  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.
