Visualizing flow vortices inside a single levitated drop

The internal flow dynamics in single liquid drops, kept in place through levitation by a counterflowing continuous fluid phase in a suitably designed glass cell, is investigated by PFG NMR techniques. The positional stability of the drops was confirmed from series of one-dimensional profiles and was found to be below the spatial resolution of the experiment. Velocity distribution functions (propagators) along all three coordinates were obtained and demonstrated the long-time stability of the internal dynamics in terms of the velocity magnitudes occurring in the systems. Finally, velocity imaging was applied to visualize the internal vortex patterns in the drops either as projections onto different planes or within thin slices of selected orientations. Two different fluid systems were investigated in order to cover the principal cases of rigid and mobile interfaces. Different fast velocity imaging techniques were employed for monitoring the vastly differing velocity ranges of both cases, and the high sensitivity of the internal three-dimensional motion to the cell geometry is demonstrated.