A simple portable magnetic resonance technique for characterizing circular couette flow of non-Newtonian fluids.

In this work, we expand on past portable magnetic resonance flow methods and propose a novel method for characterizing circular (laminar) Couette flow of non-Newtonian fluids. Symmetry of the flow system combined with a constant magnetic field gradient leads to phase interference, affecting the signal magnitude, and net phase cancellation when averaging across the excited slice, preventing the use of phase-sensitive methods. Therefore, we utilize the dependence of signal magnitude at variable echo times and shear rates to characterize rheological properties. Theoretical equations governing the velocity distributions of fluids that obey a simple power-law model are used to obtain integral expressions for signal magnitude. Integral expressions can be simplified by approximating a thin excited slice or complete excitation of the Couette cell depending on experimental parameters. With simple data acquisition and analysis procedures employed, our measurements of the flow behavior indices of non-Newtonian xanthan gum dispersions are in close agreement with conventional rheological magnetic resonance measurements.

Dynamic mechanical analysis with portable NMR.

Dynamic mechanical analysis (DMA) is an umbrella term for a variety of rheological experiments in which the response of a sample subjected to an oscillatory force is measured to characterize its dynamic properties. In this work, we present a method for DMA that employs a small unilateral three magnet array with an extended constant gradient to measure the velocity of a vibrating sample. By orienting the vibrations in the direction of the gradient, we use the motion-sensitized phase accumulation to determine the velocity. By implementing delays into the pulse sequence, we measure the phase at evenly spaced points in the vibration cycle, allowing for the acquisition of a complete velocity waveform. Using velocity waveforms, samples are characterized through differences in amplitude and phase, providing information on the magnitude of the dynamic modulus and loss-angle, respectively.

Studies of periodic seawater spray icing with unilateral NMR.

Sea water ice has a complicated pore structure consisting of brine-filled pockets within a solid ice matrix. In this work, an unilateral Nuclear Magnetic Resonance instrument was used to characterize the evolution of sea-ice brine inclusions in two types of ice: stationary seawater ice and seawater spray ice formed by periodic spraying on horizontal and vertical surfaces. The portable unilateral NMR was capable of measuring very low amounts of brine (<10% of the water-filled volume). CPMG experiments were performed to extract the information on brine content and T2 distribution at temperatures between -6 °C and -16 °C. 1D imaging was used to spatially resolve the ice brine accumulation. The results show that the seawater spray ice growth, brine content (greater for the horizontal orientation than for the vertical one), and T2 distribution (unimodal for stationary ice and bimodal for spray ice) depend on temperature and surface orientation.

MRI monitoring of sea spray freezing.

Sea spray icing is a common hazard for vessels and offshore structures in cold climates. In this paper, quantitative 3D MRI and T1 - T2 mapping of the formation of sea spray ice were performed. Three different freezing regimes were employed. During freezing, changes in both relaxation times and signal intensity were greater than an order of magnitude. Results show strong differences in brine intensity and distribution for the three freezing regimes. The observed ranges of spin densities and relaxation times during freezing are well suited to measurements with portable NMR devices. There is a considerable potential for the use of MRI in studies of sea spray ice.

Motion-sensitized SPRITE measurements of hydrodynamic cavitation in fast pipe flow.

The pressure variations experienced by a liquid flowing through a pipe constriction can, in some cases, result in the formation of a bubble cloud (i.e., hydrodynamic cavitation). Due to the nature of the bubble cloud, it is ideally measured through the use of non-optical and non-invasive techniques; therefore, it is well-suited for study by magnetic resonance imaging. This paper demonstrates the use of Conical SPRITE (a 3D, centric-scan, pure phase-encoding pulse sequence) to acquire time-averaged void fraction and velocity information about hydrodynamic cavitation for water flowing through a pipe constriction.

Magnetic Resonance Imaging measurements of a water spray upstream and downstream of a spray nozzle exit orifice.

Sprays are dynamic collections of droplets dispersed in a gas, with many industrial and agricultural applications. Quantitative characterization is essential for understanding processes of spray formation and dynamics. There exists a wide range of measurement techniques to characterize sprays, from direct imaging to phase Doppler interferometry to X-rays, which provide detailed information on spray characteristics in the "far-nozzle" region (≫10 diameters of the nozzle). However, traditional methods are limited in their ability to characterize the "near-nozzle" region where the fluid may be inside the nozzle, optically dense, or incompletely atomized. Magnetic Resonance Imaging (MRI) presents potential as a non-invasive technique that is capable of measuring optically inaccessible fluid in a quantitative fashion. In this work, MRI measurements of the spray generated by ceramic flat-fan nozzles were performed. A wide range of flow speeds in the system (0.2 to >25m/s) necessitated short encoding times. A 3D Conical SPRITE and motion-sensitized 3D Conical SPRITE were employed. The signal from water inside the nozzle was well-characterized, both via proton density and velocity measurements. The signal outside the nozzle, in the near-nozzle region, was detectable, corresponding to the expected flat-fan spray pattern up to 3mm away. The results demonstrate the potential of MRI for measuring spray characteristics in areas inaccessible by other methods.

Sensitization of a stray-field NMR to vibrations: a potential for MR elastometry with a portable NMR sensor.

An NMR signal from a sample in a constant stray field of a portable NMR sensor is sensitized to vibrations. The CPMG sequence is synchronized to vibrations so that the constant gradient becomes an "effective" square-wave gradient, leading to the vibration-induced phase accumulation. The integrating nature of the spot measurement, combined with the phase distribution due to a non-uniform gradient and/or a wave field, leads to a destructive interference, the drop in the signal intensity and changes in the echo train shape. Vibrations with amplitudes as small as 140 nm were reliably detected with the permanent gradient of 12.4 T/m. The signal intensity depends on the phase offset between the vibrations and the pulse sequence. This approach opens the way for performing elastometry and micro-rheology measurements with portable NMR devices beyond the walls of a laboratory. Even without synchronization, if a vibration frequency is comparable to 1/2TE of the CPMG sequence, the signal can be severely affected, making it important for potential industrial applications of stray-field NMR.

Oscillating gradient measurements of fast oscillatory and rotational motion in the fluids.

We demonstrate the combination of oscillating gradient waveforms with single-point imaging techniques to perform measurements of rapidly oscillating and/or rotating fluid motion. Measurements of Fourier components of motion can be performed over a wide range of frequencies, while the immunity of single-point imaging to time-evolution artefacts allows applications to systems with great susceptibility variations. The processing approaches, displacement resolution, and the diffusive attenuation are analyzed. Measurements of high-speed flow rotation in a spiral phantom, periodic displacements of oscillating gas in a thermoacoustic device and of cavitating liquid reveal a variety of motion spectra. The potential systems for study with the technique include turbulent motion, cavitation, and multiphase flows in general.

A unilateral magnet with an extended constant magnetic field gradient.

Unilateral magnetic resonance (UMR) has become, in different research areas, a powerful tool to interrogate samples of arbitrary size. Numerous designs have been suggested in the literature to produce the desired magnetic field distributions, including designs which feature constant magnetic field gradients suitable for diffusion and profiling experiments. This work presents a new approach which features extended constant magnetic field gradients with a three magnet array. Constant gradients of more than 3cm extent can be achieved in a very simple, compact and safe design. Diffusion measurements from different positions over the magnet are presented in addition to practical applications for reservoir core plug characterization. The idea of a solenoid as a probe for specific measurements in UMR is introduced. Simple profiling experiments are also presented.

Dynamics of dissolved gas in a cavitating fluid.

A strong acoustic field in a liquid separates the liquid and dissolved gases by the formation of bubbles (cavitation). Bubble growth and collapse is the result of active exchange of gas and vapor through the bubble walls with the surrounding liquid. This paper details a new approach to the study of cavitation, not as an evolution of discrete bubbles, but as the dynamics of molecules constituting both the bubbles and the fluid. We show, by direct, independent measurement of the liquid and the dissolved gas, that the motions of dissolved gas (freon-22, CHClF2 ) and liquid (water) can be quite different during acoustic cavitation and are strongly affected by filtration or previous cavitation of the solvent. Our observations suggest that bubbles can completely refresh their content within two acoustic cycles and that long-lived ( approximately minutes) microbubbles act as nucleation sites for cavitation. This technique is complementary to the traditional optical and acoustical techniques.

A compact permanent magnet array with a remote homogeneous field.

We present the design and construction of a single sided magnet array generating a homogeneous field in a remote volume. The compact array measures 11.5 cm by 10 cm by 6 cm and weights approximately 5 kg. It produces a B(0) field with a 'sweet spot' at a point 1cm above its surface, where its first and second spatial derivatives are approximately zero. Unlike other sweet spot magnets of this general type, our array has B(0) oriented parallel to its surface. This allows an ordinary surface coil to be used for unilateral measurements, giving the potential for dramatic SNR improvement.

A constant gradient unilateral magnet for near-surface MRI profiling.

The design and construction of a unilateral NMR (UMR) magnet assembly for near-surface 1D profiling is presented. The arrangement consists of a single permanent magnet topped with a shaped iron pole cap. The analytically determined profile of the pole cap shapes the field over the magnet, giving a constant gradient of 31 G/cm over a 8mm depth at a 1H frequency of 4.26 MHz in a spot approximately 5 mm wide. The moderate gradient allows 1D profiling of planar samples with a frequency encoded spin-echo experiment. The curvature of the magnetic field limits the available resolution to 100's of microm. The device is suitable for profiling planar samples in which a coarse resolution but large spatial extent is desired.

MRI measurements of an acoustically cavitated fluid in a standing wave.

Double-diffusive convection in a horizontally infinite layer of a unit height in a large-Rayleigh-number limit is considered. From linear stability analysis it is shown that the convection tends to have a form of traveling tall thin rolls with width about 30 times less than height. Amplitude equations of ABC type for vertical variations of the amplitude of these rolls and mean values of diffusive components are derived. As a result of its numerical simulation it is shown that for a wide variety of parameters considered ABC system have solutions, known as diffusive chaos, which can be useful for the explanation of fine structure generation in some important oceanographical systems like thermohaline staircases.

An analytical methodology for magnetic field control in unilateral NMR.

Traditionally, unilateral NMR systems such as the NMR-MOUSE have used the fringe field between two bar magnets joined with a yoke in a 'U' geometry. This allows NMR signals to be acquired from a sensitive volume displaced from the magnets, permitting large samples to be investigated. The drawback of this approach is that the static field (B0) generated in this configuration is inhomogeneous, and has a large, nonlinear, gradient. As a consequence, the sensitive volume of the instrument is both small and ill defined. Empirical redesign of the permanent magnet array producing the B0 field has yielded instruments with magnetic field topologies acceptable for varying applications. The drawback of current approaches is the lack of formalism in the control of B0. Rather than tailoring the magnet geometry to NMR investigations, measurements must be tailored to the available magnet geometry. In this work, we present a design procedure whereby the size, shape, field strength, homogeneity, and gradients in the sensitive spot of a unilateral NMR sensor can be controlled. Our design uses high permeability pole pieces, shaped according to the contours of an analytical expression, to control B0, allowing unilateral NMR instruments to be designed to generate a controlled static field topology. We discuss the approach in the context of previously published design techniques, and explain the advantages inherent in our strategy as compared to other optimization methods. We detail the design, simulation, and construction of a unilateral magnet array using our approach. It is shown that the fabricated array exhibits a B0 topology consistent with the design. The utility of the design is demonstrated in a sample nondestructive testing application. Our design methodology is general, and defines a class of unilateral permanent magnet arrays in which the strength and shape of B0 within the sensitive volume can be controlled.

Rapid determination of the RF pulse flip angle and spin-lattice relaxation time for materials imaging.

For samples with T1s longer than 10s, calibration of the RF probe and a measurement of T1 can be very time-consuming. A technique is proposed for use in imaging applications where one wishes to rapidly obtain information about the RF flip angle and sample T1 prior to imaging. The flip angle measurement time is less than 1s for a single scan. Prior knowledge of the RF flip angle is not required for the measurement of T1. The resulting time savings in measuring the values of flip angle and T1 are particularly significant in the case of samples with very long T1 and short T2*. An imaging extension of the technique provides RF flip angle mapping without the need for incrementing the pulse duration, i.e., RF mapping can be performed at fixed RF amplifier output.

Distortion-free single point imaging of multi-layered composite sandwich panel structures.

The results of a magnetic resonance imaging (MRI) investigation concerning the effects of an aluminum honeycomb sandwich panel on the B1 and B0 fields and on subsequent image quality are presented. Although the sandwich panel structure, representative of an aircraft composite material, distorts B0 and attenuates B1, distortion-free imaging is possible using single point (constant time) imaging techniques. A new expression is derived for the error caused by gradient field distortion due to the heterogeneous magnetic susceptibility within a sample and this error is shown not to cause geometric distortion in the image. The origin of the B0 distortion in the sample under investigation was also examined. The graphite-epoxy 'skin' of the panel is the principal source of the B0 distortion. Successful imaging of these structures sets the stage for the development of methods for detecting moisture ingress and degradation within composite sandwich structures.