Introduction In recent years significant progress has been made in the development of modern optical techniques to study and characterise 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 analysing the thermal motion of the particle it is possible to obtain quantitative information about the loss and storage moduli, G¢(x) and G¢¢(x) 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 Progr Colloid Polym Sci (2004) 123: 141–146 DOI 10.1007/b11748 Ó Springer-Verlag 2004 F. Scheffold S. Romer F. Cardinaux H. Bissig A. Stradner L.F. Rojas-Ochoa V. Trappe C. Urban S.E. Skipetrov L. Cipelletti P. Schurtenberger New trends in optical microrheology of complex fluids and gels F. Scheffold (&) Æ S. Romer F. Cardinaux Æ H. Bissig Æ A. Stradner L.F. Rojas-Ochoa Æ V. Trappe C. Urban Æ P. Schurtenberger Department of Physics, University of Fribourg, 1700 Fribourg, Switzerland e-mail: Frank.Scheffold@unifr.ch C. Urban LS Instruments, c/o Department of Physics, University of Fribourg, 1700 Fribourg, Switzerland S.E. Skipetrov Department of Physics, Moscow State University, 119899 Moscow, Russia L. Cipelletti GDPC, Universite´ Montpellier II, 34095 Montpellier Cedex 05, France Abstract We have studied various complex systems from particle and biopolymer gels to concentrated surfactant solutions using classical rheometry and optical microrheolo- gy. Optical microrheology uses dy- namic 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 trans- parent systems. Based on the local dynamics the macroscopic visco- elastic properties are predicted. We have implemented several new ap- proaches to extend the range of application for optical microrheolo- gy: Taking advantage of the recently developed ‘‘two-cell technique’’ we will show how dynamic multiple light scattering (Diffusing Wave Spectroscopy) can be used to inves- tigate 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 mul- tispeckle analysis scheme. Our experiments cover such different materials as polystyrene latex dis- persions and gels, ceramic green bodies, casein micellar gels (yogurt) and giant micelle solutions. Excel- lent 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 opti- cal microrheology, thereby opening up a wealth of new possibilities for the study of these fascinating mate- rials. Keywords Microrheology Æ Diffusing wave spectroscopy Æ Colloids Æ Biopolymers Æ Micelles