Characterising the rheology of non-Newtonian fluids using PFG-NMR and cumulant analysis

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.

Encapsulation in alginate-polymers improves stability and allows controlled release of the UFV-AREG1 bacteriophage

The bacteriophage UFV-AREG1 was used as a model organism to evaluate the encapsulation via extrusion using different hydrocolloids. Pure alginate [0.75%, 1.0%, 1.5% and 2.0% (m/v)] and mixtures of alginate [0.75% or 1.0% (m/v)] with carrageenan [1.25% (m/v)], chitosan [0.5% (m/v)], or whey protein [1.5% (m/v)] were used to produce bacteriophage-loaded beads. The encapsulating solutions presented flow behavior of non-Newtonian pseudoplastic fluids and the concentration of hydrocolloid did not influence (p > 0.05) the morphology of the beads, except for alginate-chitosan solutions, which presented the higher flow consistency index (K) and the lower flow behavior index (n). The encapsulation efficiency was about 99% and the confocal photomicrography of the encapsulated bacteriophages labeled with fluorescein isothiocyanate showed homogenous distribution of the viral particles within the beads. The phages remained viable in the beads of alginate-whey protein even when submitted to pH 2.5 for 2 h. Beads incubated directly in simulated intestinal fluid (pH 6.8) resulted in a minimal of 50% release of the UFV-AREG1 phages after 5 min, even when previously submitted to the simulated gastric fluid (pH 2.5). Encapsulation enabled phages to remain viable under refrigeration for five months. Encapsulated UFV-AREG1 phages were sensitive to dehydration, suggesting the need for protective agents. In this study, for the first-time bacteriophages were encapsulated in alginate-carrageenan beads, as well as alginate-chitosan as a bead-forming hydrocolloid. In addition, a novel procedure for encapsulating bacteriophages in alginate-whey protein was proposed. The assembled system showed efficiency in the encapsulation of UFV-AREG1 bacteriophages using different hydrocolloids and has potential to be used for the entrapment of a variety of bioactive compounds.

Applications of magnetic resonance imaging in chemical engineering

While there are many techniques to study phenomena that occur in chemical engineering applications, magnetic resonance imaging (MRI) receives increasing scientific interest. Its non-invasive nature and wealth of parameters with the ability to generate functional images and contrast favors the use of MRI for many purposes, in particular investigations of dynamic phenomena, since it is very sensitive to motion. Recent progress in flow-MRI has led to shorter acquisition times and enabled studies of transient phenomena. Reactive systems can easily be imaged if NMR parameters such as relaxation change along the reaction coordinate. Moreover, materials and devices can be examined, such as batteries by mapping the magnetic field around them.

Accessing the long-time limit in diffusion NMR: The case of singlet assisted diffusive diffraction q-space

The latest developments in the field of long-lived spin states are merged with pulsed-field gradient techniques to extend the diffusion time beyond what is currently achievable in standard q-space diffusive-diffraction studies. The method uses nearly-equivalent spin-1/2 pairs that let diffusion times of the order of many minutes to be measured allowing access to the long-time limit in cavities of macroscopic size (millimeters). A pulse sequence suitable to exploit this regime has been developed and validated with the use of numerical simulations and experiments.

Magnetic Resonance Imaging and Velocity Mapping in Chemical Engineering Applications

This review aims to illustrate the diversity of measurements that can be made using magnetic resonance techniques, which have the potential to provide insights into chemical engineering systems that cannot readily be achieved using any other method. Perhaps the most notable advantage in using magnetic resonance methods is that both chemistry and transport can be followed in three dimensions, in optically opaque systems, and without the need for tracers to be introduced into the system. Here we focus on hydrodynamics and, in particular, applications to rheology, pipe flow, and fixed-bed and gas-solid fluidized bed reactors. With increasing development of industrially relevant sample environments and undersampling data acquisition strategies that can reduce acquisition times to <1 s, magnetic resonance is finding increasing application in chemical engineering research.

PFG NMR and Bayesian analysis to characterise non-Newtonian fluids

Many industrial flow processes are sensitive to changes in the rheological behaviour of process fluids, and there therefore exists a need for methods that provide online, or inline, rheological characterisation necessary for process control and optimisation over timescales of minutes or less. Nuclear magnetic resonance (NMR) offers a non-invasive technique for this application, without limitation on optical opacity. We present a Bayesian analysis approach using pulsed field gradient (PFG) NMR to enable estimation of the rheological parameters of Herschel-Bulkley fluids in a pipe flow geometry, characterised by a flow behaviour index n, yield stress τ₀, and consistency factor k, by analysis of the signal in q-space. This approach eliminates the need for velocity image acquisition and expensive gradient hardware. We investigate the robustness of the proposed Bayesian NMR approach to noisy data and reduced sampling using simulated NMR data and show that even with a signal-to-noise ratio (SNR) of 100, only 16 points are required to be sampled to provide rheological parameters accurate to within 2% of the ground truth. Experimental validation is provided through an experimental case study on Carbopol 940 solutions (model Herschel-Bulkley fluids) using PFG NMR at a ¹H resonance frequency of 85.2MHz; for SNR>1000, only 8 points are required to be sampled. This corresponds to a total acquisition time of <60s and represents an 88% reduction in acquisition time when compared to MR flow imaging. Comparison of the shear stress-shear rate relationship, quantified using Bayesian NMR, with non-Bayesian NMR methods demonstrates that the Bayesian NMR approach is in agreement with MR flow imaging to within the accuracy of the measurement. Furthermore, as we increase the concentration of Carbopol 940 we observe a change in rheological characteristics, probably due to shear history-dependent behaviour and the different geometries used. This behaviour highlights the need for online, or inline, rheological characterisation in industrial process applications.

Stray-field NMR diffusion q-space diffraction imaging of monodisperse coarsening foams

The technique of stray field diffusion NMR is adapted to study the diffusion properties of water in monodisperse wet foams. We show for the first time, that the technique is capable of observing q-space diffusion diffraction peaks in monodisperse aqueous foams with initial bubble sizes in the range of 50-85μm. The position of the peak maximum can be correlated simply to the bubble size in the foam leading to a technique that can investigate the stability of the foam over time. The diffusion technique, together with supplementary spin-spin relaxation analysis of the diffusion data is used to follow the stability and coarsening behaviour of monodisperse foams with a water fraction range between 0.24 and 0.33. The monodisperse foams remain stable for a period of hours in terms of the initial bubble size. The duration of this stable period correlates to the initial size of the bubbles. Eventually the bubbles begin to coarsen and this is observed in changes in the position of the diffusion diffraction maxima.