ORIENTATIONAL EFFECTS IN THE CHANNEL FLOW OF FLEXIBLE AND RIGID MOLECULES

Short linear molecules exhibit rotational motion and orientational pre ferences under sheared flow. This paper describes a series of molecula r-dynamics simulations of fluids consisting of various kinds of short linear chain molecules that are forced to flow through a channel bound ed by rough, parallel walls (Poiseuille flow). The molecules are const ructed of soft spheres; these are linked in different ways to produce either fully flexible chains, stiff chains with restricted internal de grees of freedom, or rigid rodlike molecules. The channel walls act as nonslip boundaries and also serve to absorb the thermal energy genera ted by the shear motion. For each kind of molecule, the rotational mot ion and orientation distributions are explored as functions of the cro ss-stream position. The variation in orientation shows that there is a competition between the Maxwell orientation, where the molecule is al igned at 45 degrees to the flow, and a tendency for molecules close to the walls to align parallel to the direction of flow. The orientation al effects become more pronounced with increasing molecular stiffness.