158 research outputs found
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.Comment: Addendum to: Scheibert J, Leurent S, Prevost A, Debr\'egeas G. The
role of fingerprints in the coding of tactile information probed with a
biomimetic sensor. Science 2009; 323:1503?6 3 pages, 1 figur
Statics and Dynamics of Adhesion between Two Soap Bubbles
9 pagesInternational audienceAn original set-up is used to study the adhesive properties of two hemispherical soap bubbles put into contact. The contact angle at the line connecting the three films is extracted by image analysis of the bubbles profiles. After the initial contact, the angle rapidly reaches a static value slightly larger than the standard \textdegree\ angle expected from Plateau rule. This deviation is consistent with previous experimental and theoretical studies: it can be quantitatively predicted by taking into account the finite size of the Plateau border (the liquid volume trapped at the vertex) in the free energy minimization. The visco-elastic adhesion properties of the bubbles are further explored by measuring the deviation of the contact angle from the static value as the distance between the two bubbles supports is sinusoidally modulated. It is found to linearly increase with where is the radius of the central film and the amplitude of modulation of this length induced by the displacement of the supports. The in-phase and out-of-phase components of with the imposed modulation frequency are systematically probed, which reveals a transition from a viscous to an elastic response of the system with a crossover pulsation of the order rad.s. Independent interfacial rheological measurements, obtained from an oscillating bubble experiment, allow us to develop a model of dynamic adhesion which is confronted to our experimental results. The relevance of such adhesive dynamic properties to the rheology of foams is briefly discussed using a perturbative approach to the Princen 2D model of foams
MĂ©canique du contact rugueux et perception tactile
5 pages, 2 figuresNational audienceDans un contact entre solides rugueux, l'interface constitue la partie la plus déformable. Son comportement mécanique détermine les contraintes s'établissant dans les deux solides, ainsi que la dynamique de frottement. Deux nouvelles méthodes expérimentales, fondées respectivement sur une observation optique directe et sur l'utilisation d'un microcapteur de force MEMS, permettent de sonder la mécanique locale de ces interfaces. Le dispositif MEMS, qui est un analogue trÚs rudimentaire de l'extrémité du doigt humain, nous a permis de proposer un rÎle possible des empreintes digitales dans la transduction de l'information tactile
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 CM's 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 CM's 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 CM's
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.Comment: 11 page
Stick-slip instability for viscous fingering in a gel
The growth dynamics of an air finger injected in a visco-elastic gel (a
PVA/borax aqueous solution) is studied in a linear Hele-Shaw cell. Besides the
standard Saffmann-Taylor instability, we observe - with increasing finger
velocities - the existence of two new regimes: (a) a stick-slip regime for
which the finger tip velocity oscillates between 2 different values, producing
local pinching of the finger at regular intervals, (b) a ``tadpole'' regime
where a fracture-type propagation is observed. A scaling argument is proposed
to interpret the dependence of the stick-slip frequency with the measured
rheological properties of the gel.Comment: 7 pages, 4 figures. Submitted to Europhysics Letter
Deformation and flow of a two-dimensional foam under continuous shear
We investigate the flow properties of a two-dimensional aqueous foam
submitted to a quasistatic shear in a Couette geometry. A strong localization
of the flow (shear banding) at the edge of the moving wall is evidenced,
characterized by an exponential decay of the average tangential velocity.
Moreover, the analysis of the rapid velocity fluctuations reveals self-similar
dynamical structures consisting of clusters of bubbles rolling as rigid bodies.
To relate the instantaneous (elastic) and time-averaged (plastic) components of
the strain, we develop a stochastic model where irreversible rearrangements are
activated by local stress fluctuations originating from the rubbing of the
wall. This model gives a complete description of our observations and is also
consistent with data obtained on granular shear bands by other groups.Comment: 5 pages, 2 figure
Dewetting of thin polymer films near the glass transition
Dewetting of ultra-thin polymer films near the glass transition exhibits
unexpected front morphologies [G. Reiter, Phys. Rev. Lett., 87, 186101 (2001)].
We present here the first theoretical attempt to understand these features,
focusing on the shear-thinning behaviour of these films. We analyse the profile
of the dewetting film, and characterize the time evolution of the dry region
radius, , and of the rim height, . After a transient time
depending on the initial thickness, grows like while
increases like . Different regimes of growth are
expected, depending on the initial film thickness and experimental time range.Comment: 4 pages, 5 figures Revised version, published in Physical Review
Letters: F. Saulnier, E. Raphael and P.-G. de Gennes, Phys. Rev. Lett. 88,
196101 (2002
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