Markus Baer (Berlin)
Thu 28 May 2015, 12:00 - 13:00
C.H Waddington Building, Seminar room 1.08, King's Building's

If you have a question about this talk, please contact: Julie Fyffe (jfyffe)

Active fluids are complex fluids wherein energy is injected by active internal units. In this talk I consider two examples of active biological fluids – dense suspension of swimming bacteria (e. g. Bacillus subtilis) and the cytoplasm of the true slime mold Physarum polycephalum.  For dense suspensions of bacterial swimmers, we propose a simple phenomenological model that predicts regular and turbulent vortex lattices [1] and reproduces recent experimental findings of mesoscale turbulence in two- and three-dimensional suspensions of Bacillus subtilis quantitatively [2]. Recently, we have been able to reproduce the observed behavior in a minimal model based on the competition between short-range and long-range antialignment of self-propelled particles representing the bacterial swimmers [3].

Intracellular processes are controlled by biochemical substances regulating active stresses. The cytoplasm is an active material with both viscoelastic and liquid properties. Therefore, we model the cytoskeleton of as a solid matrix that together with the cytosol as interstitial fluid constitutes a poroelastic material and find different forms of mechanochemical waves [4].

To reproduce the variety of contraction patterns observed experimentally in protoplasmic droplets of Physarum, we have combined a biophysically realistic model of a calcium oscillator in Physarum with the poroelastic model of the cytosol and assume that the active tension is regulated by calcium. With the help of linear stability analysis and two-dimensional finite element simulations the model is shown to reproduce the contraction patterns observed in in protoplasmic droplets including traveling and standing waves, chaotic structures and synchronized antiphase oscillations [5].

[1] J. Dunkel, S. Heidenreich, M. Bär and R. E. Goldstein (2013). Minimal continuum theories of structure formation in dense active fluids. New. J. Phys., 045016.

[2] J. Dunkel, S. Heidenreich, K. Drescher, H. H. Wensink, M. Bär and R. E. Goldstein (2013). Fluid dynamics of bacterial turbulence.  Phys. Rev. Lett. 110, 228102.

[3]  R. Großmann, P. Romanczuk, M. Bär, and L. Schimansky-Geier (2014). Vortex Arrays and Mesoscale Turbulence of Self-Propelled Particles. Phys. Rev. Lett. 113, 258104.

[4]  M. Radszuweit, S. Alonso, H. Engel and M. Bär (2013). Intracellular mechanochemical waves in an active poroelastic model. Phys. Rev. Lett. 110, 138102-1 - 138102-5.

[5]  M. Radszuweit, H. Engel, and M. Bär (2014). An active poroelastic model for mechanochemical patterns in protoplasmic droplets of Physarum polycephalum. PLOS ONE 9, e99220.