# Filamentous Active Matter: Band Formation, Bending, Buckling, and Defects

## TL;DR

This study uses computer simulations to connect microscopic filament-motor interactions with large-scale self-organization phenomena like bundle formation, nematic phases, and defect dynamics in active filament systems.

## Contribution

It introduces a component-based simulation approach linking microscopic interactions to mesoscopic active matter behaviors, revealing universal scaling laws and defect formation mechanisms.

## Key findings

- Bundles form at low densities due to crosslinking and sliding.
- Active polar nematics emerge at high densities.
- Domain size and defect density scale with activity and motor concentration.

## Abstract

Motor proteins drive persistent motion and self-organisation of cytoskeletal filaments. However, state-of-the-art microscopy techniques and continuum modelling approaches focus on large length and time scales. Here, we perform component-based computer simulations of polar filaments and molecular motors linking microscopic interactions and activity to self-organisation and dynamics from the two-filament level up to the mesoscopic domain level. Dynamic filament crosslinking and sliding, and excluded-volume interactions promote formation of bundles at small densities, and of active polar nematics at high densities. A buckling-type instability sets the size of polar domains and the density of topological defects. We predict a universal scaling of the active diffusion coefficient and the domain size with activity, and its dependence on parameters like motor concentration and filament persistence length. Our results provide a microscopic understanding of cytoplasmic streaming in cells and help to develop design strategies for novel engineered active materials.