Active Filaments In and Out of Confinement
Creators: name: Peterson, Matthew
Publisher: name: Brandeis University
Year: 2021
Subjects: Active Filaments
Active Matter
Soft Matter
Statistical Mechanics

description: Active matter encompasses systems whose microscopic constituents consume energy at the particle scale to produce forces and motion. Novel macroscale phenomena emerge in these systems when these forces collectively organize into mesoscale `active stresses'. Harnessing these active stresses to drive particular emergent behaviors could enable a new class of materials with life-like properties that would be impossible in traditional equilibrium materials. Similarly, many biological functions, such as cytoplasmic streaming, morphogenesis, and cell migration, are driven by active stresses that emerge from active components confined within a cell. Simplified model systems of active matter have yielded key insights into the physical mechanisms underlying these functions. Building off of this idea, we use a minimal model of an active semiflexible filament to understand the emergent behaviors of systems comprising such filaments both in and out of confinement.
description: First, we use a continuum approximation of a Rouse chain in order to analytically compute various dynamical properties of isolated polar filaments. We find that, for such an idealized chain, the diffusion coefficient grows linearly with activity for sufficiently large active forces. This is a notable deviation from the behavior of, for example, active Brownian particles whose diffusion coefficient grows with the squared activity. Additionally, we find that these results generalize to semiflexible filaments by mapping the Rouse chain with bond length $b$ to a filament with Kuhn length $b' = 2l_p$, where $l_p$ is the (possibly activity-renormalized) persistence length. Thus, we are able to glean insight about the behavior of semiflexible filaments from the simple, analytically tractable Rouse model.
description: Next, we study a dry system of polar active filaments confined to an elastic vesicle. Though the system itself is very simple, a wide variety of steady-state configurations emerge, including stable polar rings, high-density caps, and surface-aggregated states. In general, these states can be understood through surprisingly simple physical arguments: a competition between the rotational and collisional timescales determine the onset of cap and ring formation, while appealing to an equilibrium-like energy minimization with an assumed activity-induced filament attraction can be used to estimate the number of caps that form. These physical insights likely extend beyond this simple dry system, and have implications in engineering applications and biology.
description: Finally, we give an overview of various preliminary results for active nematic systems, both in and out of confinement. Notably, we see that energy input due to active forces may preferentially enter as both twist and bend distortions, rather than splay. Similarly, simulations suggest that the 3D generic instability is a bend-twist instability. When these active filaments are confined to a highly-deformable fluidized vesicle, the vesicle fluctuations at low wavelengths transition from being tension-dominated to bending-dominating, indicating that activity leads to an effective softening of confining vesicles.