Taylor dispersion and molecular displacements in Poiseuille flow

We have used pulsed gradient spin echo (PGSE) NMR to measure longitudinal d isplacements of octane molecules undergoing Poiseuille flow in a 150 mu m d iameter pipe, accessing time scales which approach the Taylor dispersion li mit. We monitor the change in displacement distribution which occurs as mol ecules undergoing Brownian motion sample an increasing proportion of the en semble of streamlines, observing the effects of wall collisions and the gra dual transition of the propagator from Poiseuille to Taylor-Aris behavior. The further use of a double PGSE sequence allows the direct measurement of the stochastic part of the motion alone. [S 1063-651X(99)50310-3].