Probing the micromechanics of a multi-contact interface at the onset of frictional sliding

Digital Image Correlation is used to study the micromechanics of a multi-contact interface formed between a rough elastomer and a smooth glass surface. The in-plane elastomer deformation is monitored during the incipient sliding regime, i.e. the transition between static and sliding contact. As the shear load is increased, an annular slip region, in coexistence with a central stick region, is found to progressively invade the contact. From the interfacial displacement field, the tangential stress field can be further computed using a numerical inversion procedure. These local mechanical measurements are found to be correctly captured by Cattaneo and Mindlin (CM)s model. However, close comparison reveals significant discrepancies in both the displacements and stress fields that reflect the oversimplifying hypothesis underlying CMs scenario. In particular, our optical measurements allow us to exhibit an elasto-plastic like friction constitutive equation that differs from the rigid-plastic behavior assumed in CMs model. This local constitutive law, which involves a roughness-related length scale, is consistent with the model of Bureau textit{et al.} [Proc. R. Soc. London A textbf{459}, 2787 (2003)] derived for homogeneously loaded macroscopic multi-contact interfaces, thus extending its validity to mesoscopic scales.measurements allow for the first quantitative test of Cattaneo and Mindlin (CM) classical model of the incipient sliding of a smooth interface. Small deviations are observed and interpreted as a result of the finite compliance of the rough interface, a behavior which contrasts with Amontons law of friction assumed to be valid locally in CMs model. We illustrate how these measurements actually provide a method for probing the rheology of the rough interface, which we find to be of the elasto-plastic type.

Effect of fingerprints orientation on skin vibrations during tactile exploration of textured surfaces

In humans, the tactile perception of fine textures is mediated by skin vibrations when scanning the surface with the fingertip. These vibrations are encoded by specific mechanoreceptors, Pacinian corpuscules (PCs), located about 2 mm below the skin surface. In a recent article, we performed experiments using a biomimetic sensor which suggest that fingerprints (epidermal ridges) may play an important role in shaping the subcutaneous stress vibrations in a way which facilitates their processing by the PC channel. Here we further test this hypothesis by directly recording the modulations of the fingerpad/substrate friction force induced by scanning an actual fingertip across a textured surface. When the fingerprints are oriented perpendicular to the scanning direction, the spectrum of these modulations shows a pronounced maximum around the frequency v/lambda, where v is the scanning velocity and lambda the fingerprints period. This simple biomechanical result confirms the relevance of our previous finding for human touch.

Stress Field at a Sliding Frictional Contact: Experiments and Calculations

A MEMS-based sensing device is used to measure the normal and tangential stress fields at the base of a rough elastomer film in contact with a smooth glass cylinder in steady sliding. This geometry allows for a direct comparison between the stress profiles measured along the sliding direction and the predictions of an original textit{exact} bidimensional model of friction. The latter assumes Amontons friction law, which implies that in steady sliding the interfacial tangential stress is equal to the normal stress times a pressure-independent dynamic friction coefficient $mu_d$, but makes no further assumption on the normal stress field. Discrepancy between the measured and calculated profiles is less than 14% over the range of loads explored. Comparison with a test model, based on the classical assumption that the normal stress field is unchanged upon tangential loading, shows that the exact model better reproduces the experimental profiles at high loads. However, significant deviations remain that are not accounted for by either calculations. In that regard, the relevance of two other assumptions made in the calculations, namely (i) the smoothness of the interface and (ii) the pressure-independence of $mu_d$ is briefly discussed.

Experimental evidence of non-Amontons behaviour at a multicontact interface

We report on normal stress field measurements at the multicontact interface between a rough elastomeric film and a smooth glass sphere under normal load, using an original MEMS-based stress sensing device. These measurements are compared to Finite Elements Method calculations with boundary conditions obeying locally Amontons rigid-plastic-like friction law with a uniform friction coefficient. In dry contact conditions, significant deviations are observed which decrease with increasing load. In lubricated conditions, the measured profile recovers almost perfectly the predicted profile. These results are interpreted as a consequence of the finite compliance of the multicontact interface, a mechanism which is not taken into account in Amontons law.