Physics – Condensed Matter – Statistical Mechanics
Scientific paper
2009-07-27
J. Phys A: Math. Theor. 42, 445001 (2009)
Physics
Condensed Matter
Statistical Mechanics
33 pages, 11 figures, submitted to J. Phys. A
Scientific paper
10.1088/1751-8113/42/44/445001
Considering a gas of self-propelled particles with binary interactions, we derive the hydrodynamic equations governing the density and velocity fields from the microscopic dynamics, in the framework of the associated Boltzmann equation. Explicit expressions for the transport coefficients are given, as a function of the microscopic parameters of the model. We show that the homogeneous state with zero hydrodynamic velocity is unstable above a critical density (which depends on the microscopic parameters), signaling the onset of a collective motion. Comparison with numerical simulations on a standard model of self-propelled particles shows that the phase diagram we obtain is robust, in the sense that it depends only slightly on the precise definition of the model. While the homogeneous flow is found to be stable far from the transition line, it becomes unstable with respect to finite-wavelength perturbations close to the transition, implying a non trivial spatio-temporal structure for the resulting flow. We find solitary wave solutions of the hydrodynamic equations, quite similar to the stripes reported in direct numerical simulations of self-propelled particles.
Bertin Eric
Droz Michel
Grégoire Guillaume
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