State, rate and temperature-dependent sliding friction of elastomers

We present an experimental investigation of the non stationary frictional properties of multicontact interfaces between rough elastomers and rough hard glass at low velocities (<= 200 mu m s^{-1}). These systems, for which the deformation contribution to friction is negligible, are shown to exhibit a phenomenology which is similar to what is observed for non elastomeric materials in the same multi-contact configuration, and which are quantitatively described by the state- and rate-dependent friction laws. This permits to identify clearly the two contributions to adhesive friction which are mixed in steady sliding: the interfacial shear stress which appears as thermally activated formation and breaking of molecular bonds, and the real area of contact which evolves through viscoelastic creep of the load bearing asperities

Self-healing slip pulses and the friction of gelatin gels

We present an extensive experimental study and scaling analysis of friction of gelatin gels on glass. At low driving velocities, sliding occurs via propagation of periodic self-healing slip pulses whose velocity is limited by collective diffusion of the gel network. Healing can be attributed to a frictional instability occurring at the slip velocity $v = V_c$. For $v > V_c$, sliding is homogeneous and friction is ruled by the shear-thinning rheology of an interfacial layer of thickness of order the (nanometric) mesh size, containing a semi-dilute solution of polymer chain ends hanging from the network. Inspite of its high degree of confinement, the rheology of this system does not differ qualitatively from known bulk ones. The observed ageing of the static friction threshold reveals the slow increase of adhesive bonding between chain ends and glass. Such structural ageing is compatible with the existence of a velocity-weakening regime at velocities smaller than $V_c$, hence with the existence of the healing instability.Comment: 9 pages, 16 figure

Lossless and Lossy Minimal Redundancy Pyramidal Decomposition for Scalable Image Compression Technique

International audienceWe present a new scalable compression technique dealing simultaneously with both lossy and lossless image coding. An original DPCM scheme with refined context is introduced through a pyramidal decomposition adapted to the LAR (Locally Adaptive Resolution) method, which becomes by this way fully progressive. An implicit context modeling of the prediction errors, due to the low resolution image representation including variable block size structure, is then exploited to the for lossless compression purpose

Role of the interplay between spinodal decomposition and crystal growth in the morphological evolution of crystalline bulk heterojunctions

The stability of organic solar cells is strongly affected by the morphology of the photoactive layers, whose separated crystalline and/or amorphous phases are kinetically quenched far from their thermodynamic equilibrium during the production process. The evolution of these structures during the lifetime of the cell remains poorly understood. In this paper, a phase-field simulation framework is proposed, handling liquid-liquid demixing and polycrystalline growth at the same time in order to investigate the evolution of crystalline immiscible binary systems. We find that initially, the nuclei trigger the spinodal decomposition, while the growing crystals quench the phase coarsening in the amorphous mixture. Conversely, the separated liquid phases guide the crystal growth along the domains of high concentration. It is also demonstrated that with a higher crystallization rate, in the final morphology, single crystals are more structured and form percolating pathways for each material with smaller lateral dimensions

Interleaved S+P Pyramidal Decomposition with Refined Prediction Model

International audienceScalability and others functionalities such as the Region of Interest encoding become essential properties of an efficient image coding scheme. Within the framework of lossless compression techniques, S+P and CALIC represent the state-of-the-art. The proposed Interleaved S+P algorithm outperforms these method while providing the desired properties. Based on the LAR (Locally Adaptive Resolution) method, an original pyramidal decomposition combined with a DPCM scheme is elaborated. This solution uses the S-transform in such a manner that a refined prediction context is available for each estimation steps. The image coding is done in two main steps, so that the first one supplies a LAR low-resolution image of good visual quality, and the second one allows a lossless reconstruction. The method exploits an implicit context modelling, intrinsic property of our content-based quad-tree like representation

Crystalline Morphology Formation in Phase-Field Simulations of Binary Mixtures

Understanding the morphology formation process of solution-cast photoactive layers (PALs) is crucial to derive design rules for optimized and reliable third generation solar cell fabrication. For this purpose, a Phase-Field (PF) computational framework dedicated to the simulation of PAL processing has recently been developed. In this study focused on non-evaporating, crystallizing binary mixtures, distinct crystalline morphology formation pathways are characterized by a systematic exploration of the model's parameter space. It is identified how, depending on material properties, regular, dilution-enhanced, diffusion-limited and demixing-assisted crystallization can take place, and which associated structures then arise. A comprehensive description of the thermodynamic and kinetic mechanisms that respectively drive these separate crystallization modes is provided. Finally, comparisons with experimental results reported in the literature highlight the promising potential of PF simulations to support the determination of process-structure relationships for improved PAL production

Généralisation du JPEG standard à travers la méthode LAR (Locally Adaptive Resolution)

Cet article présente un nouveau schéma de codage pour les images fixes, combinant à la fois un codeur spatial, et un codeur spectral. Le premier est basé sur une nouvelle forme de sous-échantillonnage non uniforme simple, adaptant la résolution locale à l'activité dans l'image, et ayant pour effet d'éliminer la texture locale. Il peut être utilisé seul pour atteindre de fort taux de compression. Afin d'améliorer la qualité, l'image d'erreur peut être transmise à travers un second décodeur basé JPEG, mais pour lequel la taille des blocs est variable, et fournie par le premier codeur

Décomposition pyramidale a redondance minmale pour compression d'images sans perte

- Afin de répondre au besoin de progressivité des méthodes de compression, une nouvelle méthode de décomposition pyramidale associée à l'utilisation d'une forme originale de MICD à contexte enrichi a été mise en oeuvre. La méthode LAR (Locally Adaptive Resolution) pour du codage avec pertes a donc été adaptée à cet effet. L'image d'erreur résultante est ensuite codée sans perte par prédiction

Secured and progressive transmission of compressed images on the Internet: application to telemedicine

International audienceWithin the framework of telemedicine, the amount of images leads first to use efficient lossless compression methods for the aim of storing information. Furthermore, multiresolution scheme including Region of Interest (ROI) processing is an important feature for a remote access to medical images. What is more, the securization of sensitive data (e.g. metadata from DICOM images) constitutes one more expected functionality: indeed the lost of IP packets could have tragic effects on a given diagnosis. For this purpose, we present in this paper an original scalable image compression technique (LAR method) used in association with a channel coding method based on the Mojette Transform, so that a hierarchical priority encoding system is elaborated. This system provides a solution for secured transmission of medical images through low-bandwidth networks such as the Internet

Phase-field simulation of liquid-vapor equilibrium and evaporation of fluid mixtures

In solution-processing of thin films, the material layer is deposited from a solution composed of several solutes and solvents. The final morphology and hence the properties of the film often depend on the time needed for the evaporation of the solvents. This is typically the case for organic photoactive or electronic layers. Therefore, it is important to be able to predict the evaporation kinetics of such mixtures. We propose here a new phase-field model for the simulation of evaporating fluid mixtures and simulate their evaporation kinetics. Similar to the Hertz-Knudsen theory, the local liquid-vapor equilibrium is assumed to be reached at the film surface and evaporation is driven by diffusion away from this gas layer. In the situation where the evaporation is purely driven by the liquid-vapor equilibrium, the simulations match the behavior expected theoretically from the free energy: for evaporation of pure solvents, the evaporation rate is constant and proportional to the vaporpressure. For mixtures, the evaporation rate is in general strongly time-dependent because of the changing composition of the film. Nevertheless, for highly non-ideal mixtures, such as poorly compatible fluids or polymer solutions, the evaporation rate becomes almost constant in the limit of low Biot numbers. The results of the simulation have been successfully compared to experiments on a polystyrene-toluene mixture. The model allows to take into account deformations of the liquid-vapor interface and therefore to simulate film roughness or dewetting