Simulations of viscous shape relaxation in shuffled foam clusters

We simulate the shape relaxation of foam clusters and compare them with the time exponential expected for Newtonian fluid. Using two-dimensional Potts Model simulations, we artificially create holes in a foam cluster and shuffle it by applying shear strain cycles. We reproduce the experimentally observed time exponential relaxation of cavity shapes in the foam as a function of the number of strain steps. The cavity rounding up results from local rearrangement of bubbles, due to the conjunction of both a large applied strain and local bubble wall fluctuations

Etude en rayons X cohérents de la dynamique de suspensions concentrées de sphères dures

Les suspensions colloïdales de particules sphériques présentant des interactions de type sphères dures font partie des systèmes les plus simples et les plus largement étudiés en Matière Molle. Elles peuvent être considérées comme systèmes modèles pour tester des théories plus générales, par exemple en ce qui concerne la cristallisation [1] ou la transition vitreuse [2]. Malgré de nombreux résultats théoriques et expérientaux dans ce domaine, le comportement dynamique des suspensions de sphères dures n'a pas été complètement élucidé.La spectroscopie à corrélation de photons X (XPCS) est une technique de diffusion cohérente équivalente à la Diffusion Quasi-Elastique de la Lumière [3], qui est un des principaux outils d'investigation de la dynamique colloïdale [4]. Comparée à la luière visible, l'utilisation de rayons X procure des rensignements sur les transferts de moment de plus haute énergie, et évite les diffusions multiples - phénomène qui complique sensiblement les études en DQEL pour les échantillons concentrés. De plus, l'utilisation du détecteur 2D compteur de photons (MAXIPIX) disponible sur la ligne ID10 (ESRF) donne des renseignements sur l'évolution de la dynamique de l'échantillon au cours de l'exposition, via les fonctions de corrélation à deux temps.Dans ce travail, nous avons étudié une suspension de spheres colloïdales de PMMA (poly(méthylmétacrylate)) stériquement stabilisées. La distribution en taille des particules et leur concentration ont été obtenues par diffusion de rayons X aux petits angles (SAXS). Les expériences de XPCS effectuées aux plus grandes fractions volumiques en particules (>0.5) mettent en évidence à la fois des temps de diffusion courts et des temps longs autour des pics de Bragg. Une comparaison avec une précédente étude [5] montre, pour une petite gamme de fractions volumiques, une modification drastique de la loi d'échelle entre les temps de relaxation courts et les temps longs qui avait été initialement proposée par Segrè et Pusey [6]. L'analyse des fonctions de corrélation à deux temps révèle un comportement dynamique complexe des échantillons légèrement au-dessus de la transition vitreuse, alors qu'on n'observe aucun signe de modifications structurales via diffusion statique. Utiliser la XPCS sur des suspensions en écoulement dans des canaux cylindriques avait fait ses preuves pour renseigner à la fois sur les propriétés dynamiques et d'écoulement de suspensions diluées [7]. Ici, nous discutons les potentialités et les limites de cette méthode, en étudiant l'interaction entre les propriétés rhéologiques et dynamiques dans ces systèmes complexes modèles que sont les verres colloïdaux.[1] P. N. Pusey and W. van Megen. In: Nature 320.6060 (Mar. 1986), pp. 340 342 [2] P. N. Pusey and W. van Megen. In: Phys. Rev. Lett. 59 (18 1987), pp. 2083 2086.[3] V. A. Martinez et al. In: The Journal of Chemical Physics 134.5, 054505 (2011), p. 054505.[4] B. J. Berne and R. Pecora. Dynamic Light Scattering with application to chemistry, biology and physics. Dover Publications, New York, 2000. [5] D. Orsi et al. Dynamics in dense hard-sphere colloidal suspensions . In: Phys. Rev. E 85 (1 2012), p. 011402. doi: 10.1103/PhysRevE.85.011402. url: http://link.aps.org/doi/1 0.1103/PhysRevE.85.011402. [6] P. N. Segrè and P. N. Pusey. In: Phys. Rev. Lett. 77.4 (1996), pp. 771 774.[7] A. Fluerasu et al. In: New Journal of Physics 12.3 (2010)Colloidal suspensions of spherical particles presenting hard-sphere like interactions is one of the simplest and most widely studied systems of soft condensed matter. They can be treated as a model for testing fundamental theories, regarding e.g. crystallization [1] or glass transition [2]. Despite the long history of both theoretical and experimental research, the dynamic behavior of hard sphere suspensions still lacks a complete understanding.X-ray Photon Correlation Spectroscopy (XPCS) is a coherent scattering technique equivalent to Dynamic Light Scattering (DLS) [3], which is one of the main tools used in the study of colloidal dynamics [4]. Comparing to visible light, the use of X-rays provides access to higher momentum transfer vector values and allows to avoid multiple scattering a phenomena significantly complicating DLS measurements on concentrated samples. Moreover, the use of a fast, single photon counting area detector (MAXIPIX) available at the ID10 beamline at ESRF gives insight into the evolution of sample dynamics during the measurement time by the means of two-time correlation functions.In this work suspensions of sterically stabilized poly(methyl methacrylate) (PMMA) colloidal spheres were used. Particle size, polydispersity and volume fractions of the samples were obtained using the Small-Angle X-ray Scattering (SAXS) technique. XPCS measurements at high volume fractions (>0.5) show both short- and long-time diffusive behaviour for scattering vector values around, but not restricted to the structure factor peak position. A comparison with an earlier study [5] shows a dramatic change in the approximate scaling between the short- and long-time relaxation rates, initially proposed by Segrè and Pusey in [6], over a small range of volume fractions. The analysis of two-time correlation functions reveals complex dynamic behaviour of a sample slightly above the glass transition, while no signs of structural changes are observed in the static scattering patterns. The studies indicate the dynamics being governed by a jamming transition driven by restrictions in free volume rather than a glass transition as know from the mode-coupling theory. A combination of XPCS with flow in a cylindrical channel has demonstrated previously to give both dynamic and flow properties of dilute suspensions [7]. Here we discuss the potential and limitations of this method in the study of the interplay between rheological properties and dynamics in complex systems such as colloidal glasses. [1] P. N. Pusey and W. van Megen. In: Nature 320.6060 (Mar. 1986), pp. 340 342[2] P. N. Pusey and W. van Megen. In: Phys. Rev. Lett. 59 (18 1987), pp. 2083 2086.[3] V. A. Martinez et al. In: The Journal of Chemical Physics 134.5, 054505 (2011), p. 054505.[4] B. J. Berne and R. Pecora. Dynamic Light Scattering with application to chemistry, biology and physics. Dover Publications, New York, 2000.[5] D. Orsi et al. Dynamics in dense hard-sphere colloidal suspensions . In: Phys. Rev. E 85 (2012), p. 011402.[6] P. N. Segrè and P. N. Pusey. In: Phys. Rev. Lett. 77.4 (1996), pp. 771 774.[7] A. Fluerasu et al. In: New Journal of Physics 12.3 (2010)SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Statistical Mechanics of Two-dimensional Foams

The methods of statistical mechanics are applied to two-dimensional foams under macroscopic agitation. A new variable -- the total cell curvature -- is introduced, which plays the role of energy in conventional statistical thermodynamics. The probability distribution of the number of sides for a cell of given area is derived. This expression allows to correlate the distribution of sides ("topological disorder") to the distribution of sizes ("geometrical disorder") in a foam. The model predictions agree well with available experimental data

Volume-controlled buckling of thin elastic shells: Application to crusts formed on evaporating partially-wetted droplets

Motivated by the buckling of glassy crusts formed on evaporating droplets of polymer and colloid solutions, we numerically model the deformation and buckling of spherical elastic caps controlled by varying the volume between the shell and the substrate. This volume constraint mimics the incompressibility of the unevaporated solvent. Discontinuous buckling is found to occur for sufficiently thin and/or large contact angle shells, and robustly takes the form of a single circular region near the boundary that `snaps' to an inverted shape, in contrast to externally pressurised shells. Scaling theory for shallow shells is shown to well approximate the critical buckling volume, the subsequent enlargement of the inverted region and the contact line force.Comment: 7 pages in J. Phys. Cond. Mat. spec; 4 figs (2 low-quality to reach LANL's over-restrictive size limits; ask for high-detailed versions if required

Two-dimensional flow of foam around an obstacle: force measurements

A Stokes experiment for foams is proposed. It consists in a two-dimensional flow of a foam, confined between a water subphase and a top plate, around a fixed circular obstacle. We present systematic measurements of the drag exerted by the flowing foam on the obstacle, \emph{versus} various separately controlled parameters: flow rate, bubble volume, bulk viscosity, obstacle size, shape and boundary conditions. We separate the drag into two contributions, an elastic one (yield drag) at vanishing flow rate, and a fluid one (viscous coefficient) increasing with flow rate. We quantify the influence of each control parameter on the drag. The results exhibit in particular a power-law dependence of the drag as a function of the bulk viscosity and the flow rate with two different exponents. Moreover, we show that the drag decreases with bubble size, and increases proportionally to the obstacle size. We quantify the effect of shape through a dimensioned drag coefficient, and we show that the effect of boundary conditions is small.Comment: 26 pages, 13 figures, resubmitted version to Phys. Rev.

Anisotropic colloids through non-trivial buckling

We present a study on buckling of colloidal particles, including experimental, theoretical and numerical developments. Oil-filled thin shells prepared by emulsion templating show buckling in mixtures of water and ethanol, due to dissolution of the core in the external medium. This leads to conformations with a single depression, either axisymmetric or polygonal depending on the geometrical features of the shells. These conformations could be theoretically and/or numerically reproduced in a model of homogeneous spherical thin shells with bending and stretching elasticity, submitted to an isotropic external pressure.Comment: submitted to EPJ

Modifying effect of dual antiplatelet therapy on incidence of stent thrombosis according to implanted drug-eluting stent type

Aim To investigate the putative modifying effect of dual antiplatelet therapy (DAPT) use on the incidence of stent thrombosis at 3 years in patients randomized to Endeavor zotarolimus-eluting stent (E-ZES) or Cypher sirolimus-eluting stent (C-SES). Methods and results Of 8709 patients in PROTECT, 4357 were randomized to E-ZES and 4352 to C-SES. Aspirin was to be given indefinitely, and clopidogrel/ticlopidine for ≥3 months or up to 12 months after implantation. Main outcome measures were definite or probable stent thrombosis at 3 years. Multivariable Cox regression analysis was applied, with stent type, DAPT, and their interaction as the main outcome determinants. Dual antiplatelet therapy adherence remained the same in the E-ZES and C-SES groups (79.6% at 1 year, 32.8% at 2 years, and 21.6% at 3 years). We observed a statistically significant (P = 0.0052) heterogeneity in treatment effect of stent type in relation to DAPT. In the absence of DAPT, stent thrombosis was lower with E-ZES vs. C-SES (adjusted hazard ratio 0.38, 95% confidence interval 0.19, 0.75; P = 0.0056). In the presence of DAPT, no difference was found (1.18; 0.79, 1.77; P = 0.43). Conclusion A strong interaction was observed between drug-eluting stent type and DAPT use, most likely prompted by the vascular healing response induced by the implanted DES system. These results suggest that the incidence of stent thrombosis in DES trials should not be evaluated independently of DAPT use, and the optimal duration of DAPT will likely depend upon stent type (Clinicaltrials.gov number NCT00476957

Retrovirus-mediated gene transfer corrects DNA repair defect of xeroderma pigmentosum cells of complementation groups A, B and C

International audienceWith the aim to devise a long-term gene therapy protocol for skin cancers in individuals affected by the inherited autosomal recessive xeroderma pigmentosum we transferred the human DNA repair XPA, XPB/ERCC3 and XPC cDNAs, by using the recombinant retroviral vector LXSN, into primary and immortalized fibroblasts obtained from two XP-A, one XP-B (associated with Cockayne's syndrome) and two XP-C patients. After transduction, the complete correction of DNA repair deficiency and functional expression of the transgenes were monitored by UV survival, unscheduled DNA synthesis and recovery of RNA synthesis, and Western blots. The results show that the recombinant retroviruses are highly efficient vectors to transfer and stably express the human DNA repair genes in XP cells and correct the defect of DNA repair of group A, B and C. With our previous results with XPD/ERCC2, the present work extends further promising issues for the gene therapy strategy for most patients suffering from this cancer-prone syndrome