1. Department of Mechanical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 e-mail:
2. Department of Precision Instruments, Tsinghua University, Beijing 100084, China
3. Department of Mechanical and Aerospace Engineering, University of California, Los Angeles, CA 90095-1597
4. Professor Department of Material Science and Engineering, University of Washington, Seattle, WA 98195-2120
5. Professor Department of Mechanical Engineering, University of Washington, Seattle, WA 98195-2600