Irréversibilité et chaos dans les suspensions non-browniennes cisaillées alternativement
Mercredi 26 janvier 2011
à 11h en salle C. Brot
Many-body systems often exhibit irreversible behavior even though the governing equations of motion are reversible. Nevertheless, it is unusual to encounter a physical system in which the transition from reversible to irreversible behavior can be explored experimentally. Recent experiments on periodically sheared non-Brownian suspensions showed a sharp transition from reversible to irreversible chaotic behavior above a concentration dependent threshold strain amplitude gC . For a given particle volume fraction, two regimes are obtained depending on the shear strain amplitude. Below gC, most of the particles have reversible trajectories and return to their initial positions at the end of each shear cycle. Above gC, reversibility is lost and particles are displaced irreversibly due to shear-induced diffusion. The irreversibility of particle trajectories in suspensions is well-known. It is related to shear-induced diffusion and to chaos. The observation of such a sharp threshold however is puzzling as the initial distribution of particles is random, with no obvious mechanism for the onset of irreversibility. We develop a simple model, explored through simulation and mean field theory, which captures the salient behavior of the experiments. For small strain amplitude, the model reveals that random displacements of colliding particles can cause the system to self-organize into a reversible state that avoid further collisions. This model and additional experiments show that the strain threshold actually corresponds to the critical point of a non-equilibrium phase transition between absorbing and active fluctuating states . These results provide new insights into how microstructure can spontaneously develop and how random encounters can help a system evolve towards a stable fixed point. In particular, we will see how non-colloidal suspensions under slow periodic shear could constitute new model systems for the study of critical phenomena in dynamical systems.  Pine D.J., Gollug J.P., Brady J.F., Leshansky A.M. Nature 438, 997-1000, (2005).  Corté L., Chaikin P.M., Gollub J.P., Pine D.J. Nature Physics 4, 420-424, (2008).
invité par E. Lemaire
Voir en ligne : Center for Soft Matter Research, New York University
Fluides & Matériaux Complexes
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