Solute Mass Transport Research Articles

Deciphering the transfer mechanisms for CO2 absorption into binary blended solutions

A thorough understanding of the mechanisms involved in CO2 chemical absorption is crucial for the development of efficient CO2 capture technologies. Due to the insufficient insight into the interaction mechanism of different components in blended solutions, CO2 absorption reactions were often categorized as unidirectional transfer between absorption products. In this study, the CO2 absorption performance of binary blended solutions, including piperazine (PZ)/n-methyldiethanolamine (MDEA), ethanolamine (MEA)/MDEA, ammonia (NH3)/MDEA, PZ/potassium carbonate (K2CO3), and NH3/K2CO3, was investigated using a combination of experimental and computational methods. The CO2 absorption mechanisms of “competitive reaction”, “transfer reaction” and “parallel reaction” in the binary blended solution system were proposed. Kinetic experiments revealed that different blended solutions had varying impacts on the process of CO2 absorption. Among them, PZ/MDEA and MEA/MDEA solutions reduced the absorption rates by an average of 8% and 25%, respectively, compared to PZ or MEA component solutions. NH3/MDEA and PZ/K2CO3 solutions had absorption rates similar to those of single NH3/PZ component solutions. NH3/K2CO3 solutions, on the other hand, exhibited an average increase of 17% in absorption rates compared to NH3 solutions. Quantum mechanical (QM) methods were employed to evaluate of the absorption products and key processes in terms of kinetics and thermodynamics. Quantitative 13C NMR analyses were conducted to further investigate the interactions between components and the pathways of mass transport in blended solutions, which demonstrated proton transfer and CO2/-COO transfer between adsorption products. This study highlights an accurate description of the transfer mechanisms of various blended systems for the enhanced CO2 capture.

Effect of AC electric field in mass transport of a neutral solute in a microtube with porous wall

Electroosmotic flow actuation is one the most active research area in microfluidics. The advantages of microfluidic systems with porous walls are improvement of heat and mass transport, selective separation of solutes and tailored drug delivery. The novelty of this work is investigating impact of electroosmotic flow under AC electric field on mass transport of neutral solute through porous walled microtube. Higher frequency (522 kHz) makes the velocity field stagnant for 80 % of tube cross-section leading better permeation. Effect of frequency is marginal on permeation flux but is significant for permeate concentration. At pseudo-steady state, Sherwood number approaches 2.5 irrespective of operating conditions. It is observed that solute permeation flux increases 13 % by manipulating frequency compared to DC field. The optimum frequency for solute delivery increases with Re.Sc.d/L. The findings of this study helps in better understanding of tailored drug delivery by microfluidic channels and nutrient transport in physiological systems.

Probing the Use of Homemade Carbon Fiber Microsensor for Quantifying Caffeine in Soft Beverages.

In the development of electrochemical sensors, carbon micro-structured or micro-materials have been widely used as supports/modifiers to improve the performance of bare electrodes. In the case of carbon fibers (CFs), these carbonaceous materials have received extensive attention and their use has been proposed in a variety of fields. However, to the best of our knowledge, no attempts for electroanalytical determination of caffeine with CF microelectrode (µE) have been reported in the literature. Therefore, a homemade CF-µE was fabricated, characterized, and used to determine caffeine in soft beverage samples. From the electrochemical characterization of the CF-µE in K3Fe(CN)6 10 mmol L-1 plus KCl 100 mmol L-1, a radius of about 6 µm was estimated, registering a sigmoidal voltammetric profile that distinguishes a µE indicating that the mass-transport conditions were improved. Voltammetric analysis of the electrochemical response of caffeine at the CF-µE clearly showed that no effects were attained due to the mass transport in solution. Differential pulse voltammetric analysis using the CF-µE was able to determine the detection sensitivity, concentration range (0.3 to 4.5 µmol L-1), limit of detection (0.13 μmol L-1) and linear relationship (I (µA) = (11.6 ± 0.09) × 10-3 [caffeine, μmol L-1] - (0.37 ± 0.24) × 10-3), aiming at the quantification applicability in concentration quality-control for the beverages industry. When the homemade CF-µE was used to quantify the caffeine concentration in the soft beverage samples, the values obtained were satisfactory in comparison with the concentrations reported in the literature. Additionally, the concentrations were analytically determined by high-performance liquid chromatography (HPLC). These results show that these electrodes may be an alternative to the development of new and portable reliable analytical tools at low cost with high efficiency.

Mass transport in oscillatory electroosmotic viscoelectric flow in a hydrophobic microchannel with steric effect

We conduct a numerical study of viscoelectric and steric effects on an oscillatory electroosmotic flow (OEOF) and their impact on the mass transport of a passive solute through a hydrophobic microchannel. In many applications of electroosmosis, zeta potentials as high as 100–200 mV can be found; in such a situation, the Debye–Hückel approximation is no longer valid, and the steric effect must be considered because the crowding of finite-sized ions close to the microchannel walls. In addition to the previous effect, the local viscosity can be increased due to the viscoelectric effect for strong electric potentials induced in the electric double layer. Earlier works have studied the mass transfer caused by an OEOF; however, the combined effects’ influence has not been considered. This research suggests that under an appropriate combination of the viscoelectric and steric effects, together with the microchannel hydrophobicity, the mass transport can be controlled and notably enhanced compared with the case where such effects are disregarded. An interesting behavior occurs for relatively high values of the steric factor ν, where there is a linear dependence between the mass transport and the viscoelectric factor ; in contrast, for low values of ν, the relationship is non-linear.

Mass Transport of Dye Solutions through Porous Membrane Containing Tannic Acid/Fe3+ Selective Layer.

Tannic acid (TA)-Fe3+ membranes have received recent attention due to their sustainable method of fabrication, high water flux and organic solutes rejection performance. In this paper, we present a description of the transport of aqueous solutions of dyes through these membranes using the transport parameters of the Spiegler-Kedem-Katchalsky (SKK) model. The reflection coefficient (σ) and solute permeability (PS) of the considered TA-Fe3+ membranes were estimated from the non-linear model equations to predict the retention of solutes. The coefficients σ and PS depended on the porous medium and dye molecular size as well as the charge. The simulated rejections were in good agreement with the experimental findings. The model was further validated at low permeate fluxes as well as at various feed concentrations. Discrepancies between the observed and simulated data were observed at low fluxes and diluted feed solutions due to limitations of the SKK model. This work provides insights into the mass transport mechanism of dye solutions and allows the prediction of dye rejection by the TFC membranes containing a TA-Fe3+ selective layer using an SKK model.

The role of coupled water and solute diffusion and product shrinkage during osmotic dehydration

Osmotic dehydration (OD) is a coupled mass transfer operation where foods are partially dewatered while simultaneously impregnated with solute from the solution. Nonetheless, water loss ( W L ) and solute gain ( S G ) are treated as independent processes for modeling purposes and the effect of their coupling on the estimation of water/solute diffusivities ( D w , D s ) and equilibrium dehydration/impregnation levels ( W L e , S G e ) has not been elucidated yet. The objective of this study was to develop a new model considering the coupled mass transport of water and solute occurring during OD of foods and to evaluate the contribution of this coupling on the estimation of D w , D s , W L e , and S G e in comparison with the rigid/shrinking solid assumption. Experimental W L , S G , and volume kinetics were obtained during OD of papaya cubes in sucrose solutions (50°Bx; 45, 55 and 65 °C) and further analyzed with the proposed model solved under different assumptions: coupled and partially coupled mass transfer equations (MTEs) in a shrinking solid and uncoupled MTEs in a rigid solid (no shrinkage) with uncorrected and corrected lengths for diffusion. Properties D w , D s , W L e , and S G e with coupled MTEs and shrinkage were estimated in the ranges of (2.52–5.48) × 10 −10 m 2 /s, (3.91–5.43) × 10 −10 m 2 /s, 0.47–0.53 g/g and 0.18–0.30 g/g, respectively, with partially coupled and uncoupled MTEs leading to a significant overestimation of diffusivities under the tested experimental conditions (up to 281% for water and 75% for solute), even when shrinkage was considered in the 3D model solution. Current findings demonstrate that, like shrinkage, coupling of W L and S G is of upmost importance for a reliable estimation of mass transfer properties during OD. • A mass transfer model is proposed to analyze the OD of shrinking foods. • Mass transfer of water and solute are fully coupled by a density-related term. • OD experiments with papaya cubes were conducted to validate the model. • The effect of different modeling assumptions on mass transfer properties is analyzed. • Non-fully coupled models overestimate diffusivities even considering food shrinkage.

Convergence of Groundwater Discharge through the Hyporheic Zone of Streams.

Significant attention has been given to hyporheic water fluxes induced by hydromorphologic processes in streambeds and the effects they have on stream ecology. However, the impact of hyporheic fluxes on regional groundwater flow discharge zones as well as the interaction of these flows are much less investigated. The groundwater-hyporheic interactive flow not only governs solute mass and heat transport in streams but also controls the retention of solute and contamination following the discharge of deep groundwater, such as naturally occurring solutes and leakage from geological waste disposal facilities. Here, we applied a physically based modeling approach combined with extensive hydrologic, geologic and geographical data to investigate the effect of hyporheic flow on groundwater discharge in the Krycklan catchment, located in a boreal landscape in Sweden. Regional groundwater modeling was conducted using COMSOL Multiphysics by considering geologic heterogeneity and infiltration constraint of the groundwater circulation intensity. Moreover, the hyporheic flow was analyzed using an exact spectral solution accounting for the fluctuating streambed topography and superimposed with the regional groundwater flow. By comparing the discharge flow fields with and without consideration of hyporheic flows, we found that the divergence of the discharge was substantially enhanced and the distribution of the travel times of groundwater was significantly shifted toward shorter times due to the presence of hyporheic flow. Particularly important is that the groundwater flow paths contract near the streambed interface due to the hyporheic flow, which leads to a phenomenon that we name "fragmentation" of coherent areas of groundwater upwelling in pinhole-shaped stream tubes.

Identification of Overpotentials in Vanadium Redox Flow Battery with Reference Electrodes and Determination of Apparent Electrochemical Rate Constants

Vanadium redox flow battery (VRFB) is an amateur technology that passed pilot stages in fulfilling the stationary applications at the energy storage market with a high share of renewable power generation. The commercial applications of VRFBs require the optimization of their internal losses and developing the optimal control strategies for in-grid operation. This calls for a deep understanding of the internal VRFB processes, which greatly benefit from both the mathematical modeling and experimental testing for the model validation and refinement.However, the available experimental data for VRFBs is usually limited by open-circuit voltage (OCV) and potential (OCP), charge/discharge voltage profiles at constant load current, full cell polarization curves at the fixed state of charge (SOC), and internal stack ohmic resistances. This also complicates the decoupling and accessing the voltage loss sources, with the prevailing ones usually classified as ohmic, concentration, and activation overpotentials. As such, Aaron et al. (1) showed that decoupling of overpotentials at positive and negative half-cell requires the half-cell potentials measurements with reference electrodes (RE) in addition to full cell data. Different configurations of RE incorporation (2–4) were suggested with a limited agreement with full cell experimental data. The alternative approach with extrapolation of parameters presented in the literature is also challenging due to their strong dependence on an experimental protocol (e.g., the electrode material and its pre-treatment procedure, a membrane, and a setup geometry). As an illustration, the comprehensive review Roznyatsovskaya et al. (5) highlights the four orders of magnitude difference in reported surface area-normalized rate constants. Therefore, the development of a self-consistent experimental setup is crucial for understanding and decoupling internal losses for flow batteries. Combined with VRFB modeling, this may extend accurate predictions of its operation at extreme SOCs and load currents, where voltage losses hamper the efficiency of the battery with the dominating ones usually attributed to concentration (mass-transport in solutions) and activation (sluggish kinetics of reactions on electrodes) overpotentials.Herein, we developed the experimental setup adopting a 5cm2 single flow cell equipped with reference electrodes at positive and negative sides. The experiments were performed in symmetric and full cell designs. The polarization curves were measured at different SOCs [10 – 90 %], electrolyte flow rates [5-30 ml min-1], and load currents. The additional effort was made to compare the results of the experiments in symmetric and full cell designs and extract the apparent kinetic rate constants for positive and negative half-cells. The latter was performed within the 0D modeling approach, which is based on general mass and charge conservation principles. The key feature of the model is the approach for the determination of the integral mass-transfer coefficient on the electrode-electrolyte interface that allows describing non-linear polarization behaviour at high current densities. The effective thermodynamically consistent single-step reaction mechanisms were considered at both half cells, thus decreasing the number of free kinetic parameters. Overall, the proposed approach demonstrated the growing impact of the membrane losses at high current densities. The extracted apparent rate constants under conditions of a real flow battery provide important insights for developing reliable and efficient VRFBs applicable for energy storage systems.1 D. Aaron, Z. Tang, A. B. Papandrew and T. A. Zawodzinski, J. Appl. Electrochem., 2011, 41, 1175–1182.2 H. Lim, J. S. Yi and D. Lee, J. Power Sources, 2019, 422, 65–72.3 S. Ressel, A. Laube, S. Fischer, A. Chica, T. Flower and T. Struckmann, J. Power Sources, 2017, 355, 199–205.4 M. Cecchetti, A. Casalegno and M. Zago, J. Power Sources, 2018, 400, 218–224.5 N. Roznyatovskaya, J. Noack, K. Pinkwart and J. Tübke, Curr. Opin. Electrochem., 2020, 19, 42–48.

Production and dispersion of free radicals from transient cavitation Bubbles: An integrated numerical scheme and applications

As an advanced oxidation process with a wide range of applications, sonochemistry relies on acoustic cavitation to induce free radicals for degrading chemical contaminants. The complete process includes two critical steps: the radical production inside the cavitation bubble, and the ensuing dispersion of these radicals into the bulk solution. To grasp the physicochemical details in this process, we developed an integrated numerical scheme with the ability to quantitatively describe the radical production-dispersion behavior. It employs coupled simulations of bubble dynamics, intracavity chemical reactions, and diffusion–reaction-dominated mass transport in aqueous solutions. Applying this method to the typical case of argon and oxygen bubbles, the production mechanism for the main radicals is revealed. Moreover, the temporal-spatial distribution of the radicals in the liquid phase is presented. The results demonstrate that the enhanced radical production observed in oxygen bubbles can be traced to the initiation reaction O2 + H2O → OH+HO2, which requires relatively low activation energy. In the outside liquid region, the dispersion of radicals is limited by robust recombination reactions. The simulated penetration depth of OH is around 0.2 μm and agrees with reported experimental measurements. The proposed numerical approach can be employed to better capture the radical activity and is instrumental in optimizing the engineering application of sonochemistry.

Hydrologic Alteration and Enhanced Microbial Reductive Dissolution of Fe(III) (hydr)oxides Under Flow Conditions in Fe(III)-Rich Rocks: Contribution to Cave-Forming Processes.

Previous work demonstrated that microbial Fe(III)-reduction contributes to void formation, and potentially cave formation within Fe(III)-rich rocks, such as banded iron formation (BIF), iron ore and canga (a surficial duricrust), based on field observations and static batch cultures. Microbiological Fe(III) reduction is often limited when biogenic Fe(II) passivates further Fe(III) reduction, although subsurface groundwater flow and the export of biogenic Fe(II) could alleviate this passivation process, and thus accelerate cave formation. Given that static batch cultures are unlikely to reflect the dynamics of groundwater flow conditions in situ, we carried out comparative batch and column experiments to extend our understanding of the mass transport of iron and other solutes under flow conditions, and its effect on community structure dynamics and Fe(III)-reduction. A solution with chemistry approximating cave-associated porewater was amended with 5.0 mM lactate as a carbon source and added to columns packed with canga and inoculated with an assemblage of microorganisms associated with the interior of cave walls. Under anaerobic conditions, microbial Fe(III) reduction was enhanced in flow-through column incubations, compared to static batch incubations. During incubation, the microbial community profile in both batch culture and columns shifted from a Proteobacterial dominance to the Firmicutes, including Clostridiaceae, Peptococcaceae, and Veillonellaceae, the latter of which has not previously been shown to reduce Fe(III). The bacterial Fe(III) reduction altered the advective properties of canga-packed columns and enhanced permeability. Our results demonstrate that removing inhibitory Fe(II) via mimicking hydrologic flow of groundwater increases reduction rates and overall Fe-oxide dissolution, which in turn alters the hydrology of the Fe(III)-rich rocks. Our results also suggest that reductive weathering of Fe(III)-rich rocks such as canga, BIF, and iron ores may be more substantial than previously understood.

Cost optimization of multi-stage gap membrane distillation

Estimating the costs and performance of emerging separation technologies is technically challenging because of the complex design space and the interdependence between system and module-level design decisions. This work develops a process-based cost-optimization model that minimizes the levelized cost of water (LCOW) produced using multi-stage gap MD. The model fully captures solution properties, heat transport, and mass transport relationships within each membrane module, specifies the optimal multi-stage configuration of modules in series, and determines cost-optimal operation of each stage and the system as a whole as a function of gap type, the number of stages, feed conditions, and desired water recovery. We estimate that the LCOW for recovering 50% of a 100 g/L feed using conductive gap multi-stage MD is approximately 6.5 $ per m3 of product water. Cost-optimal designs of multi-stage systems balance operational costs (primarily energy) and capital costs (primarily membrane). This balance results in stages having a different design and operation across the system (e.g. initial stages operating with chilling and heating and end stages using only heating). Sensitivity analysis on the model results suggests that improving membrane parameters and module designs may provide modest cost reductions of <15%.

Computational modeling of drug separation from aqueous solutions using octanol organic solution in membranes

Continuous membrane separation of pharmaceuticals from an aqueous feed was studied theoretically by development of high-performance mechanistic model. The model was developed based on mass and momentum transfer to predict separation and removal of ibuprofen (IP) and its metabolite compound, i.e. 4-isobutylacetophenone (4-IBAP) from aqueous solution. The modeling study was carried out for a membrane contactor considering mass transport of solute from feed to organic solvent (octanol solution). The solute experiences different mass transfer resistances during the removal in membrane system which were all taken into account in the modeling. The model’s equations were solved using computational fluid dynamic technique, and the simulations were carried out to understand the effect of process parameters, flow pattern, and membrane properties on the removal of both solutes. The simulation results indicated that IP and 4-IBAP can be effectively removed from aqueous feed by adjusting the process parameters and flow pattern. More removal was obtained when the feed flows in the shell side of membrane system due to improving mass transfer. Also, feed flow rate was indicated to be the most affecting process parameter, and the highest solute removal was obtained at the lowest feed flow rate.

Complete Reconstruction of Hydrate Pre-Catalysts for Ultrastable Water Electrolysis in Industrial-Concentration Alkali Media

Summary Fundamental investigations of reconstruction of oxygen evolution reaction (OER) pre-catalysts and performance evaluation under realistic conditions are vital for practical water electrolysis. Here, we capture dynamic reconstruction, including the geometric/phase structure, of hydrate molybdates at oxidized potentials. Etching-reconstruction engineering endows the formed NiOOH with a sub-5-nm particle-interconnected structure, as revealed by multi-angle electron tomography. The key to complete reconstruction is the multicomponent co-leaching-induced loose reconstruction layer, conductive to solution penetration and mass transport. This unique structure avoids particle agglomeration in catalysis and promotes complete exploitation of the catalyst with 1,350 h of durability to meet industrial requirements. Upon addition of iron during reconstruction, mainstream Fe-NiOOH with a retained structure forms. Coupled with MoO2-Ni arrays in a membrane-free and two-electrode cell, it achieves stable electrolysis in industrial-concentration KOH for 260 h. This work highlights the reconstruction chemistry of hydrate oxygen-evolving systems and their performance evaluation under industrial conditions.

Supported gas membrane-based ammonia removal and recovery for a pH-dependent sink: Effect of water vapor transport

Sometimes NH3 is stripped from process/effluent streams through hydrophobic porous hollow-fiber-membranes (HFMs) via a supported-gas-membrane (SGM) process and recovered in concentrated H2SO4 solution as (NH4)2SO4. To recover relatively purified (NH4)2SO4, one can avoid excess H2SO4 with a more dilute H2SO4 strip solution. Neglect of strip-side mass-transfer resistance for low-pH strip H2SO4 solutions is not desirable with higher-pH H2SO4 strip solutions. Small hollow-fiber membrane modules (HFMMs) were used with a higher-pH H2SO4 strip solution. Mass transfer was successfully modeled using reaction-enhanced mass transport in higher-pH H2SO4 solution. Employing larger-scale crossflow HFMMs, time-dependent ammonia removal from a large tank having ammonia-containing process effluent was modeled for batch recirculation operation. The larger-scale modules employ shell-side feed liquid in crossflow with an overall countercurrent flow pattern and acid flow in the tube side. Modeling ammonia transport without water vapor transfer can cause substantial errors in batch recirculation method. Water vapor transport was considered here for low-pH and high-pH H2SO4 strip solutions for ammonia-containing feed in a large tank. Model results describe literature-based experimentally observed mass transfer behavior in industrial-treatment systems well. Model calculations were also made for continuous ammonia recovery from industrial effluents by a number of series-connected HFMMs without any batch recirculation.

Evaluation of mass transfer mechanisms involved during the adsorption of metronidazole on granular activated carbon in fixed bed column

This work is focused on the transient analysis of the different mass transfer mechanisms during the adsorption of metronidazole on activated carbon packed in a fixed-bed column. The analysis was carried out by modeling the breakthrough curves at different operation conditions with a fixed bed general model (FBGM) that considers the mass transport mechanisms of axial dispersion, convective, external and intraparticle (pore volume diffusion and surface diffusion). The results show that the FBGM model adequately simulated the dynamics of the experimental data under all operating conditions. Moreover, it was demonstrated that the convective mass transport governs the mass transport of metronidazole primarily, while the axial dispersion contributes only up to 5–10 % within the interstitial space of the packed column. The external mass transfer can be less than 5 % of the total mass transport of solute in the liquid phase. Regarding the intraparticle mass transport inside the activated carbon, it was found that surface diffusion is more important than the pore volume diffusion in the dynamic adsorption of metronidazole, and represents 80–100 % of the total intraparticle transport.

Concentration polarization over reverse osmosis membranes with engineered surface features

Creating membranes with engineered surface features has been shown to reduce membrane fouling and increase flux. Surface feature patterns can be created by several means, such as thermal embossing with hard stamps, template-based micromolding, and printing. It has been proposed that the patterns create enhanced mixing and irregular fluid flow that increases mass transfer of solutes away from the membrane. The main objective of this paper is to explore whether enhanced mixing and improved mass transfer actually do take place for reverse osmosis (RO) membranes operated in laminar flow conditions typical of full-scale applications. We analyzed velocity, concentration, shear stress, and concentration polarization (CP) profiles for flat, nanopatterned, and micropatterned membranes using computational fluid dynamics. Our methods coupled the calculation of fluid flow with solute mass transport, rather than imposing a flux, as has often been done in other studies. A correlation between Sherwood number and mass-transfer coefficient for flat membranes was utilized to help characterize the hydrodynamic conditions. These results were in good agreement with the numerical simulations, providing support for the modeling results. Models with flat, several line and groove patterns, rectangular and circular pillars, and pyramids were explored. Feature sizes ranged from zero (flat) to 512 μm. The ratio of feature length, between-feature distance, and feature height was 1:1:0.5. Results indicate that patterns greatly affected velocity, shear stress, and concentration profiles. Lower shear stress was observed in the valleys between the pattern features, corresponding to the higher concentration region. Some vortices were generated in the valleys, but these were low-velocity flow features. For all of the patterned membranes CP was between 1% and 64% higher than the corresponding flat membrane. It was found that pattern roughness correlated with boundary layer thickness and thus the patterns with higher roughness caused lower mass transfer of solute away from the surface. Rather than enhancing mixing to redistribute solute, the patterns accumulated solute in valleys and behind surface features. Despite the elevated CP, the nominal permeate flux increased by as much as 40% in patterned membranes due to higher surface area compared to flat membranes. The advantageous results seen in other studies where patterns have helped increase flux may be caused by the additional surface area that patterns provide.

Buoyancy-driven dispersion in confined drying of liquid binary mixtures

We investigate the impact of buoyancy on the solute mass transport in an evaporating liquid mixture (non-volatile solute $+$ solvent) confined in a slit perpendicular to the gravity. Solvent evaporation at one end of the slit induces a solute concentration gradient which in turn drives free convection due to the difference between the densities of the solutes and the solvent. From the complete model coupling mass transport and hydrodynamics, we first use a standard Taylor-like approach to derive a one dimensional non-linear advection-dispersion equation describing the solute concentration process for a dilute mixture. We then perform a complete analysis of the expected regimes using both scaling analysis and asymptotic solutions of this equation. The validity of this approach is confirmed using a thorough comparison with the numerical resolution of both the complete model and the 1D advection-dispersion equation. Our results show that buoyancy-driven free convection always impacts solute mass transport at long time scales, dispersing solutes in a steadily increasing length scale along the slit. Beyond this confined drying configuration, our work also provides an easy way for evaluating the relevance of buoyancy on mass transport in any other microfluidic configuration involving concentration gradients.

Mass transport of SDS and AOT solutions during a rapid surface expansion: Relaxation of surface tension

Abstract Surface tension (ST) relaxations are influential in industrial processes that are characterized by substantial surface deformations. Despite this, the dynamics of surfactant adsorption onto a dilating surface is still unclear. Recently, the ST relaxation of a non-ionic surfactant (TX-100) was reported (Lin et al. (2017) [17]), but little is known about the behavior of ionic surfactants. In this study, the relaxations of ST and surface dilational rates (λ) of ionic aerosol-OT (AOT) and sodium dodecyl sulfate (SDS) solutions were investigated by monitoring a continuously expanding bubble using a pendant bubble tensiometer. The data showed that during the rapid bubble expansion, ST of AOT solutions increased with time, while that of SDS changed negligibly. This implied that the mass transfer rate of AOT molecules from bulk to the bubble surface during expansion (at λ

Effect of water on the electrodeposition of copper on nickel in deep eutectic solvents

ABSTRACTMost studies of metal electrodeposition in ionic liquids dry the electrolyte thoroughly, as water is thought to be detrimental. In some cases, water has a beneficial effect on deposit morphology. The electrodeposition of copper has been studied in 1ChCl: 2EG-water mixtures. It is shown here that the presence of water increases the apparent brightness of the deposit due to changes in the electrodeposit surface feature size, up to a water content of approximately 20 wt-%. This study characterises speciation and mass transport in solution, and shows that diffusion can be controlled independently of speciation. It is shown that there is an optimal water content, which is thought to originate from formation of a bi-continuous micro-emulsion phase in DES-water mixtures. Additionally, the copper species remains in a predominantly ionic medium at low water content but moves to an aqueous environment when water is the main component.

Effect of electrolyte nature in mass transport of a neutral solute in a microtube with porous wall

AbstractElectroosmotic flow in a microchannel is an active area in microfluidics. Microchannels with porous wall are advantageous due to selective separation and enhanced mass transport. An economic and innovative method to fabricate hollow microtubes and their application in electrokinetics are illustrated. Effects of asymmetric electrolyte on mass transport of a neutral macrosolute in the microtube with porous wall are investigated. The combined velocity profile including both pressure‐driven and electroosmotic flow is modeled for different electrolytes. It is found from the study that the addition of higher valency asymmetric electrolyte (3:1) increases transport of neutral solutes across the porous wall compared to its symmetric counterpart (1:1). The developed model was validated with the experimental results, using a cartridge having hollow microtubes with porous wall. This study would be helpful to select an appropriate electrolyte, improving the design, and performance prediction of microfluidic devices.