A dynamical light scattering technique and its application in viscoelastic networks in soft matter

Abstract

In this paper, we present a dynamic light scattering technique using diffusing wave spectroscopy to track the dynamics of colloidal particles embedded in a complex fluid which allows us to obtain structural and dynamical information of a transparent viscoelastic material. Scattered light of a single speckle is detected by a photomultiplier tube and the time correlation function of light intensity is calculated using a temporal average. If the particles can not explore the entire phase space, temporal average and ensemble average are not the same. This is a necessary condition to relate ensemble average from the scattering by many particles to intensity temporal fluctuations. To overcome non-ergodicity for large lag times, a CCD camera is used for the acquisition of the scattered light were pixels form an array of detectors which enables us to perform thousands of simultaneous experiments. In this manner, the time correlation function is obtained directly by taking the ensamble average instead of using a temporal average. For short lag times, the non-ergodicity problem can be avoided by remixing the scattered light coming from the sample by the use of a slowly rotating diffuser disk placed before the collection optics of the photomultiplier tube. This procedure provides a true ensemble-averaged time correlation function over ~ 7-8 decades of time. As an example of the application of this technique, the dynamics of microspheres embedded in cross-linked polymer matrix, namely, an acrylamide-bisacrylamide gel is studied. This polymer network is known to swell or shrink by changing the solvent composition. The description of the arrested dynamics of the microspheres can be obtained, as well as the viscoelastic properties of the polymer network at different cage sizes.