Progress in Colloid & Polymer Science submitted 1 New trends in optical microrheology of complex fluids and gels F. Scheffold 1,* , S. Romer 1 , F. Cardinaux 1 , H. Bissig 1 , A. Stradner 1 , L.F. Rojas-Ochoa 1 , V. Trappe 1 , C. Urban 1,2 , S.E. Skipetrov 3 , L. Cipelletti 4 and P. Schurtenberger 1 1 Department of physics, University of Fribourg, CH-1700 Fribourg, www.unifr.ch/physics/mm 2 LS Instruments, c/o Department of physics, University of Fribourg, www.lsintruments.ch 3 Department of Physics, Moscow State University,119899 Moscow, Russia 4 GDPC, Université Montpellier II, 34095 Montpellier Cedex 05 Abstract: We have studied various complex systems from particle and biopolymer gels to concentrated surfactant solutions using classical rheometry and optical microrheology. Optical microrheology uses dynamic light scattering, usually in the multiple scattering regime, to obtain information about the microscopic dynamic properties of complex media. This can be done either by direct investigation or by addition of tracer particles to otherwise transparent systems. Based on the local dynamics the macroscopic viscoelastic properties are predicted. We have implemented several new approaches to extend the range of application for optical microrheology: Taking advantage of the recently developed “two-cell technique” we will show how dynamic multiple light scattering (Diffusing Wave Spectroscopy) can be used to investigate the properties of fluid and solid-like media. Furthermore we have significantly extended the range of accessible correlation times to 10 -8 -10 4 s using a CCD based multi-speckle analysis scheme. Our experiments cover such different materials as polystyrene latex dispersions and gels, ceramic green bodies, casein micellar gels (yogurt) and giant micelle solutions. Excellent quantitative agreement is found when comparing the results obtained from DWS to classical rheological measurements. However, compared to classical rheology, we were able to significantly increase the range of accessible frequencies using optical microrheology, thereby opening up a wealth of new possibilities for the study of these fascinating materials. Keywords : Microrheology, Diffusing Wave Spectroscopy, Colloids, Biopolymers, Micelles * Corresponding author: Frank.Scheffold@unifr.ch 1. Introduction In recent years significant progress has been made in the development of modern optical techniques to study and characterize the rheological properties of complex fluids [1-9]. While these techniques have been mostly restricted to fundamental research they now become increasingly available to both industrial and applied researchers [6-11]. The underlying idea of optical microrheology is to study the thermal response of small (colloidal) particles embedded in the system under study. In this case the particle can either be artificially introduced, which is then called "tracer-microrheology", or can be part of the system itself, e.g. like in the case of ceramic green bodies. By analyzing the thermal motion of the particle it is possible to obtain quantitative information about the loss and storage moduli, G'(ω) and G''(ω) over an extended range of frequencies [1-3]. One of the most popular techniques to study the thermal motion of the particles is diffusing wave spectroscopy (DWS) which is an extension of standard photon correlation spectroscopy (PCS) to turbid media. Here the analysis of (multiply) scattered laser light is used to determine the time evolution of the probe particles mean square displacement [12- 14]. DWS allows access to a broad range of time scales which results in the above mentioned large frequency range covered by DWS-based optical microrheology. The aim of this article is twofold. First we want to show how modern optical techniques