1. Department of Mechanical Engineering, Tsinghua University, A1003-1 Lizhaoji, Beijing 100084, China;
2. Department of Mechanical Engineering, University of California, Berkeley, Berkeley, CA 94720 e-mail:
3. Department of Mechanical Engineering, University of California, Berkeley, 2114 Etcheverry, Berkeley, CA 94720 e-mail:
4. Department of Mechanical Engineering, University of California, Berkeley, 354/360 Hearst Memorial Mining Building, Berkeley, CA 94720 e-mail:
5. Mem. ASME Jacobs Institute for Design Innovation, University of California, Berkeley, 2530 Ridge Road, Berkeley, CA 94720 e-mail:
6. Fellow ASME Department of Mechanical Engineering, University of California, Berkeley, 415 Sutardja Dai Hall, Berkeley, CA 94720 e-mail: