One- and two-particle microrheology in semiflexible polymer solutions Christoph F. Schmidt Vrije Universiteit Amsterdam Faculty of Sciences, Department of Physics and Astronomy Section of Physics of Complex Systems De Boelelaan 1081 1081 HV Amsterdam The Netherlands 　　　　　Microrheology uses micron sized probes to measure viscoelastic properties of 　　　　　complex fluids. There are clear advantages to the miniaturization of a rheology 　　　　　experiment: small amounts of material are necessary, spatial inhomogeneities can be 　　　　　studied and the bandwidth of the measurement can be high. We have developed a 　　　　　micorheology technique using laser interferometric tracking of the Brownian motion 　　　　　of micron-sized beads embedded in viscoelastic materials. We have studied several 　　　　　distinct systems with this technique, ranging from synthetic to biological, and from 　　　　　entangled solutions of rather flexible wormlike micelles to semiflexible fd virus 　　　　　particles and actin filaments. We have used both single-bead autocorrelation and 　　　　　dual-bead crosscorrelation methods. In the well-characterized wormlike micelle 　　　　　system, we show that (single-bead and dual-bead) micro- and macrorheology agree 　　　　　very well. In the semiflexible systems, which are a first approximation to the 　　　　　cytoskeleton of cells, we explore the rich multitude of length and time scales in the 　　　　　dynamic behavior of these networks, which are usually not accessible to 　　　　　macrorheology.