Crack path instabilities in DCDC experiments in the low speed regime

We studied the low speed fracture regime (0.1mm/s - 1nm/s) in different glassy materials (soda-lime glass, glass-ceramics) with variable but controlled length scale of heterogeneity. The chosen mechanical system enabled us to work in pure mode I (tensile) and at a fixed load on DCDC (double cleavage drilled compression) specimen. The internal residual stresses of studied samples were carefully relaxed by appropriate thermal treatment. By means of optical and atomic force (AFM) microscopy techniques fracture surfaces have been examined. We have shown for the first time that the crack front line underwent an out-of-plane oscillating behavior as a result of a reproducible sequence of instabilities. The wavelength of such a phenomenon is in the micrometer range and its amplitude in the nanometer range. These features were observed for different glassy materials providing that a typical length scale characterizing internal heterogeneities was lower than a threshold limit estimated to few nanometers. This effect is the first clear experimental evidence of crack path instabilities in the low speed regime in a uniaxial loading experiment. This phenomenon has been interpreted by referring to the stability criterion for a straight crack propagation as presented by Adda-Bedia et al. (Phys. Rev. Letters (1996) 76} p1497).Comment: 16 pages, 10 figures, submitted to Journal of Non-Crystalline Solid

Fracture of glassy materials as detected by real-time Atomic Force Microscopy (AFM) experiments

We have studied the low speed fracture regime for different glassy materials with variable but controlled length scales of heterogeneity in a carefully mastered surrounding atmosphere. By using optical and atomic force (AFM) microscopy techniques we tracked in real-time the crack tip propagation at the nanometer scale on a wide velocity range (1 mm/s and 0.1 nm/s and below). The influence of the heterogeneities on this velocity is presented and discussed. Our experiments revealed also -for the first time- that the crack advance proceeds through nucleation, growth and coalescence of nanometric damage cavities inside the amorphous phase, which generate large velocity fluctuations. The implications of the existence of such a nano-ductile fracture mode in glass are discussed.Comment: 6 pages, 5 figures, submitted to Applied surface Scienc

Glass breaks like metals, but at the nanometer scale

We report in situ Atomic Force Microscopy experiments which reveal the presence of nanoscale damage cavities ahead of a stress-corrosion crack tip in glass. Their presence might explain the departure from linear elasticity observed in the vicinity of a crack tip in glass. Such a ductile fracture mechanism, widely observed in the case of metallic materials at the micrometer scale, might be also at the origin of the striking similarity of the morphologies of fracture surfaces of glass and metallic alloys at different length scales.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Lett, few minor corrections, Fig. 1b change

Dynamic condensation of water at crack tips in fused silica glass

Water molecules play a fundamental role in the physics of slow crack propagation in glasses. It is commonly understood that, during stress-corrosion, water molecules that move in the crack cavity effectively reduce the bond strength at the strained crack tip and, thus, support crack propagation. Yet the details of the environmental condition at the crack tip in moist air are not well determined. In a previous work, we reported direct evidence of the presence of a 100 nm long liquid condensate at the crack tip in fused silica glass during very slow crack propagation (10^-9 to 10^-10 m/s). These observations are based on in-situ AFM phase imaging techniques applied on DCDC glass specimens in controlled atmosphere. Here, we discuss the physical origin of the AFM phase contrast between the liquid condensate and the glass surface in relation to tip-sample adhesion induced by capillary bridges. We then report new experimental data on the water condensation length increase with relative humidity in the atmosphere. The measured condensation lengths were much larger than what predicted using the Kelvin equation and expected geometry of the crack tip.Comment: Accepted in JNCS. In pres

A Tractable Experimental Model for Study of Human and Animal Scabies

Scabies, a neglected parasitic disease caused by the microscopic mite Sarcoptes scabiei, is a major driving force behind bacterial skin infections in tropical settings. Aboriginal and Torres Strait Islander peoples are nearly twenty times more likely to die from acute rheumatic fever and rheumatic heart disease than individuals from the wider Australian community. These conditions are caused by bacterial pathogens such as Group A streptococci, which have been linked to underlying scabies infestations. Community based initiatives to reduce scabies and associated disease have expanded, but have been threatened in recent years by emerging drug resistance. Critical biological questions surrounding scabies remain unanswered due to a lack of biomedical research. This has been due in part to a lack of either a suitable animal model or an in vitro culture system for scabies mites. The pig/mite model reported here will be a much needed resource for parasite material and will facilitate in vivo studies on host immune responses to scabies, including relations to associated bacterial pathogenesis, and more detailed studies of molecular evolution and host adaptation. It represents the missing tool to extrapolate emerging molecular data into an in vivo setting and may well allow the development of clinical interventions

Potential secondary structure at translation-initiation sites.

Since translational start codons also occur internally, more-complex features within mRNA must determine initiation. We compare the potential secondary structure of 123 prokaryotic mRNA start regions to that of regions coding for internal methionines. The latter display an unexpectedly-uniform, almost-periodic pattern of pairing potential. In contrast, sequences 5' to start codons have little self-pairing, and do not pair extensively with the proximal coding region. Pairing potential surrounding start codons was found to be less than half of that found near internal AUGs. In groups of random sequences where the distribution of nucleotides at each position, or of trinucleotides at each in-frame codon position, matched the observed natural distribution, there was no periodicity in the pairing potential of the internal sequences. Randomized internal sequences had less pairing: the ratio of pairing intensity between internals and starts was reduced from 2.0 to 1.6 by randomization. We propose that the transition from the relatively-unstructured start domains to the highly-structured internal sequences may be an important determinant of translational start-site recognition

Mesure des émissions de pesticides pendant les applications. Le cas des traitements viticoles.

During 2 years (2001 to 2002) lab and field experimentation was carried out on a vineyard in south of France to quantify pesticide deposit on soil and vegetation and pesticide emission in air in real conditions and on a field unit scale. Original sampling methodologies were developed in this project. The objectives of this work was to forecast the amount of pesticide in the three compartments (soil, vegetation and air), in relation with spraying profiles, density of foliage, weather (rain episodes included). In addition, laboratory experimentation was carried out to finely characterise some processes (i.e. spray, flow penetration, droplets deposition, rain washing of deposits). In parallel three different models were developed: a representation of the vine at a field unit scale, a model of spray penetration and deposit in the vegetation and a model on rain washing of the pesticides deposits. The selected unit to couple the models is a representative volume of foliage.Des expérimentations ont été conduites pendant 2 ans, (2001 et 2002) dans un vignoble du Sud de la France pour quantifier le depot de pesticide sur le sol et la végétation ainsi que les émissions de produit vers l'air, en conditions réelles à l'échelle d'une parcelle. Des méthodes originales d'échantillonnage ont été développées dans ce projet. L'object est de prévoir les quantités de pesticide dans les compartiments sol, air et végétation en fonction du profil de pulvérisation, de la densité de végétation et du temps (y compris les épisodes pluvieux). Des expérimentations en laboratoires ont complété cette démarche pour caractériser certains procédés (pulvérisation, pénétration du jet, dépôt des gouttelettes, entraînement par ruissellement). En parallèle, trois différents modèles ont été développés : une représentation de la vigne à l'échelle de la parcelle, un modèle pour la pénétration du jet et le dépôt dans la végétation et un modèle d'entraînement par ruissellement. Le couplage de ces trois modèles s'est fait à l'échelle d'une un volume représentatif de végétation

Infrared absorption analysis of organosilicon/oxygen plasmas in a microwave multipolar plasma excited by distributed electron cyclotron resonance

The dissociation of 4 groups of organosilicon monomers (such as hexamethydisiloxane, tetraethoxysilane, tetramethylsilane or tetramethoxysilane) in a MMP DECR reactor is analyzed by infrared absorption spectroscopy. The parent molecules are totally dissociated above 100 watts. The daughter stable molecules, such as C2H2, CH4, CO2, CO, C2H4 and OCH2 have been detected at low power but are dissociated at higher power. Each group exhibits its very own characteristics of dissociation as well as production of new species depending on its chemical composition. It seems that the DECR plasma at high energy dissociates the monomer molecule into its constituent atoms.La dissociation de 4 groupes de monomères organosiliciés (tels que l'hexaméthydisiloxane, le tétraéthoxysilane, le tétraméthylsilane ou encore le tétraméthoxysilane) dans un réacteur PMM RCER est analysée par spectroscopie d'absorption infrouge. Les molécules mères sont totalement dissociées au delà de 100 watts. Des molécules stables telles que C2H2, CH4, CO2, CO, C2H4 et OCH2 ont été détectées à faible puissance mais sont dissociées à plus forte énergie. Chaque groupe fait apparaitre ses propres charactéristiques au niveau de la dissociation ainsi que de la production de nouvelles espèces en fonction de sa composition chimique. Il semble que le plasma RCER dissocie les molécules en espèces atomiques