The transition from full film lubrication to local film breakdown is studied using molecular dynamic simulations. In particular, the behavior of the confined lubricant and the friction are analyzed as a function of the number of molecules in the contact, surface wettability and nanoroughness. When the film thickness is progressively reduced, smooth wettable surfaces feature minima and maxima in friction corresponding to ordered and disordered states of the lubricant respectively. Wall slip occurs for non wettable smooth surfaces, which limits the shear stress to a low constant value. Finally, nanorough surfaces frustrate lubricant ordering and wall slip, thus resulting in high friction, which reaches its maximum when the surfaces come in contact.

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