Event

Active matter is a fascinating new field in soft matter physics aiming to understand how interacting active particles self-organize into an intriguing set of patterns and collective non-equilibrium states. Superficially, flocks of animals, self-propelled microorganisms or cytoskeletal systems appear to display similar phenomenologies, hinting towards universal organizing principles. However, upon closer inspection, it has turned out that a more comprehensive understanding of the microscopic interaction between active particles beyond simple collision rules is needed to explain the emergent macroscopic order. In this context, the actomyosin motility assay [1] has played an important role. It is composed of two ingredients: actin filaments and molecular motor proteins. Actin filaments move on a lawn of active motors by consuming adenosine triphosphate. We have used the binary collision statistics determined in this assay [2] to predict the collective behavior by using a recently developed numerical scheme to solve the Boltzmann equation for active particles [3]. Contrary to common beliefs, we have found that the alignment effect of the binary collision statistics is too weak to account for the observed ordering transition [2]. This indicates that the dynamics of active matter requires a description that accounts for multi-filament collisions, and calls for a theoretical framework that goes beyond kinetic theories.

In this talk, we will discuss recent theoretical and experimental advances in understanding pattern formation and collective dynamics in active biopolymer systems [4]. We will explain how by tuning the polar or nematic nature of the binary interaction between active biopolymers one is able to precisely control the formation of distinct patterns. In addition, the macroscopic state of these systems exhibits an emergent symmetry that is not already dictated by the symmetry of the microscopic interaction as in equilibrium systems. Both agent-based computer simulations and experiments with the actomyosin motility assay consistently show chimera states in which polar and nematic structures coexist and are in dynamic equilibrium with each other. Our theoretical analysis indicates that multi-stability, as well as the capability of varying the emerging order upon small variations in local interactions, is a generic feature of active biopolymer liquids. Finally, we will discuss how chirality in the biopolymers’ trajectories leads to novel collective vorticity states [5].

[1]   V. Schaller, C.A Weber, C. Semmrich, E. Frey, and A.R Bausch, Nature 467, 73 (2010).

[2]   F. Thüroff, C.A. Weber, and E. Frey, Phys. Rev. X 4, 041030 (2014).

[3]   R. Suzuki, C.A. Weber, E. Frey, and A. Bausch, Nature Physics 11, 839 (2015).

[4]   L. Huber, T. Krüger, R. Suzuki, A.R. Bausch, and E. Frey, unpublished.

[5]   J. Denk, L. Huber, E. Reithmann, and E. Frey, Phys. Rev. Lett 116, 178301 (2016).