Polar patterns of driven filaments

Pattern formation in complex systems made up of many individual self-propelled components is a ubiquitous phenomenon, seen in systems as diverse as flocks of birds, colonies of microorganisms and in the cytoskeleton of living cells. Progress towards a unifying explanation of its mechanisms has been slow because of the lack of a sufficiently simple model, but now there is a candidate for the role. The new experimental system involves filaments of the protein actin propelled by motor proteins immobilized on a surface. Above a critical density the filaments self-organize to form coherently moving structures with persistent density modulations such as clusters, swirls and interconnected bands. Experimental observations combined with simulations reveal a variety of mechanisms underling assembly and disassembly of the ordered structures and show that weak and local alignment interactions are essential for pattern formation. The system's controllability and scope for extension to more complex interactions should make it well suited to studying the emergence of macroscopic order from microscopic interactions.