Molecular simulation of the transition from liquidlike to solidlike behavior in complex fluids confined to nanoscale gaps

We report molecular dynamics simulations at ambient temperature and pressure of dodecane films of thickness between three and eight molecular layers confined between mica surfaces. We use an accurate united-atom model for dodecane and an effective interaction between the dodecane and the confining mica surfaces that is consistent with the surface energy of a mica surface. At ambient normal pressure, the strong surface-fluid interaction leads to increased dodecane density as the wall spacing is narrowed, crossing into a density region corresponding to bulk solid when the confined film becomes narrower than six molecular layers. Correspondingly, we observed a dramatic transition from a liquidlike to an ordered, solidlike structure when the confined dodecane film is reduced from seven to six molecular layers, consistent with experimental observation of many orders of magnitude increase in viscosity at the same film thickness. The solidlike structure is characterized by the layering as well as the in-plane orientational order of the dodecane molecules. At an extreme confinement of three molecular layers, the solidlike confined film is able to sustain a nonzero shear stress. These results with realistic models provide an improved understanding of the solidlike behavior observed in surface force apparatus experiments. (C) 2001 American Institute of Physics.