Transitioning from Active Nematic Films to Mesoscale Turbulence in 3D Channels

Active fluids comprise a wide range of biological and synthetic materials that are driven out of equilibrium by energy injection from their microscopic elements, such as swimming bacteria or motor protein-microtubule bundles. The elongated nature of these constituents favours nematic alignment, while their activity continuously disturbs the orienational ordering leading to topological defects. Defects play an important role in the disorderly flows of 2D mesoscale turbulence and have been observed active films of microtubules, bacteria and monolayers of fibroblast and epithelial cells. However, it is less clear what role topological disclination lines play in 3D active flows. By numerically simulating a confining active nematic fluid between parallel plates, we characterize the transition from quasi-2D to confined 3D chaotic flows. At small heights, the active nematic exhibits effectively 2D mesoscale turbulence, with straight disclination lines that span the channel gap and interact as 2D defects. Upon increasing channel height, we find that a twist contortion of the disclinations leads to a transition to fully 3D chaotic flows. By defining an order parameter based on the conformation of the disclinations, we characterize the transition to 3D mesoscale turbulence.