  Connecting the large-scale emergent behaviors of active materials to the
microscopic properties of their constituents is a challenge due to a lack of
data on the multiscale dynamics and structure of such systems. We approach this
problem by studying the impact of polyethylene glycol, a crowding agent, on
bundles of microtubules and kinesin-14 molecular motors. Bundles assembled in
the presence of either low or high concentrations of polyethylene glycol
generate similar net extensile behaviors. However, as polyethylene glycol
concentration is increased, the motion of microtubules in the bundles
transition from bi-directional sliding with extension to pure extension with no
sliding. Small-angle X-ray scattering shows that the transition in microtubule
dynamics is concomitant with a rearrangement of microtubules in the bundles
from an open hexagonal to a compressed rectangular lattice. These results
demonstrate that bundles of microtubules and molecular motors can display
similar mesoscopic extensile behaviors despite having very different internal
structures and dynamics.
