Which Factors Control the Onset of the Fluid-Solid Transition of a Nanoconfined Fluid?

It is well known that the behavior of fluids confined to the order of a few nanometers may differ greatly from that of the corresponding bulk fluid.¹ For example, in the 1980s and early 1990s a large number of studies were reported on a variety of ultrathin liquid films confined between mica surfaces.² Although the fluids investigated ranged from linear alkanes³ to cyclical⁴ and globular5 molecules, they exhibited one common feature: When their confinement reached the order of several molecular diameters, a rapid many-orders-of-magnitude increase in the viscosity of the confined fluid was observed, together with behavior typical of the stick-slip response of a crystalline solid structure.⁶  Following this work, attention shifted to the nature of the transition from fluid-like to solid-like behavior.  Unfortunately, in this case, a consensus has not been achieved and, for over a decade, there has been intense debate as to whether it is a first-order (crystallization) or second-order (vitrification) order phase transition.

Previously, we presented initial results of a computer simulation study of fluids (dodecane and cyclohexane) confined between mica sheets, making use of the latest atomistically-detailed fully-flexible molecular models.⁷ These results provided compelling evidence that the transition is consistent with a first-order phase transition. Here we build on that work by applying the same highly-detailed molecular models to an investigation of factors that may explain the experimental discrepancies underlying the intense debate surrounding the effects of nanoconfinement. In particular, we consider the effects of surface shear, contamination of the confined fluid and the relative alignment of the confining surfaces.

¹       Van Alsten, J. and Granick, S., Langmuir 6, 876 (1990).

²       Alba-Simionesco, C. et al., Journal of Physics-Condensed Matter 18, R15 (2006).

³       Christenson, H. K., Gruen, D. W. R., Horn, R. G., and Israelachvili, J. N., Journal of Chemical Physics 87, 1834 (1987).

⁴       Gee, M. L., Mcguiggan, P. M., Israelachvili, J. N., and Homola, A. M., Journal of Chemical Physics 93, 1895 (1990).

⁵       Israelachvili, J. et al., Journal of Physics-Condensed Matter 2, Sa89 (1990).

⁶       Klein, J. and Kumacheva, E., Journal of Chemical Physics 108, 6996 (1998).

⁷     Docherty, H and Cummings P.T., "When Nano-Confined Between Mica Sheets, Does Dodecane Undergo a Phase Transition?" AIChE Annual Meeting 2008, Philadelphia, PA.