(289c) Active Nematic Liquid Crystals with Variable Activity and Elasticity

Actomyosin has long been believed to be an active contractile gel. Here we show for the first time that when actin filaments are sufficiently short, a two-dimensional dense sheet of actin-myosin solution will exhibit a nematic phase and spontaneously develop extensile flow. As short actin filaments slide on each other due to the action of motor protein clusters, such collective motion leads to a sustainable, cohesive flow that drives the proliferation, motion, and annihilation of topological defects, reminiscent of active nematic liquid crystals. Our hybrid lattice Boltzmann simulation based on the hydrodynamic model of active nematics can well capture the experimental observations that the orientation and velocity correlation lengths scale inversely with the activity parameter(s?). We further show that by visualizing the +1/2 defect's morphology, one can deduce the ratio of the splay and bend elastic constants of the material. As the system is driven out of equilibrium by the action of myosin clusters, the defect morphology transitions from a U-shape to a V-shape, implying an increase of the apparent bend-to-splay ratio - such change in a material's apparent elasticity is an indicator of how far the system is away from equilibrium. We also investigate how defect velocity and defect-defect interaction depend on the local director field, its material constants, and its activity parameters. Both experiments and simulations demonstrate that one can switch the interaction of $\pm1/2$ defect pairs between repulsion and attraction by simply tuning the activity parameter. We therefore propose that by controlling the spontaneous flow of active nematics, one can assemble and transport defect-carried cargos.