Diffusion of a Tracer Sphere in Fibrous Virus (fd) Networks

摘要

We have studied the long-time self diffusion of a tracer sphere in concentrated suspensions of fibrous virus (fd), which might be of relevance for drug delivery and exploration of mass transport in biological materials like soft human tissues and organs. Various sizes of tracer spheres (3 nm- 500 nm radius) are probed in both isotropic and nematic fd rod-networks, where the mesh-size of the networks varies from small to large as compared to the tracer-sphere size. When the tracer sphere is large as compared to the mesh-size of the rod-network, direct interactions between the tracer sphere and the network are important. However, for small tracer spheres, screened hydrodynamic interactions are dominant. By systematically varying the salt concentration, and thereby the Debye length, the relative contribution of direct interactions can be tuned. In this way we were able to measure the hydrodynamic screening length over a large fd-concentration range. The hydrodynamic screening length is found to be anisotropic in nematic networks, and is a strong function of the degree of alignment. Depending on the size of the tracer spheres, we have chosen different experimental methods to investigate long-time self-diffusion: for small particles, fluorescent correlation spectroscopy (FCS) is used, and for big particles video microscopy is used. For intermediate sizes of spheres, dynamic light scattering (DLS) is an appropriate method to measure both short- and long-time diffusion coefficients. It turns out that for relatively big spheres, FCS probes short-time diffusion of the tracer sphere through the fd-network, while for small spheres FCS probes long-time diffusion.