The flock of starlings acting as a swarm

Active matter is matter composed of large numbers of active "agents", each of which consumes

energy Energy () is the physical quantity, quantitative physical property, property that is transferred to a physical body, body or to a physical system, recognizable in the performance of Work (thermodynamics), work and in the form of heat and l ...

in order to move or to exert mechanical forces. Such systems are intrinsically out of

thermal equilibrium Two physical systems are in thermal equilibrium if there is no net flow of thermal energy between them when they are connected by a path permeable to heat. Thermal equilibrium obeys the zeroth law of thermodynamics. A system is said to be in t ...

. Unlike thermal systems relaxing towards equilibrium and systems with boundary conditions imposing steady currents, active matter systems break time reversal symmetry because energy is being continually dissipated by the individual constituents. Most examples of active matter are biological in origin and span all the scales of the living, from

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

and self-organising bio-polymers such as

microtubule Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nanometer, nm and have an inner diameter bet ...

s and

actin Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of ...

(both of which are part of the

cytoskeleton The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is compos ...

of living cells), to schools of fish and flocks of birds. However, a great deal of current experimental work is devoted to synthetic systems such as artificial

self-propelled particles Self-propelled particles (SPP), also referred to as self-driven particles, are terms used by physicists to describe autonomous agents, which convert energy from the environment into directed or persistent random walk. Natural systems which have ...

. Active matter is a relatively new material classification in

soft matter Soft matter or soft condensed matter is a type of matter that can be deformed or structurally altered by thermal or mechanical stress which is of similar magnitude to thermal fluctuations. The science of soft matter is a subfield of condensed ...

: 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 In physics, physical chemistry and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids – liquids and gases. It has several subdisciplines, including (the study of air and other gases in ...

, kinetic theory, and non-equilibrium

statistical physics In physics, statistical mechanics is a mathematical framework that applies statistical methods and probability theory to large assemblies of microscopic entities. Sometimes called statistical physics or statistical thermodynamics, its applicati ...

. Numerical studies mainly involve self-propelled-particles models, making use of agent-based models such as

molecular dynamics Molecular dynamics (MD) is a computer simulation method for analyzing the Motion (physics), physical movements of atoms and molecules. The atoms and molecules are allowed to interact for a fixed period of time, giving a view of the dynamics ( ...

algorithms or lattice-gas models, as well as computational studies of hydrodynamic equations of active fluids. 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 Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial ( mesothelial) tissues line the outer surfaces of man ...

tissue layers, cancer growth and embryogenesis),

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 Quincke rotators. Concepts in Active matter * Active gels ** Dense active matter * Collective motion **

Collective animal behavior Collective animal behaviour is a form of social behavior involving the coordinated behavior of large groups of similar animals as well as emergent properties of these groups. This can include the costs and benefits of group membership, the tra ...

** Collective cell migration * Motility induced phase separation * Schooling, flocking and

swarming Swarm behaviour, or swarming, is a collective animal behaviour, collective behaviour exhibited by entities, particularly animals, of similar size which aggregate together, perhaps milling about the same spot or perhaps moving ''en masse'' or a ...

* Active stress * Disordered hyperuniformity Active matter systems *

Biological tissue In biology, tissue is an assembly of similar cells and their extracellular matrix from the same embryonic origin that together carry out a specific function. Tissues occupy a biological organizational level between cells and a complete or ...

s ** Subcellular and cell mechanics * Crowd behaviour *

Self-propelled particles Self-propelled particles (SPP), also referred to as self-driven particles, are terms used by physicists to describe autonomous agents, which convert energy from the environment into directed or persistent random walk. Natural systems which have ...

and

colloid A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others exte ...

References

{{swarming Soft matter Crowds