title: Passive appendages generate drift through symmetry breaking.
abstract: Plants and animals use plumes, barbs, tails, feathers, hairs and fins to aid locomotion. Many of these appendages are not actively controlled, instead they have to interact passively with the surrounding fluid to generate motion. Here, we use theory, experiments and numerical simulations to show that an object with a protrusion in a separated flow drifts sideways by exploiting a symmetry-breaking instability similar to the instability of an inverted pendulum. Our model explains why the straight position of an appendage in a fluid flow is unstable and how it stabilizes either to the left or right of the incoming flow direction. It is plausible that organisms with appendages in a separated flow use this newly discovered mechanism for locomotion; examples include the drift of plumed seeds without wind and the passive reorientation of motile animals.
authors.affiliation: Linné Flow Centre, KTH Royal Institute of Technology, Department of Mechanics, 10044 Stockholm, Sweden.
authors.forename: U
authors.initials: U
authors.lastname: Lācis
authors.affiliation: Linné Flow Centre, KTH Royal Institute of Technology, Department of Mechanics, 10044 Stockholm, Sweden.
authors.forename: N
authors.initials: N
authors.lastname: Brosse
authors.affiliation: Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey 08544, USA.
authors.forename: F
authors.initials: F
authors.lastname: Ingremeau
authors.affiliation: 1] Department of Chemical, Civil and Environmental Engineering (DICCA), University of Genova, 16145 Genova, Italy [2] INFN and CINFAI Consortium, Genova Section, 16146 Genova, Italy.
authors.forename: A
authors.initials: A
authors.lastname: Mazzino
authors.affiliation: Linné Flow Centre, KTH Royal Institute of Technology, Department of Mechanics, 10044 Stockholm, Sweden.
authors.forename: F
authors.initials: F
authors.lastname: Lundell
authors.affiliation: Université de Bordeaux, Laboratoire Ondes et Matière d'Aquitaine (UMR 5798 CNRS), 351 cours de la Libération, 33405 Talence, France.
authors.forename: H
authors.initials: H
authors.lastname: Kellay
authors.affiliation: Linné Flow Centre, KTH Royal Institute of Technology, Department of Mechanics, 10044 Stockholm, Sweden.
authors.forename: S
authors.initials: S
authors.lastname: Bagheri
mesh_terms: D000818:Animals; D055592:Biophysical Phenomena; D005121:Extremities / Q000502:physiology*; D008124:Locomotion; D008962:Models, Theoretical; D058614:Seed Dispersal