Self-organized dynamics and the transition to turbulence of confined active nematics

Abstract

Significance Topological defects are a ubiquitous feature of diverse materials ranging from superconductors to liquid crystals. In contrast to conventional materials where defects produce static field configurations, topological defects in energy-consuming active matter acquire motility. In bulk active nematic liquid crystals, motile defects drive turbulent-like dynamics. We show that confining a model experimental active nematic converts bulk chaotic motion into coherent circulatory flows. This observation suggests the possibility of exploiting geometric design to encode the spatiotemporal dynamics of topological defects, thereby endowing synthetic materials with the self-organized capabilities heretofore mainly found in living organisms. Furthermore, qualitative differences between experimental observations and numerical solutions of hydrodynamic equations suggest improvements to widely studied but incomplete theoretical models. We study how confinement transforms the chaotic dynamics of bulk microtubule-based active nematics into regular spatiotemporal patterns. For weak confinements in disks, multiple continuously nucleating and annihilating topological defects self-organize into persistent circular flows of either handedness. Increasing confinement strength leads to the emergence of distinct dynamics, in which the slow periodic nucleation of topological defects at the boundary is superimposed onto a fast procession of a pair of defects. A defect pair migrates toward the confinement core over multiple rotation cycles, while the associated nematic director field evolves from a distinct double spiral toward a nearly circularly symmetric configuration. The collapse of the defect orbits is punctuated by another boundary-localized nucleation event, that sets up long-term doubly periodic dynamics. Comparing experimental data to a theoretical model of an active nematic reveals that theory captures the fast procession of a pair of +1/2 defects, but not the slow spiral transformation nor the periodic nucleation of defect pairs. Theory also fails to predict the emergence of circular flows in the weak confinement regime. The developed confinement methods are generalized to more complex geometries, providing a robust microfluidic platform for rationally engineering 2D autonomous flows.