Vicinity Of Surface Research Articles

Local pH Modulation during Electro-Enzymatic O2 Reduction: Characterization of the Influence of Ionic Strength by In Situ Fluorescence Microscopy.

Understanding how environmental factors affect the bioelectrode efficiency and stability is of uttermost importance to develop high-performance bioelectrochemical devices. By coupling fluorescence confocal microscopy in situ to electrochemistry, this work focuses on the influence of the ionic strength on electro-enzymatic catalysis. In this context, the 4 e-/4 H+ reduction of O2 into water by the bilirubin oxidase from Myrothecium verrucaria (MvBOD) is considered as a model. The effects of salt concentration on the enzyme activity and stability were probed by enzymatic assays performed in homogeneous catalysis conditions and monitored by UV-vis absorption spectroscopy. They were also investigated in heterogeneous catalysis conditions by electrochemical measurements with MvBOD immobilized at a graphite microelectrode. We demonstrate that the catalytic activity and stability of the enzyme both in solution and in the immobilized state at the bioelectrode were conserved with an electrolyte concentration of up to 0.5 M, both in a buffered and a non-buffered electrolyte. Relying on this, we used fluorescence confocal laser scanning microscopy coupled in situ to electrochemistry to explore the local pH of the electrolyte at the vicinity of the electrode surface at various ionic strengths and for several overpotentials. 3D proton depletion profiles generated by the interfacial electro-enzymatic reaction were recorded in the presence of a pH-sensitive fluorophore. These concentration profiles were shown to contract with increasing ionic strength, thus highlighting the need for a minimal electrolyte concentration to ensure availability of charged substrates at the electrode surface during electro-enzymatic experiments.

An interfacial gas-enrichment strategy for mitigating hydrate adhesion and blockage

Unwanted gas hydrates blockage is a threat to the safety of oil–gas pipeline systems. Deploying passive anti-hydrate surfaces is a promising approach to circumventing the long-lasting hydrate problem, and a precise understanding of the interactions between hydrate and solid surfaces is a prerequisite for the creation of such surfaces. In this work, the underlying mechanisms and the key influencing factors of hydrate adhesion are explored by large-scale molecular dynamics simulations. Hydrates with an intermediate layer (IML) containing varied gas content are brought to contact with solid surfaces having different levels of wettability and roughness. It is found that the final IML structure is dictated by the gas concentration. Enriching gas content in the vicinity of solid surfaces is crucial for lowering hydrate adhesion strength. The results indicate that forming a molecular gas layer or gas bubbles on solid surfaces can enable the automatic detachment of the eventually formed hydrate on a pipeline wall surface under the action of shear flow. This study manifests our approach of utilizing an interfacial gas-enrichment strategy (IGES) for weakening hydrate adhesion as a novel passive anti-hydrate surface design.

Enhancement of photocatalytic degradation by combination of light illumination and bias voltages toward anti‐virus coating applications

AbstractIn this work, oxidative decomposition reaction of organic reactant on TiO2 surface was investigated under soaking conditions by combination of applying voltages and light illumination. Since carriers of TiO2 thin films originate from point defects such as oxygen vacancies, reactive carriers possibly exist on TiO2 thin films even under room light illuminating condition and dark condition. Thus, enhancement degrees of photooxidation by applying bias voltages onto TiO2 thin films were compared under dark, visible light illumination and UV illumination conditions. Bias voltages were applied to localize the photogenerated carriers at the vicinity of TiO2 surface for enhancing photooxidation. Sol‐gel derived TiO2 thin films with porous nano structure exhibited a stronger oxidative decomposition reaction than sputter‐deposited TiO2 thin films.

Electrical conductance of conical nanopores: Symmetric and asymmetric salts and their mixtures.

We have studied experimentally the electrical conductance-voltage curves of negatively and positively charged conical nanopores bathed in ionic solutions with monovalent, divalent, and trivalent cations at electrochemically and biologically relevant ionic concentrations. To better understand the interaction between the pore surface charge and the mobile ions, both single salts and salt mixtures have been considered. We have paid attention to the effects on the conductance of the cation valency, the pore charge asymmetry, and the pore charge inversion phenomena due to trivalent ions, both in single salts and salt mixtures. In addition, we have described how small concentrations of multivalent ions can tune the nanopore conductance due to monovalent majority ions, together with the effect of these charges on the additivity of ionic conductance and fluoride-induced negative differential conductance phenomena. This compilation and discussion of previously presented experimental data offers significant insights on the interaction between fixed and mobile charges confined in nanoscale volumes and should be useful in establishing and checking new models for describing ionic transport in the vicinity of charged surfaces.

Hematite surfaces: Band bending and local electronic states

Hematite is a promising material for photoelectrochemical water splitting applications and understanding the complete mechanism of water oxidation reaction close to its surfaces is of great importance. Herein, we present a theoretical investigation of the electronic states of nonstoichiometric hematite(0001) films in the presence of absorbed H on the surfaces, as well as Ti doping. The calculations, performed at the $\text{DFT}+U$ level, indicate that iron can appear in different oxidation states, from ${\mathrm{Fe}}^{2+}$ to ${\mathrm{Fe}}^{4+}$ and ${\mathrm{Fe}}^{5+}$ in the vicinity of impurities or surfaces. The electron and hole distributions in hematite slabs could control the bending and local state of valence and conduction bands. The projected density of states in different atomic positions shows an upward band bending close to the surfaces, which varies with the number of absorbed H or with Ti doping. In the dark, the width of the depletion layer is computed as less than 12 \AA{} for neutral slabs, while adding excess electrons expands it to involve more atomic layers, depending on the number of absorbed H on the surface. In the neutral slabs in the presence of H, we obtain an upward band bending of 0.18, 0.42, and 0.24 eV by increasing the number of H from one to three on each side, respectively. The evolution of bands in neutral slabs, with/without Ti doping, occurs in a few layers in nearby the surface and also Fe with an oxidation state rather than ${\mathrm{Fe}}^{3+}$. According to our results, doping Ti creates a polaron and locally affects the band structure. In the charged surface with excess electrons, a metallic behavior is observed on the surface and a large band bending in the middle layers. This work shows that a substantial upward bending of the bands in hematite can exist even at neutral surfaces in contact with a vacuum.

(Invited) Effect of Atomic-Scale Surface Overcoat By Atomic Layer Deposition on Dopant Segregation of Perovskite Cathodes

Intense research has been performed to advance the solid oxide cell (SOC) technology because of their excellent efficiency for clean energy conversion and storage, but the lack of long-term durability, particularly of perovskite (ABO3)-based air electrodes, is currently the main technological bottleneck for wider commercial adoption. Long-term durability of SOC air electrodes has been mainly hampered by surface segregation of A-site dopant and morphological agglomeration. Since a deposition of an atomically thin oxide has proven to be highly effective in suppressing electrode agglomeration, a successful suppression of dopant segregation with the same approach will enhance the durability of air electrodes significantly by killing two birds with one stone. In the previous study [1], we demonstrated the effectiveness of an atomic scale overcoat in suppressing particle agglomeration. In this presentation, we demonstrate that even a few angstroms of metal oxide overcoat by atomic layer deposition (ALD) affects the behavior of Sr segregation in the underlying La0.8Sr0.2MnO3 (LSM) electrode significantly. By leveraging angle-resolved X-ray photoelectron spectroscopy (ARXPS), we provide a quantitative analysis of the relative atomic concentration on both the very surface and bulk of the electrode. Compared to bare LSM electrodes, those with 2 – 3 Å ALD overcoat show relatively high resistivity against Sr segregation when exposed to of 750 °C for 250 h. Particularly, an oxide overcoat with multi-valent cations (CeO2 and TiO2) tends to suppress Sr segregation effectively and even drive Sr species back into the bulk of LSM while an overcoat with single valent cations (ZrO2 and Y2O3) exhibits little effect on Sr segregation. It was further demonstrated that the segregation behavior is nicely aligned with the electrode performance; those coated with CeO2 and TiO2 showed better thermal stability in terms of polarization resistance. Based upon the time evolution of Sr content, cationic valence states, and oxygen defect concentration in the vicinity of surface, we conjecture that the amount of oxygen defects formed by the overcoat and the resulting chemical potential gradient at the interface between the overcoat underlying LSM surface plays a pivotal role in the Sr segregation behavior. The authors acknowledge the support from National Science Foundation (DMR 1753383) and NASA MIRO program (NNX15AQ01A).

Conceptualizing the Fe0/H2O System: A Call for Collaboration to Mark the 30th Anniversary of the Fe0-Based Permeable Reactive Barrier Technology

Science denial relates to rejecting well-established views that are no longer questioned by scientists within a given community. This expression is frequently connected with climate change and evolution. In such cases, prevailing views are built on historical facts and consensus. For water remediation using metallic iron (Fe0), also known as the remediation Fe0/H2O system, a consensus on electro-chemical contaminant reduction was established during the 1990s and still prevails. Arguments against the reductive transformation concept have been regarded for more than a decade as ‘science denial’. However, is it the prevailing concept that denies the science of aqueous iron corrosion? This article retraces the path taken by our research group to question the reductive transformation concept. It is shown that the validity of the following has been questioned: (i) analytical applications of the arsenazo III method for the determination of uranium, (ii) molecular diffusion as sole relevant mass-transport process in the vicinity of the Fe0 surface in filtration systems, and (iii) the volumetric expansive nature of iron corrosion at pH > 4.5. Item (i) questions the capability of Fe0 to serve as an electron donor for UVI reduction under environmental conditions. Items (ii) and (iii) are inter-related, as the Fe0 surface is permanently shielded by a non-conductive oxide scale acting as a diffusion barrier to dissolved species and a barrier to electrons from Fe0. The net result is that no electron transfer from Fe0 to contaminants is possible under environmental conditions. This conclusion refutes the validity of the reductive transformation concept and calls for alternative theories.

Free surface effects and the utility of a skim plate for experiments in a water towing tank at steady and unsteady model velocity

A towing tank is utilized to investigate the flow field around a two-dimensional submerged foil model operating near the free surface. Free surface effects are analyzed for steady and unsteady model velocity. The model’s submergence depth and angle of attack are varied. Tests are conducted for the model facing upside-up and upside-down. The surface deflection is recorded and the experimental results are utilized to validate an analytic model that is deployed to predict wake wave patterns at arbitrary model velocity. The flow mechanism leading to load alterations when the foil is in the vicinity of the free surface is explored in detail using experimental and analytic results. The imposed wave-induced velocity perturbations alter the effective angle of attack experienced by the foil. Flow separation is delayed when the model is facing upside-up and promoted when facing upside-down. For test cases with unsteady sinusoidal model velocity, forward traveling waves are generated, leading to a time-varying change in the inflow condition of the submerged foil. Increasing the model’s submergence depth alleviates free surface effects. A skim plate is installed in-between the free surface and model. It shows similar wave alleviating effects as obtained when increasing the model submergence depth by locally blocking wave-induced velocities. The skim plate position is varied in the longitudinal direction to determine its most advantageous position. Surface wave effects at unsteady model velocity are alleviated most effectively when the skim plate protrudes upstream of the model.Graphical abstract

Evaluation of the ITZ modification efficiency via aggregate surface coating with nano SiO2 (NS) and its influence on properties

In order to effectively refine the porous microstructure of interfacial transition zone (ITZ) region in cement-based materials, the nano SiO2 (NS) was introduced into the mixture via aggregate pre-immerging in NS suspension to directly regulate the formation of ITZ microstructure. For comparison, a corresponding content of NS was directly added to the mixture during the casting process. The macro mechanical properties and micro pore structures of the specimens with varying modification methods and NS contents were assessed. The ITZ microstructures (in terms of porosity distribution and nano mechanical properties) were quantitatively characterized via the combination of BSE-SEM and nano indentation techniques. The results demonstrate that both modification methods could refine the microstructure of the bulk paste and ITZ. The directing adding method can more effectively modify the microstructure of the bulk paste with certain NS dosage (no more than 3%). At higher dosage, due to the agglomeration of the NS, the difference between two methods was not obvious. For ITZ modification, the aggregate pre-immerging method has more profound effect, from the aspects of reducing the ITZ thickness and decreasing the porosity of region in the extreme vicinity of aggregate surface (0–10 µm).

Numerical and Experimental Investigation of the Effect of Current Density on the Anomalous Codeposition of Ternary Fe-Co-Ni Alloy Coatings.

Gradient-structured ternary Fe-Co-Ni alloy coatings electrodeposited on steel substrates at various current densities from chloride baths were numerically and experimentally investigated. The electrodeposition process, considering hydrogen evolution and hydrolysis reaction, was modelled using the finite element method (FEM) and was based on the tertiary current distribution. The experimentally tested coating thickness and elemental contents were used to verify the simulation model. Although there was a deviation between the simulation and experiments, the numerical model was still able to predict the variation trend of the coating thickness and elemental contents. The influence of the current density on the coating characterization was experimentally studied. Due to hydrogen evolution, the coating surface exhibited microcracks. The crack density on the coating surface appeared smaller with increasing applied current density. The XRD patterns showed that the deposited coatings consisted of solid-solution phases α-Fe and γ (Fe, Ni) and the metallic compound Co3Fe7; the current density in the present studied range had a small influence on the phase composition. The grain sizes on the coating surface varied from 15 nm to 20 nm. The microhardness of the deposited coatings ranged from 625 HV to 655 HV. Meanwhile, the average microhardness increased slightly as the current density increased from 5 A/dm2 to 10 A/dm2 and then decreased as the current density further increased. Finally, the degree of anomaly along with the metal ion and hydrogen atom concentrations in the vicinity of the cathodic surface were calculated to investigate the anomalous codeposition behaviour.

Study of anisotropy of convective optical underwater turbulence and the effect of the mean water temperature in the presence of a varying temperature gradient on it

In this paper, the anisotropy of optical convective underwater turbulence is investigated in terms of the variance of angle of arrival (AOA) fluctuations of a narrow laser beam propagating through it in different sections of the medium. The collimated laser beam with a wavelength 532 nm and a diameter 1 cm, which passes through a convective underwater turbulence. The turbulence is generated in a water tank with dimensions of 20 cm × 36 cm × 20 cm, which is installed on a flat surface electrical heater. During the experiments, the mean water temperature (MWT) can be changed from room temperature to 34 ∘C by increasing the heater temperature. The use of the heater also generates a temperature gradient in the medium. The laser beam propagates along a horizontal path with a length of 20 cm inside the tank at different altitudes from the heater source, as well as at different distances from one of the side walls of the turbulent medium. After passing the laser beam through the turbulent medium, the fluctuations of the AOA components in the vertical and horizontal directions are measured. From the time series of the measured AOA fluctuations, their variances are determined. The anisotropy of the medium is investigated by comparing the variance of AOA components measured in the vertical and horizontal directions. We show that the variances of both of vertical and horizontal components of the AOA fluctuations are increased with the MWT, and they are saturated at higher MWTs. In addition, different anisotropic behaviors are observed for the variances of the measured AOA fluctuations at the vicinity of the lateral wall and upper surface of the water. At the vicinity of the lateral wall the variances of the AOA fluctuations in the horizontal component are larger, but at the vicinity of the upper surface the variances of the AOA fluctuations in the vertical component are dominant. This behavior may be caused by the change of the convection motion direction in the turbulent fluid.

Numerical study of the effect of wind above irregular waves on the wave-induced load statistics

A wind-forcing model is implemented into a fully nonlinear potential flow solver for water wave propagation. The model is suited for simulations of a large number of waves and for generation of fully nonlinear wave kinematics. The direct effect of wind is achieved through Jeffreys sheltering mechanism, which models the form drag on steep waves by a varying surface pressure. We detail the implementation and examine the effect of the model parameters, which consist of a sheltering coefficient and a threshold of the spatial slope of the wave for activation of the wind model. Next, we present the calibration and application of the model for reproduction of a test campaign for the effect of wind over waves on the wave-induced loads on a vertical surface piercing cylinder. The experimental study has been reported in ( Kristoffersen et al., 2021 ) and consisted of four storm sea states, each run with a duration equivalent to 30 h (full scale), with and without a simultaneous scaled wind field above the waves in the flume. After calibration, the numerical model is able to reproduce many of the findings from the experimental study. When wind forcing is added and the significant wave height is kept fixed, the crest statistics are slightly suppressed in the tail by the presence of wind. The wind, however, leads to a pronounced increase in the wave steepness and to a larger number of breaking waves. Despite the increased number of breaking waves, the depth integrated inline force was only found to increase slightly for the most extreme events at small exceedance probabilities. In contrast, the local force in the vicinity of the free surface was found to increase significantly, at short term exceedance probabilities up to 10 −2 , which is consistent with the measured pressures. For the numerical force profile along the vertical direction of the pile, we found that the altitude of maximum wave force is lowered with the introduction of wind. All of these findings are consistent with the experimental observations. Further for the numerical wave-induced local force at short term exceedance probability of 10 −4 , an increase by a factor of 1.8 was found from the introduction of wind. Although the effect on the global inline force was less strong, this local effect is important for design. The present numerical approach offers a direct way to establish such design loads.

Lasing versus lasing without inversion in an optically thin gain medium near a metal surface

A theory of lasing in an optically thin layer of active centers disposed at a metal surface is developed from first principles. The approach is based on a rigorous account of the local field in the close vicinity of a reflective surface, which provides feedback for dipole oscillations in active centers. It is demonstrated that the gain medium thickness plays a crucial role in the lasing condition and controls switching from conventional lasing to lasing without inversion. The numerical calculations are carried out for erbium doped glass bordering a gold surface where radiation at telecom wavelength (1532 nm) can be generated.

Rheological characterization of biofouling layers developing in nanofiltration processes：macroscale rheological properties of biofouling layer and micro fluid field

Nanofiltration (NF) membrane promotes the rapid development of reclaimed water treatment technology. Biofouling significantly limits NF membranes' performance in the case of wastewater treatment. The rheological characteristics of NF biofouling layer and the microfluid field on the vicinity of membrane surface determine the strategies for alleviating biofouling. Here, we tested the biofouling layer generated on NF in a parallel plate rheometer operated in oscillation and stable shear mode, along with shear stress calculation using computational fluid dynamic simulations on the membrane surface. Results indicated that the biofouling layer was typical viscoelastic material, and the yield stress exhibited shear thinning behavior. The biofouling layer tended to show elasticity during long-term operation, and with higher COD content formed, showed stronger elasticity, larger viscosity, and yield stress. Compared with the shear stress on the membrane surface obtained using numerical simulation (0.57 Pa~1.22 Pa), the yield stress (0.59 Pa~266.98 Pa) of NF biofouling layer was relatively large. Our results suggest that the shear stress generated by water flow may not effectively cause formed biofouling layer shedding. The research will provide theoretical and technical support for biofouling control in NF membrane systems and can be applied in advanced wastewater treatment and reclamation processes.

Robust Tipless Positioning Device for Near-Field Investigations: Press and Roll Scan (PROscan).

Scanning probe microscopes scan and manipulate a sharp tip in the immediate vicinity of a sample surface. The limited bandwidth of the feedback mechanism used for stabilizing the separation between the tip and the sample makes the fragile nanoscopic tip very susceptible to mechanical instabilities. We propose, demonstrate, and characterize an alternative device based on bulging a thin substrate against a second substrate and rolling them with respect to each other. We showcase the power of this method by placing gold nanoparticles and semiconductor quantum dots on the two opposite substrates and positioning them with nanometer precision to enhance the fluorescence intensity and emission rate. Furthermore, we exhibit the passive mechanical stability of the system over more than 1 h. Our design concept finds applications in a variety of other scientific and technological contexts, where nanoscopic features have to be positioned and kept near contact with each other.

Model of interfacial term in turbulent kinetic energy equation and computation of dissipation rate for particle-laden flows

In this paper, we demonstrate that the original Troshko–Hassan model with the correction coefficient being unity should be chosen for an interfacial term in the turbulent kinetic energy equation at the statistically steady state based on the theoretical analysis and interface-resolved direct numerical simulations of the particle sedimentation in a periodic domain and an upward turbulent channel flow. In addition, the computational schemes for the viscous dissipation near the particle surface with the non-boundary-fitted mesh are examined, and the results show that the second-order single-sided differentiation for the velocity gradient at the fluid grids in the immediate vicinity of the particle surface can reduce the computational error significantly, compared to the two-sided differentiation.

Characterisation of defects in die cast aluminium parts

In this paper, the results of computed tomography and destructive testing of a die cast, gas tight aluminium part are compared and analysed with the aim of refining the porosity map. The mapping of the continuity defects detected by the two methods makes it possible to estimate the number, shape and distribution of continuity defects on and in the immediate vicinity of the machined surface, and to compare the characteristics of continuity defects detected by non-destructive and sequential fine machining. Based on the results of the CT scan of a given casting, we also performed destructive testing to find defects in a selected part of the casting by sequential fine machining. The basic criterion for the choice of the cutting parameters was to minimize the plastic deformation (smearing) on the machined surface, i.e. to clearly identify porosity. We then investigated the position, shape and other characteristics of porosity on the surfaces. The results and differences resulting from the two different methods are also analysed.

Very-high-cycle fatigue induced growth and amorphization of Si particles in additively manufactured AlSi10Mg alloy: Dependence of applied stress ratio

The microstructural evolution at the fracture surface in response to very-high-cycle fatigue (VHCF) under stress ratios (R) of −1 and 0.5 in an AlSi10Mg alloy produced by Laser Powder Bed Fusion was investigated. The results show that appreciable growth of the Si precipitates at the cellular network boundaries in the as-built microstructure was observed under R = −1. Moreover, significant amorphization of the initial crystalline Si precipitates occurred in the vicinity of the fracture surface under this condition. A layer of fine Al grains was developed in the “fish-eye” region of the fracture surface. These microstructural responses are rationalized by the generation of lattice defects including dislocations and sub-grain boundaries during cyclic pressing of crack surfaces under R = −1, which mediates the Si solute diffusion and re-precipitation in the alloy as well as the amorphization of initial Si crystalline precipitation. In contrast, far fewer dislocations were observed near the crack surfaces under R = 0.5, which is attributed to the absence of cyclic pressing of crack surfaces and severe plastic deformation in this scenario. This work provides insights into the stress ratio dependence of the microstructural evolution in the fatigued Al alloys. The obtained knowledge is useful for future understanding of the fatigue failure in Al alloys produced by additive manufacturing.

Effects of membrane polarization, steric repulsion and ion-solvent interactions on electroosmosis through a conical nanopore

The impact of solid polarization of a dielectric membrane on ion transport in a conical nanopore drilled through a membrane is studied by considering ions as finite-sized dielectric charged spheres. The excess polarization of the hydrated ions creates a dielectric decrement. A mean-field-based approach is adopted by modifying the Nernst-Planck equation to incorporate the ion steric interactions, the Born force, and the dielectrophoretic force arising due to the spatial variation of electrolyte permittivity. The viscosity of the medium is considered to vary with the local ionic concentration through the Bachelor-Green equation, which leads to a reduction in the mobility of ions near a charged surface. Governing equations are solved numerically through the control volume method along with a pressure-correction-based iterative algorithm. The solid polarization of the membrane, which is often neglected in existing studies, is found to have a strong impact on the volume flux, conductance, selectivity, and ion current rectification in the conical nanopore. The ion saturation in the vicinity of the charged surface, as governed by the present modified model, creates a pronounced alteration in the electrokinetics compared to that predicted by the standard Poisson-Nernst-Planck (PNP-model) based on the point charge consideration of ions. This discrepancy due to ion steric repulsion and ion-solvent interactions is pronounced at a higher ionic concentration as well as a highly polarizable surface. A close agreement between the predictions of the present modified model and the existing experimental data, compared with that of the standard PNP model, is encouraging. The present modified model provides an accurate analysis of the electroosmotic flow and ion transport in a dielectric charged conical nanopore beyond the low charge density and dilute solution limits.

Giant Enhancement of Optical Second Harmonic in Poled Glasses by Cold Repoling.

For the first time, we demonstrated that additional poling at room temperature (cold repoling) of a soda-lime glass thermally poled in open anode configuration causes an increase of the optical second harmonic signal by more than an order of magnitude. Performed experiments allow relating this increase in the optical nonlinearity of the glass to the orientation of hydrogenated dipoles in the vicinity of the glass surface. This indicates that both frozen electric field and dipole orientation are responsible for the nonlinearity of thermally poled alkali-containing glasses. Observed dipole orientation in glasses is of interest for molecular physics and electrodynamic studies.