Liquid dynamics in molecularly thin films

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Abstract
Using the surface forces apparatus technique the authors have measured the dynamic properties of ultrathin liquid films between two molecularly smooth solid surfaces sliding past each other. The results on several different liquids are reviewed; these reveal film properties that are profoundly different from those of the bulk liquids once the film thickness falls below five molecular diameters. For example, the liquids can now support a normal hydrostatic pressure as well as shear stresses, exhibiting upper and lower yield points (stick-slip friction). Certain molecular rearrangements can take 1010 times longer in a 10 AA film than in the bulk liquid. The experimental results are reproduced by computer simulations which indicate that liquid molecules in ultrathin films become ordered and solid-like, 'freezing' into discrete layers which also have lateral order. On applying a shear force, the film undergoes a melting transition from 'solid' to 'liquid' at the yield point. But even during slip some order remains within the film which therefore becomes more like a liquid crystal than a simple liquid. These phenomena and thin film properties do not appear to be readily describable in terms of mechanisms or concepts applicable to bulk liquids or solids.

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