1. Department of Mechanical Engineering; State Key Laboratory of Tribology; Tsinghua University; Beijing 100084 P. R. China
2. Department of Mechanical Engineering; Beijing Key Lab of Precision/Ultra-Precision Manufacturing Equipments and Control; Tsinghua University; Beijing 100084 P. R. China
3. Department of Chemical and Biomolecular Engineering; Tulane University; New Orleans LA 70118 USA