The term active matter describes diverse systems, spanning macroscopic (e.g., shoals of fish and flocks of birds) to microscopic scales (e.g., migrating cells, motile bacteria and gels formed through the interaction of nanoscale molecular motors with cytoskeletal filaments within cells). Such systems are often idealizable in terms of collections of individual units, referred to as active particles or self-propelled particles, which take energy from an internal replenishable energy depot or ambient medium and transduce it into useful work performed on the environment, in addition to dissipating a fraction of this energy into heat. These individual units may interact both directly and through disturbances propagated via the medium in which they are immersed. Active particles can exhibit remarkable collective behavior as a consequence of these interactions, including non-equilibrium phase transitions between novel dynamical phases, large fluctuations violating expectations from the central limit theorem and substantial robustness against the disordering effects of thermal fluctuations. In this chapter, following a brief summary of experimental systems, which may be classified as examples of active matter, I describe some of the principles which underlie the modelling of such systems.
KeywordsStress Tensor Active Matter Polar Order Hydrodynamic Description Nematic Order
I thank Sriram Ramaswamy and Madan Rao for many enlightening and valuable discussions concerning the physics of active matter. Conversations at various points of time with Cristina Marchetti, Tanniemola Liverpool, Jacques Prost, Karsten Kruse, Frank Julicher, David Lacoste, Ronojoy Adhikari, P. B. Sunil Kumar, Aparna Baskaran and Jean-Francois Joanny have also helped to shape the material in this chapter. This work was supported by DST (India) and by the Indo-French Centre for the Promotion of Advanced Research (CEFIPRA) [Grant No. 3502]. The hospitality of ESPCI and the Institut Henri Poncare is gratefully acknowledged.
- 7.Bray D (1992) Cell movements: From molecules to motility. Garland Publishing, New YorkGoogle Scholar
- 9.de Gennes PG, Prost J (1993) Physics of liquid crystals, 2nd edn. Clarendon, OxfordGoogle Scholar
- 12.Gruler H, Dewald U, Eberhardt M (1999) Nematic liquid crystals formed by living amoeboid cells. Eur Phys J B 11:187–192Google Scholar
- 14.Jűlicher F, Kruse K, Prost J et al (2007) Active behavior of the cytoskeleton. Phys Rep449:3–28Google Scholar