1. Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
2. Physics of Fluids Group, Max Planck Center Twente for Complex Fluid Dynamics, Department of Science and Technology, University of Twente, P.O. Box 217, Enschede 7500 AE, The Netherlands
3. Department of Physics, Johannes Gutenberg University, Staudingerweg 10, 55128 Mainz, Germany