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Collective motion
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.〔Palacci, Jeremie; Sacanna, Stefano; Steinberg, Asher Preska; Pine, David J.; Chaikin, Paul M. (2013-02-22). "Living Crystals of Light-Activated Colloidal Surfers". Science 339 (6122): 936–940. doi:10.1126/science.1230020. ISSN 0036-8075. PMID 23371555.〕〔Theurkauff, I., Cottin-Bizonne, C., Palacci, J., Ybert, C., & Bocquet, L. (2012). Dynamic clustering in active colloidal suspensions with chemical signaling. Physical Review Letters, 108(26), 268303.〕〔Buttinoni, I., Bialké, J., Kümmel, F., Löwen, H., Bechinger, C., & Speck, T. (2013). Dynamical clustering and phase separation in suspensions of self-propelled colloidal particles. Physical review letters, 110(23), 238301.〕 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,〔Feder, Toni (October 2007). "Statistical physics is for the birds". Physics Today 60 (10): 28–30. doi:10.1063/1.2800090〕 biological and synthesized self-propelled particles, simulations and theories are conducted in parallel to study these phenomena. One of the most famous model trying to exhibit such behavior is the Vicsek model introduced by Tamás Vicsek et al. in 1995.〔Vicsek, T.; Czirok, A.; Ben-Jacob, E.; Cohen, I.; Shochet, O. (1995). "Novel type of phase transition in a system of self-driven particles". Physical Review Letters 75 (6): 1226–1229. arXiv:cond-mat/0611743. Bibcode:1995PhRvL..75.1226V. doi:10.1103/PhysRevLett.75.1226. PMID 10060237.〕
==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.〔Wang, W., Duan, W., Ahmed, S., Mallouk, T., & Sen, A. (2013). Small power: Autonomous nano- and micromotors propelled by self-generated gradients. NanoToday, 8(5), 531.〕 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.〔Ibele, M., Mallouk, T., & Sen, A. (2009). Schooling behavior of light-powered autonomous micromotors in water. Angewandte Chemie International Edition, 48(18), 3308.〕 Similar behavior can also be observed with Titanium dioxide microparticles. 〔Hong, Y., Diaz, M., Córdova‐Figueroa, U., & Sen, A. (2010). Light‐Driven Titanium‐Dioxide‐Based Reversible Microfireworks and Micromotor/Micropump Systems. Advanced Functional Materials, 20(10), 1568.〕 Silver orthophosphate microparticles exhibit transitions between schooling and exclusion behaviors in response to ammonia and UV light.〔Duan, W., Liu, R., & Sen, A. (2013). Transition between collective behaviors of micromotors in response to different stimuli. Journal of American Chemical Society, 135, 1280.〕 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.〔Hong, Y., Blackmann, NMK., Kopp, ND., Sen, A., & Velegol, D. (2007). Chemotaxis of nonbiological colloidal rods. Physics Review Letters, 99, 178103.〕 Silica microparticles with Grubbs catalyst tethered to them, also move towards higher monomer concentrations.〔Ravlick, RA., Sengupta, S., McFadden, T., Zhang, H., & Sen, A. (2011). A Polymerization-Powered Motor. Angewandte Chemie International Edition, 50(40), 9374.〕 Enzymes also behave as nanomotors and migrate towards regions of higher substrate concentration.〔Sengupta, S., Dey, KK., Muddana, HS., Tabouillot, T., Ibele, M., Butler, PJ., & Sen, A. (2013). Enzyme Molecules as Nanomotors. Journal of American Chemical Society, 135(4), 1406.〕 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.
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