O. Dauchot – 09 Octobre

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.

In the present talk, I will illustrate that matter in the case of two  remarkable experimental systems, namely rolling colloids [1] and walking  grains [2,3].

[1] Bricard, A., Caussin, J.-B., Desreumaux, N., Dauchot, O. & Bartolo, D. Emergence of macroscopic directed motion in populations of motile  colloids. Nature 503, 95–98 (2013).
[2] Deseigne, J., Dauchot, O. & Chaté, H. Collective Motion of Vibrated  Polar Disks. Phys. Rev. Lett. 105, (2010).
[3] Briand, G. & Dauchot, O. Crystallization of Self-Propelled Hard Discs. Phys. Rev. Lett. 117, 098004–5 (2016).