Probing soft materials with light

Frank Scheffold
Department of Physics, University of Fribourg, CH-1700 Fribourg

Understanding structural and dynamic properties of soft materials requires information on the relevant mesoscopic length scales. Such information is often obtained using coherent laser light as a probe. Dense systems, such as colloidal suspensions and gels, however, are usually turbid and thus difficult to access. Furthermore these systems often behave as viscoelastic fluids or solids; hence do not fully relax on experimental time scales. This apparent nonergodicity previously made many interesting materials inaccessible to dynamic light scattering. I will show how recent advances in static and dynamic light scattering allow such restrictions to be overcome. Special emphasis will be given both to turbid and solid-like media such as colloidal suspensions, biopolymer solutions, glasses or gels. Based on a combination of single and multi-speckle detection schemes it is nowadays possible to cover an extended range of relaxation times from a few nanoseconds to minutes or hours corresponding to length scales below 1nm up to several microns. These new techniques go hand in hand with novel concepts in the theory of dense glassy systems, the ergodic non-ergodic transition, and micron scale dynamic properties. Significant efforts are made to conceptually link these micron-scale properties to the corresponding macroscopic elastic properties. Applications and current limitations of this so called optical microrheolgoy approach will be discussed.