COMPLEX FLUIDS, Structure, Interactions and Dynamics

This project is our basic research project where we focus on the development of new techniques or new possible applications. We want to continue our work on complex fluids, which was started with project P10628-CHE, with new objectives. We want to focus on structures up to emulsion droplets with a diameter of a micrometer and more. The possible structures range from spherical and rodlike micelles over cubic and hexagonal phases to connected cylindrical structures, bicontineous structures like L3 phases, and stiff gels. A common feature of all these complex fluids is the fact that the structures highly depend on the concentration which is usually much higher than in an ideally diluted system. We want to attack these problems with different scattering techniques. Smallangle scattering of X-rays and neutrons can be used to determine structures of complex fluids in a size range from a few nanometers up to about 100 nanometers. Static light scattering starts at about 50 nm and is the method of choice for particle sizing up to a few micrometers while dynamic light scattering can be used to measure dynamics (translational and rotational diffusion) in the whole size range covered by the different static scattering methods. In the field of small-angle scattering we plan to continue our development of a global evaluation technique based on the generalized indirect Fourier transformation. Using this technique enables us for the first time to determine the particle form factor and the structure factor (interparticle interaction) simultaneously from experimental data. In the field of dynamic light scattering we shall focus on depolarized dynamic light scattering and dynamic light scattering on nonergodic systems. Depolarized dynamic light scattering allows to measure rotational and translational diffusion coefficients from nonspherical objects, an ideal technique to follow the transition from spherical to rodlike micelles and to determine the mean length of the rods. Systems like stiff gels with partially frozen structures are called nonergodic and show different kinetic behavior in dynamic light scattering experiments when compared with fully fluid systems. Static light scattering with thin flat cells is a new technique to study dense and turbid emulsions with droplet sizes above 50 nanometers. Multiple scattering contributions can be eliminated now with a new numerical routine so that we can study such turbid systems without dilution. We plan to continue the further development of all these techniques and to perform exemplary applications to different samples like surfactant solutions, micoemulsions, gels and aggregating systems.

Complex fluids are systems made of at least two different materials like oil-water emulsions or colloidal suspensions. They may be structured in a wide regime of sizes ranging from nanometers to micrometers. Neutrons or X-rays need to be used to study nano-structured fluids while structure and dynamics of larger systems can be measured by static and dynamic light scattering. In our project we used and improved all these techniques for the characterization of complex fluids. Nano-structured complex fluids are formed by a self-assembly process which depends on concentration, i.e., these systems need to be studied at the given concentration. We have successfully developed completely new evaluation methods which avoid dilution during the investigation. With this technique we can separate information about the individual structural elements, for example emulsion droplets, from the information about their spatial arrangement driven by their interaction. We have been able to apply this new technique in basic research as well as in applications like food grade microemulsions - also known as functional food. Light scattering on micron-sized fluids is highly hindered by multiple scattering, the fluids - like milk - are turbid! We were able to overcome this essential problem by designing a new instrument using extremely thin sample cells. With this instrument we were able to study the formation of cheese in its first stages for the first time in natural concentration. Previous studies had to dilute the milk by a factor of at least one hundred, not a good start to make cheese! This technology can, of course, also be applied in basic research. So we were able the measure droplet size and interaction in micron sized emulsions for volume fractions up to 50%. Dynamic light scattering measures motions in the fluid - typically translational and rotational diffusion. This technique is best suited to study the gelation process in complex fluids without introducing external forces like in rheological measurements. The method shows the increasing amount of immobilized structures and allows to follow the changes in local diffusion dynamics during the gelation process. We have studied this process using concentrated solutions of special polymers which self-aggregate into small spherical particles leading to a transparent gel. The same polymer forms elongated, worm-like structures when the temperature is raised above 50C. We were able to follow this process by depolarized dynamic light scattering giving information about the mean length of these nano-structured objects.