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See detailSelf-organization of a monolayer of magnetized beads
Schockmel, Julien ULiege

Doctoral thesis (2019)

We present experimental results obtained with a model experimental system dedicated to the study of 2D structures and phase transitions. The system is composed of a monolayer of millimetric soft ... [more ▼]

We present experimental results obtained with a model experimental system dedicated to the study of 2D structures and phase transitions. The system is composed of a monolayer of millimetric soft ferromagnetic beads confined in a 2D horizontal cell. The beads are immersed in a vertical and homogeneous magnetic field inducing magnetic dipole-dipole interactions between beads. Due to the confinement, the repulsive interactions tend to order the system. As the system is athermal, a mechanical agitation is used to produce a Brownian motion of the beads which creates disorder. Adjusting the competition between the effects of the interaction and the agitation allows us to control the order of the system. Thanks to this experimental setup, we can study the transition from a frozen and very ordered state, called a crystal, to a disordered and dynamic state similar to a liquid. As predicted by the KTHNY theory of 2D melting, a two-step transition is highlighted, including the so-called hexatic phase between crystal and liquid. In addition, the structures observed agree quantitatively with structures obtained in colloidal systems and in numerical simulations. Our experimental system is, therefore, a suitable model to study 2D thermal structures. In this work, we focus on the study of topological defects that are crucial in low-dimensional physics. We first forced topological defects into the syttem by inducing frustration from confinement. We show that the size and the geometry of the confinement strongly affect the global order, as well as the type and the number of topological defects observed in the systems. Finally, we force pointlike defects in the structure by introducing beads of different size, called impurities. At global scale, the increase of the impurity can prevent the crystallization. At local scale, we observe that an impurity induces a very localized geometrical frustration, which creates a topological defect whose nature is directly related to the size of the impurity. [less ▲]

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See detailThreshold of gas-like to clustering transition in driven granular media in low-gravity environment
Noirhomme, Martial ULiege; Cazaubiel, Annette; Darras, Alexis ULiege et al

in Europhysics Letters (2018), 123(14003),

Strongly driven granular media are known to undergo a transition from a gas-like to a cluster regime when the density of particles is increased. However, the main mechanism triggering this transition is ... [more ▼]

Strongly driven granular media are known to undergo a transition from a gas-like to a cluster regime when the density of particles is increased. However, the main mechanism triggering this transition is not fully understood so far. Here, we investigate experimentally this transition within a 3D cell filled with beads that are driven by two face-to-face vibrating pistons in low gravity during parabolic flight campaigns. By varying large ranges of parameters, we obtain the full phase diagram of the dynamical regimes reached by the out-of-equilibrium system: gas, cluster or bouncing aggregate. The images of the cell recorded by two perpendicular cameras are processed to obtain the profiles of particle density along the vibration axis of the cell. A statistical test is then performed on these distributions to determinate which regime is reached by the system. The experimental results are found in very good agreement with theoretical models for the gas-cluster transition and for the emergence of the bouncing state. The transition is shown to occur when the typical propagation time needed to transmit the kinetic energy from one piston to the other is of the order of the relaxation time due to dissipative collisions. [less ▲]

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See detailFrustrated crystallization of a monolayer of magnetized beads under geometrical confinement
Schockmel, Julien ULiege; Vandewalle, Nicolas ULiege; Opsomer, Eric ULiege et al

in Physical Review. E ,Statistical, Nonlinear, and Soft Matter Physics (2017)

We present a systematic experimental study of the confinement effect on the crystallization of a monolayer of magnetized beads. The particles are millimeter-scale grains interacting through the short ... [more ▼]

We present a systematic experimental study of the confinement effect on the crystallization of a monolayer of magnetized beads. The particles are millimeter-scale grains interacting through the short range magnetic dipole-dipole potential induced by an external magnetic field. The grains are confined by repulsing walls and are homogeneously distributed inside the cell. A two-dimensional (2d) Brownian motion is induced by horizontal mechanical vibrations. Therefore, the balance between magnetic interaction and agitation allows investigating 2d phases through direct visualization. The effect of both confinement size and shape on the grains’ organization in the low-energy state has been investigated. Concerning the confinement shape, triangular, square, pentagonal, hexagonal, heptagonal, and circular geometries have been considered. The grain organization was analyzed after a slow cooling process. Through the measurement of the averaged bond order parameter for the different confinement geometries, it has been shown that cell geometry strongly affects the ordering of the system. Moreover, many kinds of defects, whose observation rate is linked to the geometry, have been observed: disclinations, dislocations, defects chain, and also more exotic defects such as a rosette. Finally, the influence of confinement size has been investigated and we point out that no finite-size effect occurs for a hexagonal cell, but the finite-size effect changes from one geometry to another. [less ▲]

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See detailQuantitatively mimicking wet colloidal suspensions with dry granular media
Messina, René; Aljawhari, Sarah; Bécu, Lydiane et al

in Scientific Reports (2015)

Athermal two-dimensional granular systems are exposed to external mechanical noise leading to Brownian-like motion. Using tunable repulsive interparticle interaction, it is shown that the same ... [more ▼]

Athermal two-dimensional granular systems are exposed to external mechanical noise leading to Brownian-like motion. Using tunable repulsive interparticle interaction, it is shown that the same microstructure as that observed in colloidal suspensions can be quantitatively recovered at a macroscopic scale. To that end, experiments on granular and colloidal systems made up of magnetized particles as well as computer simulations are performed and compared. Excellent agreement throughout the range of the magnetic coupling parameter Γ is found for the pair distribution as well as the bond-orientational correlation functions. This finding opens new ways to efficiently and very conveniently explore phase transitions, crystallization, nucleation, etc in confined geometries. [less ▲]

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See detailFlow of magnetic repelling grains in a two-dimensional silo
Lumay, Geoffroy ULiege; Schockmel, Julien ULiege; Henandez-Enriquez, D. et al

in Papers in Physics (2015)

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See detailMelting of a confined monolayer of magnetized beads
Schockmel, Julien ULiege; Mersch, Eric; Vandewalle, Nicolas ULiege et al

in Physical Review. E ,Statistical, Nonlinear, and Soft Matter Physics (2013)

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See detailHysteretic behavior in three-dimensional soap film rearrangements
Vandewalle, Nicolas ULiege; noirhomme, matial; Schockmel, Julien ULiege et al

in Physical Review. E ,Statistical, Nonlinear, and Soft Matter Physics (2011), 83

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