1. Department of Physical Chemistry, Barcelona University, c/ Marti i Franques 1, Pta 4, 08028 Barcelona, Spain, Department of Physics, Tampere University of Technology, Tampere, Finland, Department of Applied Mathematics, The University of Western Ontario, London (ON), Canada, Department of Applied Physics and Helsinki Institute of Physics, Helsinki University of Technology, Helsinki, Finland, and MEMPHYS-Center for Biomembrane Physics, University of Southern Denmark, DK-5230 Odense, Denmark