Robert GROSSMANN - Robert GROSSMANN (Laboratoire J.A. Dieudonné Université Cote d Azur) - Keywords: nonequilibrium statistical physics, emergence, synergetics

Robert GROSSMANN - Robert GROSSMANN (Laboratoire J.A. Dieudonné Université Cote d Azur) - Keywords: nonequilibrium statistical physics, emergence, synergetics

Contribution title: Emergent collective dynamics of active particles with alignment-interactions

The emergent pattern formation in active matter is a paradigm for self-organization. The term active matter refers to complex systems composed of particles which are characterized by the ability to convert energy of their surroundings into kinetic energy, i.e., self-propelled motion. Due to the perpetual energy influx at the particle scale, active matter belongs to the class of open systems, hence allowing for self-organized multi-scale pattern formation far from thermodynamic equilibrium. The dynamics of active particles neither conserves energy nor momentum, thereby giving reason to rethink statistical physics.

In this talk, we review our recent research on self-propelled particle systems interacting via velocity-alignment. Throughout, the focus of the investigation is thereby on the identification of essential interaction mechanisms that lead to certain large-scale structures. The modeling is based on symmetry considerations of the underlying physical interaction, analogously to spin models for magnetism. Particularly two exemplary cases are illustrated: (i) the turbulent-like vortex dynamics as observed in dense bacterial suspensions which we trace back to the interplay of competing velocity-alignment interactions; (ii) the emergence of large-scale ordering which is accompanied by the formation of large-scale density bands due to alignment-induced aggregation of particles, interpreted as a nonequilibrium phase-separation process.

The simplicity of alignment models for active particles notably enables their analytical analysis: kinetic and hydrodynamic theories, accounting for the large-scale dynamics, are systematically derived from particle-based models. The nonequilibrium character of the dynamics is reflected by the appearance of novel terms which are forbidden thermodynamic equilibrium, such as active fluxes and stresses. Predictions of those large-scale descriptions are, in turn, compared to particle-based simulations.

Finally, a microscopic justification of alignment-based models is presented by mapping a more microscopic model with anisotropic repulsion onto an effective alignment interaction.