title: Emergent vortices in populations of colloidal rollers.
abstract: Coherent vortical motion has been reported in a wide variety of populations including living organisms (bacteria, fishes, human crowds) and synthetic active matter (shaken grains, mixtures of biopolymers), yet a unified description of the formation and structure of this pattern remains lacking. Here we report the self-organization of motile colloids into a macroscopic steadily rotating vortex. Combining physical experiments and numerical simulations, we elucidate this collective behaviour. We demonstrate that the emergent-vortex structure lives on the verge of a phase separation, and single out the very constituents responsible for this state of polar active matter. Building on this observation, we establish a continuum theory and lay out a strong foundation for the description of vortical collective motion in a broad class of motile populations constrained by geometrical boundaries.
authors.affiliation: Laboratoire de Physique de l'Ecole Normale Supérieure de Lyon, Université de Lyon and CNRS, 46, allée d'Italie, Lyon F-69007, France.
authors.forename: Antoine
authors.initials: A
authors.lastname: Bricard
authors.affiliation: Laboratoire de Physique de l'Ecole Normale Supérieure de Lyon, Université de Lyon and CNRS, 46, allée d'Italie, Lyon F-69007, France.
authors.forename: Jean-Baptiste
authors.initials: JB
authors.lastname: Caussin
authors.affiliation: Department of Mechanical and Aerospace Engineering, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0411, USA.
authors.forename: Debasish
authors.initials: D
authors.lastname: Das
authors.affiliation: Laboratoire de Physique de l'Ecole Normale Supérieure de Lyon, Université de Lyon and CNRS, 46, allée d'Italie, Lyon F-69007, France.
authors.forename: Charles
authors.initials: C
authors.lastname: Savoie
authors.affiliation: Laboratoire de Physique de l'Ecole Normale Supérieure de Lyon, Université de Lyon and CNRS, 46, allée d'Italie, Lyon F-69007, France.
authors.forename: Vijayakumar
authors.initials: V
authors.lastname: Chikkadi
authors.affiliation: Department of Physics, Kyushu University 33, Fukuoka 812-8581, Japan.
authors.forename: Kyohei
authors.initials: K
authors.lastname: Shitara
authors.affiliation: 1] Department for Theoretical Physics, Odessa National University, Dvoryanskaya 2, Odessa 65026, Ukraine [2] Université Nice Sophia Antipolis, Laboratoire J.A. Dieudonné, UMR 7351 CNRS, Parc Valrose, Nice F-06108, France.
authors.forename: Oleksandr
authors.initials: O
authors.lastname: Chepizhko
authors.affiliation: Université Nice Sophia Antipolis, Laboratoire J.A. Dieudonné, UMR 7351 CNRS, Parc Valrose, Nice F-06108, France.
authors.forename: Fernando
authors.initials: F
authors.lastname: Peruani
authors.affiliation: Department of Mechanical and Aerospace Engineering, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0411, USA.
authors.forename: David
authors.initials: D
authors.lastname: Saintillan
authors.affiliation: Laboratoire de Physique de l'Ecole Normale Supérieure de Lyon, Université de Lyon and CNRS, 46, allée d'Italie, Lyon F-69007, France.
authors.forename: Denis
authors.initials: D
authors.lastname: Bartolo