One- and Two-Particle Microrheology in Entangled Solutions of fd Virus
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One- and Two-Particle Microrheology in Entangled Solutions of fd Virus Karim M. Addas1, Alex J. Levine2, Jay X. Tang1, Christoph F. Schmidt3 Department of Physics & Indiana Molecular Biology Institute, Indiana University, 727 East Third St., Bloomington, IN 47405, U.S.A. 2 Department of Chemical Engineering and Materials Research Laboratory, University of California, Santa Barbara, CA 93106-4030, U.S.A. 3 Department of Physics of Complex Systems, Vrije Universiteit Amsterdam, De Boelelaan 1081, 1081HV Amsterdam, The Netherlands. 1
ABSTRACT We have used one- and two-particle microrheology, employing µm-sized beads and laser interferometric displacement detection, to study the rheological properties of entangled solutions of the filamentous fd virus. Thermal fluctuations of the embedded probes were measured and viscoelastic parameters of the embedding medium were derived. In two-particle microrheology the correlated motions of two identical particles separated by a varying distance in the medium are analyzed, which can avoid biased results due to surface-depletion effects near the probes. INTRODUCTION Polymeric materials are mechanically characterized by their viscoelastic properties. Commonly used methods involve macroscopic probes, in plate and cone or concentric cylinder (Couette) geometries [1]. Sample volumes are on the order of milliliters in such macroscopic methods. Recently, a number of techniques, collectively called microrheology, have been developed to probe the material properties of systems ranging from polymer solutions to the interior of living cells on microscopic scales [2]. All the related methods use microscopic particles embedded in the sample to be studied and either just observe thermal fluctuations of those particles (passive method) or the response to applied forces (active method). There have been several important motivations for such developments. First, in many cases and especially in biological systems, only small volumes of material with typical dimensions of micrometers are available. Second, due to volume constraints in macroscopic rheology, only pure shear elastic deformations can be applied to the systems, whereas in microrheology both shear and compressional elastic modes can be probed. Another strong motivation particularly for biological applications has been the prospect of studying inhomogeneities, for instance in the elastic properties of the cytoskeleton of cells. The cytoskeleton consists of protein polymers with a rather large persistence length compared to their diameter, which makes them “semiflexible”. In networks of semiflexible polymers, non-linear response occurs for small strains, and another advantage of using passive microrheology is that linear response is measured by definition. Furthermore, microrheology generally makes it possible to study viscoelasticity at higher frequencies, above 1kHz or in some cases up to MHz. Finally, in materials such as polymer solutions, the ability to study them with probes spanning some of the characteristic microscopic length scale
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