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Sliding parts in nanosystems such as Nano ElectroMechanical Systems (NEMS) and nanomotors 1-9 , increasingly involve large speeds, and rotations as well as translations of the moving surfaces; yet, the physics of high speed nanoscale friction is so far unexplored. Here, by simulating the motion of drifting and of kicked Au clusters on graphite -a workhorse system of experimental relevance 10-13 -we demonstrate and characterize a novel "ballistic" friction regime at high speed, separate from drift at low speed. The temperature dependence of the cluster slip distance and time, measuring friction, is opposite in these two regimes, consistent with theory. Crucial to both regimes is the interplay of rotations and translations, shown to be correlated in slow drift but anticorrelated in fast sliding. Despite these differences, we find the velocity dependence of ballistic friction to be, like drift, viscous.The friction of adsorbed molecules and mobile clusters thermally diffusing on surfaces is gradually moving from a theoretical concept to reality as shown for example by recent Helium-3 spin-echo experiments 14 . As well known in Brownian motion 15 , everything that diffuses will slowly drift and exhibit viscous friction under a weak force; we are however much more ignorant about the frictional behaviour to expect when the adsorbate is forced to move at a higher externally imposed speed. High speed friction has long been known in the traditional macroscopic context 16 , and also in AFM experiments 17 on asperity dominated rough surface friction, with highly case-specific outcomes. Here we address the simpler and more fundamental question of speed dependent friction during sliding motion of nanosized objects on atomically flat surfaces, a situation more likely to yield a result of generic validity. To gain experience, we undertake to explore the problem by simulations. Adsorbed Au clusters on graphite 18 are known to be thermally mobile even at room temperature 10 , and make an ideal test case. Experimentally, the low speed Au cluster friction is in principle accessible in Quartz Crystal Microbalance (QCM) experiments 19-21 , while high speed friction could be realized in AFM/STM tip based setups, where it is possible to pursue kicking techniques, either mechanical 22,23 or electrical. Anticipating experiment, we simulate by realistic molecular dynamics (MD), the speed-dependent friction of small Au clusters (typically 250-500 Au atoms, as in Fig. 1) on a fully mobile graphite surface (see Methods and Supplementary Information).Canonical MD is initially used to simulate the forcefree cluster diffusion as a function of temperature T . The Au cluster center-of-mass (CM) coordinate − − → R(t) is found to diffuse positionally on graphite, with results that are close both to experiment and to previous simulations 10-13 . In addition we observe that the two dimensional CM positional random walk is closely linked to another conjugate, and so far unexplored, one dimensional random walk executed by the cluster orientation angle...

EVOLUTIONCorrection for "Flowers of Cypripedium fargesii (Orchidaceae) fool flat-footed flies (Platypezidae) by faking fungus-infected foliage," by Zong-Xin Ren, De-Zhu Li, Peter Bernhardt, and Hong Wang, which appeared in issue 18, May 3, 2011, of Proc Natl Acad Sci USA (108:7478-7480, first published April 18, 2011; 10.1073/pnas.1103384108).The authors note that, due to misidentification by the entomologist, the flat-footed fly (Agathomyia sp.) is dropped and reidentified as Cheilosia lucida Barkalov et Cheng (Diptera, Syrphidae) by K. K. Huo of Shaanxi University of Technology. The genus Cheilosia includes some fungal feeders (1). Due to this error, we cannot prove that flat-footed flies (Platypezidae) are pollen vectors of an angiosperm species. Nevertheless, the specimens of Cheilosia lucida carried conidia of Cladosporium, thus this misidentification does not affect the main conclusions or interpretations in this article.The authors thank Peter Chandler for uncovering this error, as well as K. K. Huo of Shaanxi University of Technology and C. D. Zhu of the Institute of Zoology, Chinese Academy of Sciences for reidentification of the insects.