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 pressur e of dodecane films of thickness between three and eight molecular layers c onfined between mica surfaces. We use an accurate united-atom model for dod ecane and an effective interaction between the dodecane and the confining m ica surfaces that is consistent with the surface energy of a mica surface. At ambient normal pressure, the strong surface-fluid interaction leads to i ncreased dodecane density as the wall spacing is narrowed, crossing into a density region corresponding to bulk solid when the confined film becomes n arrower than six molecular layers. Correspondingly, we observed a dramatic transition from a liquidlike to an ordered, solidlike structure when the co nfined dodecane film is reduced from seven to six molecular layers, consist ent with experimental observation of many orders of magnitude increase in v iscosity at the same film thickness. The solidlike structure is characteriz ed by the layering as well as the in-plane orientational order of the dodec ane molecules. At an extreme confinement of three molecular layers, the sol idlike confined film is able to sustain a nonzero shear stress. These resul ts with realistic models provide an improved understanding of the solidlike behavior observed in surface force apparatus experiments. (C) 2001 America n Institute of Physics.