Active matter is composed of large numbers of active "agents", each of which consumes
energy in order to move or to exert mechanical forces.[1][2] 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 material classification in
soft matter: the most extensively studied model, the
Vicsek model, dates from 1995.[3]
Research in active matter combines analytical techniques, numerical simulations and experiments. Notable analytical approaches include
hydrodynamics,[4]kinetic theory, and non-equilibrium
statistical physics. Numerical studies mainly involve
self-propelled-particles models,[5][6] making use of
agent-based models such as
molecular dynamics algorithms as well as computational studies of hydrodynamic equations of
active fluids.[4] Experiments on biological systems extend over a wide range of scales, including animal groups (e.g., bird flocks,[7] mammalian herds, fish schools and insect
swarms[8]), bacterial colonies, cellular tissues (e.g. epithelial tissue layers,[9] cancer growth and embryogenesis),
cytoskeleton components (e.g., in vitro motility assays, actin-myosin networks and molecular-motor driven filaments[10]). Experiments on synthetic systems include self-propelled colloids (e.g., phoretically propelled particles[11]), driven granular matter (e.g. vibrated monolayers[12]), swarming robots and Quinke rotators.
Animals who live in groups are inclined to demonstrate coordinated behaviors as well as emergent properties which could incorporate cost-benefit balance, information transmission and decision making process.
Collective cell migration is commonly seen in multicellular organisms whose cells are cohesive to each other and can move in shapes like sheets, strands, and tubes, etc. The environment can largely affect the process of migration and cells can sense their neighbors by adhesion.
Self-propelled particles tend to accumulate in areas where they move relatively slowly, while at high density, they may also move more slowly. This mechanism could lead to positive feedback which will cause motility induced phase separation[15].
There are many ways to simulate swarms in natural settings. For example, a very common phenomenon in school of fish, colony of bees is that agents are influenced by the mean action of their neighbors[14]. These micro-interactions between individual group members will at last emerges as a group-level behavior pattern. A simulation of such emergence is shown in the right picture.
Collective motion refers to the emergence of ordered motion in a self-propelled system. Observations has been made in such systems consisting of units ranging from macromolecules through metallic rods and robots to groups of animals and people.
There are many current research areas in active matter that are closely related to the
biophysics of living cells, micro-tissues, or subcellular processes. For example, the
cardiac cells which have typical natural beating along with interaction of adjacent neighboring cells, would demonstrate unique mechanical properties.
Self-propelled particles (SPP) is a broad concept which can refer to items from Active colloidal particles, dubbed
nanomotors which are wet-artificial SPP to most animals which find energy from their food and show locomotion strategies. In the modeling of SPP introduced in 1995 by
Tamás Vicseket al[19], the SPP are point particles moving with a constant speed. Simulations have demonstrated that an appropriate "
nearest neighbour rule", at low noise, would eventually lead to all the particles moving in the same direction or forming a swarm, no matter how those agents are arranged to each other or whether their neighbors change over time.
^Alvarado, José. "Force percolation of contractile active gels". Soft matter.
^
abYaodong, Yang. "Mean Field Multi-Agent Reinforcement Learning". 35th International Conference on Machine Learning.
^Cates M E, Tailleur J. "Motility-induced phase separation". Condens. Matter Phys.
^Tamás Vicsekab, Anna Zafeiris. "Collective motion". Physics Report.
^Liu, Alan S. "Matrix viscoplasticity and its shielding by active mechanics in microtissue models: experiments and mathematical modeling". Scientific Report.
^B.A. Camley and W.-J. Rappel. "Physical models of collective cell motility: from cell to tissue". J. Phys. D.
^Vicsek, T.; Czirok, A.; Ben-Jacob, E.; Cohen, I.; Shochet, O. "Novel type of phase transition in a system of self-driven particles". Physical Review Letters.{{
cite journal}}: CS1 maint: multiple names: authors list (
link)