1. Department of Mechanical Engineering, Northwestern University , Evanston, IL 60208
2. Division of Applied Mathematics, Brown University , Providence, RI 02912
3. Department of Mechanical Engineering, Northwestern University , Evanston, IL 60208 ; , Evanston, IL 60208
4. Theoretical and Applied Mechanics Program, Northwestern University , Evanston, IL 60208 ; , Evanston, IL 60208