Complex Fluids II - Structure, Interactions and Dynamics

Complex fluids consist of two or more components like water, surfactant and oil, or suspended particles. As a consequence of their molecular structure they can from ordered aggregates on their own ("self-assembly"). These structures are typically very small, i.e., in the nanometer regime, and the fluid has a clear appearance like pure water, even though high amounts of oil are solubilized. Structures can also be created and stabilized by input of energy to the system. Such structures are typically in the micrometer regime and are therefore usually turbid (emulsions like milk). Hierarchically organized structures can be created if self-assembly is combined with external stabilization. These systems are self-assembled in the nanometer regime and artificially made in the micrometer regime. Scattering methods using laser-, X-ray and neutron-radiation are perfectly suited for the characterization of these structures. In this project we want to study highly concentrated complex fluids, where the particles or substructures approach each other so closely that they show essential interaction with each other. The interpretation of scattering data from such dense, interacting systems is very difficult. In the previous we succeeded to develop a worldwide unique method for the interpretation of such systems using minimum a priori information. We could also apply it successfully for several problems. We plan to continue the development of this method. The new possibilities of interpretation cause the need for improvements of the experimental installations necessary for the acquisition of data with the required quality. Interactions can lead to highly ordered lamellar structures (like a staple of sheets of paper). Attractive interaction can cause the formation of a gel or of a solid, glasslike state, where external shear forces can lead to the formation of crystal-like order. Complex fluids do not only show a large variety of different spatial structures, but show also variable internal dynamics (diffusion) and kinetics at order- disorder-transitions. The study of these structures, interactions, and dynamical processes is not only of interest in terms of basic research but has also many aspects that reach deep into applications. The field of these applications is very wide and covers topics like functional food, drug delivery, plant protection and production of new materials and the improvement of technological processes like the formation of green bodies in sinter technology.

Complex fluids are a central field of soft condensed matter research: differently to a simple molecular fluid we are dealing with fluids that not only consist of a continuous fluid phase - typically a molecular fluid like water, oil or a mixture of small molecules, but in addition contain some other species solubilized in the continuous phase: either some small solid particles or small droplets which can consist of a mixture of liquids or of a liquid crystalline phase. These particles - named colloids - will interact with each other when their have a high enough concentration. The properties of such complex fluids depend highly on these interactions and it is therefore essential to understand these relations in order to create new functional materials from such complex fluids. One very important class of techniques to study structure and dynamics of such complex fluids are scattering methods. These methods are noninvasive and give a good statistical mean of the measured properties because the illuminated scattering volume is orders of magnitude larger than the structural details in these fluids. However, these methods suffer also from different drawbacks or difficulties. The interpretation of the scattering results from such dense systems is still a nontrivial task. Here we were able to develop new software routines. They are part of a software package developed during the last years in our group in Graz, which is used is used in more than hundred research groups all over the world. With this software we were able to contribute essentially to the development and understanding of the direct coagulation casting method, a new technique for ceramic production pioneered at the ETH Zürich - our cooperation partner. But it also allows now a detailed analysis of the structure of liquid crystalline samples which are, for example, used in modern food technology. Another problem of dense colloidal systems is the fact that they are usually turbid and not optically transparent - like milk - this makes investigations with light scattering very difficult due to the effect of multiple scattering. Here we could successfully develop new instrumentation. This new laser light scattering system combines for the first time three basic components: an adjustable very thin sample cell, the 3D - cross correlation technique and the echo method. The first two in combination allow the elimination the multiple scattering effects and the later allows the study the transition from a mobile fluid system into a system with arrested dynamics. Such a transition happens every day in cheese and yoghourt making and is also important when modern ceramic materials are produced. The main advantage of the scattering methods in the study is they are noninvasive - we only have to shine a laser into the sample. This has no influence on the sample. Differently to applied shear - a yoghurt is not the same after it has been steered once!