Intrinsic creep of a granular column subjected to temperature changes.

International audienceMinute external pertubations, such as temperature variations, can lead to a creep of the fragile structures that are the granular piles. We report a study, resolved in space and time, of the dynamics associated with the slow compaction of a granular column submitted to thermal cycles. Avoiding the thermal dilations of the container, we observe that the material still creeps, even in absence of external mechanical perturbations. The latter intrinsic creep of the material exhibits several surprizing features: the compaction of the material is not homogeneous along the column height and sudden collapses involve the entire system even if one could expect arches to screen the mechanical interaction between regions far apart one from another. In addition, the analysis of the dependence of the flow-rate on the frequency of the temperature cycles suggests that the creep is mainly induced by the propagation of the temperature pro file, slow temperature changes being less effi cient than rapid ones in making the material flow

Galvanic Coupling Between Pure Copper and Pure Aluminum Experimental Approach and Mathematical Model

The corrosion behavior of a pure aluminum/pure copper couple in a weakly conductive sulfate solution was investigated. Potential and current distributions on the surface of the model couple at the beginning of immersion were obtained by solving the Laplace equation using a finite element method (FEM) algorithm. The potential distribution predicted by the calculations was checked using a Ag/AgCl microreference electrode. A good agreement was found between experimental and theoretical results. It was shown that the reaction occurring at the copper electrode was oxygen reduction, while aluminum remote from the Al/Cu interface remained in the passive state. Moreover, calculations predicted a large cathodic current, related to an increase in oxygen reduction, restricted to copper at the Al/Cu interface. This led to a local pH increase reaching values higher than 9, allowing the dissolution of aluminum to occur close to the interface. Combining these data with optical and scanning electron microscope observations after 24 h of immersion in the sodium sulfate solution allowed a three-step mechanism to be proposed to explain the corrosion damage, and particularly the presence of a copper deposit on the aluminum surface, some distance from the Al/Cu interface, a phenomenon currently observed in commercial copper-rich aluminum alloys

Quantitative Single-letter Sequencing: a method for simultaneously monitoring numerous known allelic variants in single DNA samples

<p>Abstract</p> <p>Background</p> <p>Pathogens such as fungi, bacteria and especially viruses, are highly variable even within an individual host, intensifying the difficulty of distinguishing and accurately quantifying numerous allelic variants co-existing in a single nucleic acid sample. The majority of currently available techniques are based on real-time PCR or primer extension and often require multiplexing adjustments that impose a practical limitation of the number of alleles that can be monitored simultaneously at a single locus.</p> <p>Results</p> <p>Here, we describe a novel method that allows the simultaneous quantification of numerous allelic variants in a single reaction tube and without multiplexing. Quantitative Single-letter Sequencing (QSS) begins with a single PCR amplification step using a pair of primers flanking the polymorphic region of interest. Next, PCR products are submitted to single-letter sequencing with a fluorescently-labelled primer located upstream of the polymorphic region. The resulting monochromatic electropherogram shows numerous specific diagnostic peaks, attributable to specific variants, signifying their presence/absence in the DNA sample. Moreover, peak fluorescence can be quantified and used to estimate the frequency of the corresponding variant in the DNA population.</p> <p>Using engineered allelic markers in the genome of <it>Cauliflower mosaic virus</it>, we reliably monitored six different viral genotypes in DNA extracted from infected plants. Evaluation of the intrinsic variance of this method, as applied to both artificial plasmid DNA mixes and viral genome populations, demonstrates that QSS is a robust and reliable method of detection and quantification for variants with a relative frequency of between 0.05 and 1.</p> <p>Conclusion</p> <p>This simple method is easily transferable to many other biological systems and questions, including those involving high throughput analysis, and can be performed in any laboratory since it does not require specialized equipment.</p

Creep motion of a model frictional system

We report on the dynamics of a model frictional system submitted to minute external perturbations. The system consists of a chain of sliders connected through elastic springs that rest on an incline. By introducing cyclic expansions and contractions of the springs we observe a reptation of the chain. We account for the average reptation velocity theoretically. The velocity of small systems exhibits a series of plateaus as a function of the incline angle. Due to elastic e ects, there exists a critical amplitude below which the reptation is expected to cease. However, rather than a full stop of the creep, we observe in numerical simulations a transition between a continuous-creep and an irregular-creep regime when the critical amplitude is approached. The latter transition is reminiscent of the transition between the continuous and the irregular compaction of granular matter submitted to periodic temperature changes

Large Bottleneck Size in Cauliflower Mosaic Virus Populations during Host Plant Colonization

The effective size of populations (Ne) determines whether selection or genetic drift is the predominant force shaping their genetic structure and evolution. Despite their high mutation rate and rapid evolution, this parameter is poorly documented experimentally in viruses, particularly plant viruses. All available studies, however, have demonstrated the existence of huge within-host demographic fluctuations, drastically reducing Ne upon systemic invasion of different organs and tissues. Notably, extreme bottlenecks have been detected at the stage of systemic leaf colonization in all plant viral species investigated so far, sustaining the general idea that some unknown obstacle(s) imposes a barrier on the development of all plant viruses. This idea has important implications, as it appoints genetic drift as a constant major force in plant virus evolution. By co-inoculating several genetic variants of Cauliflower mosaic virus into a large number of replicate host plants, and by monitoring their relative frequency within the viral population over the course of the host systemic infection, only minute stochastic variations were detected. This allowed the estimation of the CaMV Ne during colonization of successive leaves at several hundreds of viral genomes, a value about 100-fold higher than that reported for any other plant virus investigated so far, and indicated the very limited role played by genetic drift during plant systemic infection by this virus. These results suggest that the barriers that generate bottlenecks in some plant virus species might well not exist, or can be surmounted by other viruses, implying that severe bottlenecks during host colonization do not necessarily apply to all plant-infecting viruses

High-Level Behavior Regulation for Multi-Robot Systems

International audienceWe propose a new collaborative guidance platform for a team of robots that should protect a fixed ground target from one or several threats. The team of robots performs high-level behaviors. These are hand-coded since they consist in driving the robots to some given position. However, deciding when and how to use these behaviors is much more challenging. Scripting high-level interception strategies is a complex problem and applicable to few specific application contexts. We propose to use a gene regulatory network to regulate high-level behaviors and to enable the emergence of efficient and robust interception strategies

Thermalizing a telescope in Antarctica: Analysis of ASTEP observations

The installation and operation of a telescope in Antarctica represent particular challenges, in particular the requirement to operate at extremely cold temperatures, to cope with rapid temperature fluctuations and to prevent frosting. Heating of electronic subsystems is a necessity, but solutions must be found to avoid the turbulence induced by temperature fluctua- tions on the optical paths. ASTEP 400 is a 40 cm Newton telescope installed at the Concordia station, Dome C since 2010 for photometric observations of fields of stars and their exoplanets. While the telescope is designed to spread star light on several pixels to maximize photometric stability, we show that it is nonetheless sensitive to the extreme variations of the seeing at the ground level (between about 0.1 and 5 arcsec) and to temperature fluctuations between --30 degrees C and --80 degrees C. We analyze both day-time and night-time observations and obtain the magnitude of the seeing caused by the mirrors, dome and camera. The most important effect arises from the heating of the primary mirror which gives rise to a mirror seeing of 0.23 arcsec K--1 . We propose solutions to mitigate these effects.Comment: Appears in Astronomical Notes / Astronomische Nachrichten, Wiley-VCH Verlag, 2015, pp.1-2

Effet des activités de loisirs non consommatrices sur la faune sauvage

Human leisure activities are becoming more and more various and widespread, which may increase their potential consequences for wildlife in terms of disturbance. This paper summarizes existing knowledge on the effect of non-consumptive (i.e. not hunting nor fishing) leisure activities on wildlife. The aim is not to provide an exhaustive literature review but through the selection of relevant literature to examine the various aspects of the subject. First, we present the different definitions of disturbance, list the types of activities most likely to affect wildlife, and the species or taxonomic groups generally considered as being the most susceptible. Then, we summarize the various means of measuring the effects of disturbance, highlighting the fact that, though generally most obvious, changes in animal behaviour are not necessarily the most appropriate index of disturbance. Then we present the various scales at which disturbance has been studied, from individual behaviour to population dynamics. Finally, we suggest further research priorities, especially the recognized need for more experimental studies and studies of long-term effects of disturbance on individual fitness, hence population dynamics, since it is at this scale that appropriate management measures of disturbance have to be taken in the futureLes activités de loisirs sont de plus en plus variées et répandues, ce qui peut augmenter leurs effets en termes de dérangement de la faune sauvage. Dans cet article, nous présentons une synthèse des connaissances actuelles sur l'effet des activités de loisirs non consommatrices (ne prenant pas en compte la chasse et la pêche, dont les effets ont été largement étudiés par ailleurs) pour les espèces sauvages. L'objectif n'est pas de fournir une revue exhaustive de la littérature, mais plutôt de présenter une sélection des publications pertinentes sur les différents aspects du sujet. Nous présentons d'abord les différentes définitions de la notion de dérangement, listons les activités reconnues comme les plus dérangeantes et les espèces ou groupes taxinomiques considérés comme les plus sensibles. La définition du dérangement que nous retenons ici est « toute déviation du comportement normal en réponse à des événements inattendus à proximité d'un animal ». Une grande variété d'activités de loisirs peuvent entraîner un dérangement de la faune sauvage, en particulier celles employant des véhicules à moteur ou celles amenant un contact rapproché entre le pratiquant et la faune sauvage (que ce rapprochement soit recherché ou non). La littérature démontre que tous les groupes animaux peuvent potentiellement être affectés par les activités de loisirs, même si c'est chez les oiseaux et les mammifères que le phénomène a été le plus étudié. Suite à ces définitions et revues initiales, nous résumons les différents modes de mesure du dérangement, mettant en exergue le fait que le changement de comportement des animaux, même s'il est un critère souvent évident à mesurer, n'est pas forcément le meilleur indicateur du dérangement. Car les individus commencent généralement à ressentir ses effets avant de modifier leurs activités ou de quitter la zone dérangée. Nous présentons ensuite les différentes échelles auxquelles le dérangement a été étudié, du comportement individuel à la dynamique des populations, les effets sur les populations étant évidemment les plus difficiles à mesurer, mais aussi les plus cruciaux à long terme. Nous suggérons donc quelques perspectives de recherche, en particulier le besoin reconnu de travaux expérimentaux et sur les conséquences du dérangement à long terme pour la valeur sélective des individus, donc la dynamique des populations. C'est en effet à cette échelle que doivent être prises les mesures de gestion adéquates dans le futur