From nematic defects to polar order: polar active flow of cytoskeletal filaments shaped by nematic defects.

Macroscopic materials can be affected by the presence of microscopic imperfections, called defects. For instance, passive nematic materials, composed of aligned, elongated building blocks, can contain defects in the local order (nematic defects) that induce large-scale distortions. Here we show that the presence of such defects can also play a role in the formation of active patterns in a system of active filaments. By using an experimental setup in which cytoskeletal filaments glide on a lipid membrane we enforce strong steric interactions between them. We then show that the alignment between individual filaments in these conditions leads to the formation of collective polar structures, despite a microscopic nematic symmetry. We also find that the transient formation of +1/2 defects plays a pivotal role in sorting the filaments’ polarity on the mesoscale. We then extend the system to filaments gliding inside a giant vesicle and find that spherical confinement enhances the global polarity, leading the formation of highly polar vortices. However, here, topological constraints on the alignment of filaments at high density hinder their ability to assemble effectively and lead to a “nematically jammed” state. We finally study the influence of passive defects by having filaments glide inside a nematic material. Again, we observe the formation of polar patterns whose emergence is now a direct effect of passive defects. All these results suggest strategies to tune active patterns by controlling defects.