Elastic contact to nearly incompressible coatings -- Stiffness enhancement and elastic pile-up

We have recently proposed an efficient computation method for the frictionless linear elastic axisymmetric contact of coated bodies [A. Perriot and E. Barthel, J. Mat. Res. 19 (2004) 600]. Here we give a brief description of the approach. We also discuss implications of the results for the instrumented indentation data analysis of coated materials. Emphasis is laid on incompressible or nearly incompressible materials (Poisson ratio $\nu>0.4$): we show that the contact stiffness rises much more steeply with contact radius than for more compressible materials and significant elastic pile-up is evidenced. In addition the dependence of the penetration upon contact radius increasingly deviates from the homogeneous reference case when the Poisson ratio increases. As a result, this algorithm may be helpful in instrumented indentation data analysis on soft and nearly incompressible layers

Surface Fluctuations of an Aging Colloidal Suspension: Evidence for Intermittent Quakes

We present measurements of the thermal fluctuations of the free surface of an aging colloidal suspension, Laponite. The technique consists in measuring the fluctuations of the position of a laser beam that reflects from the free surface. Analysing the data statistics, we show that, as the fluid ages, the dynamics becomes intermittent. The intermittent events correspond to large changes in the local slope of the free surface over a few milliseconds. We show that those quakes are uncorrelated, although they are kept in memory by the surface over short time scales

Contact AFM on soft surfaces: Elasticity and friction effects

We present contact AFM images of an annealed film of latex balls of 5° glass transition temperature. Surface appears to be constituted of an hexagonal arrangement of flatten spheres. Contrast is seen to depend very much on the applied load and on the scan direction: spheres appear concave or convex according to these parameters. An analysis of the friction clearly shows that there are two regimes for imaging. For high magnifications, the tip is deeply intruded in the material and images do not reflect the topography of the surface but seem to be characteristic of its mechanical properties. For higher scan domains, friction is of classical type and resolution is good. These results permit to clarify the limitations of the technique applied on soft materials.Nous montrons des images obtenues en AFM de contact d'un film recuit de billes de latex de 5° de température de transition vitreuse. La surface présente un arrangement hexagonal de sphères aplaties. Le contraste dépend fortement de la force appliquée et de la direction de balayage : les sphères apparaissent soit concaves soit convexes. Une analyse des frottements montre distinctement deux régimes de fonctionnement du microscope. Pour de forts grossissements, la pointe est profondément enfoncée dans le matériau et les images ne reflètent pas la topographie de la surface, mais semblent caractéristiques de ses propriétés mécaniques. Pour de plus grands domaines de balayage, le frottement est de type classique et la résolution est bonne. Ces résultats permettent de préciser les limitations de la technique pour des matériaux mous

Friction of viscoelastic elastomers with rough surfaces under torsional contact conditions

International audienceFrictional properties of contacts between a smooth viscoelastic rubber and rigid surfaces are investigated using a torsional contact configuration where a glass lens is continuously rotated on the rubber surface. From the inversion of the displacement field measured at the surface of the rubber, spatially resolved values of the steady state frictional shear stress are determined within the nonhomogeneous pressure and velocity fields of the contact. For contacts with a smooth lens, a velocity-dependent but pressure-independent local shear stress is retrieved from the inversion. On the other hand, the local shear stress is found to depend on both velocity and applied contact pressure when a randomly rough (sand-blasted) glass lens is rubbed against the rubber surface. As a result of changes in the density of microasperity contacts, the amount of light transmitted by the transparent multicontact interface is observed to vary locally as a function of both contact pressure and sliding velocity. Under the assumption that the intensity of light transmitted by the rough interface is proportional to the proportion of area into contact, it is found that the local frictional stress can be expressed experimentally as the product of a purely velocity-dependent term, k(upsilon), by a term representing the pressure and velocity dependence of the actual contact area, A/A(0). A comparison between k(upsilon) and the frictional shear stress of smooth contacts suggests that nanometer scale dissipative processes occurring at the interface predominate over viscoelastic dissipation at microasperity scale

20th international electric propulsion conference Proceedings

SIGLECopy held by FIZ Karlsruhe; available from UB/TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Wrinkling of a nanometric glassy skin/crust induced by drying in poly(vinyl alcohol) gels

International audienceDuring drying of a chemically crosslinked poly(vinyl alcohol) gel, we observed, by atomic force microscopy (AFM) and scanning electronic microscopy, micron-order wrinkling which remains until the end of drying. We show that while at high humidity drying is homogeneous and no surface instability is observed, fast drying at low humidity induces the formation of a glassy skin layer on the soft bulk that wrinkles because the skin is compressed as the bulk contracts. The role of the glass transition is not only to create a mismatch of the elasticity with the bulk but also to remove the in-plane tensile tension which stretches the skin before drying. We proved experimentally the presence of this thin glassy skin on the wet bulk by approach–retract curve measurements in AFM. We also comment on the effect of mechanical restraint and the observation of hierarchical wrinkling over different length scales

Columnar aggregates of crown ether substituted phthalocyanines perpendicularly anchored on a surface via a selective binding site

International audienc