1. Department of Chemistry and Biochemistry, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53211, United States
2. Laboratory of Mechanics, Faculty of Science, University of Yaounde I, Box 812, Yaounde, Cameroon
3. Laboratoire de Tribologie et Dynamique des Systèmes, CNRS UMR5513, Ecole Centrale de Lyon, Ecully F-69134 cedex, France