Granular dynamics in compaction and stress relaxation

Elastic and dissipative properties of granular assemblies under uniaxial compression are studied both experimentally and by numerical simulations. Following a novel compaction procedure at varying oscillatory pressures, the stress response to a step-strain reveals an exponential relaxation followed by a slow logarithmic decay. Simulations indicate that the latter arises from the coupling between damping and collective grain motion predominantly through sliding. We characterize an analogous "glass transition" for packed grains, below which the system shows aging in time-dependent sliding correlation functions.Comment: 5 pages, 5 figure

Domain-domain interactions in Filamin A (16-23) impose a hierarchy of unfolding forces

The quaternary structure of Filamin A (FLNa) 16-23 was recently shown to exhibit multiple domain-domain interactions that lead to a propeller-like construction. Here we present single molecule force spectroscopy experiments to show a wide variety of mechanical responses of this molecule and compare it with its linear counterpart FLNa 1-8. The compact structure of FLNa 16-23 leads to a broad distribution of rupture forces and end-to-end lengths in the force-extension mode and multiple unraveling timescales in the force-clamp mode. Moreover, a subset of force-extension trajectories reveals a mechanical hierarchy in which the rupture of domain-domain interactions at high forces (200 pN) liberates the unfolding of individual domains at low forces (100 pN). This mechanism may also explain the order of magnitude difference in the rates of the biexponential fits to the distribution of unfolding dwell times under force-clamp. Overall, FLNa 16-23 under a force of 100 pN is more compliant than the linear FLNa 1-8. Since a physiological role of FLNa is to crosslink actin filaments, this range of responses allows it to accommodate a broad spectrum of forces exerted by the cell and its environment

Model for random packing of polydisperse frictionless spheres

International audienceWe propose a statistical model for the random packing of frictionless polydisperse spheres in which the complexity of the global packing is distilled into a local stochastic process. We simplify the problem by considering the "granocentric" point of view of a single particle in the bulk, thereby reducing random packing to the assembly of nearest neighbours, followed by a random choice of contacts among them. The model is based on only two parameters, the available solid angle around each particle and the ratio of contacts to neighbors, which are both directly obtainable from experiments or simulations. As a result, the model analytically predicts the microscopic distributions of nearest neighbours and contacts, the local density fluctuations as well as the global density of the packing. We find that this granocentric view captures the essential properties of the polydisperse emulsion packing. This model suggests a general principle of organization for random packing and provides a statistical tool for quantifying the effect of the particle size distribution on the geometry of random packing in a variety of contexts of industrial relevance

Contemporary Migration Trends and Flows on the Territory of Southeast Europe

This edited volume tackles different topics con­cerning old/new conceptual, methodological and theoretical dilemmas in migra­tion studies. Papers written by ethnologists and cultural anthropologists, sociologists, geographers, and others are brought together in order to gain a better understanding of the social, economic, political, cultural and other processes connected with migration in modern European societies. While some of the papers focus on migration processes, others dwell on post-migration phenom­ena and migrants’ livelihoods in their places of immigration. Nineteen authors participated in writing thirteen papers, divided in four interrelated sections.&nbsp

Measuring the coordination number and entropy of a 3D jammed emulsion packing by confocal microscopy

Jammed matter is by definition impenetrable to light, such that little is known about the geometry of jammed systems. Using confocal microscopy to image an emulsion in 3D, we first explain the origin of the enhanced fluorescence at the droplet contacts and then determine the contact network inside the model frictionless system. This enables the experimental determination of the average coordination number which agrees with the isostatic predicted value of approximately 6. Furthermore, we calculate the entropy of the packing from the network of contacts