Active matter

A flock of starlings acting as a swarm

Active matter is composed of large numbers of active "agents", each of which consumes energy in order to move or to exert mechanical forces. Due to the energy consumption, these systems are intrinsically out of thermal equilibrium. Examples of active matter are schools of fish, flocks of birds, bacteria, artificial self-propelled particles, and self-organising bio-polymers such as microtubules and actin, both of which are part of the cytoskeleton of living cells. Most examples of active matter are biological in origin; however, a great deal of current experimental work is devoted to synthetic systems. Active matter is a relatively new field in soft matter physics: the most extensively studied model, the Vicsek model, dates from 1995.

Research in active matter combines analytical techniques, numerical simulations and experiments. Notable analytical approaches include hydrodynamics, kinetic theory, and non-equilibrium statistical physics. Numerical studies mainly involve self-propelled-particles models, making use of agent-based techniques and molecular dynamics algorithms. Experiments on biological systems extend over a wide range of scales, including animal groups (e.g., bird flocks, mammalian herds, fish schools and insect swarms), bacterial colonies, cellular tissues (e.g. epithelial tissue layers, cancer growth and embryogenesis), cytoskeleton components (e.g., in vitro motility assays, actin-myosin networks and molecular-motor driven filaments). Experiments on synthetic systems include self-propelled colloids (e.g., phoretically propelled particles), driven granular matter (e.g. vibrated monolayers), swarming robots and Quinke rotators.

Concepts in Active matter

• Active gels
  • Dense active matter
• Collective motion
  • Collective animal behavior
• Motility induced phase separation
• Schooling, flocking and swarming

Active matter systems

• Biological tissues
  • Subcellular and cell mechanics
• Crowd behaviour
• Self-propelled particles and colloids