Colloidal Dynamics in Turbid Suspensions

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Depending on the range and strength of their mutual interactions, suspensions of colloidal particles can develop complex dynamics on different time and length scales as well as a rich and fascinating phase behaviour. Charged spheres in particular have emerged as excellent models for a vast variety of charge-stabilized globular colloids encountered in chemical industry, biology, and medicine. Investigations on the dynamics of these mainly water based systems by conventional homodyne time domain dynamic light scattering (DLS) are often limited by the onset of multiple scattering at larger concentrations and by spatial and temporal heterogeneity upon structure formation. Some alternative techniques to cope with one or both of these challenges exist. Examples include cross-correlation, echo-experiments or X-ray scattering. However, most of these are instrumentally rather demanding. A facile frequency domain approach to the intermediate scattering function is, however, possible usingSuper-Heterodyning Doppler Velocimetry, which is a standard technique in electro-kinetics and flow analysis. In our preliminary work we developed an effective way of ensemble averaging as well as an empirical, post-recording, multiple scattering correction for a small angle variant of this method. Here we propose to combine this promising technique with the versatility of a conventional Dynamic Light Scattering goniometer setup for investigations over a large range of scattering vectors. Using a prototype machine, we have already given proof of concept for this approach in our preliminary work. Now, a PhD student shall construct an improved version of this instrument, develop the necessary data processing and implement suitable evaluation algorithms. In a first stage, focus will be on demonstrating its robust and reliable performance as well as on mapping out its range of accuracy and precision. Using model systems, industrial and biological systems, we will comprehensively test the machine against time domain reference experiments like conventional and cross correlation DLS. In the second stage, we will exploit the instrumental features demonstrated before, i.e. multiple scattering correction, facile ensemble averaging and accessibility of complex scale and time dependent dynamics. We will apply the new approach to two selected examples identified in our preliminary work. Both the melt dynamics during the late stages of colloidal crystallization and the relaxation processes encountered in low density charged sphere glasses pose a number of interesting open questions. Our studies will thereby also highlight the versatility of our approach and its potential applicability for a broader range of Soft Matter systems.