(284c) Glassy Dynamics for Monoatomic and Molecular Fluids Confined Between Mica Surfaces

Using a molecular model for Octamethylcyclotetrasiloxane (OMCTS), molecular dynamics simulations are carried out to study the state of OMCTS confined between extended mica surfaces at  various surface separations. The mica surfaces are equilibrated with a liquid OMCTS bath to determine the equilibrium number density between the surfaces.  OMCTS is found to form distinct layers for surface separations below 4 nm. Analysis of static structural quantities such as in-plane correlation functions did not reveal signatures of freezing even under extreme confinement of two layers. The self-intermeidate scattering function is found to decay with increasing relaxation times as the surface separation is decreased, suggesting that confined OMCTS behaves like a fluid approaching a glass transition upon confinement.  The two-step relaxation in the scattering function, a distinct signature of glassy dynamics, was clearly observed at lower temperatures. However even at 5 K above the melting point we did not observe a freezing transition. The self-diffusivity and relaxation times obtained from the Kohlrausch-Williams-Watt stretched exponential fits to the late alpha relaxation exhibit power law scalings with the packing fraction as predicted by mode coupling theory. These dynamical signatures indicate that OMCTS undergoes a slowdown akin to a fluid approaching a glass transition upon increasing confinement. We also carry out simulations with a monoatomic Lennard-Jones fluid near the bulk freezing point. These simulations show distinct glass-like dynamics for the monoatomic fluid, as reflected in the two step relaxation in the self-intermediate scattering function, with increasing confinement.