Flow Behavior Of Complex Fluids

We are studying the flow behavior of complex liquids as they occur in processes such as extrusion, fiber spinning and injection molding by using particle imaging velocimetry and rheological methods. Our experiments test new theoretical approaches with the aim of correlating microstructure, complex rheological behavior of these liquids and the properties of the finished product. We are especially focused on the occurrence of so-called “shear bands” in ultra-high molecular, semi-concentrated polymer solutions in complex pressure-driven flows.  Shear band formations refers to the coexistence of bands of different shear rates but same stress, and has been observed  in various systems such as DNA, colloids, micelles and polymer solutions. Shear bands can differ in their micro- structures but the underlying microstructural mechanism for shear band formation for the large part is not understood as yet. This phenomenon is undesirable in applications, as it can lead to inhomogeneity in processing and in material properties.

Important applications we are studying lie in the area of gel fiber spinning. In this process, ultra-high molecular, semi-concentrated polymer solutions are pressed through spin jets and are then stretched to a high degree. This pulling causes the polymers to align and orientate in the pulling direction, creating fibers of excellent tensile strength. The drawing ability of the polymer solutions depends on their microstructure. Therefore, any microstructural inhomogeneity has an effect on their drawability and consequently on the mechanical properties of the fibers.

This project is performed in cooperation with the Technical University Munich, Prof. Dr. Natalie German.