Jean-Christophe Géminard
École normale supérieure de Lyon
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Featured researches published by Jean-Christophe Géminard.
Physical Review E | 1999
Jean-Christophe Géminard; Wolfgang Losert; Jerry P. Gollub
The mechanical response of a wet granular layer to imposed shear is studied experimentally at low applied normal stress. The granular material is immersed in water and the shear is applied by sliding a plate resting on the upper surface of the layer. We monitor simultaneously the horizontal and the vertical displacements of the plate to submicron accuracy with millisecond time resolution. The relations between the plate displacement, the dilation of the layer and the measured frictional force are analyzed in detail. When slip begins, the dilation increases exponentially over a slip distance comparable to the particle radius. We find that the total dilation and the steady state frictional force do not depend on the driving velocity, but do depend linearly on the applied normal stress. The frictional force also depends linearly on the dilation rate (rather than the dilation itself), and reaches a maximum value during the transient acceleration. We find that the layer can temporarily sustain a shear stress that is in excess of the critical value that will eventually lead to slip. We describe an empirical model that describes much of what we observe. This model differs in some respects from those used previously at stresses 10(6) times larger.
Physical Review E | 2000
Wolfgang Losert; Jean-Christophe Géminard; S. Nasuno; Jerry P. Gollub
Several mechanisms cause a granular material to strengthen over time at low applied stress. The strength is determined from the maximum frictional force F(max) experienced by a shearing plate in contact with wet or dry granular material after the layer has been at rest for a waiting time tau. The layer strength increases roughly logarithmically with tau only if a shear stress is applied during the waiting time. The mechanisms of strengthening are investigated by sensitive displacement measurements, and by imaging of particle motion in the shear zone. Granular matter can strengthen due to a slow shift in the particle arrangement under shear stress. Humidity also leads to strengthening, but is found not to be its sole cause. In addition to these time dependent effects, the static friction coefficient can also be increased by compaction of the granular material under some circumstances, and by a cycling of the applied shear stress.
Physical Review Letters | 2008
Thibaut Divoux; Hervé Gayvallet; Jean-Christophe Géminard
We report a time-resolved study of the dynamics associated with the slow compaction of a granular column submitted to thermal cycles. The column height displays a complex behavior: for a large amplitude of the temperature cycles, the granular column settles continuously, experiencing a small settling at each cycle. By contrast, for a small-enough amplitude, the column exhibits a discontinuous and intermittent activity: successive collapses are separated by quiescent periods whose duration is exponentially distributed. We then discuss potential mechanisms which would account for both the compaction and the transition at finite amplitude.
Molecular Biology of the Cell | 2011
Shiqiong Hu; Emmanuelle Planus; Dan Georgess; Christophe Place; Xianghui Wang; Corinne Albiges-Rizo; Pierre Jurdic; Jean-Christophe Géminard
Podosomes are dynamic, actin-containing adhesion structures that self-organize as rings in osteoclasts. We observe that podosome rings generate tension forces and that osteoclasts migrate by periodically moving toward the actin-containing structures. Actin dynamics thus not only correlate with the cell migration, but drive it.
Physical Review Letters | 2010
M. A. Aguirre; Juan Gabriel Grande; A. Calvo; Luis A. Pugnaloni; Jean-Christophe Géminard
We experimentally demonstrate that the flow rate of granular material through an aperture is controlled by the exit velocity imposed on the particles and not by the pressure at the base, contrary to what is often assumed in previous work. This result is achieved by studying the discharge process of a dense packing of monosized disks through an orifice. The flow is driven by a conveyor belt. This two-dimensional horizontal setup allows us to independently control the velocity at which the disks escape the horizontal silo and the pressure in the vicinity of the aperture. The flow rate is found to be proportional to the belt velocity, independent of the amount of disks in the container and, thus, independent of the pressure in the outlet region. In addition, this specific configuration makes it possible to get information on the system dynamics from a single image of the disks that rest on the conveyor belt after the discharge.
Physical Review E | 2009
Thibaut Divoux; Eric Bertin; Valérie Vidal; Jean-Christophe Géminard
We report an experimental study of the intermittent dynamics of a gas flowing through a column of a non-Newtonian fluid. In a given range of the imposed constant flow rate, the system spontaneously alternates between two regimes: bubbles emitted at the bottom either rise independently one from the other or merge to create a winding flue which then connects the bottom air entrance to the free surface. The observations are reminiscent of the spontaneous changes in the degassing regime observed on volcanoes and suggest that, in the nature, such a phenomenon is likely to be governed by the non-Newtonian properties of the magma. We focus on the statistical distribution of the lifespans of the bubbling and flue regimes in the intermittent steady state. The bubbling regime exhibits a characteristic time whereas, interestingly, the flue lifespan displays a decaying power-law distribution. The associated exponent, which is significantly smaller than the value 1.5 often reported experimentally and predicted in some standard intermittency scenarios, depends on the fluid properties and can be interpreted as the ratio of two characteristic times of the system.
Physical Review E | 2009
Germán Varas; Valérie Vidal; Jean-Christophe Géminard
We report the formation of a crater at the free surface of an immersed granular bed, locally crossed by an ascending gas flow. In two dimensions, the crater consists of two piles which develop around the location of the gas emission. We observe that the typical size of the crater increases logarithmically with time, independently of the gas emission dynamics. We describe the related granular flows and give an account of the influence of the experimental parameters, especially of the grain size and of the gas flow.
Applied Physics Letters | 2007
Roberto Bernal; Chantal Tassius; Francisco Melo; Jean-Christophe Géminard
Wrinkled patterns produced by cells crawling onto elastic membranes provide a useful tool to test living cells’ locomotion. The technique, however, requires a characterization of the intrinsic properties and tension of the membrane as well as an accurate determination of the deformation induced by the cell. Here, the authors present a capillary-type technique to estimate the elastic modulus of such membranes, describe an indentation technique to measure the initial tension, and develop a simple interferometric method to measure the amplitude field of the wrinkled pattern produced, for instance, by the action of living cell.
Physical Review Letters | 2007
Thibaut Divoux; Jean-Christophe Géminard
The friction of a sliding plate on a thin immersed granular layer obeys Amonton-Coulomb law. We bring to the fore a large set of experimental results which indicate that, over a few decades of values, the effective dynamical friction coefficient depends neither on the viscosity of the interstitial fluid nor on the size of beads in the sheared layer, which bears out the analogy with the solid-solid friction in a wide range of experimental parameters. We accurately determine the granular-layer dilatancy, which dependence on the grain size and slider velocity can be qualitatively accounted by considering the rheological behavior of the whole slurry. However, additional results, obtained after modification of the grain surface by a chemical treatment, demonstrate that the theoretical description of the flow properties of dense granular matter, even immersed, requires the detailed properties of the grain surface to be taken into account.
Geophysical Research Letters | 2011
Thibaut Divoux; Valérie Vidal; Maurizio Ripepe; Jean-Christophe Géminard
Many volcanoes exhibit temporal changes in their degassing process, from rapid gas puffing to lava fountaining and long-lasting quiescent passive degassing periods. This range of behaviors has been explained in terms of changes in gas flux and/or magma input rate. We report here a simple laboratory experiment which shows that the non- Newtonian rheology of magma can be responsible, alone, for such intriguing behavior, even in a stationary gas flux regime. We inject a constant gas flow-rate Q at the bottom of a non-Newtonian fluid column, and demonstrate the existence of a critical flow rate Q* above which the system spontaneously alternates between a bubbling and a channeling regime, where a gas channel crosses the entire fluid column. The threshold Q* depends on the fluid rheological properties which are controlled, in particular, by the gas volume fraction (or void fraction) {\phi}. When {\phi} increases, Q* decreases and the degassing regime changes. Non-Newtonian properties of magma might therefore play a crucial role in volcanic eruption dynamics.