Collective motion of driven semiflexible filaments tuned by soft repulsion and stiffness

Abstract

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 exhibit activity and self-organization 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 behavior, including flocks, polar streams, swirling vortices, and single filament spirals. Recent experiments with microtubules and kinesin motor proteins found that the effective repulsive interaction between filaments can be tuned by crowding agents in solution, altering the collective behavior. Adding a crowder reduced filament crossing, promoted alignment, and led to a transition from active, isotropically oriented filaments to locally aligned polar streams. These results suggest that tunable soft repulsion can control active phase behavior, but how altering steric interactions and filament stiffness alter collective motion is not fully understood. Here 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 steady states. We identify swirling flocks, polar streams, buckling bands, and spirals, and describe the physics that govern transitions between these states. In addition to repulsion, tuning filament stiffness can promote collective behavior, and controls the transition between active isotropic filaments, locally aligned flocks, and polar streams.

Chiral self-sorting of active semiflexible filaments with intrinsic curvature

“…This is due to smaller domains having more efficient packing within the system, and therefore fewer collisions between clusters result in fewer opportunities for filaments to intermix between sorted domains. Filament flexibility weakens chiral sorting due to filament deformation, consistent with our observations that flexibility weakens structural order in systems of driven filaments [32]. Flexible filaments collide and form heterochiral flocks, which perturb chiral clusters with collisions, increasing mixing.…”

“…This packing reduces interweaving between filament layers, and thereby decreases the stability of large flocks, even in entirely homochiral systems (see ESI). Therefore, filament curvature causes rigid filaments to have lower long-range structural order than flexible filaments, a reversal of what has been previously reported for achiral semiflexible filament behavior [32,42].…”

“…In the absence of curvature, achiral driven filaments aggregate into well-known polar flocks [32,[36][37][38][39][40][41], with structural order that is dependent on filament bending rigidity, as shown in previous work [32,42]. Driven flexible filaments can form an active disordered phase of swirling defects [17,43,44].…”

“…Our Brownian dynamics simulations are two-dimensional with periodic boundary conditions of N filaments with intrinsic curvature. Following previous work [31,32], the filaments in our simulations are modeled as discretized wormlike chains [33] composed of n sites and n − 1 rigid segments of fixed length a. We adopt the algorithm of Montesi et.…”