Signatures of nanoemulsion jamming and unjamming in stimulated-echo NMR.

The unjamming of elastic concentrated nanoemulsions into viscous dilute nanoemulsions, through dilution with the continuous phase, offers interesting opportunities for a pulsed-field gradient (PFG) NMR, particularly if the nanoemulsion is designed to take advantage of the nuclear specificity offered by NMR. Here, we make and study size-fractionated oil-in-water nanoemulsions using a perfluorinated copolymer silicone oil that is highly insoluble in the aqueous continuous phase. By studying these nanoemulsions using ^{19}F stimulated-echo PFG-NMR, we avoid any contribution from the aqueous continuous phase, which contains a nonfluorinated ionic surfactant. We find a dramatic change in the ^{19}F PFG-NMR decays at high field-gradient strengths as the droplet volume fraction, ϕ, is lowered through dilution. At high ϕ, observed decays as a function of field-gradient strength exhibit decay-to-plateau behavior indicating the jamming of nanodroplets, which contain ^{19}F probe molecules, in an elastic material reminiscent of a nanoporous solid. In contrast, at lower ϕ, only a simple decay is observed, indicating that the nanodroplets have unjammed and can diffuse over much larger distances. Through a comparison with bulk mechanical rheometry, we show that this dramatic change coincides with the loss of low-frequency shear elasticity of the nanoemulsion.

Impact of Xylose on Dynamics of Water Diffusion in Mesoporous Zeolites Measured by NMR.

Zeolites are known to be effective catalysts in biomass converting processes. Understanding the mesoporous structure and dynamics within it during such reactions is important in effectively utilizing them. Nuclear magnetic resonance (NMR) T2 relaxation and diffusion measurements, using a high-power radio frequency probe, are shown to characterize the dynamics of water in mesoporous commercially made 5A zeolite beads before and after the introduction of xylose. Xylose is the starting point in the dehydration into furfural. The results indicate xylose slightly enhances rotational mobility while it decreases translational motion through altering the permeability, K, throughout the porous structure. The measurements show xylose inhibits pure water from relocating into larger pores within the zeolite beads where it eventually is expelled from the bead itself.

Mechanisms of water permeation and diffusive API release from stearyl alcohol and glyceryl behenate modified release matrices.

This work aims to develop complimentary analytical tools for lipid formulation selection that offer insights into the mechanisms of in-vitro drug release for solid lipid modified release excipients. Such tools are envisioned to aide and expedite the time consuming process of formulation selection and development. Two pharmaceutically relevant solid lipid excipients are investigated, stearyl alcohol and glyceryl behenate, which are generally known to exhibit faster and slower relative release rates, respectively. Nuclear magnetic resonance spectroscopy and diffusometry are used, along with water uptake and dissolution experiments to help distinguish between two proposed in-vitro release mechanisms for crystalline caffeine from these matrices: 1) rate limiting movement of the wetting front through the particle, and 2) rate limiting diffusive release of the active from the wetted particle. Findings based on water permeation rates, API diffusion coefficients and kinetic modeling suggest that the rate limiting steps for caffeine release from these matrices are different, with stearyl alcohol being co-rate limited by movement of the wetting front and diffusive release of API, whereas glyceryl behenate is more strictly limited by diffusive release of API from the wetted matrix. A Peclet-like number is proposed to describe the different regimes of rate limitation for drug release. NMR spectroscopy and diffusometry are demonstrated to be useful tools for elucidating mechanisms of API release from crystalline drug/lipid mixtures and have significant potential value as screening tools in MR formulation development.

Characterizing the structure of aerobic granular sludge using ultra-high field magnetic resonance.

Despite aerobic granular sludge wastewater treatment plants operating around the world, our understanding of internal granule structure and its relation to treatment efficiency remains limited. This can be attributed in part to the drawbacks of time-consuming, labor-intensive, and invasive microscopy protocols which effectively restrict samples sizes and may introduce artefacts. Time-domain nuclear magnetic resonance (NMR) allows non-invasive measurements which describe internal structural features of opaque, complex materials like biofilms. NMR was used to image aerobic granules collected from five full-scale wastewater treatment plants in the Netherlands and United States, as well as laboratory granules and control beads. T1 and T2 relaxation-weighted images reveal heterogeneous structures that include high- and low-density biofilm regions, water-like voids, and solid-like inclusions. Channels larger than approximately 50 µm and connected to the bulk fluid were not visible. Both cluster and ring-like structures were observed with each granule source having a characteristic structural type. These structures, and their NMR relaxation behavior, were stable over several months of storage. These observations reveal the complex structures within aerobic granules from a range of sources and highlight the need for non-invasive characterization methods like NMR to be applied in the ongoing effort to correlate structure and function.

Heterogeneous diffusion in aerobic granular sludge.

Aerobic granular sludge (AGS) technology allows simultaneous nitrogen, phosphorus, and carbon removal in compact wastewater treatment processes. To operate, design, and model AGS reactors, it is essential to properly understand the diffusive transport within the granules. In this study, diffusive mass transfer within full-scale and lab-scale AGS was characterized with nuclear magnetic resonance (NMR) methods. Self-diffusion coefficients of water inside the granules were determined with pulsed-field gradient NMR, while the granule structure was visualized with NMR imaging. A reaction-diffusion granule-scale model was set up to evaluate the impact of heterogeneous diffusion on granule performance. The self-diffusion coefficient of water in AGS was ∼70% of the self-diffusion coefficient of free water. There was no significant difference between self-diffusion in AGS from full-scale treatment plants and from lab-scale reactors. The results of the model showed that diffusional heterogeneity did not lead to a major change of flux into the granule (<1%). This study shows that differences between granular sludges and heterogeneity within granules have little impact on the kinetic properties of AGS. Thus, a relatively simple approach is sufficient to describe mass transport by diffusion into the granules.

Microbial growth rates and local external mass transfer coefficients in a porous bed biofilm system measured by 19 F magnetic resonance imaging of structure, oxygen concentration, and flow velocity.

19 F nuclear magnetic resonance (NMR) oximetry and 1 H NMR velocimetry were used to noninvasively map oxygen concentrations and hydrodynamics in space and time in a model packed bed biofilm system in the presence and absence of flow. The development of a local oxygen sink associated with a single gel bead inoculated with respiring Escherichia coli was analyzed with a phenomenological model to determine the specific growth rate of the bacteria in situ, returning a value (0.66 hr-1 ) that was close to that measured independently in planktonic culture (0.62 hr-1 ). The decay of oxygen concentration in and around the microbiologically active bead was delayed and slower in experiments conducted under continuous flow in comparison to no-flow experiments. Concentration boundary layer thicknesses were determined and Sherwood numbers calculated to quantify external mass transfer resistance. Boundary layers were thicker in no-flow experiments compared to experiments with flow. Whereas the oxygen concentration profile across a reactive biofilm particle was symmetric in no-flow experiments, it was asymmetric with respect to flow direction in flow experiments with Sherwood numbers on the leading edge (Sh = 7) being larger than the trailing edge (Sh = 3.5). The magnitude of the experimental Sh was comparable to values predicted by a variety of correlations. These spatially resolved measurements of oxygen distribution in a geometrically complex model reveal in innovative detail the local coupling between microbial growth, oxygen consumption, and external mass transfer.

A two-region transport model for interpreting T1-T2 measurements in complex systems.

A 1D two region coupled pore model with discrete pore coupling is developed to elucidate the eigenmode interactions in regions with different surface relaxivity. Numerical solution of the model and simulation of the correlation experiment for varying surface relaxivity, pore connectivity and pore size ratio indicate the role of negative eigenmodes and overlap of T1 and T2 eigenmodes in generating a time domain signal increase with inversion recovery time, t1. The eigenmodes and eigenfunctions are considered in detail providing connection between the mathematical model and the diffusion dynamics and spin physics of the system. Physical systems, i.e. a microporous glass bead pack, a cyclopentane/water hydrate former, and beeswax, showing experimentally measured T1-T2 time domain signal rise are considered within the limitations of the model.

Non-invasive imaging of oxygen concentration in a complex in vitro biofilm infection model using 19 F MRI: Persistence of an oxygen sink despite prolonged antibiotic therapy.

PURPOSE: Oxygen availability is a critical determinant of microbial biofilm activity and antibiotic susceptibility. However, measuring oxygen gradients in these systems remains difficult, with the standard microelectrode approach being both invasive and limited to single-point measurement. The goal of the study was to develop a 19 F MRI approach for 2D oxygen mapping in biofilm systems and to visualize oxygen consumption behavior in real time during antibiotic therapy. METHODS: Oxygen-sensing beads were created by encapsulating an emulsion of oxygen-sensing fluorocarbon into alginate gel. Escherichia coli biofilms were grown in and on the alginate matrix, which was contained inside a packed bed column subjected to nutrient flow, mimicking the complex porous structure of human wound tissue, and subjected to antibiotic challenge. RESULTS: The linear relationship between 19 F spin-lattice relaxation rate R1 and local oxygen concentration permitted noninvasive spatial mapping of oxygen distribution in real time over the course of biofilm growth and subsequent antibiotic challenge. This technique was used to visualize persistence of microbial oxygen respiration during continuous gentamicin administration, providing a time series of complete spatial maps detailing the continued bacterial utilization of oxygen during prolonged chemotherapy in an in vitro biofilm model with complex spatial structure. CONCLUSIONS: Antibiotic exposure temporarily causes oxygen consumption to enter a pseudosteady state wherein oxygen distribution becomes fixed; oxygen sink expansion resumes quickly after antibiotic clearance. This technique may provide valuable information for future investigations of biofilms by permitting the study of complex geometries (typical of in vivo biofilms) and facilitating noninvasive oxygen measurement.

Probing diffusion dynamics during hydrate formation by high field NMR relaxometry and diffusometry.

High-field nuclear magnetic resonance (NMR) relaxometry and diffusometry along with magnetic resonance imaging were used to monitor phase transition molecular dynamics during hydrate formation occurring in water droplets dispersed in liquid cyclopentane. 1D T2 relaxation measurements indicate the extent of hydrate formation as well as a reduction in water droplet size with progression of hydrate growth. MRI intensity maps and T2 relaxation maps indicate spatially dependent hydrate formation rates due to the heterogeneity of the system. Spectrally resolved diffusion measurements indicate a reduction in the porosity of the hydrate agglomerate as the hydrate shell increases in thickness. A novel signal rise observed in two dimensional T1-T2 relaxation correlation experiments indicates complex diffusion dynamics due to coupling between regions with varying relaxation and diffusion. These results indicate the ability to monitor hydrate growth and phase transition molecular dynamics due to evolution of the porous hydrate agglomerate by means of high-field NMR.

Glass Dynamics and Domain Size in a Solvent-Polymer Weak Gel Measured by Multidimensional Magnetic Resonance Relaxometry and Diffusometry.

Nuclear magnetic resonance measurements of rotational and translational molecular dynamics are applied to characterize the nanoscale dynamic heterogeneity of a physically cross-linked solvent-polymer system above and below the glass transition temperature. Measured rotational dynamics identify domains associated with regions of solidlike and liquidlike dynamics. Translational dynamics provide quantitative length and timescales of nanoscale heterogeneity due to polymer network cross-link density. Mean squared displacement measurements of the solvent provide microrheological characterization of the system and indicate glasslike caging dynamics both above and below the glass transition temperature.

Spatiotemporal mapping of oxygen in a microbially-impacted packed bed using 19F Nuclear magnetic resonance oximetry.

19F magnetic resonance has been used in the medical field for quantifying oxygenation in blood, tissues, and tumors. The 19F NMR oximetry technique exploits the affinity of molecular oxygen for liquid fluorocarbon phases, and the resulting linear dependence of 19F spin-lattice relaxation rate R1 on local oxygen concentration. Bacterial biofilms, aggregates of bacteria encased in a self-secreted matrix of extracellular polymers, are important in environmental, industrial, and clinical settings and oxygen gradients represent a critical determinant of biofilm function. However, measurement of oxygen distribution in biofilms and biofouled porous media is difficult. Here the ability of 19F NMR oximetry to accurately track oxygen profile development in microbial impacted packed bed systems without impacting oxygen transport is demonstrated. Time-stable and inert fluorocarbon containing particles are designed which act as oxygen reporters in porous media systems. Particles are generated by emulsifying and entrapping perfluorooctylbromide (PFOB) into alginate gel, resulting in oxygen-sensing alginate beads that are then used as the solid matrix of the packed bed. 19F oxygenation maps, when combined with 1H velocity maps, allow for insight into the interplay between fluid dynamics and oxygen transport phenomena in these complex biofouled systems. Spatial maps of oxygen consumption rate constants are calculated. The growth characteristics of two bacteria, a non-biofilm forming Escherichia coli and Staphylococcus epidermidis, a strong biofilm-former, are used to demonstrate the novel data provided by the method.

NMR Relaxometry to Characterize the Drug Structural Phase in a Porous Construct.

Nuclear magnetic resonance (NMR) frequency spectra and T2 relaxation time measurements, using a high-power radio frequency probe, are shown to characterize the presence of an amorphous drug in a porous silica construct. The results indicate the ability of non-solid-state NMR methods to characterize crystalline and amorphous solid structural phases in drugs. Two-dimensional T1- T2 magnetic relaxation time correlation experiments are shown to monitor the impact of relative humidity on the drug in a porous silica tablet.

Flow velocity maps measured by nuclear magnetic resonance in medical intravenous catheter needleless connectors.

This work explains the motivation, advantages, and novel approach of using velocity magnetic resonance imaging (MRI) for studying the hydrodynamics in a complicated structural biomedical device such as an intravenous catheter needleless connector (NC). MRI was applied as a non-invasive and non-destructive technique to evaluate the fluid dynamics associated with various internal designs of the NC. Spatial velocity maps of fluid flow at specific locations within these medical devices were acquired. Dynamic MRI is demonstrated as an effective method to quantify flow patterns and fluid dynamic dependence on structural features of NCs. These spatial velocity maps could be used as a basis for groundtruthing computational fluid dynamics (CFD) methods that could impact the design of NCs.

Quantifying NMR relaxation correlation and exchange in articular cartilage with time domain analysis.

Measured nuclear magnetic resonance (NMR) transverse relaxation data in articular cartilage has been shown to be multi-exponential and correlated to the health of the tissue. The observed relaxation rates are dependent on experimental parameters such as solvent, data acquisition methods, data analysis methods, and alignment to the magnetic field. In this study, we show that diffusive exchange occurs in porcine articular cartilage and impacts the observed relaxation rates in T1-T2 correlation experiments. By using time domain analysis of T2-T2 exchange spectroscopy, the diffusive exchange time can be quantified by measurements that use a single mixing time. Measured characteristic times for exchange are commensurate with T1 in this material and so impacts the observed T1 behavior. The approach used here allows for reliable quantification of NMR relaxation behavior in cartilage in the presence of diffusive fluid exchange between two environments.

Effective Rheology of Two-Phase Flow in Three-Dimensional Porous Media: Experiment and Simulation.

We present an experimental and numerical study of immiscible two-phase flow of Newtonian fluids in three-dimensional (3D) porous media to find the relationship between the volumetric flow rate (Q) and the total pressure difference ([Formula: see text]) in the steady state. We show that in the regime where capillary forces compete with the viscous forces, the distribution of capillary barriers at the interfaces effectively creates a yield threshold ([Formula: see text]), making the fluids reminiscent of a Bingham viscoplastic fluid in the porous medium. In this regime, Q depends quadratically on an excess pressure drop ([Formula: see text]). While increasing the flow rate, there is a transition, beyond which the overall flow is Newtonian and the relationship is linear. In our experiments, we build a model porous medium using a column of glass beads transporting two fluids, deionized water and air. For the numerical study, reconstructed 3D pore networks from real core samples are considered and the transport of wetting and non-wetting fluids through the network is modeled by tracking the fluid interfaces with time. We find agreement between our numerical and experimental results. Our results match with the mean-field results reported earlier.

NMR investigation of water diffusion in different biofilm structures.

Mass transfer in biofilms is determined by diffusion. Different mostly invasive approaches have been used to measure diffusion coefficients in biofilms, however, data on heterogeneous biomass under realistic conditions is still missing. To non-invasively elucidate fluid-structure interactions in complex multispecies biofilms pulsed field gradient-nuclear magnetic resonance (PFG-NMR) was applied to measure the water diffusion in five different types of biomass aggregates: one type of sludge flocs, two types of biofilm, and two types of granules. Data analysis is an important issue when measuring heterogeneous systems and is shown to significantly influence the interpretation and understanding of water diffusion. With respect to numerical reproducibility and physico-chemical interpretation, different data processing methods were explored: (bi)-exponential data analysis and the Γ distribution model. Furthermore, the diffusion coefficient distribution in relation to relaxation was studied by D-T2 maps obtained by 2D inverse Laplace transform (2D ILT). The results show that the effective diffusion coefficients for all biofilm samples ranged from 0.36 to 0.96 relative to that of water. NMR diffusion was linked to biofilm structure (e.g., biomass density, organic and inorganic matter) as observed by magnetic resonance imaging and to traditional biofilm parameters: diffusion was most restricted in granules with compact structures, and fast diffusion was found in heterotrophic biofilms with fluffy structures. The effective diffusion coefficients in the biomass were found to be broadly distributed because of internal biomass heterogeneities, such as gas bubbles, precipitates, and locally changing biofilm densities. Thus, estimations based on biofilm bulk properties in multispecies systems can be overestimated and mean diffusion coefficients might not be sufficiently informative to describe mass transport in biofilms and the near bulk.

Impact of Mineral Precipitation on Flow and Mixing in Porous Media Determined by Microcomputed Tomography and MRI.

Precipitation reactions influence transport properties in porous media and can be coupled to advective and dispersive transport. For example, in subsurface environments, mixing of groundwater and injected solutions can induce mineral supersaturation of constituents and drive precipitation reactions. Magnetic resonance imaging (MRI) and microcomputed tomography (µ-CT) were employed as complementary techniques to evaluate advection, dispersion, and formation of precipitate in a 3D porous media flow cell. Two parallel fluids were flowed concentrically through packed glass beads under two relative flow rates with Na2CO3 and CaCl2 in the inner and outer fluids, respectively. CaCO3 became supersaturated and formed a precipitate at the mixing interface between the two solutions. Spatial maps of changing local velocity fields and dispersion in the flow cell were generated from MRI, while high resolution µ-CT imaging visualized the precipitate formed in the porous media. Formation of a precipitate minimized dispersive and advective transport between the two fluids and the shape of the precipitation front was influenced by the relative flow rates. This work demonstrates that the combined use of MRI and µ-CT can be highly complementary in the study of reactive transport processes in porous media.

Simultaneous Gaussian and exponential inversion for improved analysis of shales by NMR relaxometry.

Nuclear magnetic resonance (NMR) relaxometry is commonly used to provide lithology-independent porosity and pore-size estimates for petroleum resource evaluation based on fluid-phase signals. However in shales, substantial hydrogen content is associated with solid and fluid signals and both may be detected. Depending on the motional regime, the signal from the solids may be best described using either exponential or Gaussian decay functions. When the inverse Laplace transform, the standard method for analysis of NMR relaxometry results, is applied to data containing Gaussian decays, this can lead to physically unrealistic responses such as signal or porosity overcall and relaxation times that are too short to be determined using the applied instrument settings. We apply a new simultaneous Gaussian-Exponential (SGE) inversion method to simulated data and measured results obtained on a variety of oil shale samples. The SGE inversion produces more physically realistic results than the inverse Laplace transform and displays more consistent relaxation behavior at high magnetic field strengths. Residuals for the SGE inversion are consistently lower than for the inverse Laplace method and signal overcall at short T2 times is mitigated. Beyond geological samples, the method can also be applied in other fields where the sample relaxation consists of both Gaussian and exponential decays, for example in material, medical and food sciences.

Anomalous preasymptotic colloid transport by hydrodynamic dispersion in microfluidic capillary flow.

The anomalous preasymptotic transport of colloids in a microfluidic capillary flow due to hydrodynamic dispersion is measured by noninvasive nuclear magnetic resonance (NMR). The data indicate a reduced scaling of mean squared displacement with time from the 〈z(t)(2)〉(c) ∼ t(3) behavior for the interaction of a normal diffusion process with a simple shear flow. This nonequilibrium steady-state system is shown to be modeled by a continuous time random walk (CTRW) on a moving fluid. The full propagator of the motion is measured by NMR, providing verification of the assumption of Gaussian jump length distributions in the CTRW model. The connection of the data to microrheology measurements by NMR, in which every particle in a suspension contributes information, is established.