Self-propelled Worm-like Filaments: Spontaneous Spiral Formation, Structure, and Dynamics

TL;DR

This study uses Brownian dynamics simulations to explore how self-propelled worm-like filaments form spirals and behave dynamically, revealing the influence of propulsion strength and flexibility on their structure and motion.

Contribution

It introduces a comprehensive simulation analysis of self-propelled worm-like filaments, highlighting spontaneous spiral formation and activity-dependent dynamic properties.

Findings

Strong propulsion and flexibility lead to stable spiral formation.

Active propulsion significantly affects rotational and translational diffusion.

Structural properties remain largely unaffected unless spirals form.

Abstract

Worm-like filaments that are propelled homogeneously along their tangent vector are studied by Brownian dynamics simulations. Systems in two dimensions are investigated, corresponding to filaments adsorbed to interfaces or surfaces. A large parameter space covering weak and strong propulsion, as well as flexible and stiff filaments is explored. For strongly propelled and flexible filaments, the free-swimming filaments spontaneously form stable spirals. The propulsion force has a strong impact on dynamic properties, such as the rotational and translational mean square displacement and the rate of conformational sampling. In particular, when the active self-propulsion dominates thermal diffusion, but is too weak for spiral formation, the rotational diffusion coefficient has an activity-induced contribution given by $v_c/\xi_P$, where $v_c$ is the contour velocity and $\xi_P$ the persistence length. In contrast, structural properties are hardly affected by the activity of the system, as long as no spirals form. The model mimics common features of biological systems, such as microtubules and actin filaments on motility assays or slender bacteria, and artificially designed microswimmers.