Logo PTB
Gateway for economy

Controlling active fluids

Turbulence in bacterial suspensions and other active fluids described theoretically for the first time

PTBnews 1.2021
07.01.2021
Especially interesting for

biology, medicine (e.g. formation of biofilm)

technology (e.g. new materials)

In a cooperation project between PTB and TU Berlin, the experimentally observed turbulence formation in the novel class of active fluids was successfully described, and motion in such fluids was controlled.

Stabilized square vortex lattice (L: lattice constant of periodically arranged pillars; ROI: region of interest in which the vortex statistics in the experiment was compared to theoretical predictions)

Swarming bacteria can determine the motion of the entire suspension with their flagella. These active fluids are therefore not driven macroscopically in the form of external gradients, but on the microscopic scale of the active particles. Active fluids thus open up an intriguing new field of physics, far away from thermodynamic equilibrium.

In bacterial suspensions, turbulence- like dynamics called meso-scale turbulence can build up. At PTB, it was shown that this turbulence corresponds to a disordered dynamic vortex lattice with a typical mean spacing between neighboring vortices.

What is interesting for future applications is not only the correct theoretical description of this class of novel fluids, but also the possibility of controlling the collective motion. This was achieved in collaboration with TU Berlin within the scope of the Collaborative Research Center 910, “Controlling Self-Organized Systems”. In an experiment conducted by the Argonne National Laboratory with bacterial suspensions, periodically arranged obstacles (“pillars”) were generated on a glass plate. These pillars caused the stabilization of a nearly perfect square vortex lattice. This observation was successfully reproduced with a new theoretical description. One of the challenges hereby was the derivation of the correct boundary conditions around the pillars. Moreover, the model allowed us to predict that stable vortex lattices with a different symmetry (e.g. hexagonal) are also feasible if appropriately sized pillars are used. A so-called Kagome lattice with a circular flow then forms. In analogy to solid-state physics, one can talk about a topological active fluid in this case. The simulations have confirmed that stabilization only takes place if the obstacle lattice has a spatial period in the range of the characteristic length scale of the meso-scale turbulence.

This shows how turbulence in active fluids can be controlled and transformed into a regular motional pattern. The formation of undesired biofilms could thus be hampered by using specific nanostructured surfaces.

Contact

Sebastian Heidenreich
Department 8.4
Mathematical Modelling and Data Analysis
Phone: +49 30 3481-7726
Opens local program for sending emailsebastian.heidenreich(at)ptb.de

Scientific publication

H. Reinken, D. Nishiguchi, S. Heidenreich, A. Sokolov, M. Bär, S. H. L. Klapp, I. S. Aranson: Organizing bacterial vortex lattices by periodic obstacle arrays. Commun. Phys. 3 (2020)

[Opens external link in new windowDOI: 10.1038/s42005-020-0337-z]