Heterodyne near-field scattering: a technique for complex fluids.

We implemented the heterodyne near-field scattering (HNFS) technique [Appl. Phys. Lett. 81, 4109 (2002)]], showing that it is a fairly valid alternative to traditional elastic low-angle light scattering and quite suitable for studying complex fluids such as colloidal systems. With respect to the original work, we adopted a different data reduction scheme, which allowed us to improve significantly the performance of the technique, at levels of sensitivity and accuracy much higher than those achievable with classical low-angle light scattering instrumentation. This method also relaxes the requirements on the optical/mechanical stability of the experimental setup and allows for a real time analysis. The HNFS technique has been tested by using calibrated colloidal particles and its capability of performing accurate particle sizing was ascertained on both monodisperse and bimodal particle distributions. Nonstationary samples, such as aggregating colloidal solutions, were profitably studied, and their kinetics quantitatively characterized.