1. Swiss Federal Institute for Environmental Science and Technology (EAWAG), CH-8600 Duebendorf, Switzerland, Swiss Federal Institute for Environmental Science and Technology (EAWAG), CH-6047 Kastanienbaum, Switzerland, and Insitut de Mécanique des Fluides et des Solides, UMR 7507, Université Louis Pasteur-CNRS, 2 Rue Boussingault, 67000 Strasbourg, France