Twisted by fate, or screwing up purpose: self-limiting fibers from achiral vs. chiral filament assembly

Twisted bundles of cohesive filaments or chains are common architectures in a range of supramolecular materials, from protein filaments to self-stacking, organogel fibers. Several theoretical models have shown that assembly twist can lead to a thermodynamic self-limitation of bundles due to the build-up of the costs of geometric frustration with lateral diameter. Common to all these models is the assumption of intrinsic chirality of the constituent filaments as the driving force for collective twist. However, recent experimental and simulation results point to the possibility of self-limitation by the spontaneous twisting of achiral fibers. In this talk, I describe a generic continuum elastic framework for comparing formation of twisted fibers of chiral vs. achiral filament bundles of various states of order (e.g. columnar, solid, polymer nematic). I will contrast predictions for the evolution of pitch with lateral size as well as possible ranges of self-limitation. This analysis argues that self-limited fibers of chiral bundles may reach mesoscopic (multi-filament) dimensions, while self-limitation of spontaneously twisting fibers (achiral filaments) should be expected only up to diameter of a few filaments wide.