a Molecular Dynamics Study of a Two-Dimensional Diatomic Lennard-Jones System Under Shear Stress.

The dynamical response of monatomic and diatomic liquid systems to large and suddenly applied external shearing forces is investigated. In these nonlinear situations it is difficult to carry out laboratory investigations that probe the microscopic origin of the macroscopic behavior; consequently a molecular dynamics computer simulation approach is utilized to examine the structural behavior of liquids abruptly subjected to large shear rates. A two-dimensional system of particles interacting via a Lennard-Jones potential is considered. While it precludes making any direct statements about the response of a "real" liquid, this two-dimensional simplification proves useful in visualizing the structural changes that occur in a sheared liquid. Moreover, it helps to establish molecular theories of liquid behavior that can perhaps be generalized to three-dimensional situations, i.e., to real liquids. The time evolution of shear stress, internal energy and pressure in monatomic and diatomic systems in response to a suddenly applied couette flow pattern was computed for shear rates ranging over more than two orders of magnitude. The structural dynamics were characterized by the time dependence of the pair distribution function and the density fluctations in the liquid. As an aid in visualizing the liquids' microscopic response, computer graphics were employed to generate a pictorial record of the molecular motion, which was stored on videotape. The asymmetry in the shape of the molecules used in the non-equilibrium simulation of the diatomic system introduces a significant feature to the liquid response. The most obvious distinction between the diatomic and the monatomic systems is the coupling of the translational and rotational degrees of freedom of a di-atom. This allows for a transfer of energy such that di-atoms can reorient themselves in a manner that creates relatively stable structural groupings within the flow pattern. The appearance of a thermodynamic instability at low shear rates in the monatomic system is known to be characteristic of the behavior of a two -dimensional fluid. That a qualitatively identical instability was observed in the diatomic system suggests that current theories, developed initially to describe the thermodynamic of monatomic liquids, are also applicable to molecular systems.