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Updated: May 17, 2026

An Analog Macroscopic Technique for Studying Molecular Hydrodynamic Processes in Dense Gases and Liquids
Published on: December 4, 2017
Microscopically reversible kinetic theory of flocking
1University of Amsterdam, University of Amsterdam, University of Amsterdam, Institute for Theoretical Physics, 1090 GL Amsterdam, The Netherlands; Dutch Institute for Emergent Phenomena, 1090 GL Amsterdam, The Netherlands; and Institute for Advanced Study, Oude Turfmarkt 147, 1012 GC Amsterdam, The Netherlands.
Abstract:
We formulate a kinetic theory of two species of hard spheres undergoing reactive collisions that convert chemical energy into kinetic energy. The model describes an active species interacting with a passive background, labeled as "birds" and "air," respectively, with the reactive collisions representing self-propulsion. Microscopic reversibility of the reactive dynamics is imposed, and a chemostat is introduced to drive the system out of equilibrium. When the chemostat is sufficiently strong, and one restricts to grazing interspecies collisions, we find that the bird momentum damping coefficient can change sign, giving rise to a flocking transition.
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