Collective motion is defined as the spontaneous
emergence of ordered movement in a system consisting of many
self-propelled agents. It can be observed in everyday life, for example in
flocks of birds,
schools of fish, herds of animals and also in crowds and car traffic. It also appears at the microscopic level: in colonies of bacteria, motility assays and 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 motion. Natural systems which have insp ...
. The scientific community is trying to understand the
universality of this phenomenon. In particular it is intensively investigated in
statistical physics
Statistical physics is a branch of physics that evolved from a foundation of statistical mechanics, which uses methods of probability theory and statistics, and particularly the mathematical tools for dealing with large populations and approxim ...
and in the field of
active matter
Active matter is matter composed of large numbers of active "agents", each of which consumes energy in order to move or to exert mechanical forces. Such systems are intrinsically out of thermal equilibrium. Unlike thermal systems relaxing towa ...
. Experiments on animals, biological and synthesized
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 motion. Natural systems which have insp ...
, simulations and theories are conducted in parallel to study these phenomena. One of the most famous models that describes such behavior is the
Vicsek model The Vicsek model is a mathematical model used to describe active matter. One motivation of the study of active matter by physicists is the rich phenomenology associated to this field. Collective motion and swarming are among the most studied pheno ...
introduced by
Tamás Vicsek
Tamás Vicsek (, born 10 May 1948, Budapest) is a Hungarian scientist with research interests in numerical studies of dense liquids, percolation theory, Monte Carlo simulation of cluster models, aggregation phenomena, fractal growth, pattern fo ...
et al. in 1995.
Collective behavior of Self-propelled particles
Just like biological systems in nature,
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 motion. Natural systems which have insp ...
also respond to external gradients and show collective behavior. Micromotors or
nanomotor
A nanomotor is a molecular or nanoscale device capable of converting energy into movement. It can typically generate forces on the order of piconewtons.
While nanoparticles have been utilized by artists for centuries, such as in the famous Lycu ...
s can interact with self-generated gradients and exhibit schooling and exclusion behavior. For example, Ibele, ''et al.'' demonstrated that silver chloride micromotors, in the presence of UV light, interact with each other at high concentrations and form schools. Similar behavior can also be observed with titanium dioxide microparticles. Silver orthophosphate microparticles exhibit transitions between schooling and exclusion behaviors in response to ammonia, hydrogen peroxide, and UV light. This behavior can be used to design a NOR gate since different combinations of the two different stimuli (ammonia and UV light) generate different outputs. Oscillations between schooling and exclusion behaviors are also tunable via changes in hydrogen peroxide concentration. The fluid flows generated by these oscillations are strong enough to transport microscale cargo and can even direct the assembly of close-packed colloidal crystal systems.
Micromotors and nanomotors can also move preferentially in the direction of externally applied chemical gradients, a phenomenon defined as
chemotaxis. Chemotaxis has been observed in self-propelled Au-Pt nanorods, which diffuse towards the source of hydrogen peroxide, when placed in a gradient of the chemical. Silica microparticles with Grubbs catalyst tethered to them, also move towards higher monomer concentrations. Enzymes also behave as
nanomotor
A nanomotor is a molecular or nanoscale device capable of converting energy into movement. It can typically generate forces on the order of piconewtons.
While nanoparticles have been utilized by artists for centuries, such as in the famous Lycu ...
s and migrate towards regions of higher substrate concentration, which is known as enzyme chemotaxis. One interesting use of enzyme
nanomotor
A nanomotor is a molecular or nanoscale device capable of converting energy into movement. It can typically generate forces on the order of piconewtons.
While nanoparticles have been utilized by artists for centuries, such as in the famous Lycu ...
chemotaxis is the separation of active and inactive enzymes in microfluidic channels. Another is the exploration of
metabolon formation by studying the coordinated movement of the first four enzymes of the glycolysis cascade: hexokinase, phosphoglucose isomerase, phosphofructokinase and aldolase. More recently, enzyme-coated particles and enzyme-coated liposomes have shown similar behavior in gradients of reactants in microfluidic channels. In general, chemotaxis of biological and synthesized
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 motion. Natural systems which have insp ...
provides a way of directing motion at the microscale and can be used for drug delivery, sensing,
lab-on-a-chip
A lab-on-a-chip (LOC) is a device that integrates one or several laboratory functions on a single integrated circuit (commonly called a "chip") of only millimeters to a few square centimeters to achieve automation and high-throughput screening. ...
devices and other applications.
See also
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Bacteria collective motion
Bacteria collective motion refers to the collective behavior of bacteria observed in the bacteria world.
Collective motion
Collective motion (or collective behavior) is a common phenomenon in our daily life. From bird flocks to the human gat ...
*
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 ...
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Collective cell migration Collective cell migration describes the movements of group of cells and the emergence of collective behavior from cell-environment interactions and cell-cell communication. Collective cell migration is an essential process in the lives of multicellu ...
*
Microswimmer
A microswimmer is a microscopic object with the ability to move in a fluid environment. Natural microswimmers are found everywhere in the natural world as biological microorganisms, such as bacteria, archaea, protists, sperm and microanimals. Sinc ...
Notes
Further references
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External links
Physicists come together to explore mechanics of collective motion''The Guardian'', 13 January 2014.
{{swarming
Multi-agent systems
Crowds