various types and slopes, and on the basis of two different sets of isochrones. They are available to the scientific community on the MILES web page. We checked the internal consistency of our models and compared their colour predictions to those of other models that are available in the literature. Optical and near infrared colours that are measured from our models are found to reproduce the colours well that were observed for various samples of early-type galaxies. Our models will enable a detailed analysis of the stellar populations of observed galaxies.Comment: 9 pages, 10 figures, published in A&

Observation of resonant interactions among surface gravity waves

We experimentally study resonant interactions of oblique surface gravity waves in a large basin. Our results strongly extend previous experimental results performed mainly for perpendicular or collinear wave trains. We generate two oblique waves crossing at an acute angle, while we control their frequency ratio, steepnesses and directions. These mother waves mutually interact and give birth to a resonant wave whose properties (growth rate, resonant response curve and phase locking) are fully characterized. All our experimental results are found in good quantitative agreement with four-wave interaction theory with no fitting parameter. Off-resonance experiments are also reported and the relevant theoretical analysis is conducted and validated.Comment: 11 pages, 7 figure

Oxydation en voie humide de la pollution organique aqueuse par le peroxyde d'hydrogène Procédé « Wet Peroxide Oxidation » (WPO®) Étude de nouveaux catalyseurs

Les effluents aqueux pollués par des matières organiques provenant d'industries chimiques présentent souvent une faible biodégradabilité. Dans certains domaines de concentration (DCO = 0,5 - 15 g/l), le procédé WPO® développé au laboratoire se substitue avantageusement à l'incinération pour traiter ce type d'effluents. La réaction, qui met en œuvre le réactif de Fenton à température élevée, conduit parfois à la formation de quantités importantes d'acides carboxyliques légers. Nous avons donc développé des systèmes catalytiques originaux remplaçant les sels de fer et conduisant à une oxydation totale des acides carboxyliques. Le système le plus efficace constitué de sels de fer, de cuivre et de manganèse permet d'obtenir, en 1 h à 100 °C, l'oxydation totale d'un mélange synthétique de ces acides (COT = 5 g/l) avec 1,5 fois la quantité de peroxyde théoriquement nécessaire à l'oxydation. Le catalyseur précipité et séparé en fin de traitement peut être recyclé et conserve la môme activité. Les unités industrielles permettant d'effectuer le traitement WPO® avec les nouveaux catalyseurs, recyclés ou non, seront similaires à celle déjà réalisée pour le traitement de « points noirs » industriels.There is an important concern about the problems occuring with wastes elimination, specially the industrial liquid wastes. Te face the problem of organic aqueous wastes coming front various branches of industry, the WPO® (wet peroxide oxidation) process was developed at the laboratory. In the WAO process (wet air oxidation), which uses gaseous oxygen, the limiting step is usually oxygen transfer. In this new process, this problem is suppressed by using a liquid oxidising agent (hydrogen peroxide). This process is adapted from the classical Fenton's reaction and iron salts are used as the catalyst in order to promote the formation of •OH radicles which are the main active species. But the reaction is carried out at about 120 °C; so, a very significant TOC (total organic carton) removal efficiency is obtained (60 to 90 %) in comparison with the low efficiency of the classical Fenton's reagent (typically 25 % at room temperature).Significant amounts of free fatty acids are formed during the reaction. They are namely oxalic, malonic, succinic and acetic acids, which are common by products obtained during audition of most industrial organic pollutants. In order to comply with the regulations requirements, it was necessary to improve the efficiency of the original process. It was also very important to obtain an efficient elimination at a temperature not greater than 100 °C in order to avoid to pressurize the treatment reactor. This could be obtained by using new catalysts which are described in this paper.Because of the related field, precious metals like Pt and potentially toxic ones like Cr were not considered. One needs a treatment process as cheap and as reliable as possible. So, only Fe, Cu, Co, Ni and Mn were used as salts in order to test their calalytic activity in the treatment by hydrogen peroxide of a synthetical mixture of oxalic, malonic, succinic and acetic acids (O, M, S, A). The experimental device is a stirred tank reactor where the organics and the catalyst are batch loaded. It is continuously fed, for 1 hour, with hydrogen peroxide. The total amount injected is 1.5 the stoechiometric amount. In table 1, it can be seen that any metal has a satisfactory activity when used alone (TOC removal efficiency cannot exceed 22 %). In table 2, it is clear that, in soma cases, the association of two or three metals with each other can lead to very important synergetic effects. When using a mixture of Fe, Cu and Mn, the removal efficiency can increase to 91 %. This Fe/Cu/Mn catalyst is studied with further details in table 4. It appears to have its best efficiency at about 100 °C because of a parasitic decomposition of the peroxide at higher temperatures. For an organic mixture coutaining 5 g TOC/l, 100 ppm of each metal is a convenient concentration. This new catalyst still needs an acidic pH, from 3 to about 5, but the dependency is not so strict than with Fe alone (original process) which needs a value from 3 to 3.5. In addition, it was observed that the treatment time could be easily reduced (down to 45 minutes) as well as the amount of peroxide injected.Very similar results have been obtained with synthetic solutions of pollutants and with real industrial ones, thus establishing the ability of the Fe/Cu/Mn mixture to catalyse the oxidation of a large variety of species and not only carboxilic acids. The difference between the efficiency of this new catalyst and the conventional one is shown in table 5. Figures 1 and 2 are related to an Industrial WPO® unit which is commonly used with the conventional catalyst (Fe). It has been possible to improve its efficiency by using the new one without any significant modification. The Fe/Cu/Mn catalyst can be easily separated alter reaction (coprecipitation effect). Thus, the treated water meets the regulation requirements and the recovered catalyst can be easily resolubilized and recycled

Observation of intermittency in wave turbulence

We report the observation of intermittency in gravity-capillary wave turbulence on the surface of mercury. We measure the temporal fluctuations of surface wave amplitude at a given location. We show that the shape of the probability density function of the local slope increments of the surface waves strongly changes across the time scales. The related structure functions and the flatness are found to be power laws of the time scale on more than one decade. The exponents of these power laws increase nonlinearly with the order of the structure function. All these observations show the intermittent nature of the increments of the local slope in wave turbulence. We discuss the possible origin of this intermittency.Comment: new version to Phys. Rev. Let

On the origin of intermittency in wave turbulence

Using standard signal processing tools, we experimentally report that intermittency of wave turbulence on the surface of a fluid occurs even when two typical large-scale coherent structures (gravity wave breakings and bursts of capillary waves on steep gravity waves) are not taken into account. We also show that intermittency depends on the power injected into the waves. The dependence of the power-law exponent of the gravity-wave spectrum on the forcing amplitude cannot be also ascribed to these coherent structures. Statistics of these both events are studied.Comment: To be published in EP

Dendrochronological dating of coal mine workings at the Joggins Fossil Cliffs, Nova Scotia, Canada

Joggins, Nova Scotia was one of the first places in North America where coal was mined. In this paper we employ dendrochronology to date timber pit props preserved within relic coal mine workings on the closely adjacent Fundy and Dirty seams. These remains comprise a system of adits created through ‘room and pillar’ mining. Of the seventy-three samples collected, forty-eight were successfully cross-dated against a local red spruce (Picea rubens Sarg.) master chronology thereby establishing the year in which each individual sample was cut as a live tree. Results indicate cut dates of 1849-1875 which are generally consistent with written archival records of mining activity on these coal seams. Our analysis of fourteen separate adits allows us to distinguish two phases of mining. Most adits (numbers 1-9 and 11-12 with cut dates of 1849-1868) are relics of an initial operation by the General Mining Association (1865-1871), which opened a mine entered at beach level. Dendrochronological dates preceding the opening of this mine may suggest that timber stockpiled from the nearby Joggins Mine (opened 1847) was used in its construction. The remaining adits (numbers 10 and 13-14 with cut dates of 1873-1875) are probably relics of a later mine opened by the Joggins Coal Mining Company (1872-1877). Although this mine was centered ~500 m inland, its western peripheral workings passed through the earlier workings to the shore. Findings improve knowledge of the industrial archaeology of the UNESCO World Heritage Site and help refine the regional master red spruce chronology for future dendrochronological studies