Chaotic control of active nematics*

Active fluids are a class of materials in which individual subunits consume locally available energy to create coherent motion at larger scales. In active nematics, the phase exhibits liquid crystal-like ordering and as a result gives rise to mobile topological defects. These defects can be considered as virtual stirring rods, driving chaotic mixing in the active phase. In recent work, our group has characterized an active nematic comprised of biological filaments and molecular motors. We have developed analysis techniques inspired by non-linear dynamics and chaos theory to describe the system. In this presentation, I will introduce experiments that demonstrate how geometrical confinement can influence the braiding dynamics of the topological defects. Notably, we show that confinement in cardioid-shaped wells leads to realization of the golden braid, a maximally efficient mixing state. The active nematic system naturally settles into the state of maximal fluid stretching per unit time when the boundary is appropriately engineered. Increasing the size of the confining cardioid produces a transition from the golden braid, to the fully chaotic, or active turbulent state. We show that this transition can be described using different measures of topological entropy and explore new concepts for control of defect braiding.