Friction is an extremely complex phenomenon, common to many technological, geological and biological applications. To understand friction, one needs to be able to characterize what is going on the 'single asperity' level i.e. at the molecular level [1]. Experiments using the surface forces apparatus (SFA), which monitor the frictional response of a fluid confined between smooth mica sheets, play a key role in identifying the microscopic mechanisms underlying this phenomenon. Atomistic molecular dynamics (MD) simulations provide another route to analyze this phenomenon at the molecular level. However, a direct comparison between simulation and experiment still remains impossible. This is because the lowest shear rates accessible by current MD methods are at least 4 orders of magnitude larger than those typically used in experiments. Current MD methods are therefore unable to shed light on a number of recent experimental measurements on films of about 5-8 molecular layers of aqueous solutions [2] and alkanes [3].

Using the transient-time correlation function, we show how MD simulations can be extended to study systems subjected to a realistic shear rate [4]. We apply this approach by studying the frictional response of fluids of spherical and chain molecules confined to a film of a few molecular diameters and compare our results to experimental findings.

[1] M. Urbakh, J. Klafter, D. Gourdon and J. Israelachvili, Nature 430, 325 (2004). [2] U. Raviv, P. Laurat and J. Klein, Nature 413, 51 (2001). [3] Y. Zhu and S. Granick, Phys. Rev. Lett. 93, 096101 (2004). [4] J. Delhommelle and P. T. Cummings, Phys. Rev. B 72, 172201 (2005).