Abstract

Polar flocks, understood in the Vicsek/Toner-Tu sense of the collective flow of self-propelled particles aligning their velocities, 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. Recently, evidence started accumulating that this ordered phase is both fragile and metastable. This talk will be mostly devoted to reviewing these findings and discussing their implications.