Eur. Phys. J. E (2013) 36: 135
https://doi.org/10.1140/epje/i2013-13135-7

Regular Article

Active polar two-fluid macroscopic dynamics

¹ Max Planck Institute for Polymer Research, 55021, Mainz, Germany
² Department of Physics, Faculty of Mathematics and Physics, University of Ljubljana, 1111, Ljubljana, Slovenia
³ Theoretische Physik III, Universität Bayreuth, 95440, Bayreuth, Germany

^* e-mail: pleiner@mpip-mainz.mpg.de

Received: 2 July 2013
Accepted: 12 November 2013
Published online: 28 November 2013

Abstract

We study the dynamics of systems with a polar dynamic preferred direction. Examples include the pattern-forming growth of bacteria as well as shoals of fish, flocks of birds and migrating insects. Due to the fact that the preferred direction only exists dynamically, but not statically, the macroscopic variable of choice is the macroscopic velocity associated with the motion of the active units, which are typically biological in nature. We derive the macroscopic equations for such a system and discuss novel static, reversible and irreversible cross-couplings connected to a second velocity as a variable. We analyze in detail how the macroscopic behavior of an active system with a polar dynamic preferred direction compares to other systems with two velocities including immiscible liquids and electrically neutral quantum liquids such as superfluid ⁴He and ³He . We critically discuss changes in the normal mode spectrum when comparing uncharged superfluids, immiscible liquids and active system with a polar dynamic preferred direction. We investigate the influence of a macroscopic hand (collective effects of chirality) on the macroscopic behavior of such active media.