A new instrument for time-resolved static and dynamic light-scattering experiments in turbid media

We present a new 3D cross-correlation instrument that not only allows for static and dynamic scattering experiments with turbid samples but measures at four angles simultaneously. It thus extends the application of cross-correlation light scattering to time-resolved studies where we can, for example, efficiently investigate the temporal evolution of aggregating or phase separating turbid dispersions. The combination of multiangle 3D and on-line transmission measurements is an essential prerequisite for such studies. This not only provides time-resolved information about the overall size and shape of the particles through measurements of the mean apparent radius of gyration and hydrodynamic radius, but also on the weight-average apparent molar mass via the absolute forward scattering intensity. We present an efficient alignment strategy based on the novel design of the instrument and then the application range of the instrument using well-defined model latex suspensions. The effectiveness of the cross-correlation multiangle technique to monitor aggregation processes in turbid suspensions is finally shown for the acidification of skim milk during the yoghurt-making process. Due to the self-assembled nature of the casein micelles an understanding of the sol-gel process induced by the acidification is only feasible if time-resolved light-scattering experiments on an absolute scale are possible under industrially relevant conditions, where the casein solutions are highly turbid.