Time-of-Flight Flow Imaging of Two-Component Flow inside a Microfluidic Chip

Department of Chemistry, University of California, Berkeley, Berkeley, California, United States
Physical Review Letters (Impact Factor: 7.51). 02/2007; 98(1):017601. DOI: 10.1103/PhysRevLett.98.017601
Source: PubMed


Here we report on using 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. Any species with resolvable chemical-shift signatures can be separately monitored in a single experiment, irrespective of the optical properties of the fluids, thereby eliminating the need for foreign tracers. This is demonstrated on a chip with a mixing geometry in which two fluids converge from separate channels, and is generally applicable to any microfluidic device through which fluid flows within the nuclear spin-lattice relaxation time.

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    • "In addition to this, a probeless technique is preferable if it is applied to mini-or micro-scale measurements because the probe could influence the flow of such scale. One of possible techniques that fulfill such requirements is the nuclear magnetic resonance (NMR) [23] [24] [25] [26] [27]. By employing NMR, the flow velocity, distribution or dispersion of both gas and liquid can be measured [23] if the fluids to be measured are NMR-active or can be detected indirectly by contrast agents or other sensors [27]. "
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    ABSTRACT: Coherent anti-Stokes Raman scattering (CARS) microscope system was built and applied to a non-intrusive gas concentration measurement of a mixing flow in a millimeter-scale channel. Carbon dioxide and nitrogen were chosen as test fluids and CARS signals from the fluids were generated by adjusting the wavelengths of the Pump and the Stokes beams. The generated CARS signals, whose wavelengths are different from those of the Pump and the Stokes beams, were captured by an EM-CCD camera after filtering out the excitation beams. A calibration experiment was performed in order to confirm the applicability of the built-up CARS system by measuring the intensity of the CARS signal from known concentrations of the samples. After confirming that the measured CARS intensity was proportional to the second power of the concentrations as was theoretically predicted, the CARS intensities in the gas mixing flow channel were measured. Ten different measurement points were set and concentrations of both carbon dioxide and nitrogen at each point were obtained. Consequently, it was observed that the mixing of two fluids progressed as the measurement point moved downstream. The results show the applicability of CARS to the non-intrusive concentration measurement of gas flows without any preprocess such as gas absorption into liquid or solid.
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    • "While there may be ways to fabricate chips and coils to overcome this magnetic susceptibility problem they are generally incompatible with well-established protocols for chip fabrication already in place. Detection off the chip allowed us to simultaneously image the flow of two fluids in a simple T-shaped chip as they converge into the outlet channel irrespective of the homogeneity of the magnetic field on the chip itself ([11]). However, the time resolution of the fluid flow is determined by the observation time of the spins inside the detection coil. "
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    ABSTRACT: Microfluidics has advanced to become a complete lab-on-a-chip platform with applications across many disciplines of scientific research. While optical techniques are primarily used as modes of detection, magnetic resonance (MR) is emerging as a potentially powerful and complementary tool because of its non-invasive operation and analytical fidelity. Two prevailing limitations currently inhibit MR techniques on microfluidic devices: poor sensitivity and the relatively slow time scale of dynamics that can be probed. It is commonly assumed that the time scale of observation of one variable limits the certainty with which one can measure the complementary variable. For example, short observation times imply poor spectral resolution. In this article, we demonstrate a new methodology that overcomes this fundamental limit, allowing in principle for arbitrarily high temporal resolution with a sensitivity across the entire microfluidic device several orders of magnitude greater than is possible by direct MR measurement. The enhancement is evidenced by recording chemically resolved fluid mixing through a complex 3D microfluidic device at 500 frames per second, the highest recorded in a magnetic resonance imaging experiment. The key to this development is combining remote detection with a time ‘slicing’ of its spatially encoded counterpart. Remote detection circumvents the problem of insensitive direct MR detection on a microfluidic device where the direct sensitivity is less than 10-5 relative to traditional NMR, while the time slicing eliminates the constraints of the limited observation time by converting the time variable into a spatial variable through the use of magnetic field gradients. This method has implications for observing fast processes, such as fluid mixing, rapid binding, and certain classes of chemical reactions with sub millisecond time resolution and as a new modality for on-chip chromatography.
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