title: Turbulence of swarming sperm.
abstract: Collective motion of self-sustained swarming flows has recently provided examples of small-scale turbulence arising where viscous effects are dominant. We report the first observation of universal enstrophy cascade in concentrated swarming sperm consistent with a body of evidence built from various independent measurements. We found a well-defined k^{-3} power-law decay of a velocity field power spectrum and relative dispersion of small beads consistent with theoretical predictions in 2D turbulence. Concentrated living sperm displays long-range, correlated whirlpool structures of a size that provides an integral scale of turbulence. We propose a consistent explanation for this quasi-2D turbulence based on self-structured laminated flow forced by steric interactions and alignment, a state of active matter that we call "swarming liquid crystal." We develop scaling arguments consistent with this interpretation.
authors.affiliation: University of Toulouse, INPT-UPS, IMFT, Institut de Mécanique des Fluides de Toulouse, France, and IMFT-CNRS, UMR 5502, 1, Allée du Professeur Camille Soula, 31400 Toulouse, France.
authors.forename: Adama
authors.initials: A
authors.lastname: Creppy
authors.affiliation: University of Toulouse, INPT-UPS, IMFT, Institut de Mécanique des Fluides de Toulouse, France, and IMFT-CNRS, UMR 5502, 1, Allée du Professeur Camille Soula, 31400 Toulouse, France.
authors.forename: Olivier
authors.initials: O
authors.lastname: Praud
authors.affiliation: INRA, CNRS, UMR, F-37380 Nouzilly, France.
authors.forename: Xavier
authors.initials: X
authors.lastname: Druart
authors.affiliation: INRA, CNRS, UMR, F-37380 Nouzilly, France.
authors.forename: Philippa L
authors.initials: PL
authors.lastname: Kohnke
authors.affiliation: University of Toulouse, INPT-UPS, IMFT, Institut de Mécanique des Fluides de Toulouse, France, and IMFT-CNRS, UMR 5502, 1, Allée du Professeur Camille Soula, 31400 Toulouse, France.
authors.forename: Franck
authors.initials: F
authors.lastname: Plouraboué
mesh_terms: D000465:Algorithms; D000818:Animals; D005453:Fluorescence; D007091:Image Processing, Computer-Assisted; D008297:Male; D008954:Models, Biological; D009038:Motion; D012212:Rheology; D012756:Sheep; D013094:Spermatozoa; D014741:Video Recording; D014783:Viscosity