Manipulation of meniscus rise for <i>in situ</i> multiphase nuclear magnetic resonance spectroscopy

Our earlier work demonstrated that nuclear magnetic resonance (NMR) spectroscopy is a viable technique for in situ analysis of vapor–liquid equilibria (VLE) of fluid mixtures and that inserting a capillary into the NMR sample tube allows the simultaneous collection of separate liquid- and vapor-phase spectra for composition determination. This is the result of a thin film of liquid wicking up due to capillary forces into the vapor space, which was not possible with the undisturbed meniscus in the absence of a capillary. The present study investigates the meniscus rise (i.e., capillary action) effect in detail. We use three-dimensional (3D) computational fluid dynamics to investigate the flow regime of the liquid wicking in the NMR tube considering the flow as inviscid, low-viscosity laminar, or viscous-laminar. Thus, we can predict the liquid-volume fraction as a function of the capillary diameter and fluid properties. These results are compared to, and validated against, x-ray computed tomography experiments, which provided detailed (8 μ m resolution) 3D imaging of the meniscus of decane in an NMR tube. Phase determination of liquid and vapor cyclopentane in an NMR spectrometer provided additional validation, although integrated over a 10-mm high detection volume. Both meniscus height and thickness are significant variables to modify; thus, in situ VLE studies with NMR can be improved. We demonstrate that the prediction of the effect of capillary geometry on NMR signals for liquid and vapor phases of a pure compound can be used for rapid optimization in NMR VLE measurements.