Jamming transition of a granular pile below the angle of repose

We study experimentally the relaxation towards mechanical equilibrium of a granular pile which has just experienced an avalanche and discuss it in the more general context of the granular jamming transition. Two coexisting dynamics are observed in the surface layer: a short time exponential decay consisting in rapid and independent moves of grains and intermittent bursts consisting in spatially correlated moves lasting for longer time. The competition of both dynamics results in long-lived intermittent transients, the total duration of which can late more than a thousand of seconds. We measure a two-time relaxation function, and relate it via a simple statistical model to a more usual two-time correlation function which exhibits strong similarities with auto-correlation functions found in aging systems. Localized perturbation experiments also allow us to test the pile surface layer receptivity.Comment: 9 pages, 10 figure

The Analysis of Employee Stock Ownership Plans Within Asymmetric Information, Insider Ownership, and Insider Trading Frameworks.

This dissertation examines the effects of employee stock ownership plans (ESOPs) on shareholder wealth, corporate control, and insider trading. In the first of three essays, a revision of the asymmetric information hypothesis is applied to the securities issuance structure of the ESOP to provide an explanation of the ESOP announcement\u27s effect on shareholder wealth. Essay two examines the effects of the firm\u27s (and its state of incorporation\u27s) supermajority provisions, level of pre- and post-announcement managerial voting power, and takeover attractiveness on the announcement-day reaction. The final essay examines the managers\u27 insider trading activities around the announcement of the ESOP. The first essay provides evidence that for announcements made in the absence of takeover activity, the market\u27s reaction to ESOPs partially structured with previously unissued common stock and convertible preferred equity is significantly and positively affected by the presence of a simultaneous repurchase announcement. In contrast, a similar analysis of ESOPs announced without a simultaneous repurchase finds an insignificant market reaction to ESOPs structured with common stock and a weakly significant and negative reaction to ESOPs structured with convertible preferred equity. This finding is supportive of the revised asymmetric information hypothesis. The results of the second essay suggest that the market discriminates between firms that are attractive and unattractive for takeover. A significant negative market reaction is observed when the attractive firms announce an ESOP that increases the managers\u27 voting power from a low to high level of entrenchment. In contrast, insignificant results are found in a similar low-to-high examination of unattractive firms. The final essay examines the managers\u27 insider trading activities around the announcement of the ESOP and argues that these trading activities are related to the structure of the ESOP. Insiders significantly reduce the number, and dollar value, of their shares sold in the months immediately surrounding the announcement of ESOPs structured with repurchased equity. Similar results are noted for the sample of ESOPs that experience a significant positive announcement-day reaction

Measuring order in the isotropic packing of elastic rods

The packing of elastic bodies has emerged as a paradigm for the study of macroscopic disordered systems. However, progress is hampered by the lack of controlled experiments. Here we consider a model experiment for the isotropic two-dimensional confinement of a rod by a central force. We seek to measure how ordered is a folded configuration and we identify two key quantities. A geometrical characterization is given by the number of superposed layers in the configuration. Using temporal modulations of the confining force, we probe the mechanical properties of the configuration and we define and measure its effective compressibility. These two quantities may be used to build a statistical framework for packed elastic systems.Comment: 4 pages, 5 figure

A comparative study of crumpling and folding of thin sheets

Crumpling and folding of paper are at rst sight very di erent ways of con ning thin sheets in a small volume: the former one is random and stochastic whereas the latest one is regular and deterministic. Nevertheless, certain similarities exist. Crumpling is surprisingly ine cient: a typical crumpled paper ball in a waste-bin consists of as much as 80% air. Similarly, if one folds a sheet of paper repeatedly in two, the necessary force becomes so large that it is impossible to fold it more than 6 or 7 times. Here we show that the sti ness that builds up in the two processes is of the same nature, and therefore simple folding models allow to capture also the main features of crumpling. An original geometrical approach shows that crumpling is hierarchical, just as the repeated folding. For both processes the number of layers increases with the degree of compaction. We nd that for both processes the crumpling force increases as a power law with the number of folded layers, and that the dimensionality of the compaction process (crumpling or folding) controls the exponent of the scaling law between the force and the compaction ratio.Comment: 5 page

Flow rule, self-channelization and levees in unconfined granular flows

Unconfined granular flows along an inclined plane are investigated experimentally. During a long transient, the flow gets confined by quasistatic banks but still spreads laterally towards a well-defined asymptotic state following a nontrivial process. Far enough from the banks a scaling for the depth averaged velocity is obtained, which extends the one obtained for homogeneous steady flows. Close to jamming it exhibits a crossover towards a nonlocal rheology. We show that the levees, commonly observed along the sides of the deposit upon interruption of the flow, disappear for long flow durations. We demonstrate that the morphology of the deposit builds up during the flow, in the form of an underlying static layer, which can be deduced from surface velocity profiles, by imposing the same flow rule everywhere in the flow.Comment: 4 pages, 5 figure

Aimé Vingtrinier, un imprimeur lyonnais du XIXème siècle

Mémoire du Master 2 Cultures de l\u27écrit et de l\u27image portant sur Aimé Vingtrinier, un imprimeur lyonnais du XIXème siècle

L’Art des armes à travers les siècles et ses traités d’escrime (XVIe-XIXe) dont les différentes éditions du traité de Lafaugère.

Mémoire de Master portant sur l\u27art des armes et les traités d\u27escrime du XVIe au XIXe siècles

Flows of suspensions of particles in yield stress fluids

International audienceWe study the rheological behavior of suspensions of noncolloidal spheres in yield stress fluids (concentrated emulsions). These are good model systems for understanding, e.g., the rheology of fresh concrete or debris flows, and more generally, the behavior of particles dispersed in any nonlinear material. We use magnetic resonance imaging techniques to investigate the flows of these yield stress suspensions in a concentric-cylinder Couette geometry. We extend the theoretical approach of Chateau et al. [J. Rheol. 52, 489–506 (2008)], valid for isotropic suspensions, to describe suspensions in simple shear flows, in which an anisotropic spatial distribution of particles is induced by flow. Theory and experiments show that the suspensions can be modeled by a Herschel–Bulkley behavior of same index as their interstitial fluid. We characterize the increase of their consistency and their yield stress with the particle volume fraction / in the 0%–50% range. We observe a good agreement between the experimental variations of the consistency with / and the theoretical prediction. This shows that the average apparent viscosity of the sheared interstitial material is correctly estimated and taken into account. We also observe shear-induced migration with similar properties as in a Newtonian fluid, which we predict theoretically, suggesting that particle normal stresses are proportional to the shear stress. However, the yield stress at flow stoppage increases much less than predicted. We also show that new features emerge in the rheology of the yield stress fluid when adding particles. We predict and observe the emergence of a nonzero normal stress difference at the yielding transition. We observe that the yield stress at flow start can differ from the yield stress at flow stoppage, and depends on flow history. It is likely a signature of a shear-dependent microstructure, due to the nonlinear behavior of the interstitial fluid, which makes these materials different from suspensions in Newtonian media. This is confirmed by direct characterization of shear-rate-dependent pair distribution functions using X-ray microtomography. This last observation explains why the theory predictions for the consistency can be correct while failing to model the yield stress at flow stoppage: a unique microstructure was indeed assumed as a first approximation. More sophisticated theories accounting for a shear-dependent microstructure are thus needed

Dynamics of grain ejection by sphere impact on a granular bed

The dynamics of grain ejection consecutive to a sphere impacting a granular material is investigated experimentally and the variations of the characteristics of grain ejection with the control parameters are quantitatively studied. The time evolution of the corona formed by the ejected grains is reported, mainly in terms of its diameter and height, and favourably compared with a simple ballistic model. A key characteristic of the granular corona is that the angle formed by its edge with the horizontal granular surface remains constant during the ejection process, which again can be reproduced by the ballistic model. The number and the kinetic energy of the ejected grains is evaluated and allows for the calculation of an effective restitution coefficient characterizing the complex collision process between the impacting sphere and the fine granular target. The effective restitution coefficient is found to be constant when varying the control parameters.Comment: 9 page