The flow behavior of polymeric liquids can be traced back to the complex conformational dynamics of polymer molecules in shear flow, which poses a major challenge to theory and experiment alike due to the inherently large number of degrees of freedom. Here we directly determine the configurational dynamics of individual actin filaments with varying lengths in a well defined shear geometry by combining microscopy, microfluidics, and a semiautomated moving stage. This allows the identification of the microscopic mechanisms and the derivation of an analytical model for the dynamics of individual filaments based on the balance of drag, bending, and stochastic forces.
Living cells exhibit an enormous bandwidth of mechanical and morphological properties that are mainly determined by the cytoskeleton. In metazoan cells this composite network is constituted of three different types of filamentous systems: actin filaments, microtubules and intermediate filaments. Keratin-type intermediate filaments are an essential component of epithelial tissues, where they comprise networks of filaments and filament bundles. However, the underlying mechanisms leading to this inherently polymorphic structure remain elusive. Here, we show that keratin filaments form kinetically trapped networks of bundles under near-physiological conditions in vitro. The network structure is determined by the intricate interplay between filament elongation and their lateral association to bundles and clusters.
The assembly of colloidal particles at a liquid/liquid interface is a useful technique for the formation of a large variety of structures. Recently, we created a new method which uses liquid/liquid interfaces to assemble recombinant silk proteins into thin-shelled microcapsules. These microcapsules are mechanically stable and well suited to applications such as enzyme therapy and artificial cells. In this paper the permeability properties of these microcapsules are investigated using a novel measurement technique. It is found that the microcapsules are polydisperse in their permeabilities, but for all measured microcapsules the permeability is in the range required to protect encapsulants from immunoglobulin proteins, while allowing small molecules to enter the capsule freely.
One of the great goals in polymer
physics is to relate the various
macroscopic features of polymeric fluids with the microscopic behavior
of single chains. Here, we directly visualize the conformational dynamics
of individual semiflexible polymers in a semidilute solution above
the overlap concentration under shear. We observe that the tumbling
dynamics are significantly slowed down, in marked contrast to the
case of a dilute solution, due to steric interactions with neighboring
filaments. The observed macroscopic shear thinning effect can be rationalized
by a simple model based on the single filament dynamics.
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