title: Direct visualization of flow-induced conformational transitions of single actin filaments in entangled solutions.
abstract: While semi-flexible polymers and fibres 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 behaviour of entangled F-actin solutions, which aids the understanding of the flow behaviour of complex fluids containing semi-flexible polymers.
authors.affiliation: ICS-7, Forschungszentrum Jülich, D-52425 Jülich, Germany.
authors.forename: Inka
authors.initials: I
authors.lastname: Kirchenbuechler
authors.affiliation: ICS-3, Forschungszentrum Jülich, D-52425 Jülich, Germany.
authors.forename: Donald
authors.initials: D
authors.lastname: Guu
authors.affiliation: FOM Institute AMOLF, 1098XG Amsterdam, The Netherlands.
authors.forename: Nicholas A
authors.initials: NA
authors.lastname: Kurniawan
authors.affiliation: FOM Institute AMOLF, 1098XG Amsterdam, The Netherlands.
authors.forename: Gijsje H
authors.initials: GH
authors.lastname: Koenderink
authors.affiliation: 1] ICS-3, Forschungszentrum Jülich, D-52425 Jülich, Germany [2] Department of Physics and Astronomy, Laboratory for Acoustics and Thermal Physics, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium.
authors.forename: M Paul
authors.initials: MP
authors.lastname: Lettinga
mesh_terms: D008841:Actin Cytoskeleton / Q000648:ultrastructure*; D055595:Mechanical Phenomena; D018613:Microscopy, Confocal; D011108:Polymers; D012996:Solutions
chemical_list: D011108:Polymers; D012996:Solutions