Building active liquid crystals with tunable defect dynamics and variable elasticity

Constituents of active materials convert ambient free energy into directional motion giving rise to striking dynamical phenomena. Here we perform experiments on a two-dimensional sheet of F-actin filaments of length ranging from 1-2 μm, driven by myosin-II motors and show that it acts as an extensile active nematic liquid crystal. We demonstrate that the interaction between ±1/2 defect pairs can be switched from attraction to effective repulsion of strength linearly scaling with motor concentration, φ. To characterize the emerging complex flows, we measure velocity-velocity and orientational correlation lengths and confirm their scaling of φ^-1/2. We show that the existence of hydrodynamic shear flows along the symmetry axis of the +1/2 defect leads to its shape change, signifying a lowered effective bend modulus in the active state determined through the defect morphology. We present a novel method of estimating active stress by quantifying the bending of microtubule bundles as probe particles. All our experimental observations are captured by continuum simulations based on the hydrodynamic model of active nematic liquid crystals. Our experiments demonstrate active liquid crystals with tunable defect dynamics and variable effective elasticities in the non-equilibrium state.