S. Fraden

The Role of Boundaries in 2D Active Nematics

We present experimental and theoretical studies of 2D active nematics confined to a disk with parallel boundary conditions and a topological charge of +1. For large diameters, the nematic is unconfined, exhibiting turbulent flow with pairs of +1/2 and -1/2 defects created and destroyed at a steady state. As confinement is increased, the director adopts a yin yang pattern characterized by a pair of co-rotating +1/2 defects, which undergo spontaneous and continuous flow. Upon further confinement the system transitions from the yin yang state to a dipolar configuration of two +1/2 defects resembling a confined passive nematic. Theory predicts this configuration to be static, but in experiment the director field in the center of the disk rotates like a solid body.

We compute the dynamics in disks with total topological charge of +1, 0 and -1. In contrast to the case of passive nematics, for active nematics the yin yang pattern of two +1/2 defects in the center of the disk and circulating flow is persistent for all three charges, with the topologically required -1/2 defects relegated to the boundary. This insensitivity to topological constraints distinguishes active from passive liquid crystals.