1. Mesoscale Chemical Systems Group, MESA+ Institute, TechMed Centre and Faculty of Science and Technology, University of Twente, P.O. Box 217, 7500AE Enschede, The Netherlands
2. Physics of Fluids Group, MESA+ Institute, TechMed Centre, University of Twente, P.O. Box 217, 7500AE Enschede, The Netherlands
3. Optical Science Group, MESA+ Institute and Faculty of Science and Technology, University of Twente, P.O. Box 217, 7500AE Enschede, The Netherlands
4. BioInstrumentation Laboratory, Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA