How geometric frustration shapes twisted fibers, inside and out: Competing morphologies of chiral filament assembly

TL;DR

This paper investigates how geometric frustration influences the formation and morphology of twisted filament bundles using continuum and discrete models, revealing multiple equilibrium structures shaped by key physical parameters.

Contribution

It introduces a comprehensive phase diagram of chiral filament assembly, identifying four main classes of equilibrium structures and validating predictions with simulations.

Findings

Four classes of equilibrium structures identified

Bundle morphology depends on surface energy, twist, and stiffness

Discrete simulations confirm continuum theory predictions

Abstract

Chirality frustrates and shapes the assembly of flexible filaments in rope-like, twisted bundles and fibers by introducing gradients of both filament shape (i.e. curvature) and packing throughout the structure. Previous models of chiral filament bundle formation have shown that this frustration gives rise to several distinct morphological responses, including self-limiting bundle widths, anisotropic domain (tape-like) formation and topological defects in the lateral inter-filament order. In this paper, we employ a combination of continuum elasticity theory and discrete filament bundle simulations to explore how these distinct morphological responses compete in the broader phase diagram of chiral filament assembly. We show that the most generic model of bundle formation exhibits at least four classes of equilibrium structure -- finite-width, twisted bundles with isotropic and anisotropic shapes, with and without topological defects, as well as bulk phases of untwisted, columnar assembly (i.e. "frustration escape"). These competing equilibrium morphologies are selected by only a relatively small number of parameters describing filament assembly: bundle surface energy, preferred chiral twist and stiffness of chiral filament interactions, and mechanical stiffness of filaments and their lateral interactions. Discrete filament bundle simulations test and verify continuum theory predictions for dependence of bundle structure (shape, size and packing defects of 2D cross section) on these key parameters.