The ubiquity of collective motions observed at all scales in biological systems has driven a surge of scientific activity. Within physics, important theoretical progress was achieved by studying microscopic point-particles models and their continuous descriptions. Among the landmark results are the possibility of a true long-range polar ordered collective motion as well as of a Motility Induced Phase Separation (MIPS). The robustness of these observations against the numerous factors integrated out in the above effective models is a matter of crucial importance.
This is where human-designed model experimental systems have a key role to play. Janus colloids, swimming droplets or walking grains are amazing experimental realization of self propelled particles. They are far more simple than their biological inspiration, and already contain important realistic factors, such as hydrodynamics effects and pairwise force interactions, which, at least in principle, can be controlled.
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3 Briand, G. & Dauchot, O. Crystallization of Self-Propelled Hard Discs. Phys. Rev. Lett. 117, 098004–5 (2016).