1. Films in Fluids, Department of Science and Technology; MESA Institute for Nanotechnology, University of Twente; PO Box 217 7500 AE Enschede The Netherlands
2. Present address: BASF SE; D-67056 Ludwigshafen Germany
3. Physics of Interfaces and Nanomaterials, Department of Science and Technology; MESA Institute for Nanotechnology, University of Twente; PO Box 217 7500 AE Enschede The Netherlands