1. Laboratoire des Colloïdes, Verres et Nanomatériaux (UMR CNRS-UMII 5587), cc26, Université Montpellier II, 34 095 Montpellier Cedex 5, France, and Department of Physics, Faculty of Sciences, Ochanomizu University, 2-1-1 Otsuka, Bunkyo-ku, Tokyo 112-0012, Japan