(68a) Area 1A Keynote Address: Direct Simulation-Based Evidence for Fluid-Solid Transition in Nanoconfined Non-Polar Fluids

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]. In all cases, irrespective of the fluid being studied, a common feature was noted: When the 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 [3]. Unfortunately, as attention shifted to the nature of the transition from fluid-like to solid-like behavior a similar consensus was not 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. Whilst experimental attempts have been made to clarify this issue, and end this debate, they have been severely hampered by the extreme difficulty of observing directly what occurs at this scale [4].

In contrast to the difficulties involved with experimental observation, molecular simulations techniques have an inherent atomic resolution and, as such, are ideally suited to the study of nanoscale phenomena. Because of this, a number of simulation studies have been undertaken with the aim of elucidating the nature of nanoconfinement induced phase transitions. Unfortunately, these studies have typically made use of fairly simplistic models (E.g. mica represented by an fcc lattice of Lennard-Jones spheres) [5.6], or have performed the simulations in a manner that may be criticized as biased towards the formation of such structures, leading to concerns that they fail to accurately represent the experimental systems under investigation. Thus, with the aim of elucidating the nature of any change that may occur upon nanoconfinement, here we present unprecedentedly detailed molecular dynamics simulation results for dodecane, and cyclohexane confined between molecularly-smooth, but fully atomistically-detailed mica 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. Klein, J. and Kumacheva, E., Journal of Chemical Physics 108, 6996 (1998).

4. Bae, S. C. et al., Philosophical Transactions of the Royal Society a-Mathematical Physical and Engineering Sciences 366, 1443 (2008).

5. Cui, S. T., Cummings, P. T., and Cochran, H. D., Molecular simulation of the transition from liquidlike to solidlike behavior in complex fluids confined to nanoscale gaps. Journal of Chemical Physics 114 (16), 7189 (2001).

6. Jabbarzadeh, A., Harrowell, P., and Tanner, I., Low friction lubrication between amorphous walls: unraveling the contributions of surface roughness and in-plane disorder. Journal of Chemical Physics 125, 034703 (2006).