A combination of photoemission, atomic force, and scanning tunneling microscopy/spectroscopy measurements shows that excess electrons in the TiO2 anatase (101) surface are trapped at step edges. ...Consequently, steps act as preferred adsorption sites for O2. In density functional theory calculations electrons localize at clean step edges, this tendency is enhanced by O vacancies and hydroxylation. The results show the importance of defects for the wide‐ranging applications of titania.
In step: Step edges on the TiO2 anatase (101) surface act as exclusive charge trapping centers. While the electron trapping is not favorable at (101) terraces, it is possible at the steps. It results in a higher reactivity of the steps towards some adsorbates, as illustrated for the example of O2 adsorption.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Geologic sequestration in sedimentary formations has been identified as a potential technology to prevent climate‐change inducing carbon dioxide (CO2) from being emitted to the atmosphere. To achieve ...safe and effective storage underground, accurate understanding, and predictions of supercritical CO2 (scCO2) behavior in subsurface rock formations is required; including quantifying how much scCO2 is trapped within pore spaces by capillarity (vs. how much remains mobile), and constraining the occurrence of physio‐chemical reactions between scCO2 and the mineral matrix. Experiments where multiple cycles of scCO2 and brine are injected into rock samples have produced conflicting results regarding the consistency of trapping as cycles progress; likely due to differences in mineral content, pressure‐temperature conditions, aqueous chemistry parameters, and experimental setups. We present a new set of experiments, replicating the conditions of a previous study, but with a new experimental design, apparatus, and timeline. We confirm previous results that demonstrated shifts in injection pressure and scCO2 trapping behavior over multiple injection cycles, and we conduct additional analyses to discern the fluid‐fluid macroscopic contact angle, interface mean and Gaussian curvatures, scCO2 interfacial area, and topology of trapped scCO2 ganglia. We also performed lattice‐Boltzmann simulations approximating experimental conditions where solid wettability was systematically altered over multiple injections cycles; trends in scCO2 ganglia characteristics compare well between experiment and simulation. The results indicate that this system undergoes a transition to a “patchy” mixed‐wet state, and we observe that this wettability alteration renders scCO2 more stable in the rock pore space, increasing capillary trapping over four injection cycles.
Key Points
Experimental results from four cycles of CO2 and brine injections into Bentheimer sandstone
X‐ray tomography imaged data exhibit evolution of fluid configuration and wettability state
Transition to “patchy” mixed‐wet state increases residual trapping of CO2
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
In elastic wave systems, combining the powerful concepts of resonance and spatial grading within structured surface arrays enable resonant metasurfaces to exhibit broadband wave trapping, mode ...conversion from surface (Rayleigh) waves to bulk (shear) waves, and spatial frequency selection. Devices built around these concepts allow for precise control of surface waves, often with structures that are subwavelength, and utilise Rainbow trapping that separates the signal spatially by frequency. Rainbow trapping yields large amplifications of displacement at the resonator positions where each frequency component accumulates. We investigate whether this amplification, and the associated control, can be used to create energy harvesting devices; the potential advantages and disadvantages of using graded resonant devices as energy harvesters is considered. We concentrate upon elastic plate models for which the A0 mode dominates, and take advantage of the large displacement amplitudes in graded resonant arrays of rods, to design innovative metasurfaces that trap waves for enhanced piezoelectric energy harvesting. Numerical simulation allows us to identify the advantages of such graded metasurface devices and quantify its efficiency, we also develop accurate models of the phenomena and extend our analysis to that of an elastic half-space and Rayleigh surface waves.
Fluid invasion, displacement of one fluid by another in porous media, is important in a large number of industrial and natural processes. Of special interest is the trapping of gas and oil clusters. ...We study the impact of wettability on fluid pattern formation and capillary trapping in three‐dimensional glass beads packs (dmean = 1 mm) during fluid invasion at capillary numbers of 10−7 using μ‐CT. The invading fluid was water, and the defending fluid was air. The contact angle of the glass beads was altered between 5° and 115° using Piranha cleaning and silanization. We analyzed the front morphology of the invading fluid, the residual gas saturation, the fluid occupation frequency of pores, and the morphology and statistics of the trapped gas clusters. We found a sharp transition (crossover) at a critical contact angle θc = 96°. Below θc the morphology of the displacement front was flat and compact caused by the strong smoothing effect of cooperative filling. Above θc the morphology of the displacement front was fractal and ramified caused by single bursts (Haines jumps). Across this dynamical phase transition the trapping efficiency changes from no trapping to maximal trapping. For θ > θc the experimental results show that invasion percolation governs the fluid displacement. Strong indicators are the universal scaling behavior of the size distribution of large clusters (relative data error εdata < 1%) and their linear surface‐volume relationship (R2 = 0.99).
Key Points
Wettability‐controlled dynamical phase transition in trapping efficiency for 3‐D porous media (1‐mm glass beads)
Trapping efficiency changes from no trapping to maximal trapping: Above the critical contact angle θc smaller pores are occupied by gas
For θ > θc the results show that invasion percolation governs the fluid displacement: universal scaling for the trapped gas clusters
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Direct acyl radical formation of linear aldehydes (RCH2-CHO) and subsequent hydroacylation with electron-deficient olefins can be effected with various types of metal and nonmetal catalysts/reagents. ...In marked contrast, however, no successful reports on the use of branched aldehydes have been made thus far because of their strong tendency of generating alkyl radicals through the facile decarbonylation of acyl radicals. Here, use of a hypervalent iodine(III) catalyst under visible light photolysis allows a mild way of generating acyl radicals from various branched aldehydes, thereby giving the corresponding hydroacylated products almost exclusively. Another characteristic feature of this approach is the catalytic use of hypervalent iodine(III) reagent, which is a rare example on the generation of radicals in hypervalent iodine chemistry. PUBLICATION ABSTRACT
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
3D printing and numerical analysis are combined to design a new class of architected materials that contain bistable beam elements and exhibit controlled trapping of elastic energy. The proposed ...energy‐absorbing structures are reusable. Moreover, the mechanism of energy absorption stems solely from the structural geometry of the printed beam elements, and is therefore both material‐ and loading‐rate independent.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The radical intermediates formed upon UVA irradiation of titanium dioxide suspensions in aqueous and non-aqueous environments were investigated applying the EPR spin trapping technique. The results ...showed that the generation of reactive species and their consecutive reactions are influenced by the solvent properties (e.g., polarity, solubility of molecular oxygen, rate constant for the reaction of hydroxyl radicals with the solvent). The formation of hydroxyl radicals, evidenced as the corresponding spin-adducts, dominated in the irradiated TiO2 aqueous suspensions. The addition of 17O-enriched water caused changes in the EPR spectra reflecting the interaction of an unpaired electron with the 17O nucleus. The photoexcitation of TiO2 in non-aqueous solvents (dimethylsulfoxide, acetonitrile, methanol and ethanol) in the presence of 5,5-dimethyl-1-pyrroline N-oxide spin trap displayed a stabilization of the superoxide radical anions generated via electron transfer reaction to molecular oxygen, and various oxygen- and carbon-centered radicals from the solvents were generated. The character and origin of the carbon-centered spin-adducts was confirmed using nitroso spin trapping agents.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
This study investigates fines migration and mineral reactions as a mechanism for CO2 residual trapping. We perform imbibition experiments using a sintered glass core and seven Berea sandstone cores. ...The cores receive four injection stages: water, CO2-saturated water, water-saturated CO2, and finally water or CO2-saturated water. During the second injection stage, the quantity of CO2-saturated water is altered to induce various degrees of fines migration and mineral reactions. These effects are found to yield residual CO2 saturations of 16%, 22% and 23% for zero, 25 and 50 pore volumes of CO2-saturated water injection, respectively. These percentages are 6–7% greater than if neither fines migration nor mineral reactions were present. This is attributed to pore plugging caused by fines migration and mineral reactions, impeding the imbibing water from displacing CO2 in the plugged pores. In addition, CO2-saturated water imbibition is found to increase residual CO2 saturation by 26–30% over that resulting from water imbibition. This is attributed to the CO2 dissolution effect during water imbibition. We therefore conclude that fines migration and mineral reactions is a CO2 residual trapping mechanism during CO2 sequestration.
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•Experiments are performed on glass and Berea cores to study CO2 residual trapping.•Fines migration and mineral reactions lead to plugging of pores.•Pores plugged by fines migration and mineral reactions trap CO2.•Both water and CO2-saturated water imbibition experiments are performed.•Water imbibition yields less residual CO2 due to dissolution.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The term ‘biomedical engineering’ refers to the application of the principles and problem-solving techniques of engineering to biology and medicine. Biomedical engineering is an interdisciplinary ...branch, as many of the problems health professionals are confronted with have traditionally been of interest to engineers because they involve processes that are fundamental to engineering practice. Biomedical engineers employ common engineering methods to comprehend, modify, or control biological systems, and to design and manufacture devices that can assist in the diagnosis and therapy of human diseases.
This Special Issue of Fluids aims to be a forum for scientists and engineers from academia and industry to present and discuss recent developments in the field of biomedical engineering. It contains papers that tackle, both numerically (Computational Fluid Dynamics studies) and experimentally, biomedical engineering problems, with a diverse range of studies focusing on the fundamental understanding of fluid flows in biological systems, modelling studies on complex rheological phenomena and molecular dynamics, design and improvement of lab-on-a-chip devices, modelling of processes inside the human body as well as drug delivery applications. Contributions have focused on problems associated with subjects that include hemodynamical flows, arterial wall shear stress, targeted drug delivery, FSI/CFD and Multiphysics simulations, molecular dynamics modelling and physiology-based biokinetic models.
•Residual trapping ability decreases with porosity and increases with heterogeneity.•Variance in the drainage saturation field best predicts residual gas trapping.•Correlation length and lamination ...direction have no effect on residual trapping.•Mesoscale trapping mechanism can be as important as pore-scale trapping mechanism.•Voxel-level residual trapping amount is correlated with permeability not porosity.
To increase confidence in the long-term security of CO2 geologic storage, reliable predictions of the level of post-injection CO2 residual trapping are needed. In this study, we conduct CO2/water coreflooding experiments at reservoir conditions on nine core samples with different degrees and types of heterogeneity to find the best petrophysical properties for predicting sandstone CO2 residual trapping ability. We are able to extract petrophysical properties such as porosity, permeability, degree of mesoscale heterogeneity, and spatial correlation lengths of petrophysical property fields in different directions using a CT scanner. Experimental results show that CO2 residual trapping ability decreases with porosity and increases with the degree of heterogeneity. A number of metrics for heterogeneity are evaluated, including the Dykstra-Parsons coefficient and the variance in voxel-level CO2 drainage saturation fields as well as the porosity and permeability fields. The variance of the saturation distribution during drainage provides the best predictor of residual gas trapping. By extrapolating the relationship between the degree of heterogeneity and the linear trapping coefficient, we show that pore-scale trapping mechanisms account for 46–97% of the residually trapped CO2 and the mesoscale capillary heterogeneity trapping mechanism accounts for 3–54% of the residually trapped CO2 for the nine sandstone samples tested.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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