Interfacial Concentration Gradients Research Articles

Mitigating the interfacial concentration gradient by negatively charged quantum dots toward dendrite-free Zn anodes

Aqueous zinc (Zn) batteries have shown great promise in energy storage systems as an intrinsically safe battery configuration. However, Zn dendrite and side reactions may impede their practical application. Here, we propose to mitigate interfacial concentration gradient by introducing N, S-doped carbon quantum dots (NSQDs) as an additive to achieve uniform Zn deposition. The dangling oxygen-containing groups of the NSQDs would be reduced by a Zn foil, thus interacting with the oxidized Zn2+ to form Zn-O bonds on the Zn surface. The formed NSQD layer provides abundant nucleation sites for Zn ions pre-seeding. Meantime, the NSQD layer in negative charge decreases the ion concentration gradient and uniformizes the electric field distribution near the Zn-electrolyte interfaces, facilitating dendrite-free Zn deposition, which is well evidenced by finite simulation results. Together with a decreased interfacial concentration gradient, the Zn metal anode exhibits good rate performances (20 mA cm−2 for nearly 6000 cycles). In addition, the assembled pouch full cell exhibits excellent cycling stability, and 81% of capacity retention is achieved after 250 cycles. This work not only offers a simple approach to modulate the interfacial concentration gradient for uniform Zn deposition but also paves the way for a practical dendrite-free aqueous Zn battery.

Interfacial Concentration Gradient as a Source of Anomalous Photoeffect in Photoelectrodes

Interfacial Concentration Gradient as a Source of Anomalous Photoeffect in Photoelectrodes

Molecular dynamics simulations on evaporation of a suspended binary mixture nanodroplet

Mixture working fluids have the advantages of active design and adjustable physical properties, therefore it has a promising future in many power and refrigeration applications. Studies on the evaporation process of mixture nanodroplets are helpful to reveal the mechanism of heat transfer at the interface of mixture working fluids. However, at present, there are few studies on the evaporation of mixture droplet, and the conclusions on the pure fluids evaporation cannot be directly applied to the mixture working fluids, thus it is necessary to carry out specific research on the evaporation of mixtures. In this study, molecular dynamics simulation was used to study the evaporation process of R32/R1234yf nanodroplets with different compositions (xR32=0, 0.2, 0.4, 0.6, 0.8, 1) in a large space. The dynamic changes of evaporation rates of droplets with different components were compared, and the evaporation characteristics of droplet with different concentrations were analyzed in combination with the density, temperature and concentration field distribution near the liquid-vapor interface during the evaporation process. The results show that the evaporation rate of the mixture nanodroplets is lower than the linear interpolation with those of the two components, which is similar to the heat transfer deterioration of the mixture working fluids in pool or flow boiling. The prediction deviation of R32/R1234yf mixture nanodroplet evaporation by the D2 law is around 80%. Considering the scale effect and the interfacial concentration gradient, the prediction accuracy of the D2 law can be improved.

Metal/Oxide Heterojunction Boosts Fuel Cell Cathode Reaction at Low Temperatures

AbstractAmong several types of low‐temperature solid oxide fuel cells, hydrogen‐permeable metal‐supported fuel cells (HMFCs) are devices that can achieve outputs of approximately 1.0 W cm−2 at 400 °C. This work clarifies the mechanism for promoting the cathode reaction on proton‐conducting ceramics at such low temperatures. Combined numerical and electrochemical analyses demonstrate that blocking minor oxide ion conduction at metal/oxide heterojunctions promotes proton transfer at the cathode/electrolyte interfaces, thereby enhancing the turnover frequency of the cathode reaction at the triple‐phase boundary. The electrolyte membrane in HMFCs is forced to gain extra protons to compensate for the charge of oxide ions that accumulate because of the blocking, resulting in an increment of the proton concentration gradients near the cathode/electrolyte interfaces so as to eject the excess amount of proton. The interfacial proton concentration gradient increases and thus the cathode polarization resistance of HMFCs decrease with the cell bias. An HMFC with a highly oxygen‐deficient BaZr0.5Sc0.5O3−δ electrolyte accumulates a large amount of oxide ions, thereby developing large concentration gradients. Thus, it achieves a cathode reaction resistance of 0.54 Ω cm2 at 400 °C with conventional cathode materials, La0.6Sr0.4Co0.2Fe0.8O3−δ. These findings demonstrate that HMFCs can efficiently utilize overpotential.

Using Liquid Crystals for In Situ Optical Mapping of Interfacial Mobility and Surfactant Concentrations at Flowing Aqueous-Oil Interfaces.

Flow-induced states of fluid interfaces decorated with amphiphiles underlie phenomena such as emulsification, foaming, and spreading. While past studies have shown that interfacial mass transfer, the kinetics of surfactant adsorption and desorption, interfacial mobility, and surfactant reorganization regulate the dynamic properties of surfactant-laden interfaces, few simple methods permit simultaneous monitoring of this interplay. Here, we explore the optical responses of micrometer-thick films of oils (4-cyano-4'-pentylbiphenyl, 5CB) with a liquid crystalline order in contact with flowing aqueous phases of soluble [e.g., sodium dodecyl sulfate (SDS)] or insoluble (e.g., 1,2-dilauroyl-sn-glycero-3-phosphocholine) amphiphiles. We observe the onset of flow of 0.5 mM SDS solutions within a millifluidic channel (area-average velocity of 200 mm/s) to transform a liquid crystal (LC) film with an alignment along the interface normal into a bright birefringent state (average LC tilt angle of 30°), consistent with an initially mobile interface that shears and thus tilts the LC along the flow direction. Subsequently, we observed the LC film to evolve to a steady state (over ∼10 s) with position-dependent optical retardance controlled by gradients in surfactant concentration and thus Marangoni stresses. For 0.5 mM SDS solutions, by using particle tracking and a simple hydrodynamic model, we reveal that the dominant role of the flow-induced interfacial surfactant concentration gradient is to change the mobility of the interface (and thus shear rate of LC) and not to change the easy axis (equilibrium orientation) or anchoring energy (orientation-dependent interfacial energy) of the LC. At lower surfactant concentrations (0.015 mM SDS), however, we show that the LC directly maps flow-induced interfacial surfactant concentration gradients via a change in the local easy axis of the LC. When combined with additional measurements obtained with simple salts and insoluble amphiphiles, these results hint that LC oils may offer the basis of general and facile methods that permit mapping of both interfacial mobilities and surfactant distributions at flowing interfaces.

Measuring trapped DNA at the liquid-air interface for enhanced single molecule sensing.

Nanopore sensing is a promising tool with widespread application in single-molecule detection. Borosilicate glass nanopores are a viable alternative to other solid-state nanopores due to low noise and cost-efficient fabrication. For dielectric materials, including borosilicate glass, the capacitive noise is one of the major contributors to noise, which depends on the wall thickness and the surface area submerged in an ionic solution. Here, we investigated the root mean square (IRMS) noise and ionic conductance for borosilicate nanopores in different depths (i.e., tip submersion depth) ranging from the solution surface (assumed to be zero) to 5000 μm. Our findings demonstrate a decrease in IRMS noise as the pipette moves toward the surface. We further demonstrate that borosilicate nanopores can detect single lambda DNA (λ-DNA) molecules with a high signal-to noise ratio close to the liquid-air interface. Specifically, our results indicate a higher signal to noise ratio as the submersion depth is reduced owing to the reduced surface area and thus capacitive noise. Further, our experimental results show higher DNA capture frequency at the air-water interface due to a combined effect of evaporation and an evaporation-induced thermal gradient at the surface. Therefore, our findings demonstrate that borosilicate glass nanopores are suitable for studying interfacial concentration gradients of molecules, specifically DNA, with a higher signal to noise.

Dynamic mobility of surfactant-stabilized nano-drops: unifying equilibrium thermodynamics, electrokinetics and Marangoni effects

A theoretical analysis of the dynamic electrophoretic mobility of surfactant-stabilized nano-drops is undertaken. Whereas the theory for rigid spherical nanoparticles is well developed, its application to nano-drops is questionable due to fluid mobility of the interface and of the surfactant molecules adsorbed there. At zero frequency, small drops with surface impurities are well known to behave as rigid spheres due to concentration-gradient-induced Marangoni stresses. However, at the megahertz frequencies of electroacoustic (and other spectral-based) diagnostics, the interfacial concentration gradients are dynamic, coupling electromigration, advection and diffusion fluxes. This study addresses a parameter space that is relevant to anionic-surfactant-stabilized oil–water emulsions, using sodium-dodecylsulfate-stabilized hexadecane as a specific example. The drop size is several hundred nanometres, much larger than the diffuse-layer thickness, thus motivating thin-double-layer approximations. The theory demonstrates that fluid mobility and fluctuating Marangoni stresses can have a profound influence on the magnitude and phase of the dynamic mobility. We show that the drop interface transits from a rigid/immobile one at low frequency to a fluid one at high frequency. The model unifies electrokinetics and equilibrium interfacial thermodynamics. Therefore, with knowledge of how the interfacial tension varies with electrolyte composition (oil, surfactant and added salt concentrations), the particle radius might be adopted as the primary fitting parameter (rather than the customary -potential) from an experimental measure of the dynamic mobility. This theory is general enough that it might be applied to aerosols and bubbly dispersions (at sufficiently high frequencies).

Reduced air–sea CO2 exchange in the Atlantic Ocean due to biological surfactants

Ocean CO2 uptake accounts for 20–40% of the post-industrial sink for anthropogenic CO2. The uptake rate is the product of the CO2 interfacial concentration gradient and its transfer velocity, which is controlled by spatial and temporal variability in near-surface turbulence. This variability complicates CO2 flux estimates and in large part reflects variable sea surface microlayer enrichments in biologically derived surfactants that cause turbulence suppression. Here we present a direct estimate of this surfactant effect on CO2 exchange at the ocean basin scale, with derived relationships between its transfer velocity determined experimentally and total surfactant activity for Atlantic Ocean surface seawaters. We found up to 32% reduction in CO2 exchange relative to surfactant-free water. Applying a relationship between sea surface temperature and total surfactant activity to our results gives monthly estimates of spatially resolved ‘surfactant suppression’ of CO2 exchange. Large areas of reduced CO2 uptake resulted, notably around 20° N, and the magnitude of the Atlantic Ocean CO2 sink for 2014 was decreased by 9%. This direct quantification of the surfactant effect on CO2 uptake at the ocean basin scale offers a framework for further refining estimates of air–sea gas exchange up to the global scale. Biologically produced surfactants in the sea surface microlayer reduce air–sea exchange of CO2 in the North Atlantic Ocean, according to tank and ocean measurements.

Bicarbonate Is Not a General Acid in Au-Catalyzed CO2 Electroreduction.

We show that bicarbonate is neither a general acid nor a reaction partner in the rate-limiting step of electrochemical CO2 reduction catalysis mediated by planar polycrystalline Au surfaces. We formulate microkinetic models and propose diagnostic criteria to distinguish the role of bicarbonate. Comparing these models with the observed zero-order dependence in bicarbonate and simulated interfacial concentration gradients, we conclude that bicarbonate is not a general acid cocatalyst. Instead, it acts as a viable proton donor past the rate-limiting step and a sluggish buffer that maintains the bulk but not local pH in CO2-saturated aqueous electrolytes.

The Impact of Dead Li on Mass Transport, Voltage Profiles, Capacity, and Failure of Lithium Metal Anodes

Currently, Li metal anode development is restricted by low Coulombic efficiency, poor cycle life, and safety concerns. Each of these challenges stem from the extreme reactivity of Li metal and the resulting undesirable interactions at the electrode/electrolyte interface. Dendrites grow during electrodeposition, consuming electrolyte, leading to dead Li formation and capacity fade, and eventually causing short circuiting. The limited knowledge of the dynamically evolving Li metal surface during extended cycling has prevented rational design of methods to mitigate these undesirable behaviors. Our recent work elucidated the correlation of galvanostatic voltage traces with morphology evolution on Li metal anodes during early cycles, providing a “window” into cells which traditionally are difficult to monitor with operando techniques [1]. The present work builds on that understanding by studying the evolution of the voltage trace shape and the dead Li layer over extended cycling in both Li-Li symmetric and full cells containing Li metal anodes (Figure 1). In this work [2], we focus on understanding: 1) the evolution of the voltage trace from the peaking shape seen in Figure 1a to the arcing seen in Figure 1b during extended cycling in both half and full cells, 2) how the accumulation of a dead Li layer impacts mass transport, and 3) why dead Li causes capacity fade and failure of full cells containing Li metal anodes. Continuous accumulation of dead Li on the anode surface leads to the formation of a tortuous porous interphase. This layer creates larger interfacial concentration gradients which lead to large overpotentials. Based on experimental results and modeling, we provide a detailed understanding of the effects that mass transport through complex interphases has on capacity fade, voltage traces, and failure of Li metal anodes in half and full cell configurations. We demonstrate that these effects at the anode have significant impacts on full cell performance, in many cases driving failure. This result underscores the importance of understanding the dynamic morphology evolution on anode surfaces and shows that Li-Li symmetric cells are a powerful tool to study the underlying physical phenomena occurring during cycling. This study enables evaluation of methods of reducing dead Li formation and rational design of solutions for improved performance in Li-metal batteries, an important step towards making Li metal rechargeable batteries a commercial reality.

The history effect in bubble growth and dissolution. Part 1. Theory

The term ‘history effect’ refers to the contribution of any past mass transfer events between a gas bubble and its liquid surroundings towards the current diffusion-driven growth or dissolution dynamics of that same bubble. The history effect arises from the (non-instantaneous) development of the dissolved gas concentration boundary layer in the liquid in response to changes in the concentration at the bubble interface caused, for instance, by variations of the ambient pressure in time. Essentially, the history effect amounts to the acknowledgement that at any given time the mass flux across the bubble is conditioned by the preceding time history of the concentration at the bubble boundary. Considering the canonical problem of an isolated spherical bubble at rest, we show that the contribution of the history effect in the current interfacial concentration gradient is fully contained within a memory integral of the interface concentration. Retaining this integral term, we formulate a governing differential equation for the bubble dynamics, analogous to the well-known Epstein–Plesset solution. Our equation does not make use of the quasi-static radius approximation. An analytical solution is presented for the case of multiple step-like jumps in pressure. The nature and relevance of the history effect is then assessed through illustrative examples. Finally, we investigate the role of the history effect in rectified diffusion for a bubble that pulsates under harmonic pressure forcing in the non-inertial, isothermal regime.

The role of surfactants on the mechanism of the long-wave instability in liquid film flows

AbstractThe analysis for the physical mechanism of the long-wave instability in liquid film flow is extended to take into account the presence of a surfactant of arbitrary solubility. The Navier–Stokes equations are supplemented by mass balances for the concentrations at the interface and in the bulk, by a Langmuir model for adsorption kinetics at the interface, and are expanded in the limit of long-wave disturbances. The longitudinal flow perturbation, known to result from the perturbation shear stress which develops along the deformed interface, is shown to contribute a convective flux that triggers an interfacial concentration gradient. This gradient is, at leading order, in phase with the interfacial deformation, and as a result produces Marangoni stresses that stabilize the flow. The strength of the interfacial concentration gradient is shown to be maximum for an insoluble surfactant and to decrease with increasing surfactant solubility. The decrease is explained in terms of the spatial phase of mass transfer between interface and bulk, which mitigates the interfacial flux by the flow perturbation and leads to the attenuation of Marangoni stresses. Higher-order terms are derived, which provide corrections for disturbances of finite wavelength.

Controlling thread formation during tipstreaming through an active feedback control loop

Microscale tipstreaming is a hydrodynamic phenomenon capable of producing submicron sized droplets within a microfluidic device. The tipstreaming process results in the drawing of a thin thread from a highly curved interface and occurs as a result of interfacial surfactant concentration gradients that develop due to elongational flows generated within flow focusing geometries. However, in conventional microfluidic devices, the thread formation is periodically interrupted by the formation of larger primary droplets. This study presents an active feedback control loop capable of eliminating the production of primary droplets and producing a continuous thread, and therefore a continuous droplet stream. A proportional controller is used to successfully control the position of the interface and generate a continuous thread. A derivative component is incorporated in an attempt to increase controller stability, but this component is found to be ineffective. Analysis of the tip position as a function of time is performed to determine the optimal proportional gain constant and set point value for the proportional controller that minimize fluctuations in the produced droplet sizes. The generation of a continuous thread facilitates the use of tipstreaming in several applications, including nanoparticle synthesis, chemical detection, and enzyme activity studies.

Numerical simulation of species transfer across fluid interfaces in free-surface flows using OpenFOAM

This paper presents the Continuous-Species-Transfer (CST) method, which enables interface capturing techniques – a group among Computational Multi-Fluid Dynamics (CMFD) methods that rely upon a smooth interface representation – to deal with species transfer. In this study we examine realistic species transfer across fluid interfaces, taking into account both the steep interfacial concentration gradients (at high Schmidt numbers) and the sharp interfacial concentration jump (at high Henry coefficients due to different species solubilities). Thus, the main objective is to establish the CST method for species transfer across fluid interfaces of arbitrary morphology in free-surface flows at high viscosity and density ratios.Detailed numerical simulations of single rising bubbles have been performed at high resolutions. Results were compared to experimental data and correlations derived thereof.

THE INTERFACIAL THERMAL STABILITY AND ELEMENT DIFFUSION MECHANISM OF SiCf/TC17 COMPOSITE

SiCf /TC17 composites were fabricated by a method of precursor wire with magnetron sputtering using a vacuum hot pressing (VHP) process and then exposed in vacuum at 973, 1023, 1073 and 1123 K for different times, respectively. The results show that element diffusions include interdiffusion caused by interfacial reaction and concentration gradient, and phase transformation diffusion in matrix. C and Ti mainly carry on reaction diffusion which is the reason of formation and growing up of reaction layer. Si, Al, Mo, Cr, Zr and Sn carry on downhill diffusion at interface of C-coating layer and reaction layer, but this type of diffusion is not obvious. Phase transformation diffusion in matrix lead to that Al diffuse to α phase, Mo and Cr diffuse to β phase, and Sn diffuse to Ti3AlC, so the interfacial interdiffusions of these elements is suppressed. The results of the interfacial thermal stability show that the activation energy of reaction lay growing up is 138 kJ/mol, and the interface of SiCf /TC17 composite is stable for long time while it is used not above 973 K.

Electrodiffusion of Molecules in Aqueous Media: A Robust, Discretized Description for Electroporation and Other Transport Phenomena

Electrically driven transport of molecules and ions within aqueous electrolytes is of long-standing interest, with direct relevance to applications that include the delivery/release of biologically active solutes to/from cells and tissues. Examples include iontophoretic and electroporation-mediated drug delivery. Here, we describe a robust method for characterizing electrodiffusive transport in physiologic aqueous media. Specifically, we treat the case of solute present in sufficiently low concentration as to negligibly contribute to the total ionic current within the system. In this limiting case, which applies to many systems of interest, the predominant electrical behavior due to small ions is decoupled from solute transport. Thus, electrical behavior may be characterized using existing methods and treated as known in characterizing electrodiffusive molecular transport. First, we present traditional continuum equations governing electrodiffusion of charged solutes within aqueous electrolytes and then adapt them to discretized systems. Second, we examine the time-dependent and steady-state interfacial concentration gradients that result from the combination of diffusion and electrical drift. Third, we show how interfacial concentration gradients are related to electric field strength and duration. Finally, we examine how discretization size affects the accuracy of these methods. Overall these methods are motivated by and well suited to addressing an outstanding goal: estimation of the net ionic and molecular transport facilitated by electroporation in biological systems.

Poiseuille flow and drop circulation in microchannels

Microfluidics aims to control precisely the transport of fluids and suspended particles or drops. Two characteristics of such transport in rectangular microchannels are addressed here, as a function of the cross-sectional aspect ratio. First, we highlight a convenient expression for the ratio of the centerline to bulk flow velocities, which is relevant for controlling the flow of suspended or flow-focused objects. Then, using the theory of Nadim and Stone, the droplet circulation fountain-flow pattern in such channels is evaluated explicitly, and implications for interfacial mobility measurements are discussed. For example, when the interface is retarded, part of the fountain reverses direction, thus alleviating stagnation, promoting mixing, and reducing interfacial concentration gradients.

Observation of Hydronium Ions at the Air−Aqueous Acid Interface: Vibrational Spectroscopic Studies of Aqueous HCl, HBr, and HI

The air−liquid interface of aqueous hydrogen-halide solutions is examined using vibrational sum frequency generation spectroscopy. Infrared and Raman spectroscopies are used to compare the effects of the ions on the water structure in the bulk solutions to that of the interface. The addition of HCl, HBr, and HI to water is found to cause a significant disruption in the hydrogen-bonding network at the air−liquid interface, similar to that which is observed for sodium halides. However, a convolution of additional effects are observed for acids at the air−aqueous interface: interfacial H3O+ (and H5O2+) which gives rise to surface potential changes, an increase in interfacial depth from interfacial concentration gradients, and a decrease in the number of dangling OH bonds relative to the neat water surface. Additionally, and moreover, sum frequency spectra reveal a surface proton continuum at frequencies below 3000 cm-1, indicating that hydronium ions (and Zundel ions) exist at the air−aqueous interface.

Nonlinear parametrically excited surface waves in surfactant-covered thin liquid films

The effect of gravity modulation on the nonlinear evolution of long-wavelength disturbances at the free surface of a surfactant-covered thin liquid layer is studied. The surfactants, which are assumed to be insoluble, give rise to interfacial concentration gradients and associated Marangoni flow in the underlying liquid film. A coupled system of lubrication-theory-based evolution equations for the film height and surfactant concentration is solved numerically using spectral methods

Sediment‐water exchange of total mercury and monomethyl mercury in the San Francisco Bay‐Delta

Five field trips were conducted in the San Francisco Bay‐Delta between May 2000 and October 2001 to investigate the sediment‐water exchange of total mercury (Hg) and monomethyl mercury (MMHg). Solid‐phase Hg averaged ~1 nmol g−1 and did not show any variability with depth or time or among sites. In contrast, solid‐phase MMHg showed considerable vertical, temporal, and spatial variability (0.4–66 pmol g−1), with the highest values occurring at a peat‐rich environment in May 2001, suggesting that MMHg production was largely controlled by temporal factors and habitat type. In pore water, both Hg and MMHg concentrations were generally elevated near the sediment‐water interface during warm months. Sediment‐water exchange flux of MMHg, determined with benthic chamber deployments, ranged from −92 to 850 pmol m−2 d−1, with higher values occurring in May. In most cases, diffusional fluxes of Hg and MMHg, estimated with the use of interfacial concentration gradients, constituted only a minor portion of the measured fluxes, suggesting the importance of advective processes on sediment‐water exchange. Surface‐water transect and time series studies conducted in Franks Tract support the commonly held belief that wetland and marsh regions are major sources for MMHg within the Delta. The integrated sediment‐water fluxes of Hg and MMHg in the study area were estimated to be 130 and 6 mmol d−1, respectively, and the benthic input was as important a source of Hg and MMHg as the riverine input within the Delta during low‐flow months.