Alladi Ramakrishnan Hall

Active Turbulence

Sumesh Thampi

Oxford University

Dense active systems spontaneously generate turbulent-like flow states where the velocity field
is highly disordered, with a continually changing pattern of vortices. Examples include mixtures
of microtubules and molecular motors (the machinery that acts as a cellular engine or cells
themselves), bacteria, vibrating granular rods, flocks of birds and schools of fish. We model
such active systems using continuum theory of an active nematic to show that active turbulence
develops in two stages: (i) ordered regions undergo an intrinsic hydrodynamic instability
generating lines of strong deformations, (ii) the lines relax by forming oppositely charged pairs
of topological defects. Defects and distortions in the director field act as sources of
vorticity, and thus vorticity is strongly correlated to the director field. Topological defects
can be continuously created and destroyed in biological systems and it seems that this dynamics
underlies the turbulent patterns in active materials.