Seminário extra 07/12: Fragility of polar flocks

Polar flocks, understood in the Vicsek/Toner-Tu sense of the collective flow of self-propelled particles aligning their velocities, have been and remain central in active matter studies. They are a limit case of real situations: the fluid surrounding particles is ignored (“dry”), the particles are pointlike (“dilute”), so that alignment is the only interaction. Even though their relevance in the real world is limited (but not nil), they must be studied thoroughly since understanding them is crucial to approach more complicated and realistic systems. One key property of polar flocks is that they show true long-range (polar) orientational order even in 2D. It is also well-known that a coexistence phase separates homogeneous flocks from the disordered gas phase. This coexistence phase also displays LRO, but it is inhomogeneous, typically made of dense, ordered, traveling bands. Recently, evidence started accumulating that the orientational order of polar flocks is fragile to various kinds of perturbations, whereas the band phase is more robust. I will review these findings and discuss their implications, considering successively the influence of:

*spatial anisotropy1

*spatial quenched disorder2

*population chirality disorder3

*one small obstacle4

following the recent publications listed below, whose co-authors are warmly thanked.

1A. Solon, H. Chaté, J. Toner, J. Tailleur, Phys. Rev. Lett. 128, 208004 (2022)

2Y. Duan, B. Mahault, Y-q Ma, X-q Shi, H. Chaté, Phys. Rev. Lett. 126, 178001 (2021)

3B. Ventejou, H. Chaté, R. Montagne, X-q Shi, Phys. Rev. Lett. 127, 238001 (2021)

4J. Codina, B. Mahault, H. Chaté, J. Dobnikar, I. Pagonabarraga, X-q Shi, Phys. Rev. Lett. 128, 218001 (2022)