Ultrafast MRI of immiscible fluids

Understanding the dynamics of immiscible fluids is critical for a wide variety of applications, but imaging their flows is notoriously difficult. Most conventional techniques are intrusive—introducing tracer particles, for example—or require optical access. Magnetic resonance imaging (MRI) is an attractive alternative: It can quantitatively map velocity distributions and is inherently sensitive to different molecular species. But as usually implemented, MRI is slow, and schemes to reduce data acquisition times can introduce artifacts if multiple chemical species are present. The use of MRI for multiphase flows, which can exhibit such transient phenomena as shape oscillations and vortex formation, has thus been limited. Now Andy Sederman, Lynn Gladden, and colleagues at the University of Cambridge have demonstrated a way to obtain high-speed MRI images that clearly differentiate between chemical species in immiscible fluid flow. They adapted a sophisticated mathematical trick, compressed sensing, in such a way that when combined with a fast two-dimensional acquisition scheme known as spiral imaging, they could extract a quantitative image of multiple chemical species from a small subset of standard MRI data. By applying the technique to a test system of silicone oil droplets rising through a 2-cm-diameter column of water, the team could follow the evolution of the system, including the internal dynamics of the droplets, at 188 frames per second with a resolution of 385 μm. This sample image shows the velocity fields obtained for the water and droplet (inset) as the droplet passed through the imaging plane. The approach could offer promise for the quantitative study of hydrodynamics and chemical reactions in multiphase systems. (A. B. Tayler et al., Phys. Rev. E 89, 063009, 2014 doi:10.1103/PhysRevE.89.063009 .)