Dynamics of liquid-crystalline emulsion droplets arrested in hydrogels: Addressing the multiple scattering problem in turbid systems

• DLS instrument with multispeckle detector and brute force detection is described.
• A series of DLS results for turbid colloidal suspensions are presented.
• Diffusion of lipid droplets entrapped in polymeric hydrogels is studied.
• Polymer concentration necessary to arrest lipid droplets is determined.

Performing dynamic light scattering (DLS) measurements on turbid samples is still a great challenge. A standard DLS instrument will provide reliable measurements as long as the sample is transparent and ergodic. Measurements on highly concentrated systems, on the other hand, such as glasses or gels, show partially frozen dynamics or incomplete relaxation processes. Such systems are called nonergodic. Nonergodic systems commonly appear in various practical applications and are therefore of great scientific as well as industrial interest. Many of these systems show high turbidity caused by the multiple scattering. The contribution of multiple scattering accompanied by low light transmission often leads to false DLS results for such systems, since standard DLS theory is only valid for single-scattered light. We describe here a DLS instrument that has been modified to address this problem. We use a charge-coupled device (CCD) camera as a multispeckle detector, and combine single-mode fiber optics with a photomultiplier to determine both ensemble and time-averaged autocorrelation functions. The instrument features a thin, flat sample cell with a translation stage. We show some DLS results for a series of turbid colloidal suspensions, where the lipid droplets are entrapped in polymeric hydrogels of different polymer concentrations. We focus on the transition from ergodicity to nonergodicity using ensemble-averaged DLS-intensity autocorrelation functions.

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