Igor V. Mastikhin

Moisture migration in soft-panned confections during engrossing and aging as observed by magnetic resonance imaging

A driving force for moisture migration, namely, a gradient in water chemical potential from the shell to the center in freshly panned jellybeans, was revealed by water activity measurements. Subsequent three-dimensional imaging by a nuclear magnetic resonance technique (single point ramped imaging T1 enhanced, SPRITE), especially suited for detection of components with restricted mobility as in low moisture foods, demonstrated the migration of moisture from the shell to and through the center over a 48 h period following engrossing. During this period, nuclear relaxation times were longer in the shell than in the center, where strong magnetic interactions with macromolecules are probably enhanced by proton exchange and hydrogen bonding. While some mass transfer occurs between the shell and the center, measurements of total NMR image intensity suggest a net loss of moisture from the jellybean to the atmosphere. This was verified by gravimetric measurements, which also indicated that the process is diffusion-limited. It appears that attempts to reduce the aging period between engrossing and polishing of jellybeans should be geared toward factors that increase the diffusion of moisture.

Designing Static Fields for Unilateral Magnetic Resonance by a Scalar Potential Approach

We present a method for designing single-sided magnets suitable for unilateral magnetic resonance (UMR) measurements. The method uses metal pole pieces to shape the field from permanent magnets in a target region. The pole pieces are shaped according to solutions to Laplaces equation, and can be designed using a combination of analytical methods and numerical optimization. The design leads to analytical expressions for the pole piece shapes and magnetic field. Here, we develop the method in Cartesian, polar, and spherical coordinates, and discuss the merits of each system. Finite magnet size has a substantial effect on the field quality in many cases, according to our simulations. We found that in order to achieve a compact magnet in which the static field closely matches that specified, a full 3-D design approach is necessary. A magnet designed by our method produces a static field with a constant gradient over a region 2 cm in diameter and 2 mm thick. This leads to a compact cylindrical magnet just over 11 cm in diameter, topped with a single metal pole piece. The design is validated through simulation. The simulated field is found to agree closely with that specified analytically through the design procedure

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.

Magnetic susceptibility based magnetic resonance estimation of micro-bubble size for the vertically upward bubbly flow

The approach originally developed for the Nuclear Magnetic Resonance analysis of stable micro-bubbles is applied to studies of vertical bubbly flows. A very fast dispersion (diffusion) of water in bubbly flows extends the fast diffusion limit down to short (2-10 ms) measurement times, permitting the use of the simplified analytical expression to extract the micro-bubble size information both in bulk and spatially resolved. The observed strong bubble-induced reduction in T(2)(*) necessitates the use of very short encoding times and pure phase encoding methods to accurately measure the void fraction. There was an expected underestimation of bubble sizes at faster flow rates due to the limitations of the theory derived for small bubble sizes and non-interacting spherical bubbles (low void fractions and slow flow rates). This approach lends itself to studies of bubbly flows and cavitating media characterized by small bubble sizes and low void fractions.

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.

MR relaxometry of micro-bubbles in the vertical bubbly flow at a low magnetic field (0.2 T)

Measurements of the vertical bubbly flow were performed at a low magnetic field of 0.2T. The void fraction data were acquired. The susceptibility-induced changes in T2 relaxation time were analyzed using the previously introduced approaches by Sukstanskii et al. and Ziener et al., originally developed for the Magnetic Resonance analysis of randomly distributed and isolated spherical inclusions, and a simple model of a spherical particle, respectively. The CPMG signal decay due to the presence of spherical inclusions was approximated as linear vs. CPMG inter-echo times to extract the average inclusions size information. Two equations were derived for a simplified analysis of gas-liquid systems with basic T2 measurements, and without prior knowledge on the gas-liquid susceptibility or a need for the magnetic gradient setup. They can provide estimates for the void fraction and the average inclusion size, provided the CPMG inter-echo time requirements are met. For the control samples, there was a good agreement with the theory. For the bubbly flows, a good agreement was observed between the Magnetic Resonance and optics-based estimates for the slowest airflow rate. The deviation, however, increased for higher airflow rates. The introduced approach lends itself to the characterization of multi-phase systems such as cavitating media and well-separated bubbly flows.

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.

Motion 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 work investigates 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. Direct Magnetic Resonance Imaging measurements of dynamics of the liquid and the dissolved gas show that the motions of dissolved gas (freon‐22, CHClF2) and liquid (water) can be very different during acoustic cavitation and are strongly affected by the ultrasonic power, filtration, or previous cavitation of the solvent. The observations suggest that bubbles can completely refresh their content within two acoustic cycles and that long‐lived (∼minutes) microbubbles act as nucleation sites for cavitation. The technique is complementary to the traditional optical and acoustical tec...

Magnetic Resonance Imaging of Acoustic Streaming in Cavitating Fluid

Acoustic streaming (AS) is a bulk flow caused by attenuation of an acoustic wave propagating in the medium. When cavitating bubbles are present in the fluid, they actively absorb acoustic waves, generating acoustic streaming. Therefore, measurements of acoustic streaming can provide information on the cavitation field. In this work, Magnetic Resonance Imaging was applied to studies of cavitating fluid in a standing acoustic wave at an acoustic frequency of 31kHz. A spin echo Pulsed Field Gradient sequence was employed to sensitize the measurement to motion. Velocity spectra, kinetic energy maps and maps of the hydrodynamic dispersion coefficient were obtained for air‐saturated water, water with surfactant ([SDS] = 1 mM) and water with SDS/NaCl ([NaCl] = 0.1 M). Cavitation bubbles cause an increase in dispersion coefficient and acoustic streaming. These effects are not observed in degassed samples. Streaming was most developed in samples with surfactants, which also demonstrate a pronounced anisotropy of t...

Modulated single‐bubble sonoluminescence: Dependence of phase of flashes, their intensity and rise/decay times on viscosity, the modulation strength, and frequency

The single‐bubble sonoluminescence (SBSL) signal was studied for the case of driving frequency modulated by lower frequency with an offset. In our work, the driving frequency of 28 kHz and the modulation frequencies of 25–1000 Hz were used. The modulation strength of 0.2, 0.5, and 0.8 was defined as the difference of highest and lowest pressures over modulation period. The measurements were performed for water–glycerol mixtures of various viscosities. The measured SBSL signal appeared as a train of flashes for modulation frequencies below 250 Hz, and as a continuous modulated signal for higher frequencies. At the same frequency, the flashes covered similar phase intervals for different modulation strengths and, accordingly, pressure ranges. At higher glycerol concentrations (up to 24%) both the intensity and the stability of flashes increased, due to damped shape instabilities and reduced dancing; however, the phase interval of flashes remained about the same. Such phase‐locked behavior can be explained b...