Twist-induced networking and fast propagation of defects in three-dimensional active nematics

Owing to the sustained energy input, active nematics can spontaneously generate topological defects, which generally undergo chaotic motions ruled by active hydrodynamics. Here, we combine theory and simulations to investigate the structure and dynamics of topological defects in three-dimensional (3D) active nematics under boundary twisting, similar to twisted bilayer graphene. We show that increasing twist angle will lead to the transition of defect patterns from wedge-twist defect loops to pure-twist defect lines. These defect lines can organize into ordered networks, which oscillate persistently and propagate as a wave. Intriguingly, the wave speed of defect networks is significantly faster than that of local active flows, in contrast to the traditional active defects. We identify that the fast propagation of 3D topological defects results from the activity-driven defect pattern transition and the vertical oscillations mediated by the nematic elasticity. These findings reveal the marked influence of boundary perturbations on active nematics and could also provide a simple strategy to program 3D defect dynamics.