Analysis for Polymeric Flows based on Molecular Dynamics

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This work provides a first step in attempting to analyze polymeric flows based on the molecular dynamics. In this analysis dilute suspensions of high-aspect ratio fibers in Newtonian fluids are represented as a simpler model of polymer liquids,then both the fiber orientation in simple flows and the flow patterns in complex entry flows of dilute fiber suspensions are studied. The fiber suspension flows are also compared with polymeric entry flows. Fibers, which are almost randomly oriented, change rapidly their orientations to the streamlines in a start-up simple shear flow of fiber suspensions. The rapid evolution of fiber orientation may cause the transient flow properties, such as stress overshoot, which are not exhibited in Newtonian flows. However, the suspensions of rigid straight fibers are used in the experiments, thus the drastic stress overshoot exhibited in start-up flows of polymer liquids can not be observed in fiber suspension flows. From the viewpoint of molecular dynamic … More s,large shear stress seems to be necessary in order to unravel considerably the polymer molecules close to equilibrium immediately after the start of flow.However,evidence of polymer chain unravelling is not available, thus the further research work should be required. The presence of fibers drastically changes the entry flow field: the salient corner vortex grows as the volume fraction and/or aspect ratio of fibers increase. However, even in large growth of the corner vortex, the vortex boundary becomes rather straight in rigid fiber suspension flows. This vortex boundary shape presents striking contrast to the vortex boundary being more convex in flexible molecule systems. Furthermore, the length of the salient corner vortex is almost independent of flow rate and it decreases as the Reynolds number is further increased in fiber suspension flows. On the other hand, the salient corner vortex increases in size rapidly with an increase in flow rate for flexible polymer systems. Thus, a next step in our experimental program will be the examination of flexible macroscopic fiber systems and an introduction of the effects of the change of fiber configuration during flow is necessary to develop better and simplified model fluids of polymer liquids. Less