  In active matter systems, self-propelled particles can self-organize to
undergo collective motion, leading to persistent dynamical behavior out of
equilibrium. In cells, cytoskeletal filaments and motor proteins self-organize
into complex structures important for cell mechanics, motility, and division.
Collective dynamics of cytoskeletal systems can be reconstituted using filament
gliding experiments, in which cytoskeletal filaments are propelled by
surface-bound motor proteins. These experiments have observed diverse dynamical
states, including flocks, polar streams, and single-filament spirals. Recent
experiments with microtubules and kinesin motor proteins found that the
collective behavior of filaments can be tuned by altering the concentration of
the crowding macromolecule methylcellulose in solution. Increasing the
methycellulose concentration reduced filament crossing, promoted alignment, and
led to a transition from active, isotropically oriented filaments to locally
aligned polar streams. This emergence of collective motion is typically
explained as an increase in alignment interactions by Vicsek-type models of
active polar particles. However, it is not yet understood how steric
interactions and bending stiffness modify the collective behavior of active
filaments. We use simulations of driven filaments with tunable soft repulsion
and rigidity in order to better understand how the interplay between filament
flexibility and steric effects can lead to different active dynamic states. We
find that increasing filament stiffness decreases the probability of filament
alignment, yet increases collective motion and long-range order, in contrast to
the assumptions of a Vicsek-type model. In addition to repulsion and driving,
tuning filament stiffness can promote collective behavior, and controls the
transition between active isotropic filaments, locally aligned flocks, and
polar streams.
