  Microtubules and motor proteins are building blocks of self-organized
subcellular biological structures such as the mitotic spindle and the
centrosomal microtubule array. These same ingredients can form new "bioactive"
liquid-crystalline fluids that are intrinsically out of equilibrium and which
display complex flows and defect dynamics. It is not yet well understood how
microscopic activity, which involves polarity-dependent interactions between
motor proteins and microtubules, yields such larger scale dynamical structures.
In our multiscale theory, Brownian dynamics simulations of polar microtubule
ensembles driven by crosslinking motors allow us to study microscopic
organization and stresses. Polarity sorting and crosslink relaxation emerge as
two polar-specific sources of active destabilizing stress. On larger length
scales, our continuum Doi-Onsager theory captures the hydrodynamic flows
generated by polarity-dependent active stresses. The results connect local
polar structure to flow structures and defect dynamics.
