Polar patterns of driven filaments.

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Active systems like flocks of animals, self-propelled micro-organisms or the cytoskeleton constitute an intriguing class of non-equilibrium systems. Locally generated internal forces together with interactions between the constituents are the cause for remarkable self-organisation processes which lead to structures as diverse as cohesive flocks of birds, swarming microorganisms or aster-like structures in cytoskeletal systems. Besides their active nature those systems show further similarities like the inherent polarity of the constituents, a density-dependent transition to ordered phases or huge density fluctuations.

These similarities suggest universal organizing principles underlying pattern formation in these systems; an idea followed by theoretical models on all levels of description: Micro- or mesoscopic models directly map local forces and interactions via only a few and preferably simple interaction rules, while macroscopic approaches rely on the systems’ generic symmetries.

Cluster of high density actin motility
Galaxy formation on the nanoscale
Emergence of swirls

To test the predictions of these theories our team analyzed the emergence of collective motion in the high density motility assay - a minimal pattern forming system consisting of highly concentrated actin filaments propelled by immobilized molecular motors in a planar geometry. We could show that above a critical density the filaments self-organize to form coherently moving structures with persistent density modulations such as clusters, swirls and interconnected bands (see videos 1-3). These polar nematic structures are long-lived and can span length scales orders of magnitudes larger than their constituents.

The molecular nature of the approach not only permits truly large system sizes and possibly high particle densities with only a few and easily adjustable key parameters but also offers the possibility to develop tailored theoretical models and compare them to existing more generic theories. This is done in close collaboration with the group of Prof. Frey (LMU).

Further questions that are currently addressed comprise the investigation of the systems behaviour in confinement and the relevance of hydrodynamic interactions for the observed dynamics.

You can get high-quality videos of the patterns observed in the high density motility assay here.