The rheological properties of a medium can be inferred from the Brownian motion of colloidal tracer particles using the microrheology procedure. The tracer motion can be characterized by the mean-squared displacement (MSD). It can be calculated from the intermediate scattering function determined by Differential Dynamic Microscopy (DDM). Here we show that DDM together with the empirical Cox-Merz rule is particularly suited to measure the steady-shear viscosity, i.e. the viscosity towards zero frequency, due to its ability to provide reliable information on long time and length scales and hence small frequencies. This method, η-DDM, is tested and illustrated using three different systems: Newtonian fluids (glycerol-water mixtures), colloidal suspensions (protein samples) and a viscoelastic polymer solution (aqueous poly(ethylene oxide) solution). These tests show that common lab equipment, namely a bright-field optical microscope, can be used as a convenient and reliable microliter viscometer. Because η-DDM requires much smaller sample volumes than classical rheometry, only a few microliters, it is particularly useful for biological and soft matter systems.

We characterize the translational and rotational dynamics of birefringent spherical colloidal particles by depolarized light scattering in the far- and near-field regimes. For this purpose, we use depolarized dynamic light scattering and propose an extension of dynamic heterodyne near-field light scattering that takes into account the polarization state of the light. Such a combination of methods allows to access colloidal dynamics in an extended q-range and permits to evaluate different modes of particles motion in suspension. Furthermore, we obtain a good agreement between results from the far- and near-field approaches thus validating our proposal and opening the possibility to investigate simultaneously the subtle interplay between translational and rotational motions of anisotropic colloidal particles in length-scales from the order of the particle size to several interparticle distances.

Abstract. We show that the spectral speckle intensity correlation (SSIC) technique can be profitably exploited to recover the path length distribution of photons scattered in a random turbid medium. We applied SSIC to the study of Teflon slabs of different thicknesses and were able to recover, via the use of the photon diffusion approximation theory, the characteristic transport mean free path * and absorption length s a of the medium. These results were compared and validated by means of complementary measurements performed on the same samples with standard pulsed laser time of flight techniques.