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Collective motion is defined as the spontaneous emergence of ordered movement in a system compound of a large number of 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. The scientific community is trying to understand the universality of this phenomenon. In particular it is intensively investigated in statistical physics and more precisely in the field of active matter. Experiments on animals, biological and synthesized self-propelled particles, simulations and theories are conducted in parallel to study these phenomena. One of the most famous models that attempt to exhibit such behavior is the Vicsek model introduced by Tamás Vicsek et al. in 1995.
Collective behavior of Self-propelled particles
Just like biological systems, Self-propelled particles also respond to external gradients and show collective behavior. Micromotors or nanomotors can interact with self-generated gradients and exhibit schooling and exclusion behavior. For example, Ibele et. al. demonstrated that silver chloride micromotors, in 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 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.
Micromotors and nanomotors can also move preferentially in the direction of externally applied chemical gradient, 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 nanomotors and migrate towards regions of higher substrate concentration. Chemotaxis provides a way of directing motion at the microscale and can be used for drug delivery, sensing, lab-on-a-chip devices and other applications.