Chemical gradients coupled to self-generated flow exhibit large-scale pattern formation in active fluids.

Active materials are composed of a dense ensemble of motile particles that consume chemical energy on the microscopic scale to drive collective material reorganization and autonomous flows. We investigate the mechano-chemical coupling between spatial distributions of energy sources and self-organized flows. We build a model system composed of microtubule-based active fluids. Into such systems, we embed colloidal beads which act as sources of fuel that are advected by the active flow. The resulting connection between material flows and the spatial gradients in fuel generate spatiotemporal patterns. We parameterize the behavior of our system by changing motor, fuel, and colloidal bead concentration. Varying these, we identify two qualitatively distinct limits of organization. In one extreme, active flows are localized, leaving the network largely undisturbed. In the other, material flows overlap, homogeneously fluidizing the system. Between these, there is a steady-state regime characterized by emergence of large-scale density patterns.