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Selective oil-phase rheo-MRI velocity profiles to monitor heterogeneous flow behavior of oil/water food emulsions
Publikationstyp
Journal Article
Publikationsdatum
2019-09
Sprache
English
TORE-URI
Enthalten in
Volume
57
Issue
9
Start Page
766
End Page
770
Citation
Magnetic Resonance in Chemistry 57 (9): 766-770 (2019-09)
Publisher DOI
Scopus ID
PubMed ID
30515894
Food emulsions are common to a great variety of natural and processed products encountered in everyday life. Well known are the so-called oil/water emulsions, where the poor miscibility between both fluids is overcome by dispersing one phase into the other. Examples of these systems are found not only in dairy products such as cream cheese but also in mayonnaise and dressings. At high oil concentration, these dispersions exhibit complex rheological behavior that cannot be completely understood from simple rheological measurements. Because the emulsion may display heterogeneous flow behavior under deformation, it is necessary to couple these experiments with localized shear rate measurements in order to determine a constitutive flow curve of the material. Spatially flow encoded profiles can be obtained by employing optical methods such as particle imaging/tracking velocimetry and laser doppler velocimetry (LDV). These techniques are widely known to have a high temporal and spatial resolution, but because they rely on having an optical access of the sample, they cannot be implemented on opaque systems. In contraposition, magnetic resonance imaging (MRI) uses the sample's molecules as intrinsic probes, allowing the noninvasive imaging of velocity under flowing conditions.
In the past years, MRI has proven to be a suitable technique for the characterization of multiphase flows, such as high-voidage bubbly flow, high-speed gas flows, fluidized beds, liquid–liquid systems, and oil–water multiphase flow. More in particular, MRI velocimetry experiments performed on samples under deformation (commonly known as rheo-MRI) have been successfully applied to the characterization of diluted and dense emulsions, identification of non-local effects, sedimentation, and shear-induced migration of droplets. It has become clear that the understanding of the complex rheological behavior of emulsions will depend on the accurate spatially resolved determination of the droplets' phase concentration and velocity under shear. In short, we will refer to these spatially resolved images as concentration and velocity profiles.
In order to acquire an oil concentration profile under flowing conditions, it is necessary to distinguish the oil from the water signal in a spatially resolved manner. Generally, these two components can be discriminated by their relaxation times (T1 and T2) and the difference in chemical shift between water and CH2 protons, equal to 3.5 ppm. Even though a relaxation time contrast was recently employed as a way to distinguish water and oil signal, these parameters strongly depend on fluid composition, mobility, and temperature, which hampers its generic use in rheo-MRI.
A chemical shift discrimination of oil protons can be also accomplished by employing phase encoding methods. Even though this technique is less sensitive to magnetic field inhomogeneities, usually a few experiments are needed to achieve a proper separation of oil and water images. This compromise its use for rheo-MRI applications, where a snap-shot of the flowing fluid is required.
On the other hand, a chemical shift selective (CSS) contrast can be also achieved either by exciting only oil protons within the sample or by suppressing the water signal and using the remaining magnetization to acquire a flow encoded image. One of the advantages of employing suppression techniques is they are not considerably affected by magnetic field distortions, because no net magnetization of the unwanted component is retained before image acquisition.
In this work, we propose a method to acquire chemically selective oil concentration and velocity profiles of an oil/water emulsion under shear. Following a proper validation of the technique, the method is applied to the monitoring of shear-induced migration of oil droplets as a proof of concept.
In the past years, MRI has proven to be a suitable technique for the characterization of multiphase flows, such as high-voidage bubbly flow, high-speed gas flows, fluidized beds, liquid–liquid systems, and oil–water multiphase flow. More in particular, MRI velocimetry experiments performed on samples under deformation (commonly known as rheo-MRI) have been successfully applied to the characterization of diluted and dense emulsions, identification of non-local effects, sedimentation, and shear-induced migration of droplets. It has become clear that the understanding of the complex rheological behavior of emulsions will depend on the accurate spatially resolved determination of the droplets' phase concentration and velocity under shear. In short, we will refer to these spatially resolved images as concentration and velocity profiles.
In order to acquire an oil concentration profile under flowing conditions, it is necessary to distinguish the oil from the water signal in a spatially resolved manner. Generally, these two components can be discriminated by their relaxation times (T1 and T2) and the difference in chemical shift between water and CH2 protons, equal to 3.5 ppm. Even though a relaxation time contrast was recently employed as a way to distinguish water and oil signal, these parameters strongly depend on fluid composition, mobility, and temperature, which hampers its generic use in rheo-MRI.
A chemical shift discrimination of oil protons can be also accomplished by employing phase encoding methods. Even though this technique is less sensitive to magnetic field inhomogeneities, usually a few experiments are needed to achieve a proper separation of oil and water images. This compromise its use for rheo-MRI applications, where a snap-shot of the flowing fluid is required.
On the other hand, a chemical shift selective (CSS) contrast can be also achieved either by exciting only oil protons within the sample or by suppressing the water signal and using the remaining magnetization to acquire a flow encoded image. One of the advantages of employing suppression techniques is they are not considerably affected by magnetic field distortions, because no net magnetization of the unwanted component is retained before image acquisition.
In this work, we propose a method to acquire chemically selective oil concentration and velocity profiles of an oil/water emulsion under shear. Following a proper validation of the technique, the method is applied to the monitoring of shear-induced migration of oil droplets as a proof of concept.