Catching a Falling Drop by NMR: Correlation of Position and Velocity

Abstract

The spin density profile and the statistical probability density of displacements of a drop free falling through a vertical-bore magnet was measured in all three spatial directions, both separated and in a correlated way employing NMR imaging methods and in particular pulsed field gradient (PFG) NMR techniques. The falling motion was monitored by double encoding of vertical position at successive times separated by Δ within a single two-dimensional (2D) PFG NMR experiment i.e. position exchange spectroscopy (POXSY). 2D images along the cross-section of the drop, mapping the z- velocity component in each pixel, verifies the constant z-velocity throughout the total drop volume. 2D images encoded with transverse velocities contain structures characteristic of velocity distributions which can be attributed either to vortex-motions inside the drop or coherent oscillations of the whole drop. The fact that the net transverse velocity over the cross-section is zero proves that no net motions of the drop itself contribute to the results. The essential challenge has been to implement one- and multi-dimensional NMR imaging techniques within the residence time of the falling drop inside the resonator of about 10 ms. The solution was to accumulate information from many drops by triggering every single acquisition to another falling drop. The regularity and uniformity of the dripping process was clearly demonstrated, which was the absolute requirement for signal accumulation as well as for multidimensional NMR experiments. Given that the experimentally determined velocity profile of the drop by NMR imaging methods was reproducible and reliable, the basis for mapping motion of and inside a drop in a direct, three-dimensional and non-invasive way by NMR flow imaging methods is given.