Characterization of slow dynamics in turbid colloidal systems by a cross-correlation scheme based on echo dynamic light scattering.

We describe the implementation of echo dynamic light scattering in a cross-correlation detection scheme, which enables the study of slow dynamics in moderately turbid colloidal systems by adapting a commercial light scattering device. Our setup combines a 3D cross-correlation detection scheme (3DDLS), which allows for suppression of multiple scattering, with the speckle echo technique for dynamic light scattering. The recorded cross-correlation echoes provide precise ensemble-averaged results that appropriately describe sample dynamics of ergodic and non-ergodic colloidal systems of different turbidities. Additionally, the high mechanical stability achieved in our setup makes possible an absolute estimation of the scattering intensity correlation function (ICF) directly from the height of echoes, thus making unnecessary any correction for imperfect rotation of the sample or of any ad hoc assumption regarding the correspondence between the absolute values of echo height and ICF. Furthermore, we find that zeroth-order echo height represents the coherence factor of the 3DDLS experiment.