1. Univ Lyon, École normale supérieure de Lyon, Univ Claude Bernard Lyon 1, C.N.R.S., Laboratoire de Physique, F-69342 Lyon, France
2. Physics of Fluids Group, Max Planck UT Center for Complex Fluid Dynamics, Faculty of Science and Technology, MESA+ Institute, and J.M. Burgers Centre for Fluid Dynamics, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands