Compaction of granular mixtures

in Granular Matter (2006), 8(2), 87-91

We propose a lattice model for studying the compaction of granular mixtures under taps. Two granular species are considered: small grains with low mobilities and T-shaped grains characterized by a larger ... [more ▼]

We propose a lattice model for studying the compaction of granular mixtures under taps. Two granular species are considered: small grains with low mobilities and T-shaped grains characterized by a larger ability to move. When those grains are mixed together, the compaction dynamics is mainly controlled by the volume fraction x of the small grains. Segregation have been found on the top layers of the pile. The grain mobilities have been studied for different values of the volume fraction x. An effective grain mobility has been defined for the mixture. This mobility is given by the linear combination of the mobility of both pure species minus a non-linear interaction term. Finally, the compaction speed depends on the fraction of the anisotropic grains. [less ▲]

Dancing droplets onto liquid surfaces

Poster (2006, March)

Experimental evidences of a structural and dynamical transition in fish school

in Physica A: Statistical Mechanics and its Applications (2006), 367

We have developed a video tracking system in order to determine all the trajectories of young fish (Oreochromis niloticus L.) within a school. Both individual and collective behaviours have been studied ... [more ▼]

We have developed a video tracking system in order to determine all the trajectories of young fish (Oreochromis niloticus L.) within a school. Both individual and collective behaviours have been studied as a function of the number of fish per unit area. By studying distributions of distances between fish and distributions of relative orientations, structural effects and cooperative motions have been evidenced. Signatures of a phase transition have been found, as predicted by some numerical models. This work opens new perspectives in the study of collective phenomena in biological systems since it is the first time that such measurements are possible. [less ▲]

Dancing droplets onto liquid surfaces

in Physics of Fluids (2006), 18

Reversing the Brazil nut effect

in European Physical Journal E -- Soft Matter (2005), 18(4), 367-372

We propose a lattice model for studying the Brazil Nut Effect (BNE), i.e. the phase segregation occuring when a granular material is vertically shaked. The model considers the tap intensity and the ... [more ▼]

We propose a lattice model for studying the Brazil Nut Effect (BNE), i.e. the phase segregation occuring when a granular material is vertically shaked. The model considers the tap intensity and the mobility mu of the grains as the main physical parameters. Different mobilities for different grain species lead to segregation (BNE) patterns, reverse segregation (RBNE) patterns, "sandwhich" layered structures or vertical domains. A phase diagram (decompaction chi, mobility difference between both species Delta mu) is obtained in which the different phases are emphasized. In a narrow region of the diagram, different phases coexist. It is shown that the BNE segregation could be reversed by increasing the tap intensity or the characteristics of the grains. Numerical results are compared with earlier experimental works. [less ▲]

Capillary rise in foams under microgravity

in Colloids and Surfaces A : Physicochemical and Engineering Aspects (2005), 261(1-mars Sp. Iss. SI), 131-134

The motion of liquid into a foam under low-gravity conditions is studied both experimentally and theoretically. The foam is confined to two dimensions in a Hele-Shaw cell and the liquid fraction measured ... [more ▼]

The motion of liquid into a foam under low-gravity conditions is studied both experimentally and theoretically. The foam is confined to two dimensions in a Hele-Shaw cell and the liquid fraction measured by image analysis. The foam imbibition is shown to be a diffusive process. Two models are analysed, corresponding to the limits of rigid and mobile interfaces in the Plateau borders. Liquid fraction profiles are compared to experimental ones. (c) 2004 Elsevier B.V. All rights reserved. [less ▲]

A new automatic video multitracking system able to follow a large school.

Poster (2005)

A new automatic video multitracking system able to follow a large school

in Noldus, L.P.P.J.; Grieco, F.; Loijens, L. W. S. (Eds.) et al Proceedings of Measuring Behavior 2005 (2005)

The study of shoaling behaviours is tackled by scientists by three approaches: the field, laboratory and computer modelling approach. The field studies are complicated and permit rarely an analysis of ... [more ▼]

The study of shoaling behaviours is tackled by scientists by three approaches: the field, laboratory and computer modelling approach. The field studies are complicated and permit rarely an analysis of individual positions. The third approach, consisting of development of mathematic models, is artificial. It requires a comparison with empiric results to validate and to determine the explications’ degree of each model. Thanks to the developments of digital imaging techniques, the laboratory studies on fish are able to measure the behavioural parameters with precision, impossible to realize with manual recording. Multitracking systems are rare and of recent development. These are insufficient to study the real size of shoals, often composed of hundreds of .shes. We have developed a new system of multitracking thanks to collaboration with the G.R.A.S.P. The G.R.A.S.P. has before realised other multitracking systems to follow some hundreds physical objects. Our video multitracking system is able to detect and to track more than a hundred unmarked fishes by gray scaling technique during some minutes. It permits an analysis as well at the group level as at the individual level. The multitraking program is able to attribute a number at each fish and to follow each one during the whole duration of the track. Our system permits the analysis of the movement of each individual, even if the trajectories of two fishes cross each other. It is possible thanks to the theoretical estimation of trajectory of each fish, compared with the real trajectory (analysis with feedback). The human eye is always necessary to detect the errors in identification but reediting is possible. However, if the fishes don’t move too rapidly, a rate of 25 images /s gives very good results, without too much visual expertise from the experimenter. If the fishes are too fast (i.e. escape response), it is better to use a high speed camera. [less ▲]

Bubble and granular flows: differences and similarities

in Physica A: Statistical Mechanics and its Applications (2004), 344(3-4), 424-430

We have experimentally studied the dense flow of identical bubbles below planes inclined at an angle theta. The flow is driven by the fast motion of dislocations. As a function of the density of ... [more ▼]

We have experimentally studied the dense flow of identical bubbles below planes inclined at an angle theta. The flow is driven by the fast motion of dislocations. As a function of the density of dislocations (controlled by the parameter theta), a transition occurs between a simple collective block motion and a granular-like surface flow regime. (C) 2004 Elsevier B.V. All rights reserved. [less ▲]

Effect of friction in a toy model of granular compaction

in Physical Review E (2004), 70(5, Pt 1), 51304

A toy model of granular compaction which includes some resistance due to granular arches is proposed. In this model, the solid/solid friction of contacting grains is a key parameter and a slipping ... [more ▼]

A toy model of granular compaction which includes some resistance due to granular arches is proposed. In this model, the solid/solid friction of contacting grains is a key parameter and a slipping threshold omega(c) is defined. Realistic compaction behaviors have been obtained. Two regimes separated by a critical point omega(c)* of the slipping threshold have been emphasized: (i) a slow compaction with lots of paralyzed regions and (ii) an inverse logarithmic dynamics with a power-law scaling of grain mobility. Below the critical point omega(c)*, the physical properties of this frozen system become independent of omega(c). Above the critical point omega(c)*-i.e., for low friction values-the packing properties behave as described by the classical Janssen theory for silos. [less ▲]