Annular cracks in thin films of nanoparticle suspensions drying on a fiber

We report an experimental study of the crack pattern formed during the drying of a colloidal suspension. A horizontal fiber, which provides a one dimensional, boundary-free substrate, is coated by a film of micronic thickness. The geometry imposes a remarkable annular crack pattern and allowing precise measurements of the crack spacing over a short range of film thickness (between 2 and 10 $\mu$m) which varies linearly with the film height. We compare our experimental data with a model proposed by Kitsunezaki which suggests that the variation of the crack spacing with the film thickness depends on the ratio between a critical stress at cracking and a critical stress for slipping on the substrate. By measuring the friction force of the colloidal gels on a hydrophobic surface through a cantilever technique, we can deduce the critical crack stress for these colloidal gels simply by measuring the crack spacing of the pattern.Comment: Accepted in EP

Instability and morphology of polymer solutions coating a fiber

We report an experimental study on the dynamics of a thin film of polymer solution coating a vertical fiber. The liquid film has first a constant thickness and then undergoes the Rayleigh-Plateau instability which leads to the formation of sequences of drops, separated by a thin film, moving down at a constant velocity. Different polymer solutions are used, i.e. xanthan solutions and polyacrylamide (PAAm) solutions. These solutions both exhibit shear-rate dependence of the viscosity, but for PAAm solutions, there are strong normal stresses in addition of the shear-thinning effect. We characterize experimentally and separately the effects of these two non-Newtonian properties on the flow on the fiber. Thus, in the flat film observed before the emergence of the drops, only shear-thinning effect plays a role and tends to thin the film compared to the Newtonian case. The effect of the non-Newtonian rheology on the Rayleigh-Plateau instability is then investigated through the measurements of the growth rate and the wavelength of the instability. Results are in good agreement with linear stability analysis for a shear-thinning fluid. The effect of normal stress can be taken into account by considering an effective surface tension which tends to decrease the growth rate of the instability. Finally, the dependence of the morphology of the drops with the normal stress is investigated and a simplified model including the normal stress within the lubrication approximation provides good quantitative results on the shape of the drops.Comment: Accepted in Journal of Fluid Mechanic

Case-control study of arsenic in drinking water and lung cancer in California and Nevada.

Millions of people are exposed to arsenic in drinking water, which at high concentrations is known to cause lung cancer in humans. At lower concentrations, the risks are unknown. We enrolled 196 lung cancer cases and 359 controls matched on age and gender from western Nevada and Kings County, California in 2002-2005. After adjusting for age, sex, education, smoking and occupational exposures, odds ratios for arsenic concentrations ≥85 µg/L (median = 110 µg/L, mean = 173 µg/L, maximum = 1,460 µg/L) more than 40 years before enrollment were 1.39 (95% CI = 0.55-3.53) in all subjects and 1.61 (95% CI = 0.59-4.38) in smokers. Although odds ratios were greater than 1.0, these increases may have been due to chance given the small number of subjects exposed more than 40 years before enrollment. This study, designed before research in Chile suggested arsenic-related cancer latencies of 40 years or more, illustrates the enormous sample sizes needed to identify arsenic-related health effects in low-exposure countries with mobile populations like the U.S. Nonetheless, our findings suggest that concentrations near 100 µg/L are not associated with markedly high relative risks

Dupont et les modèles systémiques

Les membres du groupe DUPONT ont adopté comme cadre théorique, la théorie des systèmes. Mais surtout, ils ont construit des modèles systémiques. Ils participent à l’essor de modèles qualitatifs, graphiques, conceptuels et d’apprentissage automatique. Ils ont rarement employés les micro-modèles de simulation, et furent plus attirés par les macro-modèles, peu diffusés en géographie.The members of the DUPONT group have adopted the theory of systems. Most importantly, they built systemic models. They participate in the development of qualitative, graphic, conceptual and machine learning models. They rarely used simulation micro-models, and were more attracted by macro-models, which are not widely available in geography

La franc-maçonnerie et l’évasion d’Henri Rochefort

La cause paraît entendue depuis longtemps. De l’avis général, la franc-maçonnerie a joué un rôle actif, sinon déterminant, dans l’évasion réussie du déporté Henri Rochefort et de cinq de ses compagnons, qui ont faussé compagnie à leurs gardiens de l’enceinte fortifiée en mars 1874. Un événement sensationnel eu égard à la célébrité du brillant polémiste. L’auteur se propose de reconsidérer cette affaire en présentant de nouveaux éléments qui vont à l’encontre de la thèse couramment admise. Il s’attache ensuite à comprendre comment et pourquoi s’est forgée une telle certitude, pour rappeler en conclusion quelques règles de bon sens que tout historien sérieux se devrait de respecter.It has always been taken for granted that Freemasonry played a prominent part in the success of Henri Rochefort’s flight, as well as 5 of his fellow-sufferers, as they escaped from a fortified place in march 1874.A thrilling event due to the bright polemist’s fame. The author’s aim is to reconsider the whole matter in the light of new data, which go against the common idea. He, then, pays particular attention in understanding how and why such a conviction may have developed, and, as a conclusion, he recalls some common sense rules, which any serious historian should respect

Influence of humidity on granular packings with moving walls

A significant dependence on the relative humidity H for the apparent mass (Mapp) measured at the bottom of a granular packing inside a vertical tube in relative motion is demonstrated experimentally. While the predictions of Janssen's model are verified for all values of H investigated (25%< H <80%), Mapp increases with time towards a limiting value at high relative humidities (H>60%) but remains constant at lower ones (H=25%). The corresponding Janssen length is nearly independent of the tube velocity for H>60% but decreases markedly for H=25%. Other differences are observed on the motion of individual beads in the packing. For H=25%, they are almost motionless while the mean particle fraction of the packing remains constant; for H>60% the bead motion is much more significant and the mean particle fraction decreases. The dependence of these results on the bead diameter and their interpretation in terms of the influence of capillary forces are discussed.Comment: 6 pages, 6 figure

Liquid film coating a fiber as a model system for the formation of bound states in active dispersive-dissipative nonlinear media

We analyze the coherent-structure interaction and the formation of bound states in active dispersivedissipative nonlinear media using a viscous film coating a vertical fiber as a prototype. The coherent structures in this case are droplike pulses that dominate the evolution of the film.We study experimentally the interaction dynamics and show evidence for formation of bound states. A theoretical explanation is provided through a coherent-structures theory of a simple model for the flow