Solute Concentration Difference Research Articles

The effects of specimens geometry on solidification conditions and microstructure formation in laser powder bed fusion fabricated Ti–6Al–4V alloys

Laser powder bed fusion (LPBF) excels at creating intricately shaped parts, which will result in variations in solidification conditions, microstructure formation and mechanical properties. To address this issue, three distinct shapes: cubic, pyramidal and inversed pyramidal Ti–6Al–4V alloy specimens were fabricated using LPBF technology. The effects of specimen geometry on microstructure formation, transformation as well as its effects on mechanical properties were investigated in the present study. Our results show that specimen geometry significantly affects the thermal gradient, influencing nucleation and growth during fabrication. EBSD (Electron Backscatter Diffraction) characterization reveals that the primary β-phase grows in columnar grains in cubic specimens, coarse columnar grains along lateral faces in pyramidal specimens, and nearly equiaxed grains in inverted pyramidal specimens, leading to a significant reduction in texture. Nanoindentation tests show higher nanohardness in the top parts of cubic and pyramidal specimens compared to the bottom parts, while inverted pyramidal specimens have consistent nanohardness throughout. This variation is due to differences in solid solute concentration, melt pool undercooling, and thermal histories during and post solidification processes respectively. TEM results indicate that the fine lath phase on the top parts has transformed into β-phase, while the bottom parts remain as α′ phase, highlighting a disparity due to different thermal cycling times and temperatures.

Experimental investigation of two-stage NH3–H2O resorption heat storage system with solution concentration difference

Widespread application of solar energy is severely restricted to its instability in time and space. Heat storage system is considered to be one of the most effective pathways to achieve long-term heat supply. Among all the heat storage technologies, solution concentration difference storage technologies stand out for low heat loss and high energy density. By substituting condenser and evaporator in conventional absorption cycle with a high-pressure absorber and low-pressure generator, a resorption cycle could be established. Compared with absorption heat storage cycle, working pressure of resorption heat cycle is lower, making it possible to utilize low-pressure heat source. Two-stage heat source cycle is further proposed to lower working pressure and upgrading supply water temperature. Resorption energy storage cycle with solution concentration difference is established and studied. A prototype based on two-stage ammonia-water resorption heat storage cycle was set up and tested. The result shows that the constraint relation between ambient temperature and heat source temperature of the proposed system. In single-stage mode, the lowest working ambient temperature is 5 °C when heat source temperature is 90 °C. The lowest working ambient temperature could be declined to 0 °C when heat source temperature rises to 120 °C. In two-stage mode, the lowest working ambient temperature is 0 °C when heat source temperature is only 110 °C, which proves the adaptability of two-stage cycle. The highest ESCOP appears in the 15 °C–120 °C single-stage mode, which is 0.742. The highest COP is 1.324 under the same condition. The highest energy density occurs in two-stage mode, which is 467.07 kJ/kg when ambient and heat source is set as 15 °C, 120 °C. The advantage that two-stage mode could enlarge solution concentration difference is verified.

Thicket and Mesh: How the Outer Membrane Can Resist Tension Imposed by the Cell Wall.

The cell envelope of Gram-negative bacteria is composed of an outer membrane (OM) and an inner membrane (IM) and a peptidoglycan cell wall (CW) between them. Combined with Braun's lipoprotein (Lpp), which connects the OM and the CW, and numerous membrane proteins that exist in both OM and IM, the cell envelope creates a mechanically stable environment that resists various physical and chemical perturbations to the cell, including turgor pressure caused by the solute concentration difference between the cytoplasm of the cell and the extracellular environment. Previous computational studies have explored how individual components (OM, IM, and CW) can resist turgor pressure although combinations of them have been less well studied. To that end, we constructed multiple OM-CW systems, including the Lpp connections with the CW under increasing degrees of strain. The results show that the OM can effectively resist the tension imposed by the CW, shrinking by only 3-5% in area even when the CW is stretched to 2.5× its relaxed area. The area expansion modulus of the system increases with increasing CW strain, although the OM remains a significant contributor to the envelope's mechanical stability. Additionally, we find that when the protein TolC is embedded in the OM, its stiffness increases.

Effect of solute concentration on microstructure evolution during static recrystallization in Mg-0.2%Ce alloy using cellular automata

A new cellular automata model has been developed to evaluate the effect of solute drag on the microstructure and texture evolution during static recrystallization of Mg and its alloys in the temperature range of 200 – 400 °C. In the present model, the site saturation nucleation was assumed and all the nuclei are present at the grain boundaries. The growth of each nucleus depends on the solute concentration and dislocation density difference between neighboring grains. The microstructural analysis of the newly developed cellular automata model showed the evolution of grain size at different temperatures compared to the equilibrium microstructure containing the equilibrium concentration of solute atoms at the grain boundaries. Further, the texture evolution showed that there is an increase in the texture strength with an increase in the annealing temperature when the solute concentration was lower than the equilibrium concentration at the boundaries, whereas the texture strength decreased with increasing the annealing temperature when the solute concentration was higher than the equilibrium solute concentration at the boundaries. The present model shows that solute drag pressure increased continuously with increasing the annealing temperature when the solute concentration was lower than the equilibrium concentration at the boundaries. However, the solute drag pressure decreased with an increase in the annealing temperature when the solute concentration was higher than the equilibrium solute concentration at the boundaries.

Characterization of Mass Transfer within the Crystal-Solution Boundary Layer of l-Alanine {120} Faces Using Laser Interferometry during Growth and Dissolution.

Crystallization and dissolution are important processes to consider in drug development as well as many other industrial processes. Many current growth and dissolution models are based on bulk solution properties and do not implicitly consider concentration variation close to the crystal surface-solution interface and how this is mediated by solute diffusive mass transfer. Solution boundary layer thickness and concentration distribution, for the {120} crystal habit face of single crystals of l-alanine in saturated aqueous solutions during both growth and dissolution processes, is measured as a function of super/undersaturation using a Mach-Zehnder optical interferometer system. Further analysis allows determination of the diffusion coefficient and mass flux within the boundary layer as well as whether the processes are controlled by mass transfer or crystal interfacial kinetics. The measurement of this study revealed that the {120} face was not saturated at its surface during growth or dissolution meaning both processes were somewhat limited by their crystal interfacial kinetics. Growth was limited by crystal interfacial kinetics at all supersaturations to the same degree, whereas dissolution had a mixed dependency on crystal interfacial kinetics and mass transfer at lower undersaturations becoming more limited by mass transfer at higher undersaturations. Boundary layer thickness increased with super/undersaturation but to a lesser degree than the increase in the concentration difference between the crystal surface and bulk solution leading to a higher mass flux of solute molecules through the boundary layer. At the same relative super/undersaturation mass flux of solute molecules was faster during dissolution which was concurrent with its increased surface to bulk solution concentration difference and boundary layer thickness.

Permeability and diffusion resistance of porous membranes: Analytical theory and its numerical test.

This study is devoted to the transport of neutral solutes through porous flat membranes, driven by the solute concentration difference in the reservoirs separated by the membrane. Transport occurs through membrane channels, which are assumed to be non-overlapping, identical, straight cylindrical pores connecting the reservoirs. The key quantities characterizing transport are membrane permeability and its diffusion resistance. Such transport problems arising in very different contexts, ranging from plant physiology and cell biology to chemical engineering, have been studied for more than a century. Nevertheless, an expression giving the permeability for a membrane of arbitrary thickness at arbitrary surface densities of the channel openings is still unknown. Here, we fill in the gap and derive such an expression. Since this expression is approximate, we compare its predictions with the permeability obtained from Brownian dynamics simulations and find good agreement between the two.

Thicket and mesh: Understanding turgor pressure resistance of outer membrane-cell wall system.

The cell envelope of Gram-negative bacteria is composed of three layers: the outer membrane (OM), the peptidoglycan cell wall (PG), and the inner membrane. Together, the cell envelope provides a physical and chemical barrier that protects against the turgor pressure generated by the solute concentration difference between the inside and outside of bacteria, which can range from 0.2 atm to 5 atm. Previously, we measured the contribution of each component to see how it responds to different strains and determined that most of the cell envelope's strength comes from the OM and PG. However, the combined contribution from the OM and the PG, especially considering the Braun's lipoprotein (Lpp) that connects the two, remains to be elucidated. To address this, we constructed and simulated a combined OM-PG system that contains both OM and PG connected by Lpp. We also imposed different initial PG strain levels by stretching it by 2.0X and 2.5X its original size. The results showed that OM can resist imposed tensions by PG up to 2.0x its resting size. Moreover, we explored how the OM/PG-spanning protein TolC can affect the overall resistance to tension, through both enhancement of the connections between the layers and weakening them individually.

Thermal fluctuations and osmotic stability of lipid vesicles.

Biological membranes constantly change their shape in response to external stimuli, and understanding the remodeling and stability of vesicles in heterogeneous environments is therefore of fundamental importance for a range of cellular processes. One crucial question is how vesicles respond to external osmotic stresses, imposed by differences in solute concentrations between the vesicle interior and exterior. Previous analyses of the membrane bending energy have predicted that micron-sized giant unilamellar vesicles (GUVs) should become globally deformed already for nanomolar concentration differences, in contrast to experimental findings that find deformations at much higher osmotic stresses. In this article, we analyze the mechanical stability of a spherical vesicle exposed to an external osmotic pressure in a statistical-mechanical model, including the effect of thermally excited membrane bending modes. We find that the inclusion of thermal fluctuations of the vesicle shape changes renders the vesicle deformation continuous, in contrast to the abrupt transition in the athermal picture. Crucially, however, the predicted critical pressure associated with global vesicle deformation remains the same as when thermal fluctuations are neglected, approximately six orders of magnitude smaller than the typical collapse pressure recently observed experimentally for GUVs. We conclude by discussing possible sources of this persisting dissonance between theory and experiments.

Crystallization by selective evaporation using membrane pervaporation: Application to l-glutamic acid to control polymorphism

Crystallization is an important unit operation in process industries, driven by supersaturation, i.e. the solute concentration difference between solution and equilibrium. Hence, to finely control the product qualities, controlling mass and/or heat transfer is crucial. Membrane pervaporation, based on a selective evaporation technique, appears as a process able to limit phase transition and control polymorphism. In this work, a semi-continuous system is developed for the crystallization of l-glutamic acid, which has two polymorphic forms. Hydrophilic HybSi® membranes are chosen. The influence of different operating parameters on process performance in terms of solvent separation, solute crystallization and control of polymorphism is investigated. Results validate the proof of concept and highlight the preferential crystallization of a polymorphic form over the other. Hence, the α-form is usually favoured except when the β-form is seeded, or at high temperature. Temperature appears as the parameter influencing mostly the membrane selectivity and the polymorph generated. However, concentration polarization and fouling, due to the deposit of l-glutamic acid crystals on the membrane surface, are reported and decrease the process performance. Both phenomena are caused by a high local supersaturation close to the membrane surface in addition to a local cooling due to the water evaporation during pervaporation.

Effect of membrane mean pore diameter on water and solute flux in forward osmosis processes

Several forward osmosis (FO) experiments were carried out using aqueous solutions of sodium chloride (NaCl) in different concentrations, as draw solution (DS), and freshwater, as feed water (FW). The experiments were conducted at a constant temperature of ~20°C and using a symmetric cellulosic membrane in two grades different in their values of the molecular weight cut-off (MWCO), which is related to the mean pore diameter. This study investigates the effect of the membrane mean pore diameter on water flux and DS solute diffusional flux across the membrane. The value of water flux indicates the performance of the FO process, while solute flux is an important factor that determines its practicality. A proportionality factor is used to specify the relationship between water flux and solute flux for each membrane type and operational conditions. The results of this study show that water flux and solute flux increase as the solute concentration difference across the membrane increases. It is also found that, in the range of the tested membranes, membranes with larger mean pore diameters have lower water flux and higher solute flux.

Convective Heat and Mass Transport in Casson Fluid Flow in Curved Corrugated Cavity with Inclined Magnetic Field.

Convection in fluids produced by temperature and solute concentration differences is known as thermosolutal convection. It has valuable utilization in wide industrial and technological procedures such as electronic cooling, cleaning, and dying processes, oxidation of surface materials, storage components, heat exchangers, and thermal storage systems. In view of such prominent physical significance, focus is made to explicate double (thermal and solutal)-diffusive transport in viscoelastic fluid characterized by the Casson model enclosed in a curved enclosure with corrugations. An incliningly directed magnetic field is employed to the flow domain. A uniformly thermalized and concentrated circular cylinder is installed at the center of the enclosure to measure transport changes. Dimensionally balanced governing equations are formulated in 2D, representing governed phenomenon. Finite element-based open-sourced software known as COMSOL is utilized. The domain of the problem is distributed in the form of triangular and quadrilateral elements. Transport distributions are interpolated by linear and quadratic polynomials. The attained non-linear system is solved by a less time and computation cost consuming package known as PARDISO. Convergence tests for grid generation and validation of results are executed to assure credibility of work. The influence of involved physical parameters on concerned fields are revealed in graphical and tabular manner. Additionally, heat and mass fluxes, along with, kinetic energy variation are also evaluated.

Blocker Effect on Diffusion Resistance of a Membrane Channel: Dependence on the Blocker Geometry.

Being motivated by recent progress in nanopore sensing, we develop a theory of the effect of large analytes, or blockers, trapped within the nanopore confines, on diffusion flow of small solutes. The focus is on the nanopore diffusion resistance which is the ratio of the solute concentration difference in the reservoirs connected by the nanopore to the solute flux driven by this difference. Analytical expressions for the diffusion resistance are derived for a cylindrically symmetric blocker whose axis coincides with the axis of a cylindrical nanopore in two limiting cases where the blocker radius changes either smoothly or abruptly. Comparison of our theoretical predictions with the results obtained from Brownian dynamics simulations shows good agreement between the two.

Competition between continuous and discontinuous precipitation in L12-strengthened high-entropy alloys

Understanding of the competition between continuous precipitation (CP) and discontinuous precipitation (DP) is crucial for tailoring the microstructure and mechanical properties of precipitation-strengthened high-entropy alloys (HEAs). In this study, the effects of aging temperature, aging time, and grain size on the CP and DP behaviors of L12-strengthened HEAs were systematically investigated. Our results reveal that low temperatures favor the DP reaction while high temperatures facilitate the CP reaction, which is related to the nucleation site and diffusion path of the two precipitation modes. At intermediate temperatures, both the CP and DP occur simultaneously and compete with each other, and the solute concentration difference across the boundaries between the two precipitation regions serves as the driving force for the sweeping of the CP regions by the DP reaction. In addition, grain size refinement can promote the DP reaction by providing more nucleation sites and more fast diffusion paths, leading to the formation of the DP-dominant microstructure. This study not only sheds light on the fundamental understanding of the CP and DP mechanisms but also provides useful guidelines for the microstructure design of advanced L12-strengthened HEAs.

Intrinsic diffusion resistance of a membrane channel, mean first-passage times between its ends, and equilibrium unidirectional fluxes.

Diffusive flux of solute molecules through a membrane channel driven by the solute concentration difference on the two sides of the membrane is inversely proportional to the channel diffusion resistance. We show that the intrinsic, channel proper, part of this resistance is the ratio of the sum of the mean first-passage times of the molecule between the channel ends and the molecule partition function in the channel. This is derived without appealing to any specific model of the channel and, therefore, is applicable to transport in channels of arbitrary shape and tortuosity and at arbitrary interaction strength of solute molecules with the channel walls.

Concentration of “Mysterious Solute” in CoCrFeNi high entropy alloy

The appearance of serrated flow on stress-strain curves in single-phase high entropy alloys (HEAs) suggests that their lattices are arranged and behave differently than those in pure metals. In this study, a micromechanical model linking serration amplitude and solute concentration difference at the dislocation line was built to quantify the concentration of “mysterious solute” in HEAs. Using the model, the concentration of “mysterious solute” in a model HEA, CoCrFeNi, was determined to be ∼34 at.%. Despite that certain facets of the physical nature of the solute remains to be further delved into, our present findings indeed suggest a quantitative distinction between the solvent and solute in single-phase HEAs. As opposed to commonly treating these concentrated solid solutions as an average “medium”, our work paves the avenue of separating out the solute and solvent, which is beneficial to unveiling many key questions in HEAs (e.g., obstruction of the solute to dislocation motion).

Stability of double-diffusive natural convection in a vertical fluid layer

The stability of basic buoyant flow in a vertical fluid layer induced by temperature and solute concentration differences between the vertical boundaries is investigated. The linear dynamics of the perturbed flow is formulated as an eigenvalue problem and solved numerically by employing the Chebyshev collocation method. The validity of Squire's theorem is proved, and therefore, two-dimensional motions are considered. The neutral stability curves defining the threshold of linear instability and the critical values of the thermal Grashof number and wave number at the onset of instability are determined for various values of the Prandtl number Pr, the solute Grashof number GS, and the Lewis number Le. The magnitude of the Prandtl number at which the transition from stationary to travelling-wave mode occurs can be either increased or decreased by tuning the values of GS and Le. For certain combinations of the parameters, there exist one or two closed disconnected travelling-wave neutral curves emphasizing the necessity of multiple thermal Grashof numbers to embark upon the stability of fluid flow, a result of contrast to that of the single-diffusive fluid layer. The mechanism of modal instability is deciphered by using the method of energy budget and four different modes of instability are identified, one of which is new and due entirely to the presence of solutal buoyancy.

Modeling acid-base transport in hemodialyzers

One important objective of the hemodialysis treatment is the neutralization of interdialytic acid generation by transport of bicarbonate and other buffer bases from dialysis fluid to the patient via the hemodialyzer. Quantification of solute transport in hemodialyzers, in general, employs the concept of dialysance, a parameter that is often constant for given flow conditions, smaller than both the blood and dialysate flow rates, and independent of the solute concentration difference between blood and dialysate. This approach has been applied to bicarbonate transport in hemodialyzers, but such an approach neglects the transport of dissolved carbon dioxide (CO2) between dialysate and plasma, chemical equilibrium between bicarbonate and CO2, and other acid-base chemical reactions within blood. We describe a novel, one-dimensional model of bicarbonate and CO2 transport in hemodialyzers. The model equations were solved numerically and fitted to published data to estimate mass transfer-area coefficients for the relevant chemical species. Base excess in blood was assumed constant in the hemodialyzer. Simulations were performed for a dialysate bicarbonate concentration of 32 mEq/L at constant blood and dialysate flow rates and different plasma bicarbonate concentrations at the inlet of the hemodialyzer, both with and without CO2 transport. In the latter case, the bicarbonate mass transfer-area coefficient was adjusted to achieve the same total carbon dioxide transport. Calculated dialysance for CO2 exceeded the blood flow rate due to its conversion from bicarbonate in the hemodialyzer, and all calculated dialysances varied with inlet plasma bicarbonate concentration. We concluded that acid-base transport in hemodialyzers cannot be universally characterized by dialysances that are always less than the blood flow rate and independent of the concentration difference between dialysate and blood.

In vivo characterization of the structures of films of a fatty acid and an alcohol adsorbed on the skin surface

An in vivo analysis of stearyl alcohol and stearic acid films on the skin surface using polarized infrared-external reflection spectroscopy revealed that whether the sample molecules adopt an energetically stable conformation and orientation strongly depends on the molecular functionalities and sample preparation conditions. For stearic acid, even the difference in solute concentration between 0.1 and 0.5 wt% results in a different molecular conformation and orientation. This illustrates that the molecular organization of the adsorbate on the skin surface is sensitively determined by the kinetics of the sample film growth, not by the simple thermodynamic equilibrium with the skin temperature.

Temperature Effects and Entropy Generation of Pressure Retarded Osmosis Process

Pressure retarded osmosis (PRO) is considered as one of the promising and new techniques to generate power. In this work, a numerical model was used to study the effect of the flow streams temperature on the performance of the PRO process and entropy generation. The variation of the feed solution and draw solution temperatures, pressure difference, concentration difference, and flow rates on the power density and entropy generation were discussed. The model results were validated with experimental measurements obtained from literature and showed a good agreement with the model predictions. It was found that the power density increases by about 130% when both feed solution and draw solution temperatures increase from 20 °C to 50 °C. The feed solution temperature has more impact on the power density than that of the draw solution. This is due to the direct effect of the feed solution temperature on the water permeability and diffusion coefficient. The effect of the feed solution temperature becomes significant at higher concentration differences. Whereas, at low concentrations, the power density slightly increases with the feed temperature. Furthermore, it is found that there is an optimum volumetric flow in the channels that maximizes the power density and minimizes the entropy generation when fixing other operating conditions.

Numerical Modelling of consolidation-induced solute transport in unsaturated soil with dynamic hydraulic conductivity and degree of saturation

The effects of the pore pressure related dynamic hydraulic conductivity and dynamic degree of saturation on the consolidation-induced solute transport in deformable unsaturated soils are investigated. The storage equation and solute transport equations are revised to account for the dependence of these two soil parameters on pore pressures. Three dynamic models were conducted for dynamic hydraulic conductivity, dynamic degree of saturation, and both. Compared with the conventional model, the simulation results showed that both hydraulic conductivity and degree of saturation increased near the soil surface where pore pressure exceeded air-entry value. Dynamic hydraulic conductivity results in a slightly slower solute transport while dynamic degree of saturation accelerates the migration of contaminants. Including both dynamic effects produced limited differences in solute concentration, while consolidation results were affected significantly. Although dynamic degree of saturation has some influences on consolidation-induced solute transport, the correlation of the dynamic path is less significant. The air-entry value is an important parameter to determine when soil parameters become dynamic, and it affects the soil consolidation process in certain extent. This study provides new methods to incorporate in-homogeneous soil parameters, which are not only spatiotemporal variables, but also dynamic variations with pore pressure.