We fluidize a granular medium through localized stirring and probe the mechanical response of quiescent regions far away from the main flow. In these regions the material behaves like a liquid: high-density probes sink, low-density probes float at the depth given by Archimedes' law, and drag forces on moving probes scale linearly with the velocity. The fluidlike character of the material is set by agitations generated in the stirred region, suggesting a nonlocal rheology: the relation between applied stress and observed strain rate in one location depends on the strain rate in another location.

We report experiments on the deformation and transport of an elastic fiber in a viscous cellular flow, namely a lattice of counter-rotative vortices. We show that the fiber can buckle when approaching a stagnation point. By tuning either the flow or fiber properties, we measure the onset of this buckling instability. The buckling threshold is determined by the relative intensity of viscous and elastic forces, the elasto-viscous number Sp. Moreover we show that flexible fibers escape faster from a vortex (formed by closed streamlines) compared to rigid fibers. As a consequence, the deformation of the fiber changes its transport properties in the cellular flow.PACS numbers: * present adress: Kamerlingh Onnes Lab, Universiteit Leiden, Postbus 9504,

X-ray Photon Correlation Spectroscopy is used here to probe for the first time a glass-forming dispersion of repulsive particles spherical and 10 nm sized. The highly concentrated colloidal system is based on the magnetic and charge-stabilized nanoparticles of a ferrofluid. Their out-of-equilibrium translational dynamics capture on nanoscale generic features usually observed on microscale: namely i) stretched exponential α relaxation with an exponent around 1.5 and ii) age-dependent characteristic time τ evolving from exponential at young ages to a slower behavior at large ages. τ is here scaling as Q −1 in the whole experimental Q-range, below, on and above the scale of the interparticle distance, the driving force for aging in this glass-forming nanosystem being associated to larger scale rearrangements.

We probe the three dimensional flow structure and rheology of gravitational (non-density matched) suspensions for a range of driving rates in a split-bottom geometry. We establish that for sufficiently slow flows, the suspension flows as if it were a dry granular medium, and confirm recent theoretical modelling on the rheology of split-bottom flows. For faster driving, the flow behavior is shown to be consistent with the rheological behavior predicted by the recently developed "inertial number" approaches for suspension flows.PACS numbers: 83.80. Fg, 82.70.Kj, 47.57.Gc Flows of granular materials submersed in a liquid of unequal density have started to attract considerable attention [1][2][3][4][5] and are relevant in many practical applications [6]. These materials, which we will refer to as "gravitational" suspensions, clearly differ from density matched suspensions, which have been studied in great detail [7][8][9][10]. Gravitational suspensions exhibit sedimentation, large packing fractions and jamming of the material, which suggests a description similar to dry granular matter [11,12].In the last two decades, various flow regimes have been identified for dry granular matter. Sufficiently slow flows are frictional: the ratio of shear (driving) to normal (confining) stresses becomes independent of flow rate if the material is allowed to dilate [12,13]. Faster flows are referred to as inertial: here the effective friction coefficient µ depends on the so-called "inertial" number I, which is a non-dimensional measure of the local flow rate [12,14,15].For gravitational suspensions, the presence of liquid instead of gas as interstitial medium strongly affects the microscopic picture -how should we think of the flow of such suspensions? Pouliquen and coworkers proposed that the ratio of the strain rate and settling time, I S , would play a similar role as the inertial number in dry granular flows [5]. They furthermore conjectured a dependence of the effective friction coefficient µ on I S similar to the dry case, and applied this rheological law to capture the behavior of underwater avalanches [16].Here we test this picture by combining 3D imaging and rheological measurements of the flow of gravitational suspensions in a so-called split-bottom geometry (Fig. 1). This geometry has two main advantages. First, the flow rate, which is the key control parameter in the inertial number framework, can be varied over several orders of magnitude, allowing us to access slow flows as seen in plane shear [3,17], faster flows as seen in gravity driven flows [5,18], and the crossover regime in between -some- thing not achieved in previous studies of gravitational suspensions [3,5,17,18]. Second, extensive experimental and numerical work [19][20][21][22][23][24] has shown that the split bottom geometry produces highly nontrivial slow dry granular flows. A simple frictional picture is not sufficient to capture these flows [25,26], so that testing whether these profiles also arise in slowly sheared gravitational suspensions is a ...

Dry solid friction is often accompanied by force modulations originating from stick-slip instabilities. Here a distinct, quasi-static mechanism is evidenced leading to quasi-periodic force oscillations during sliding contact between an elastomer block, whose surface is patterned with parallel grooves, and finely abraded glass slides. The dominant oscillation frequency is set by the ratio between the sliding velocity and the period of the grooves. A mechanical model is proposed that provides a quantitative prediction for the amplitude of the force modulations as a function of the normal load, the period of the grooves and the roughness characteristics of the substrate. The model's main ingredient is the non-linearity of the friction law. Since such non-linearity is ubiquitous for soft solids, this "fingerprint effect" should be relevant to a large class of frictional configurations and might in particular have important consequences in human (or humanoid) active digital touch.

We report on normal contact and friction measurements of model multicontact interfaces formed between smooth surfaces and substrates textured with a statistical distribution of spherical microasperities. Contacts are either formed between a rigid textured lens and a smooth rubber, or a flat textured rubber and a smooth rigid lens. Measurements of the real area of contact A versus normal load P are performed by imaging the light transmitted at the microcontacts. For both interfaces, A(P ) is found to be sub-linear with a power law behavior. Comparison to two multi-asperity contact models, which extend Greenwood-Williamson (J. Greenwood, J. Williamson, Proc. Royal Soc. London Ser. A 295, 300 (1966)) model by taking into account the elastic interaction between asperities at different length scales, is performed, and allows their validation for the first time. We find that long range elastic interactions arising from the curvature of the nominal surfaces are the main source of the non-linearity of A(P ). At a shorter range, and except for very low pressures, the pressure dependence of both density and area of micro-contacts remains well described by Greenwood-Williamson's model, which neglects any interaction between asperities. In addition, in steady sliding, friction measurements reveal that the mean shear stress at the scale of the asperities is systematically larger than that found for a macroscopic contact between a smooth lens and a rubber. This suggests that frictional stresses measured at macroscopic length scales may not be simply transposed to microscopic multicontact interfaces.

We report on the multi-contact frictional dynamics of model elastomer surfaces rubbed against bare glass slides. The surfaces consist of layers patterned with thousands spherical caps (radius of curvature 100 µm) distributed both spatially and in height, regularly or randomly. Use of spherical asperities yields circular micro-contacts whose radius is a direct measure of the contact pressure distribution. In addition, optical tracking of individual contacts provides the in-plane deformations of the tangentially loaded interface, yielding the shear force distribution. We then investigate the stick-slip frictional dynamics of a regular hexagonal array. For all stick phases including the initial one, slip precursors are evidenced. They are found to propagate quasi-statically, normally to the isopressure contours. A simple quasi-static model relying on the existence of interfacial stress gradients is derived and predicts qualitatively the position of slip precursors.PACS numbers: 46.55.+d, 68.35.Ct, 81.40.Pq In recent years, our understanding of the transition from static to dynamic friction has been markedly changed with the development of new imaging techniques to probe spatially the interfacial dynamics at the onset of sliding [1][2][3][4]. Slip phases were found to involve the propagation of a series of dynamical rupture fronts, far from the classical Amontons-Coulomb's picture. Using true contact area imaging with evanescent illumination of a 1D Plexiglas-Plexiglas plane contact, Fineberg and coauthors [1] measured in particular slow fronts with velocities orders of magnitude lower than the Rayleigh wave velocity, along with sub-Rayleigh and fast intersonic fronts. Slow fronts were also reported to propagate at soft elastomer-roughened glass spherical 2D contacts [5] by tracking optically markers positioned on the surface of the elastomer. Brörman et al.[6] extended such studies to micro-structured elastomer surfaces in the form of hexagonal arrays of cylindrical micro-pillars in contact with glass slides, and found again a similar phenomenology. During stick phases, slow slip precursors were also observed well before macroscopic slippage occurs [2]. In all these experiments, a single physical quantity is measured, either the real area of contact directly related to the local normal stress, or the local interfacial stress using displacement measurements. In a recent work [7], Ben-David and Fineberg provided both types of measurement in a system treated as a 1D interface. Using an array of strain gauges sensors distributed directly above the interfacial plane, these authors reported strong correlations between the characteristics of the fronts and the ratio of the measured tangential to normal local stresses. For an extended 2D contact, simultaneous measurements of both pressure and tangential interfacial fields is still lacking and out of reach using Ben-David and Fineberg's approach. In addition, it remains unclear what physical mechanism underlies the existence of slip precursors in the stick phase and thei...

We present here experimental results obtained by time resolved x-ray correlation spectroscopy, revealing the heterogeneous nature of the dynamics down to the nanoscale in a dense dispersion of magnetic nanoparticles. The dynamical susceptibility, which quantifies the amplitude of the dynamical fluctuations, is investigated in a repulsive colloidal glass. Its value is an order of magnitude smaller than the ones found in gels and is length scale independent in the Q-range of the x-ray scattering experiment.