(435f) 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.