Bonded straight and helical flagellar filaments form ultra-low-density glasses

TL;DR

This study investigates how the three-dimensional shape of rigid filaments, such as straight and helical bacterial flagella, influences their microscopic dynamics and the rheological behavior of entangled semi-dilute suspensions, revealing shape-dependent glass formation.

Contribution

It demonstrates that filament shape critically affects dynamics and rheology, showing shape-diblock copolymers become permanently jammed at very low densities, enabling design of suspension properties.

Findings

Straight rods diffuse mainly along their axis with limited rotation.

Helical filaments escape confinement by corkscrewing.

Shape-diblocks become permanently jammed at low densities.

Abstract

We study how the three-dimensional shape of rigid filaments determines the microscopic dynamics and macroscopic rheology of entangled semi-dilute Brownian suspensions. To control the filament shape we use bacterial flagella, which are micron-long helices assembled from flagellin monomers. We compare the dynamics of straight rods, helical filaments, and shape diblock copolymers composed of seamlessly joined straight and helical segments. Caged by their neighbors, straight rods preferentially diffuse along their long axis, but exhibit significantly suppressed rotational diffusion. Entangled helical filaments escape their confining tube by corkscrewing through the dense obstacles created by other filaments. By comparison, the adjoining segments of the rod-helix shape-diblocks suppress both the translation and the corkscrewing dynamics, so that shape-diblocks become permanently jammed at exceedingly low densities. We also measure the rheological properties of semi-dilute suspensions and relate their mechanical properties to the microscopic dynamics of constituent filaments. In particular, rheology shows that an entangled suspension of shape rod-helix copolymers forms a low-density glass whose elastic modulus can be estimated by accounting for how shear deformations reduce the entropic degrees of freedom of constrained filaments. Our results demonstrate that the three-dimensional shape of rigid filaments can be used to design rheological properties of semi-dilute fibrous suspensions.