  Achieving control and tunability of lyotropic materials has been a
long-standing goal of liquid crystal research. Here we show that the elasticity
of a liquid crystal system consisting of a dense suspension of semiflexible
biopolymers can be manipulated over a relatively wide range of elastic moduli.
Specifically, thin films of actin filaments are assembled at an oil-water
interface. At sufficiently high concentrations, one observes the formation of a
nematic phase riddled with $\pm1/2$ topological defects, characteristic of a
two-dimensional nematic system. As the average filament length increases, the
defect morphology transitions from a U-shape into a V-shape, indicating the
relative increase of the material's bend over splay modulus. Furthermore,
through the sparse addition of rigid microtubule filaments, one can further
control the liquid crystal elasticity. We show how the material's bend constant
can be raised linearly as a function of microtubule filament density, and
present a simple means to extract absolute values of the elastic moduli from
purely optical observations. Finally, we demonstrate that it is possible to
predict not only the static structure of the material, including its
topological defects, but also the evolution of the system into dynamically
arrested states. Despite the non-equilibrium nature of the material, our
continuum model, which couples structure and hydrodynamics, is able to capture
the annihilation and movement of defects over long time scales. Thus, we have
experimentally realized a lyotropic liquid crystal system that can be truly
engineered, with tunable mechanical properties, and a theoretical framework to
capture its structure, mechanics, and dynamics.
