K.R.P. acknowledges support from a National Science Foundation Graduate Research Fellowship and a National Defense Science and Engineering Graduate fellowship from the Department of Defense. N.V. acknowledges funding from the Leopoldina Fellowship Program. C.C.P. acknowledges the support of an Edward, Frances and Shirley B. Daniels and Wyss Fellowship while at the Radcliffe Institute for Advanced Study, 2012−2013. The authors thank A. V. Shneidman for helpful discussions. This work was funded with support from the Air Force Office of Scientific Research (AFOSR) under Award FA9550-09-0669-DOD35CAP, and it was performed in part at the Center for Nanoscale Systems (CNS) at Harvard University.