1. Mesoscale Chemical Systems Group, MESA+ Institute and Faculty of Science and Technology, University of Twente, P. O. Box 217, 7500AE Enschede, The Netherlands
2. BioInstrumentation Laboratory, Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
3. Physics of Fluids group and Max Plank Center Twente, Mesa + Institute and Faculty of Science and Technology, J. M. Burgers Centre for Fluid Dynamics and Max Plank Center Twente for Complex Fluid Dynamics, University of Twente, P. O. Box 217, 7500AE Enschede, The Netherlands