Séminaire PMMH - Sophie Ramananarivo (LadHyX, École Polytechnique)

Vendredi 18 janvier 11:00-12:00 - Salle réunion PMMH 1

Emergent order, from fish schools to colloidal crystals

It has long been thought that fish form into schools and birds into flocks so that each individual can take advantage of the flows produced by others. Using physical experiments that mimic the movements of fins or wings, we discover that flapping bodies not only swim or fly faster when grouped together but that the flows also spontaneously organize the group into patterns with specific spacings. These findings suggest a powerful analogy between animal groups and states of matter, in that a school might be viewed as a ‘swimming crystal' of fish organized by flows.

In the first part of this talk, the surrounding fluid mediates interaction between individuals, thus promoting ordering. In the second part, we conversely make use of swimmers to assist the assembly and organization of its environment. We show that introducing a small amount of microswimmers in a monolayer of passive beads massively accelerates the relaxation to an ordered crystal. The use of such active dopants offers potential to control the properties of matter microscopically and in real time.

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