The emergence of collective motion exhibited by systems ranging from flocks of animals to self-propelled microorganisms to the cytoskeleton is a ubiquitous and fascinating self-organization phenomenon. Similarities between these systems, such as the inherent polarity of the constituents, a density-dependent transition to ordered phases or the existence of very large density fluctuations, suggest universal principles underlying pattern formation. This idea is followed by theoretical models at all levels of description: micro- or mesoscopic models directly map local forces and interactions using only a few, preferably simple, interaction rules, and more macroscopic approaches in the hydrodynamic limit rely on the systems' generic symmetries. All these models characteristically have a broad parameter space with a manifold of possible patterns, most of which have not yet been experimentally verified. The complexity of interactions and the limited parameter control of existing experimental systems are major obstacles to our understanding of the underlying ordering principles. Here we demonstrate the emergence of collective motion in a high-density motility assay that consists of highly concentrated actin filaments propelled by immobilized molecular motors in a planar geometry. Above a critical density, the filaments self-organize to form coherently moving structures with persistent density modulations, such as clusters, swirls and interconnected bands. These polar nematic structures are long lived and can span length scales orders of magnitudes larger than their constituents. Our experimental approach, which offers control of all relevant system parameters, complemented by agent-based simulations, allows backtracking of the assembly and disassembly pathways to the underlying local interactions. We identify weak and local alignment interactions to be essential for the observed formation of patterns and their dynamics. The presented minimal polar-pattern-forming system may thus provide new insight into emerging order in the broad class of active fluids and self-propelled particles.

The unique mechanical performance of animal cells and tissues is attributed mostly to their internal biopolymer meshworks. Its perplexing universality and robustness against structural modifications by drugs and mutations is an enigma in cell biology and provides formidable challenges to materials science. Recent investigations could pinpoint highly universal patterns in the soft glassy rheology and nonlinear elasticity of cells and reconstituted networks. Here, we report observations of a glass transition in semidilute F-actin solutions, which could hold the key to a unified explanation of these phenomena. Combining suitable rheological protocols with high-precision dynamic light scattering, we can establish a remarkable rheological redundancy and trace it back to a highly universal exponential stretching of the single-polymer relaxation spectrum of a ''glassy wormlike chain.'' By exploiting the ensuing generalized time-temperature superposition principle, the time domain accessible to microrheometry can be extended by several orders of magnitude, thus opening promising new metrological opportunities.biopolymers ͉ light scattering ͉ nonlinear rheology ͉ wormlike chain A major strategy in the investigation of complex materials is to reassemble them step by step from simpler subunits to trace back their material properties on the macroscopic scale to those of their elementary constituents. This strategy has been successfully applied to the biopolymer networks that constitute the basic scaffolding structure of animal cells, known as the cytoskeleton (1). Filamentous actin (F-actin), the major loadbearing element of the cytoskeleton has received particular attention. The success of simple viscoelastic models in the quantitative analysis of the rheological behavior of in vitro reconstituted biopolymer networks (1-5) has rendered them an attractive paradigm for rationalizing the mechanics of live cells (6, 7). Indeed, there appear to be striking mechanical analogies between cells and prestressed reconstituted networks (7-9) and strong correlations of cell functions with the viscoelastic parameters, suggesting the latter as viable indices for clinical diagnosis (10). Recently, this widely shared view has been challenged by observations (11) that live cells obey the highly universal and comparatively featureless pattern of ''soft glassy rheology'' that is ubiquitous in soft condensed matter (12). These observations seem to imply that the pertinent linear mechanical properties of a cell might be composed in a single number, the so-called ''noise temperature'' (13) of the glass, and on this basis it has indeed been suggested that we are all built of glass (14). Subsequently, it has become a central task in cell biophysics to understand how the two conflicting paradigms might be integrated into a unified picture (15-17).Here, we aim to resolve the apparent puzzle by demonstrating that highly purified in vitro polymerized semidilute F-actin solutions undergo a glass transition as a function of various physiologically relev...

In the presence of condensing agents such as nonadsorbing polymer, multivalent counter ions, and specific bundling proteins, chiral biopolymers typically form bundles with a finite thickness, rather than phase-separating into a polymer-rich phase. Although shortrange repulsive interactions or geometrical frustrations are thought to force the equilibrium bundle size to be limited, the precise mechanism is yet to be resolved. The importance of the tight control of biopolymer bundle size is illustrated by the ubiquitous cytoskeletal actin filament bundles that are crucial for the proper functioning of cells. Using an in vitro model system, we show that size control relies on a mismatch between the helical structure of individual actin filaments and the geometric packing constraints within bundles. Small rigid actin-binding proteins change the twist of filamentous actin (F-actin) in a concentrationdependent manner, resulting in small, well defined bundle thickness up to Ϸ20 filaments, comparable to those found in filopodia. Other F-actin cross-linking proteins can subsequently link these small, well organized bundles into larger structures of several hundred filaments, comparable to those found in, for example, Drosophila bristles. The energetic tradeoff between filament twisting and cross-linker binding within a bundle is suggested as a fundamental mechanism by which cells can precisely adjust bundle size and strength. chiral aggregates ͉ finite size ͉ thickness control ͉ cytoskeleton organization

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