Adhesive contact of elastomers: effective adhesion energy and creep function

For the adhesive contact of elastomers, we propose expressions to quantify the impact of viscoelastic response on effective adhesion energy as a function of contact edge velocity. The expressions we propose are simple analytical functionals of the creep response and should be suitable for experimental data analysis in terms of measured rheologies. We also emphasize the role of the coupling between local stress field at the contact edge and the macroscopic remote loading (far field). We show that the contrast between growing and receding contact originates from the impact of viscoelastic response on coupling, while the separation process at the contact edge is similarly affected by viscoelasticity in both cases.Comment: 17 pages, 7 Figures, 45 references, regular pape

Determination of viscoelastic moduli at a submicrometric scale

From an analysis of the static friction regime of the tip of an atomic force microscope, a method is proposed for determining the viscoelastic modulus of materials which are viscoelastic at the temperature of the experiment. Present experiments consist in displacing the sample in its plane, periodically or at a constant velocity, while measuring the tangential force response of the contact to this shear solicitation. The proposed model permits to determine the relaxation and complex moduli at a sub-micrometric scale. Several experimental confirmations of the description are presented and it is shown that the local moduli measurements compare favorably with macroscopical results

Monocouches de billes de latex ou de silice : utilisation pour la calibration et pour le contrôle des pointes en AFM

We propose the use of latex or silica balls deposited on mica or silicon to control the calibration of the scanners in AFM. A statistical method for the height measurement, based on an histogram analysis is given. Moreover, it is shown that the same samples can be used for tip characterisation.Nous proposons d'utiliser des billes de latex ou de silice déposées sur mica ou sur silicium pour contrôler la calibration des têtes de déplacement en AFM. Une méthode statistique de mesure des hauteurs par histogramme est donnée. De plus, il est montré que les mêmes échantillons peuvent servir à caractériser la qualité de la pointe

POLARIZATION EFFECTS IN XANES OF LAYERED MATERIALS : ALKALI-GRAPHITE INTERCALATION COMPOUNDS STUDY

Les spectres d'absorption des rayons X au voisinage des seuils de composés d'insertion alcalins du graphite sont présentés. Ces matériaux MC8 et MC24 (M = K, Rb, Cs) sont lamellaires et leurs coefficients d'absorption présentent une forte dépendance angulaire. Une analyse par régression linéaire des spectres à chaque énergie pour différents angles d'incidence (θ ≤ 40°) a permis d'extrapoler le cas limite θ = 90°. Pour les composés du potassium, dont la durée de vie du trou en couche profonde est grande, la comparaison avec des structures de bandes publiées met en évidence les effets des règles de sélection.X-ray absorption spectra in the near edge region of alkali graphite intercalation compounds are reported. MC8 and MC24 (M = K, Rb, Cs) compounds are lamellar and their spectra exhibit a high orientation dependence. At each energy, a linear regression analysis of the absorption for different incidence angles (θ ≤ 40°) permits to obtain the limiting case spectrum θ = 90°. In the case of potassium, where the core hole lifetime is large, implying a small broadening of the spectra, comparison is made with published band structures, showing selection rules effects

Measurement of thermal properties of bulk materials and thin films by modulated thermoreflectance (MTR)

International audienceThermal conductivity and thermal diffusivity of materials must be known at high accuracy for thermal engineering applications, in order to understand energy dissipation in devices and engines. Thermal conductivity libraries can be unreliable since their reported values may not reflect the complexities of the samples under investigation, and new materials may not yet be listed. Over the past three decades, modulated thermoreflectance (MTR) has emerged and evolved as a reliable, non-contact, and noninvasive technique to measure the thermal properties of a wide range of bulk and thin film materials and their interfaces. This Tutorial discusses the basis of, and recent advances in, the MTR technique, and its applications in the thermal characterization of a variety of materials. MTR experiments use two micrometer-wide laser beams. Experimental data (amplitude and phase vs the offset between the pump and the probe) give information about heat diffusion around the heat source along several tens of micrometers. Heat diffusion equations are used to fit the experimental data and extract the required thermal properties. Importantly, best fit procedures are not always needed because some analytical approximations provide evidence of the required thermal properties. We present many examples (bulk materials, layered sample, etc.) which illustrate this

Étude EXAFS en polarisation de composés lamellaires

Dans les expériences d’absorption X, on peut mettre à profit le caractère polarisé du rayonnement synchrotron en faisant simplement varier l’orientation de l’échantillon lamellaire par rapport au faisceau incident. L’étude des seuils permet alors d’obtenir des informations sur les structures de bandes et l’EXAFS peut fournir des informations angulaires en plus de l’habituelle fonction de distribution radiale

BOND ANGLE DETERMINATION BY ANGULAR EXAFS STUDY AND DEBYE-WALLER ANISOTROPY IN 2D GRAPHITE INTERCALATION COMPOUNDS

Les spectres d'un échantillon orienté de RbC8 ont été enregistrés pour des angles compris entre O° et 40°. Pour chaque pic de la transformée de Fourier. le rapport des amplitudes permet de calculer l'angle du vecteur [MATH] correspondant par rapport au plan. L'étude en fonction de la température entre 300 K et 35 K indique une anisotropie très importante du facteur de Debye-Waller, les amplitudes de vibration étant très importantes parallèlement au plan. Le second stade RbC24 montre un désordre statique très important par rapport au premier stade RbC8.Spectra of an oriented sample of RbC8 were recorded for a set of angles from 0° up to 40°. For each peak in the Fourier transform, the amplitude ratio permits to calculate the angle of the corresponding [MATH] vector with respect to the plane. The thermal study from 300 K down to 35 K shows a high anisotropy of the Debye-Waller factor, the amplitude of the vibration being larger in the plane. The second stage RbC24 shows a higher static disorder than the first stage RbC8

Modification of the Fluctuation Dynamics of Ultrathin Wetting Films

We report on the effect of intermolecular forces on the fluctuations of supported liquid films. Using an optically-induced thermal gradient, we form nanometer-thin films of wetting liquids on glass substrates, where van der Waals forces are balanced by thermocapillary forces. We show that the fluctuation dynamics of the film interface is strongly modified by intermolecular forces at lower frequencies. Data spanning three frequency decades are in excellent agreement with theoretical predictions accounting for van der Waals forces. Our results emphasize the relevance of intermolecular forces on thermal fluctuations when fluids are confined at the nanoscale.Comment: Accepted for publication in Phys. Rev. Let