# Active matter

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{{Short description|Matter behavior at system scale}}
[[File:The flock of starlings acting as a swarm. - geograph.org.uk - 124593.jpg|thumb|upright=1.3|A flock of [starling](/source/common_starling)s acting as a [swarm](/source/swarm)]]
{{microbial and microbot movement|collective}}

'''Active matter''' is matter composed of large numbers of active "agents", each of which consumes [energy](/source/energy) in order to move or to exert mechanical forces.<ref>{{cite journal |last1=Ramaswamy |first1=Sriram |title=The Mechanics and Statistics of Active Matter |journal=Annual Review of Condensed Matter Physics |date=10 August 2010 |volume=1 |issue=1 |pages=323–345 |doi=10.1146/annurev-conmatphys-070909-104101 |author-link=Sriram Ramaswamy |arxiv=1004.1933 |bibcode=2010ARCMP...1..323R }}</ref><ref name="Marchetti2012">{{cite journal | last1=Marchetti | first1=M. C. | last2=Joanny | first2=J.F. | last3=Ramaswamy | first3=S. | last4=Liverpool | first4=T. B. | last5=Prost | first5=J. | last6=Rao | first6=M. | last7=Adita Simha | first7=R. | year=2012 | title=Hydrodynamics of soft active matter | journal=[Reviews of Modern Physics](/source/Reviews_of_Modern_Physics) | volume=85 | issue=3 | pages=1143–1189 | arxiv=1207.2929| doi=10.1103/RevModPhys.85.1143 | bibcode=2013RvMP...85.1143M}}</ref><ref>{{cite journal |last1=Bechinger |first1=Clemens |last2=Di Leonardo |first2=Roberto |last3=Löwen |first3=Hartmut |last4=Reichhardt |first4=Charles |last5=Volpe |first5=Giorgio |last6=Volpe |first6=Giovanni |title=Active Particles in Complex and Crowded Environments |journal=Reviews of Modern Physics |date=23 November 2016 |volume=88 |issue=4 |article-number=045006 |doi=10.1103/RevModPhys.88.045006 |arxiv=1602.00081 |bibcode=2016RvMP...88d5006B }}</ref><ref>{{cite journal |last1=Bowick |first1=Mark J. |last2=Fakhri |first2=Nikta |last3=Marchetti |first3=M. Cristina |last4=Ramaswamy |first4=Sriram |title=Symmetry, Thermodynamics, and Topology in Active Matter |journal=Physical Review X |date=11 February 2022 |volume=12 |issue=1 |article-number=010501 |doi=10.1103/PhysRevX.12.010501 |arxiv=2107.00724 |bibcode=2022PhRvX..12a0501B }}</ref> Such systems are intrinsically out of [thermal equilibrium](/source/Thermodynamic_equilibrium). Unlike thermal systems relaxing towards equilibrium and systems with boundary conditions imposing steady currents, active matter systems break [time reversal symmetry](/source/T-symmetry) because energy is being continually dissipated by the individual constituents.<ref>{{cite journal |last1=Najafi |first1=Ali |last2=Golestanian |first2=Ramin |title=Simple swimmer at low Reynolds number: Three linked spheres |journal=Physical Review E |date=16 June 2004 |volume=69 |issue=6 |article-number=062901 |doi=10.1103/PhysRevE.69.062901 |pmid=15244646 |arxiv=cond-mat/0402070 |bibcode=2004PhRvE..69f2901N }}</ref><ref>{{cite arXiv |last1=Berthier |first1=Ludovic |last2=Kurchan |first2=Jorge |author2-link=Jorge Kurchan |title=Lectures on non-equilibrium active systems |date=7 June 2019 |class=cond-mat.stat-mech |eprint=1906.04039}}</ref><ref>{{cite journal |last1=Cates |first1=Michael E. |last2=Tailleur |first2=Julien |title=Motility-Induced Phase Separation |journal=Annual Review of Condensed Matter Physics |date=March 2015 |volume=6 |issue=1 |pages=219–244 |doi=10.1146/annurev-conmatphys-031214-014710 |arxiv=1406.3533 |bibcode=2015ARCMP...6..219C }}</ref> Most examples of active matter are biological in origin and span all the scales of the living, from [bacteria](/source/bacteria) and self-organising [bio-polymers](/source/Biopolymer) such as [microtubule](/source/microtubule)s and [actin](/source/actin) (both of which are part of the [cytoskeleton](/source/cytoskeleton) 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](/source/self-propelled_particles).<ref name=":1">{{cite journal |last1=Howse |first1=Jonathan R. |last2=Jones |first2=Richard A. L. |last3=Ryan |first3=Anthony J. |last4=Gough |first4=Tim |last5=Vafabakhsh |first5=Reza |last6=Golestanian |first6=Ramin |title=Self-Motile Colloidal Particles: From Directed Propulsion to Random Walk |journal=Physical Review Letters |date=27 July 2007 |volume=99 |issue=4 |article-number=048102 |doi=10.1103/PhysRevLett.99.048102 |pmid=17678409 |arxiv=0706.4406 |bibcode=2007PhRvL..99d8102H }}</ref><ref>{{cite journal |last1=Bricard |first1=Antoine |last2=Caussin |first2=Jean-Baptiste |last3=Desreumaux |first3=Nicolas |last4=Dauchot |first4=Olivier |last5=Bartolo |first5=Denis |title=Emergence of macroscopic directed motion in populations of motile colloids |journal=Nature |date=6 November 2013 |volume=503 |issue=7474 |pages=95–98 |doi=10.1038/nature12673 |pmid=24201282 |arxiv=1311.2017 |bibcode=2013Natur.503...95B }}</ref><ref>{{cite journal |last1=Theurkauff |first1=I. |last2=Cottin-Bizonne |first2=C. |last3=Palacci |first3=J. |last4=Ybert |first4=C. |last5=Bocquet |first5=L. |title=Dynamic Clustering in Active Colloidal Suspensions with Chemical Signaling |journal=Physical Review Letters |date=26 June 2012 |volume=108 |issue=26 |article-number=268303 |doi=10.1103/PhysRevLett.108.268303 |pmid=23005020 |arxiv=1202.6264 |bibcode=2012PhRvL.108z8303T }}</ref> Active matter is a relatively new material classification in [soft matter](/source/soft_matter_physics):  the most extensively studied model, the [Vicsek model](/source/Vicsek_model), dates from 1995.<ref name="Vicsek1995">{{cite journal |last1=Vicsek |first1=Tamás |last2=Czirók |first2=András |last3=Ben-Jacob |first3=Eshel |last4=Cohen |first4=Inon |last5=Shochet |first5=Ofer |title=Novel Type of Phase Transition in a System of Self-Driven Particles |journal=Physical Review Letters |date=7 August 1995 |volume=75 |issue=6 |pages=1226–1229 |doi=10.1103/PhysRevLett.75.1226 |bibcode=1995PhRvL..75.1226V |pmid=10060237 |arxiv=cond-mat/0611743 }}</ref>

Research in active matter combines analytical techniques, numerical simulations and experiments. Notable analytical approaches include [hydrodynamics](/source/Fluid_dynamics),<ref name=":0">{{cite journal |last1=Toner |first1=John |last2=Tu |first2=Yuhai |last3=Ramaswamy |first3=Sriram |title=Hydrodynamics and phases of flocks |journal=Annals of Physics |date=July 2005 |volume=318 |issue=1 |pages=170–244 |doi=10.1016/j.aop.2005.04.011 |bibcode=2005AnPhy.318..170T |url=http://eprints.iisc.ac.in/3397/1/A89.pdf }}</ref> [kinetic theory](/source/kinetic_theory_of_gases), and non-equilibrium [statistical physics](/source/statistical_physics). Numerical studies mainly involve [self-propelled-particles](/source/Self-propelled_particles) models,<ref>{{cite journal |last1=Vicsek |first1=Tamás |last2=Czirók |first2=András |last3=Ben-Jacob |first3=Eshel |last4=Cohen |first4=Inon |last5=Shochet |first5=Ofer |title=Novel Type of Phase Transition in a System of Self-Driven Particles |journal=Physical Review Letters |date=7 August 1995 |volume=75 |issue=6 |pages=1226–1229 |doi=10.1103/PhysRevLett.75.1226 |pmid=10060237 |arxiv=cond-mat/0611743 |bibcode=1995PhRvL..75.1226V }}</ref><ref>{{cite journal |last1=Chaté |first1=Hugues |last2=Ginelli |first2=Francesco |last3=Grégoire |first3=Guillaume |last4=Raynaud |first4=Franck |title=Collective motion of self-propelled particles interacting without cohesion |journal=Physical Review E |date=18 April 2008 |volume=77 |issue=4 |article-number=046113 |doi=10.1103/PhysRevE.77.046113 |pmid=18517696 |arxiv=0712.2062 |bibcode=2008PhRvE..77d6113C }}</ref> making use of [agent-based](/source/Agent-based_model) models such as [molecular dynamics](/source/molecular_dynamics)  algorithms or [lattice-gas models](/source/BIO-LGCA),<ref>{{cite journal |last1=Bussemaker |first1=Harmen J. |last2=Deutsch |first2=Andreas |last3=Geigant |first3=Edith |title=Mean-Field Analysis of a Dynamical Phase Transition in a Cellular Automaton Model for Collective Motion |journal=Physical Review Letters |date=30 June 1997 |volume=78 |issue=26 |pages=5018–5021 |doi=10.1103/physrevlett.78.5018 |arxiv=physics/9706008 |bibcode=1997PhRvL..78.5018B }}</ref> as well as computational studies of hydrodynamic equations of [active fluid](/source/active_fluid)s.<ref name=":0" /> Experiments on biological systems extend over a wide range of scales, including animal groups (e.g., [bird flock](/source/bird_flock)s,<ref>{{cite journal |last1=Ballerini |first1=M. |last2=Cabibbo |first2=N. |last3=Candelier |first3=R. |last4=Cavagna |first4=A. |last5=Cisbani |first5=E. |last6=Giardina |first6=I. |last7=Lecomte |first7=V. |last8=Orlandi |first8=A. |last9=Parisi |first9=G. |last10=Procaccini |first10=A. |last11=Viale |first11=M. |last12=Zdravkovic |first12=V. |title=Interaction ruling animal collective behavior depends on topological rather than metric distance: Evidence from a field study |journal=Proceedings of the National Academy of Sciences |date=29 January 2008 |volume=105 |issue=4 |pages=1232–1237 |doi=10.1073/pnas.0711437105 |pmid=18227508 |pmc=2234121 |arxiv=0709.1916 |bibcode=2008PNAS..105.1232B |doi-access=free }}</ref> mammalian herds, [fish school](/source/fish_school)s and [insect swarm](/source/insect_swarm)s<ref>{{cite journal |last1=Buhl |first1=J. |last2=Sumpter |first2=D. J. T. |last3=Couzin |first3=I. D. |last4=Hale |first4=J. J. |last5=Despland |first5=E. |last6=Miller |first6=E. R. |last7=Simpson |first7=S. J. |title=From Disorder to Order in Marching Locusts |journal=Science |date=2 June 2006 |volume=312 |issue=5778 |pages=1402–1406 |doi=10.1126/science.1125142 |pmid=16741126 |bibcode=2006Sci...312.1402B }}</ref>), [bacterial colonies](/source/Colony_(biology)), [cellular tissue](/source/cellular_tissue)s (e.g. [epithelial](/source/epithelial) tissue layers,<ref>{{cite journal |last1=Trepat |first1=Xavier |last2=Wasserman |first2=Michael R. |last3=Angelini |first3=Thomas E. |last4=Millet |first4=Emil |last5=Weitz |first5=David A. |last6=Butler |first6=James P. |last7=Fredberg |first7=Jeffrey J. |title=Physical forces during collective cell migration |journal=Nature Physics |date=June 2009 |volume=5 |issue=6 |pages=426–430 |doi=10.1038/nphys1269 |bibcode=2009NatPh...5..426T |doi-access=free}}</ref> cancer growth and embryogenesis), [cytoskeleton](/source/cytoskeleton) components (e.g., ''in vitro'' motility assays, actin-myosin networks and molecular-motor driven filaments<ref>{{cite journal |last1=Keber |first1=Felix C. |last2=Loiseau |first2=Etienne |last3=Sanchez |first3=Tim |last4=DeCamp |first4=Stephen J. |last5=Giomi |first5=Luca |last6=Bowick |first6=Mark J. |last7=Marchetti |first7=M. Cristina |last8=Dogic |first8=Zvonimir |last9=Bausch |first9=Andreas R. |title=Topology and dynamics of active nematic vesicles |journal=Science |date=5 September 2014 |volume=345 |issue=6201 |pages=1135–1139 |doi=10.1126/science.1254784 |pmid=25190790 |pmc=4401068 |arxiv=1409.1836 |bibcode = 2014Sci...345.1135K }}</ref>). Experiments on synthetic systems include self-propelled colloids (e.g., phoretically propelled particles<ref name=":1" /><ref>{{cite journal |last1=Palacci |first1=Jeremie |last2=Sacanna |first2=Stefano |last3=Steinberg |first3=Asher Preska |last4=Pine |first4=David J. |last5=Chaikin |first5=Paul M. |title=Living Crystals of Light-Activated Colloidal Surfers |journal=Science |date=22 February 2013 |volume=339 |issue=6122 |pages=936–940 |doi=10.1126/science.1230020 |pmid=23371555 |bibcode=2013Sci...339..936P }}</ref>), driven granular matter (e.g. vibrated monolayers<ref>{{cite journal |last1=Deseigne |first1=Julien |last2=Dauchot |first2=Olivier |last3=Chaté |first3=Hugues |title=Collective Motion of Vibrated Polar Disks |journal=Physical Review Letters |date=23 August 2010 |volume=105 |issue=9 |article-number=098001 |doi=10.1103/PhysRevLett.105.098001 |pmid=20868196 |bibcode=2010PhRvL.105i8001D |arxiv=1004.1499 }}</ref>), [swarming robots](/source/swarm_robotics),<ref>{{cite journal|	journal=Science Robotics|title= Morphological computation and decentralized learning in a swarm of sterically interacting robots|date=Feb 2023|volume=8|issue=75|last1=Ben Zion|first1=Matan Yah}}</ref> and Quincke rotators.<ref>{{cite journal|	journal=IEEE Transactions on Industry Applications|title= Quincke Rotation of Spheres|date=July 1984|volume=IA-20|issue=4|last1=Jones|first1=Thomas B.|pages= 845–849|doi= 10.1109/TIA.1984.4504495}}</ref>

'''Concepts in Active matter'''
* Active gels
** Dense active matter
* [Collective motion](/source/Collective_motion)
** [Collective animal behavior](/source/Collective_animal_behavior)
** [Collective cell migration](/source/Collective_cell_migration)
* Motility induced phase separation
* [Schooling](/source/Shoaling_and_schooling), [flocking](/source/Flocking_(behavior)) and [swarming](/source/Swarm_behaviour)
* Active [stress](/source/Stress_(mechanics)) 
* [Disordered hyperuniformity](/source/Disordered_hyperuniformity)

'''Active matter systems'''
* [Biological tissue](/source/Biological_tissue)s
** Subcellular and cell mechanics
* [Crowd behaviour](/source/Crowd_simulation)
* [Self-propelled particles](/source/Self-propelled_particles) and [colloid](/source/colloid)s

==References==
{{reflist|32em}}

{{swarming}}

Category:Soft matter
Category:Crowds

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Adapted from the Wikipedia article [Active matter](https://en.wikipedia.org/wiki/Active_matter) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Active_matter?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.