Direct visualization of flow-induced conformational transitions of single actin filaments in entangled solutions

TL;DR

This study visualizes how shear flow affects the three-dimensional conformations of single actin filaments in entangled solutions, revealing flow-induced structural transitions that influence the fluid's rheological properties.

Contribution

It provides direct 3D imaging of actin filament conformations under shear flow, elucidating the microscopic mechanisms behind shear-thinning and strain-softening in entangled semi-flexible polymers.

Findings

Entangled filaments form orientationally ordered hairpins in flow.

Shear flow stretches and aligns filament tails.

Filament length distribution remains unchanged under shear.

Abstract

While semi-flexible polymers and fibers are an important class of material due to their rich mechanical properties, it remains unclear how these properties relate to the microscopic conformation of the polymers. Actin filaments constitute an ideal model polymer system due to their micron-sized length and relatively high stiffness that allow imaging at the single filament level. Here we study the effect of entanglements on the conformational dynamics of actin filaments in shear flow. We directly measure the full three-dimensional conformation of single actin filaments, using confocal microscopy in combination with a counter-rotating cone-plate shear cell. We show that initially entangled filaments form disentangled orientationally ordered hairpins, confined in the flow-vorticity plane. In addition, shear flow causes stretching and shear alignment of the hairpin tails, while the filament length distribution remains unchanged. These observations explain the strain-softening and shear-thinning behavior of entangled F-actin solutions, which aids the understanding of the flow behavior of complex fluids containing semi-flexible polymers.