Time-of-Flight Flow Imaging Of Two-Component Flow Inside A Microfluidic Chip.pdf

The authors report on using Nuclear Magnetic Resonance (NMR) imaging and spectroscopy in conjunction with time-of-flight tracking to noninvasively tag and monitor nuclear spins as they flow through the channels of a microfluidic chip. This method allows for the separate monitoring of any species with resolvable chemical-shift signatures, regardless of the optical properties of the fluids, eliminating the need for foreign tracers. The authors demonstrate this on a chip with a mixing geometry, where two fluids converge from separate channels, and show that this method is generally applicable to any microfluidic device through which fluid flows within the nuclear spin-lattice relaxation time. The authors use remote detection NMR, which physically separates the encoding and detection steps of conventional MR experiments, to individually optimize each. This method is especially well-suited for studies of flow as it correlates spectroscopic or imaging information about the fluid or fluid environment with the time of flight of the encoded nuclear spins from any location inside the sample to the detection region. The authors demonstrate the potential of this approach by studying mixing in a simple microfluidic chip, where pure ethyl alcohol flows through one channel and distilled water through the other. They show that the flow from each species can be differentiated and imaged in a single remote experiment, as long as resonances from different fluids are sufficiently resolvable and each fluid reaches the detection coil on the order of T1 after encoding. The authors also show that the width of each travel curve is a measure of the axial dispersion of the fluid flow, while the signal minimum is the most abundant TOF for travel from the encoding to detection region.