Quantitative analysis of macroscopic solute transport in the murine brain

Background Understanding molecular transport in the brain is critical to care and prevention of neurological disease and injury. A key question is whether transport occurs primarily by diffusion, or also by convection or dispersion. Dynamic contrast-enhanced (DCE-MRI) experiments have long reported solute transport in the brain that appears to be faster than diffusion alone, but this transport rate has not been quantified to a physically relevant value that can be compared to known diffusive rates of tracers. Methods In this work, DCE-MRI experimental data is analyzed using subject-specific finite-element models to quantify transport in different anatomical regions across the whole mouse brain. The set of regional effective diffusivities ($$D_{eff}$$ D eff ), a transport parameter combining all mechanisms of transport, that best represent the experimental data are determined and compared to apparent diffusivity ($$D_{app}$$ D app ), the known rate of diffusion through brain tissue, to draw conclusions about dominant transport mechanisms in each region. Results In the perivascular regions of major arteries, $$D_{eff}$$ D eff for gadoteridol (550 Da) was over 10,000 times greater than $$D_{app}$$ D app . In the brain tissue, constituting interstitial space and the perivascular space of smaller blood vessels, $$D_{eff}$$ D eff was 10–25 times greater than $$D_{app}$$ D app . Conclusions The analysis concludes that convection is present throughout the brain. Convection is dominant in the perivascular space of major surface and branching arteries (Pe > 1000) and significant to large molecules (> 1 kDa) in the combined interstitial space and perivascular space of smaller vessels (not resolved by DCE-MRI). Importantly, this work supports perivascular convection along penetrating blood vessels.

Deconvolution of Gas Diffusion Polarization in Ni/Gadolinium-Doped Ceria Fuel Electrodes

The deconvolution of physicochemical processes in impedance spectra of SOCs with nickel/ceria fuel electrodes is challenging as gas diffusion strongly overlaps with the electrochemical processes at fuel and air electrode. To overcome this issue, symmetrical cells were applied and the gas diffusion process at the fuel electrode was quantified by altering the inert component (nitrogen / helium) in a ternary fuel gas mixture. An effective gas transport parameter considering microstructural and geometrical features was derived, enabling a precise quantification of polarization resistances related to gas diffusion and hydrogen electrooxidation. The obtained values were applied to parameterize a dc cell model. The model validation in fuel cell and electrolyzer mode showed an excellent agreement between measured and simulated current/voltage characteristics over a wide range of technically meaningful gas compositions and operating temperatures.

Simulating the feedback between corrosive gas generation and water availability for the evaluation of radionuclide mobility in the context of radioactive waste disposal

It has been recently recognized that the availability of liquid water may be a controlling factor in the feedback between the physical processes of variably saturated liquid and gas flow on the one hand, and various chemical processes such as metal corrosion in an underground storage facility for radioactive waste on the other hand (e.g., Huang et al., 2021, and reference therein). Iron corrosion in anoxic conditions produces hydrogen gas and consumes water, as expressed by the following stylized chemical equation (e.g., Diercks and Kassner, 1988; Senior et al., 2021): 3Fe+4H2O⟶Fe3O4+4H2 Since water is an educt the corrosion reaction may be suspended or suppressed by the scarcity of water near the corroding surfaces. At the same time, gas pressure build-up through hydrogen generation may limit further water ingress. We developed a model that focuses on the close coupling between gas generation through iron corrosion and water availability. The feedback between iron corrosion, gas generation and liquid phase flow is considered by implementing the corrosion reaction in the subsurface flow and transport simulator PFLOTRAN (Hammond et al., 2012; Lichtner et al., 2015, 2020) making use of its coding provisions to implement source/sink terms for water and gas. These source/sink terms reflect the kinetics of the iron corrosion and its dependence on the educts, where the availability of water is approximated by the local liquid saturation. The model was applied to evaluate the mobility of radionuclides in, and their release from a hypothetical geological storage facility for radioactive waste. The radionuclides are traced through the emplacement chambers and drift by means of advective and diffusive transport. Parameter variations illustrate the influence of crucial modelling parameters on the simulation results.

Quantitative Analysis of Macroscopic Solute Transport in the Murine Brain

Background: Understanding molecular transport in the brain is critical to care and prevention of neurological disease and injury. A key question is whether transport occurs primarily by diffusion, or also by convection or dispersion. Dynamic contrast-enhanced (DCE) MRI offers a whole-brain view of transport and the potential for quantitative analysis to determine fundamental transport parameters. However, few DCE-MRI studies have utilized this potential, instead reporting parameters with arbitrary units disconnected from fundamental transport processes. Methods: In this work, DCE-MRI experimental data is combined with subject-specific finite-element models to quantify transport parameters in different anatomical regions across the whole mouse brain. Effective diffusivity ( ), a transport parameter combining all mechanisms of transport, is determined for each region by minimizing the root mean square error between simulations and data. The resulting sets are compared to apparent diffusivity ( ) to draw conclusions about dominant transport mechanisms in each region. Results: In the perivascular regions of major arteries, was over 10,000 times greater than . In the brain tissue, constituting interstitial space and the perivascular space of smaller blood vessels, was 10-25 times greater than .Conclusions: The analysis concludes that convection is present throughout the brain. Convection is dominant in the perivascular space of major surface and branching arteries (Pe > 10,000) and significant to large molecules (>1 kDa) in the combined interstitial space and perivascular space of smaller arteries (not resolved by DCE-MRI). Importantly, this work supports periarterial convection along penetrating and smaller arteries.

Ionic and Thermal Transport in Na-Ion-Conducting Ceramic Electrolytes

We have studied the ionic and thermal transport properties along with the thermodynamic key properties of a Na-ion-conducting phosphate ceramic. The system Na1+xAlxTi2−x(PO4)3 (NATP) with x = 0.3 was taken as a NASICON-structured model system which is a candidate material for solid electrolytes in post-Li energy storage. The commercially available powder (NEI Coorp., USA) was consolidated using cold isostatic pressing before sintering. In order to compare NATP with the “classical” NASICON system, Na1+xZr2(SiO4)x(PO4)3−x (NaZSiP) was synthesized with compositions of x = 1.7 and x = 2, respectively, and characterized with regard to their ionic and thermal transport behavior. While ionic conductivity of the NaZSiP compositions was about more than two orders of magnitude higher than in NATP, the thermal conductivity of the NASICON compound showed an opposite behavior. The room temperature value was about a factor two higher in NATP compared to NaZSiP. While the thermal conductivity decreases with increasing temperature in NATP, it increases with increasing temperature in NaZSiP. However, the overall change of this thermal transport parameter over the measured temperature range from room temperature up to 800 °C appeared to be relatively small.

Transport Parameter Correlations for Digitally Created PEFC Gas Diffusion Layers by Using OpenPNM

A polymer electrolyte fuel cell (PEFC) is an electrochemical device that converts chemical energy into electrical energy and heat. The energy conversion is simple; however, the multiphysics phenomena involved in the energy conversion process must be analyzed in detail. The gas diffusion layer (GDL) provides a diffusion media for reactant gases and gives mechanical support to the fuel cell. It is a complex medium whose properties impact the fuel cell’s efficiency. Therefore, an in-depth analysis is required to improve its mechanical and physical properties. In the current study, several transport phenomena through three-dimensional digitally created GDLs have been analyzed. Once the porous microstructure is generated and the transport phenomena are mimicked, transport parameters related to the fluid flow and mass diffusion are computed. The GDLs are approximated to the carbon paper represented as a grouped package of carbon fibers. Several correlations, based on the fiber diameter, to predict their transport properties are proposed. The digitally created GDLs and the transport phenomena have been modeled using the open-source library named Open Pore Network Modeling (OpenPNM). The proposed correlations show a good fit with the obtained data with an R-square of approximately 0.98.

The intimate relationship between structural relaxation and the energy landscape of monatomic liquid metals

The characteristic property of a liquid, discriminating it from a solid, is its fluidity, which can be expressed by a velocity field. The reaction of the velocity field on forces is enshrined in the transport parameter viscosity. In contrast, a solid reacts to forces elastically through a displacement field, the particles are trapped in their potential minimum. The flow in a liquid needs enough thermal energy to overcome the changing potential barriers, which is supported through a continuous rearrangement of surrounding particles. Cooling a liquid will decrease the fluidity of a particle and the mobility of the neighbouring particles, resulting in an increase of the viscosity until the system comes to an arrest. This process with a concomitant slowing down of collective particle rearrangements might already start deep inside the liquid state. The idea of the potential energy landscape provides an attractive picture for these dramatic changes. However, despite the appealing idea there is a scarcity of quantitative assessments, in particular, when it comes to experimental studies. Here we present results on a monatomic liquid metal through a combination of ab initio molecular dynamics, neutron spectroscopy and inelastic x-ray scattering. We investigated the collective dynamics of liquid aluminium to reveal the changes in dynamics when the high temperature liquid is cooled towards solidification. The results demonstrate the main signatures of the energy landscape picture, a reduction in the internal atomic structural energy, a transition to a stretched relaxation process and a deviation from the high-temperature Arrhenius behavior of the relaxation time. All changes occur in the same temperature range at about $$1.4 \cdot T_{melting}$$ 1.4 · T melting , which can be regarded as the temperature when the liquid aluminium enters the landscape influenced phase and enters a more viscous liquid state towards solidification. The similarity in dynamics with other monatomic liquid metals suggests a universal dynamic crossover above the melting point.

Separation of Boron from Geothermal Waters with Membrane System

This study presents the separation and recovery of boron from geothermal waters with a polymeric membrane system and suggests a transport mechanism. The optimum relative parameters of the transport were examined. The recovery value of boron was 60.46% by using polymeric membrane system from prepared aquatic solution to the acceptor phase. The membrane’s capacity and selectivity of the transport process were examined. Kinetics values were calculated for each transport parameter. The optimum kinetic values were 1.4785 × 10−6 (s−1), 7.3273 × 10−8 (m/s), 13.5691 × 10−8 (mol/m2.s), 5.8174 × 10−12 (m2/s) for constant rate, permeability coefficient, flux, and diffusion coefficient, respectively. Boron was transported selectively and successfully from geothermal waters in the presence of other metal cations with 59.85% recovery value. This study indicates the application of real samples in polymeric membrane systems, which are very practical, economic, and easy to use for large-scale applications. The chemical and physical properties of polymer inclusion membranes (PIMs) offer the opportunity to be specially designed for specific applications.