Formation mechanism of plug-like flow in nanoconfined polymer melts: Molecular dynamics study

The formation mechanism of plug-like flow in polymer melts between smooth and rough silicon walls was investigated through molecular dynamics simulations. As the driving velocity increased, plug-like flow occurred between rough silicon walls, while smooth silicon walls exhibited Poiseuille flow. The formation process of pluglike flow involved sprouting, shaping, and forming stages. Initially, the rising driving velocity in the rough silicon channel gradually increased shear stress on confined melts above a critical value. This initiated the elongation of polymer molecules near the wall from a curled to a horizontally straightened state, reducing internal viscous resistance and creating a velocity gradient gap between the near-wall and center areas, marking the sprouting of plug-like flow. In the shaping stage, horizontally straightened polymer molecules expanded from the near-wall area to the transition area, further widening the velocity gradient gap. Finally, steady plug-like flow formed when the velocity gradient gap exceeded a critical value, namely when nearly all polymer molecules in both the near-wall and transition areas were horizontally straightened. Conversely, shear stress on melts confined between smooth walls did not reach the critical value required for plug-like flow initiation. Furthermore, simulation results showed that the elongation of polymer molecules played a more significant role than boundary slip in the formation of plug-like flow-for polymer melts.