I am interested in two-phase media (microbubbles, microdroplets) and how to measure them quantitatively with MRI. MRI can be sensitive to a wide variety of physical and chemical parameters. This very sensitivity, however, can be an obstacle in realistic two-phase flows: the magnetic susceptibility difference between the gas and liquid will generate magnetic field gradients at the gas-liquid boundary, precluding quantitative studies. Designing new, (mostly) single-point imaging based sequences permits extraction of information about parameters and processes very difficult or impossible to quantify with other means (1-5). We can, in fact, exploit the bubble-generated gradients to our advantage: in our latest paper [1], we demonstrate how, in the “fast diffusion” limit, a carefully performed quantitative mapping of void fraction, dispersion, and NMR T2 * relaxation yields a spatially resolved bubble size map for bubbly flows.

Our research into quantitative characterization of two-phase flows at short timescales was mostly focused on acoustic cavitation where a strong sound wave generates violently collapsing, chemically active gaseous bubbles. In a series of papers [3-5], we have investigated cavitation both at macro- and microscales, obtaining 3D information on the void fraction and mobility of liquid in vicinity of cavitation bubbles [5]. Measurements of velocity fields of cavitating media demonstrated how bubble interactions determine the energy transfer from the acoustic field via the bubbles to the liquid motion. Details of this transfer were further studied with the newly developed approach [2] where we measured turbulent motion spectra by scanning liquid motion with oscillating magnetic field gradients at a wide frequency range.

A very different perspective on cavitation processes can be obtained by measuring the NMR signal from the gas dissolved in the liquid prior to cavitation [4]. The 19F NMR and MRI of fluorinated gases yields estimates of average residence times of gas molecules in bubbles and provides information on interaction between the cavitation bubble clouds and with the liquid.

My other research interests are in the development of portable, unilateral NMR instruments with controlled parameters (sensitive volume size and location, magnetic field gradient) (in collaboration with Drs. B.J.Balcom and B.Colpitts), and how to use them to measure various interesting and useful things. Over several years, our then graduate student A.Marble has been very successful in this development that resulted in several research papers [6-9, plus a few others] and patents; he was the recipient of the 2007 NSERC Innovation Challenge Award and the 2008 NSERC Doctoral Prize.

1. Arbabi, A., Mastikhin, I. V. Magnetic susceptibility-based Magnetic Resonance estimation of micro-bubble size for the vertically upward bubbly flow. J Magn Reson 225, 36-45 (2012).
2. Mastikhin, I. V., Hetherington, N. L., Emms, R. Oscillating gradient measurements of fast oscillatory and rotational motion in the fluids. J Magn Reson 214, 189–199 (2012).

3. Mastikhin, I. V., Newling, B. MRI measurements of an acoustically cavitated fluid in a standing wave. Phys. Rev. E 72, 056310 (2005).
4. Mastikhin, I. V., Newling, B. Dynamics of dissolved gas in a cavitating fluid. Phys. Rev. E 78, 066316 (2008).
5. Mastikhin, I. V., Arbabi, A., Newling, B., Hamza, A., Adair, A. Magnetic resonance imaging of velocity fields, the void fraction and gas dynamics in a cavitating liquid. Exp

Fluids 52, 95–104 (2012).
6. Marble, A., Mastikhin, I. V., Colpitts, B., Balcom, B. An analytical methodology for magnetic field control in unilateral NMR. J Magn Reson 174, 78–87 (2005).
7. Marble, A., Mastikhin, I. V., Colpitts, B., Balcom, B. A constant gradient unilateral magnet for near-surface MRI profiling. J Magn Reson 183, 228–234 (2006).
8. Marble, A., Mastikhin, I. V., Colpitts, B., Balcom, B. A compact permanent magnet array with a remote homogeneous field. J Magn Reson 186, 100–104 (2007).
9. Marble, A., Mastikhin, I. V., Colpitts, B., Balcom, B. Designing static fields for unilateral magnetic resonance by a scalar potential approach. IEEE Transactions on Magnetics 43, 1903–1911 (2007).