13/12/2019 – S. Ramananarivo – Emerging order, from crystals of colloids to fish schools

Séminaire IUSTI – 13 déc. 2019 – 11h salle 250

Emerging order, from crystals of colloids to fish schools

Sophie Ramananarivo – LadHyx, Palaiseau

Living systems constantly exploit active fluctuations in their processes, to boost transport or assist assembly. Self-propelled colloids, that consume energy to move hold the same potential for man-made assembly of microparticles. We show here that introducing a small amount of microswimmers in a monolayer of passive beads massively accelerates the relaxation to an ordered crystal. Activity-induced internal agitation allows to overcome kinetic barriers and activate the annealing. The use of such active dopants offers potential to control the properties of matter in space and time. In the first part of this talk, we make use of swimmers to assist the assembly and organization of its environment. In the second part (at a much bigger scale), a surrounding fluid environment conversely mediates interaction between fish or birds, thus promoting ordering into schools and flocks. 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.