The Eulerian velocity vector defined by the expansion rate at the top element of a streakline is able to represent the template and the motion of the potential mixing pattern which is intrinsic to the flow field and independent to the initial pattern. When this new velocity vector is used, its streamline represents the dynamical pattern concerning to the velocity and the direction of the lamella in the potential mixing pattern. On the other hand, its streakline represents the static geometric pattern of the lamella.