The mixing of coffee and milk after stirring or the formation of raindrops in clouds are just two phenomena in which turbulent flows play a decisive role. The mechanisms of turbulent flows are not understood.

Eberhard Bodenschatz and co-workers, Max Planck Institute for Dynamics and Self-Organization, Göttingen, Germany, used time-lapsed recordings to image polystyrene particles illuminated by a laser to track a single particle in a turbulent flow vortex. Analyzing the images, they singled out a particle surrounded by three others in a tetrahedron. The tetrahedron changed in shape and rotation — named the pirouette effect — like an ice-skater bending their arms to increase the speed of rotation. Stretching and faster rotation of the vortices resulted in instability causing them to break up. This enabled the transfer of energy from a large vortex to ever smaller ones.

This increased understanding will improve computer modeling of flows relevant to the transport of particles or chemicals, such as mixing, or the formation of clouds.

• The pirouette effect in turbulent flows
  H. Xu, A. Pumir, E. Bodenschatz,
  Nature Physics 2011.
  DOI: 10.1038/nphys2010