Vapor Surface Tension Research Articles

Influence of molecular parameters on the representativeness of interfacial properties of simple fluids.

New parameterizations for the Lennard-Jones 12/6 potential capable of reproducing the vapor pressure and surface tension with sufficient precision, but not the liquid-vapor equilibrium densities for the case of simple fluids that include Ar, Kr, Xe, Ne, and CH4 are presented in this work. These results are compared with those derived from the family of Mie(n, 6) potentials, which adequately reproduce the coexistence curve and the vapor pressure, leaving aside the surface tension. In addition, a detailed analysis is presented on different parameterizations and methodologies, which have been developed in recent decades to estimate the interfacial properties of interest here for simple fluids, such as argon, which is a molecule that is, in principle, "simple" to study but that clearly reveals the enormous discrepancy between the results reported in the literature throughout these years. These facts undoubtedly reveal one of the fundamental problems in the context of molecular thermodynamics of fluids: reproducing different thermodynamic properties with sufficient precision from a single set of free parameters for some interaction potential. In order to show the scope of the parameterizations presented for the Lennard-Jones model, they were successfully applied to the case of binary mixtures, which included Ar-Kr, Ar-CH4, and Xe-Kr. Finally, and with the aim of showing a possible solution to the problem posed in this research, results of the same interfacial properties above mentioned for argon and methane were presented in this work by using a set of molecular interactions, called ANC2s, whose flexibility allowed to reproduce the experimental evidence with just one parameterization. The results reported in this work were generated using molecular dynamics simulations.

Characteristics and mechanism of the shift in condition- and time-dependent spray patterns induced by oxygenated fuel addition

As renewable energy sources, ethanol and butanol are usually added to fuels to reduce the fossil fuel consumption and soot emissions. Gasoline direct injection with multi-hole injectors provides accurate plume control, enabling ejection toward the desired location, while spray pattern shift leads to deviation in the injection direction. The spray shift needs to be modulated to realize the desired spray pattern, which is complicated by oxygenated fuel addition. To illustrate the shift induced by oxygenated fuel addition, numerical simulations and experimental studies were performed. The macroscopic and microscopic characteristics of the spray under various operating conditions were obtained via the diffuse backlight illumination and phase Doppler anemometry methods and the simulation of the internal three-phase flow. Moreover, typical single-component and binary fuels were employed to define the general envelope of the multi-component nature for auxiliary analysis. The shift in condition- and time-dependent spray patterns under both flashing and non-flashing conditions were illustrated. The spray morphologies were characterized by rectangular- and trapezoid-like trends. The addition of oxygenated fuels resulted in a smoother trend due to the change of the vapor pressure and surface tension of the mixed fuel. Moreover, it resulted in the variation of the critical width, which is crucial for the spray pattern shift. Additionally, the correlation between the critical width and ambient pressure was obtained. For the non-flashing spray, the internal flow and jet instabilities both influenced the spray width, and the correlation between gas ingestion and turbulence characteristics was obtained. This study aimed to provide guidance for better utilization of oxygenated fuels.

Effect of molecular branching and surface wettability on solid-liquid surface tension and line-tension of liquid alkane surface nanodroplets

HypothesisSurface nanodroplets have important technological applications. Previous experiments and simulations have shown that their contact angle deviates from Young's equation. A modified version of Young's equation considering the three-phase line tension (τ) has been widely used in literature, and a wide range of values for τ are reported. We have recently shown that molecular branching affects the liquid–vapour surface tension γlv of liquid alkanes. Therefore, the wetting behaviour of surface nanodroplets should be affected by molecular branching. This study conducted molecular dynamics (MD) simulations to gain insight into the wetting behaviour of linear and branched alkane nanodroplets on oleophilic and oleophobic surfaces. We aim to examine the Young equation's validity and branching's effect on fundamental properties, including solid–liquid surface tension γsl and line tension τ. SimulationsThe simulations were performed on a linear alkane, triacontane (C30H62), as well as four of its branched isomers: 2,6,13,17-tetrapropyloctadecane,2,6,9,10,13,17-hexaethyloctadecane, 2,5,7,8,11,12,15-heptaethylhexadecane and 2,3,6,7,10,11-hexapropyldodecane. Nanodroplets with a diameter of approximately 15 nm were released onto the surfaces, and their contact angles were measured. Additionally, using a novel approach, the solid–liquid surface tension (γsl), the validity of Young's equation and line tension for all alkane and surface combinations are determined. FindingsIt was discovered that the calculated γsl, deviated from the theoretical γsl,Young predicted from Young's equation for all alkanes on oleophilic surfaces. However, this deviation was minimal for branched alkanes on the oleophobic surfaces but more significant for the linear alkane. The findings indicated that γsl < 0 for oleophilic surfaces and γsl > 0 for oleophobic surfaces. Moreover, it was observed that |γsl| was lower for branched molecules and decreased as branching increased. Line tension values were then determined through a novel method, showing τ was positive for oleophilic surfaces ranging from 1.30 × 10-10 to 6.27 × 10-11N. On an oleophobic surface, linear alkane shows a negative line tension of −1.15 × 10-10N and branched alkanes up to two orders of magnitude lower values ranging from −2.09 × 10-12 to 2.43 × 10-11N. Line tension values between −1.15 × 10-10 and + 1.1 × 10-10N are calculated for various linear alkane and surface combinations. These findings show the dependence of line tension on the contact angle and branching, demonstrating that for linear alkanes, τ is significant, whereas, for branched alkanes, line tension is smaller or negligible for large contact angles.

A mechanistic model to predict saturated pool boiling Critical Heat Flux (CHF) in a confined gap

Boiling enables high rates of heat transfer from a surface made possible at a relatively low thermal resistance motivating the use of two-phase cooling for increasingly compact thermal management solutions. However, extreme geometrical confinement of the liquid above the boiling surfaces is known to have detrimental effects on maximum heat transfer rate by inducing premature onset of film boiling. Moreover, previously developed critical heat flux (CHF) models for confined geometries focused on triggering mechanisms associated with unconfined pool boiling and, thus, are not generalizable. This work proposes a new mechanistic model for predicting CHF during boiling within in narrow gap, specifically developed to account for confinement effects on the triggering mechanism. The model postulates that occurrence of CHF coincides with the irreversible growth of a dry spot on the boiling surface. Three competing forces govern the two-phase interface dynamics, namely vapor momentum, surface tension, and hydrostatic forces. Dryout is triggered when the vapor momentum force due to vaporization at the two-phase interface balances the combined surface tension and hydrostatic forces leading to irreversible growth of the dry spot. The present work offers a predictive confined CHF model that accounts for confined boiling surface shape, size, orientation, confinement gap spacing, and working fluid properties, with a single fluid-specific fitting coefficient that represents the ratio of vapor area to the confinement opening area near CHF conditions. Notably, the developed CHF model is also effective in predicting the threshold gap below which confinement reduces pool boiling CHF. The model is compared to 197 experimentally measured confined CHF data points available from 10 studies in the literature that represent 7 different working fluids and a range of boiling surface inclinations and shapes. The model predicts the confinement-reduced CHF values with a root mean square error of 21%, which is less than half of the error compared to all other available predictive models. This clarification of the triggering mechanism and improved prediction accuracy of CHF, as offered by the current study, will enable broader practical system implementation of compact two-phase cooling technologies.

Review of Visual Measurement Methods for Metal Vaporization Processes in Laser Powder Bed Fusion.

Laser powder bed fusion (LPBF) is of great importance for the visual measurement and analysis of the metallization process, which is the process of solid, liquid, and gas phase transformations of metal powders under high-energy laser irradiation due to the low boiling point/high saturated vapor pressure. Since the evaporation of metals involves the interaction of driving forces such as vapor back pressure, surface tension, and gravity, the movement of the melt pool is not stable. At the same time, it also produces vaporization products such as vapor plumes and sprays, which cause defects such as bubbles, porosity, lack of fusion, inclusions, etc., during the manufacturing process of the parts, affecting the performance and manufacturing quality of the parts. More and more researchers are using imaging technologies, such as high-speed X-ray, high-speed visible light cameras, and high-speed schlieren imaging, to perform noncontact visual measurements and analyses of the melt pool, vapor plume, and spatter during the metal evaporation process, and the results show that the metal evaporation process can be suppressed by optimizing the process parameters and changing the processing atmosphere, thereby reducing part defects and improving part performance and built part quality. This paper reviews the research on metal evaporation mechanisms and visual measurement methods of metal evaporation, then discusses the measures of metal evaporation, and finally summarizes and prospects the future research hotspots of LPBF technology, according to the existing scholars' research on numerical simulation analysis and visual measurement methods of the metal evaporation process.

Quantifying the mechanisms of keyhole pore evolutions and the role of metal-vapor condensation in laser powder bed fusion

Key to quality control for laser powder bed fusion (L-PBF) is the reduction of porosity in built parts. However, understanding the mechanisms of a complete cycle of keyhole pore evolutions, including the processes of the keyhole pore generation, collapse, and splitting, and the role of metal-vapor condensation, remains a great challenge. In this study, we employ a high-fidelity computational tool considering multiphase interactions and thermal-induced phase changes to reproduce the key observations and identify the critical physics underlying keyhole instability and the ensuing keyhole pore generation, collapse, and splitting. We demonstrate that the dynamic fluctuation of keyhole and keyhole pores is dictated by five interdependent factors: vapor condensation, liquid vortex, recoil pressure, surface tension, and keyhole morphology. The occurrence of protrusions inside the keyhole wall enhances the fluctuation of keyhole by re-directing the reflected laser rays and changing the transport of high-temperature liquid flows surrounding the keyhole. The locally generated liquid vortex joins with the overall melt pool dynamics to snap the lower portion of a keyhole to form a keyhole pore and further drive its motion, conditions for which are quantified from the numerical results. We further show that vapor condensation is the major mechanism that may cause two high-speed microjets of pores and result in pore collapse and splitting. Finally, we propose an optimization strategy based on a parametric study of the condensation rate to potentially eliminate keyhole pores during laser melting.

Prediction Models of Saturated Vapor Pressure, Saturated Density, Surface Tension, Viscosity and Thermal Conductivity of Electronic Fluoride Liquids in Two-Phase Liquid Immersion Cooling Systems: A Comprehensive Review

As the carriers of massive data, data centers are constantly needed to process and calculate all kinds of information from various fields and have become an important infrastructure for the convenience of human life. Data centers are booming around the world, accompanied by the problems of high power consumption and poor heat dissipation. One of the most effective solutions to these problems is to adapt a two-phase liquid immersion cooling technology, which is a more energy-saving and efficient method than the traditional cooling methods; the reason for this is mainly that in two-phase liquid immersion cooling technology, the heat transfer caused by the phase change of liquid coolants (electronic fluoride liquids) helps to cool and improve the temperature uniformity of electronic components. However, the requirements for the electronic fluoride liquids used in two-phase liquid immersion cooling systems are strict. The thermophysical properties (saturated vapor pressure, density, surface tension, viscosity, thermal conductivity and latent heat of vaporization, etc.) of the liquid coolants play a very key role in the heat dissipation capacity of two-phase liquid immersion cooling systems. However, it is not always easy to obtain new electronic fluoride liquids under many actual conditions and reasonable prediction models of their thermophysical properties could contribute to the preliminary screening of the coolants. Thus, the prediction models of their key thermophysical properties (saturated vapor pressure, saturation density, surface tension, viscosity and thermal conductivity) are reviewed, and the accuracy and practicality of these prediction models in predicting the thermophysical properties of electronic fluoride liquids (FC-72, HFE-7100 and Novec 649) are evaluated. This work will provide a valuable reference for actual engineering applications.

Condensation heat transfer of binary and ternary mixtures inside multiport tubes

This study investigated the condensation heat transfer of pure refrigerants and binary and ternary mixtures inside horizontal multiport tubes with multiple square minichannels. The effects of mass flux, vapor quality, heat flux, channel size, and channel shape were evaluated using pure refrigerants R32 and R1234yf; binary mixtures R32/R1234yf, R32/R1123, R32/R13I1, and R410A; and ternary mixtures R1123/R32/R1234yf and CO2/R32/R1234yf. For pure refrigerants, the heat transfer coefficient in the annular and churn flow regimes decreased monotonically with decreasing vapor quality. By contrast, the heat transfer coefficient in the plug flow regime was higher over a wide vapor quality range because of the thin condensate film formed by surface tension. Smaller channels exhibited higher heat transfer coefficients. The square minichannels exhibited higher heat transfer coefficients than the circular minichannels, except in the region with high mass flux and high vapor quality. For binary and ternary mixtures, the heat transfer coefficients significantly decreased with decreasing mass flux and vapor quality and were strongly deteriorated at lower mass fluxes. Compared with the annular and churn flow regimes, in the plug and slug-annular flow regimes, where heat transfer through the thin liquid film was dominant, the effect of mass diffusion resistance in deteriorating heat transfer was the most pronounced. A new heat transfer model for pure refrigerants inside multiport tubes with multiple rectangular minichannels was developed considering the contributions of vapor shear stress and surface tension. The proposed model was verified using a dataset of pure refrigerants, and a mean absolute percentage error (MAPE) of 11.5% was achieved. Furthermore, a new heat transfer model that considers the heat transfer deterioration of non-azeotropic refrigerant mixtures was proposed, which could predict 508 datapoints with an MAPE of 13.3%.

Molecular interactions at the metal-liquid interfaces.

We reported molecular simulations of the interactions among water, an epoxy prepolymer diglycidic ether of bisphenol A (DGEBA), and a hardener isophorone diamine (IPDA) on an aluminum surface. This work proposes a comprehensive thermodynamic characterization of the adhesion process from the calculation of different interfacial tensions. The cross-interactions between the atoms of the metal surface and different molecules are adjusted so as to reproduce the experimental work of adhesion. Water nanodroplets on the metal surface are then simulated to predict their contact angle. Liquid-vapor surface tensions of the epoxy prepolymer (DGEBA) and hardener (IPDA) and the solid-vapor surface tension of the aluminum surface are also calculated to provide the solid-liquid interfacial tension that remains very difficult to obtain from the mechanical definition.

CFD study on film boiling features of cryogenic fluid influenced by heat structures and gravity levels

Understanding bubble behaviors and thermal characteristics under various working conditions is of great significance to the efficient utilization of cryogenic propellants. In the present study, simulation models of pool film boiling at horizontal plate and cylinder surface are proposed with Volume of Fluid (VOF) method combined with Lee’s phase change model. Based on the physical and thermal performance, the gravity levels, physical properties, and heater structure dimensions are investigated respectively, which could satisfy the common application conditions of cryogenic propellants. The results show that the movement of bubbles at gas–liquid interface is critical to the pool film boiling heat transfer, and also the heat transfer mechanism affects the bubble motion behaviors. The critical wavelength and the most dangerous wavelength are significant length scales for the bubble behaviors in pool film boiling. When the cylindrical heater size is larger than the most dangerous wavelength, the bubble behaviors as well as the heat flux appear the similar characteristics with that at the plate heater surface. Moreover, the critical wavelength is approximately two times of the bubble departure diameter in the present cryogenic liquid boiling events, and the parameters, including liquid–vapor density difference, surface tension, and gas thermal conductivity, could affect the boiling heat flux significantly. In addition, under 0.03g condition, the bubble departure diameter could reach 27 mm and the boiling heat flux decreases to 800 W/m2, which are significantly different from that in normal gravity. In general, the present CFD model could execute a film boiling study accurately, and a series of detailed results on the bubble behaviors as well as the heat flux could be revealed.

Effect of water content in ethanol on spray formation at subcooled and flash-boiling conditions

Direct-injection spark-ignition engines can be more sensitive to fuel type as the time available for spray formation and mixing is rather limited. The challenge is further complicated by fuel stock which includes a significant bio-derived component in some markets to manage fuel sustainability. The thermophysical properties of bio-components like ethanol differ markedly from typical hydrocarbons found in gasoline like iso-octane. Moreover, the production of anhydrous ethanol fuel involves separating water and ethanol by distillation and dehydration in an energy intensive process. This study presents results from an optical investigation into the spray formation process of hydrous ethanol fuels with 4–15% water content per volume in direct comparison to anhydrous ethanol and iso-octane. Injection tests were carried out with 20–110°C fuel temperature at 1.0 bar and 0.5 bar gas pressure to mimic thermodynamic conditions of cold-start and fully-warm engine at low-load and wide-open-throttle operation. High-speed spray imaging was conducted along with droplet sizing by phase Doppler anemometry. The thermophysical properties of all fuels, including vapour pressure, density, viscosity and surface tension, as well as distillation curves, were obtained over a range of temperatures and the respective Reynolds, Weber and Ohnesorge numbers were considered in the analysis. At 20°C iso-octane and anhydrous ethanol exhibited similar spray plume penetration, whilst increasing water content in ethanol reduced spray penetration. At higher temperatures up to 90°C, anhydrous ethanol showed lower penetration than iso-octane whilst the hydrous fuels still exhibited lower penetration than anhydrous ethanol in sequence of increasing water content. At 110°C, the ethanol fuels showed signs of plume merging and spray collapse, particularly at 0.5 bar gas pressure, with clear differences from iso-octane. The degree of plume merging and collapse was stronger with increased water content. The cone angle for the hydrous fuels was higher than that of pure ethanol, with the 4% water content ethanol that was close to the azeotropic point showing wider cone angle at temperatures higher than 20°C, being distinctly higher at 90°C. Iso-octane had consistently smaller droplet Sauter mean diameter than the ethanol fuels. As the percentage of water content in ethanol increased there was an increase in droplet sizes at all conditions.

Effects of surface wettability on pool boiling process: Dynamic and thermal behaviors of dry spots and relevant critical heat flux triggering mechanism

In this paper, we investigate experimentally and numerically the influences of surface wettability on the boiling process from nucleate boiling to critical heat flux (CHF). The boiling surfaces with four different wettabilities (apparent contact angles) are examined. Using a simple optic setup, sequential phase distribution images on the boiling surfaces were acquired, and we analyzed the dynamics of microlayer, bulk liquid region, and dry spots with the processed images. The variation of surface wettability significantly affected the liquid and vapor phase behaviors on the boiling surface, such as the growth and shrinkage of the microlayer and dry spot, wetting velocity and area portion of the dry spot. As a result, the CHF increased with the surface wettability, and we found the CHF is triggered from a local dry spot where the liquid wets the spot no longer and is violently scattered by strong evaporation at the triple contact line. Our numerical simulation results, in which the three-dimensional transient heat diffusion equation was solved for the boiling substrate using the phase distribution images as boundary conditions, indicated that certain dry spots (critical dry spot) can be highly overheated under high heat flux condition, and they lead significant heat transfer degradation and, eventually, the CHF. With the numerical results and observation of the phase distribution images at the CHF conditions, we postulated the allowable wetting temperature and formulated it as a function of surface wettability through considering the balance between vapor recoil and surface tension forces. The wetting temperature increased with surface wettability, and it was confirmed by the numerical results. Finally, we proposed a qualitative relation to elucidate the influence of surface wettability on the CHF based on the heat conduction via the vapor on a critical dry spot and heat flux partitioning concept.

Temperature, Pressure, and Length-Scale Dependence of Solvation in Water-like Solvents. II. Large Solvophovic Solutes.

We use molecular simulation to determine solvation free energies, isochoric solvation energies and entropies, isobaric solvation enthalpies and entropies, partial molecular volumes, and isothermal density derivatives of the solvation free energy as a function of temperature and pressure for hard-sphere solutes with diameters ranging from 4 to 36 Å in TIP4P/2005 and Jagla water-like solvents exhibiting unusual thermodynamics. An important piece of our discussion focuses on the nanometer-sized solutes, for which simulation results are found to be accounted for by the most basic classical thermodynamic treatment contemplating bulk and interfacial contributions to the solvation free energy. Thus, since water's liquid-vapor surface tension is only special inasmuch as it takes unusually large values, solvent's water-like unusual thermodynamics manifests through a term proportional to the pressure in the solvation free energy. As a result, such solvent's unusual thermodynamics is found to be relevant to the temperature and pressure dependence of the isochoric solvation energy and entropy as well as to the isothermal density derivative of the solvation free energy. This sharply contrasts with the findings of the first part of this series indicating that the solvation free energy of small hard spheres responds to temperature and pressure changes as solvent's density does, with such a contrasting picture embodying a "pressure-density dichotomy." As for the length-scale dependence, we find the zero nominal pressure and the solvent's temperature of the maximum density as singular conditions for cavity surface-area size scaling of large solutes to occur for all solvation quantities. We finally argue that the overall study undertaken in this series suggests that water's unusual thermodynamics may be relevant to the thermodynamic stability of clusters of solvophobic units in the temperature-pressure plane. Some comments on the role of solute-solvent attractive interactions are also depicted.

Spray development of iso-octane, ethanol, hydrous ethanol and water from a multi-hole injector under ultra cold fuel temperature conditions

Multi-hole injectors for direct-injection spark-ignition engines offer atomisation benefits and flexibility in fuel targeting by selection of the number and orientation of the nozzle’s holes. However, the spray formation process from multi-hole injectors has been mostly studied for hot fuel conditions and no quantitative data really exist with fuel temperatures representative of engine operation at cold-start. The challenge is further complicated by the predicted fuel stock which will include a significant bio-derived component presenting the requirement to manage fuel sustainability. The thermophysical properties of bio-components like ethanol differ markedly from typical hydrocarbons found in gasoline like iso-octane. Moreover, the production of anhydrous ethanol fuel involves separating water and ethanol by way of distillation and dehydration in an energy intensive process. This work presents results from an optical investigation into the effect of atomisation for iso-octane, anhydrous ethanol, hydrous ethanol with 10% water content per volume and water. Tests were first carried out at ambient conditions of 20 °C, 1 bar with 150 bar injection pressure using high-speed spray imaging. Then the fuel temperature was lowered to −5 °C, −10 °C and −15 °C to focus on spray formation analysis at realistic cold-start engine conditions. Droplet sizing was also conducted by Phase Doppler Anemometry (PDA). The thermophysical properties of all fuels, including vapour pressure, density, viscosity and surface tension, as well as distillation curves, were obtained over a range of temperatures, and the Reynolds, Weber and Ohnesorge numbers were considered in the analysis. The results revealed the degree of suppression of atomisation for each type of fuel at cold temperatures and effects on spray penetration and cone angle, with ethanol fuels exhibiting the strongest effect in the form of longer initial delay out of the nozzle and shorter penetration.

Antisurfactant (Autophobic) Behavior of Superspreader Surfactant Solutions.

Surfactants are often added to water to increase the wetting of hydrophobic surfaces. We previously showed that most surfactant solutions behave identically to simple liquids with the same surface tension, indicating that the surfactants do not change the wettability of the solid surface itself. Here, we show that the superspreading surfactant Silwet results in a systematically higher contact angle on a hydrophobic surface than other surfactant solutions of comparable liquid–vapor surface tension. We also experimentally observe this “antisurfactant” behavior for CTAB on hydrophilic substrates. Supported by sum-frequency generation spectroscopy results, we suggest that this effect is due to charge-binding of the surfactant with the substrate.

A comprehensive investigation on Spirulina platensis – Part I: Cultivation of biomass, thermo–kinetic modelling, physico–chemical, combustion and emission analyses of bio–oil blends in compression ignition engine

The present work involves an investigation of Spirulina platensis bio–oil (SPO) with its engine performance in the compression–ignition engine. Spirulina platensis biomass was successfully cultivated in standard Zarrouke medium for few decades, hence in this work S. platensis is cultivated in Zarrouke and economic Zarrouke media show the maximum growth rate of 0.30 and 0.29 g/L/d respectively. The calculated average activation energy (Ea) values are 100.76, 157.21, 153.88, 167.67, 291.38, and 186.60 kJ/mol for Kissinger, Kissinger–Akahira–Sunose, Tang, Flynn–Wall–Ozawa, Friedman, and Coast Redfern (Jander 3D diffusion) models respectively. Friedman model shows highest Ea (365.81 kJ/mo1 at α = 0.7) value with more accuracy than others, hence this model is most favored for S. platensis biomass pyrolysis process. Positive ΔG and ΔH values imply that the pyrolysis process follows spontaneity with endothermic nature. A single–stage fixed bed reactor is utilized to extract SPO with 20% yield at 600 °C and its characterized by physico–chemical, spectral (GC–MS & FTIR), elemental, heat of vaporization, Reid vapor pressure, dynamic viscosity and surface tension methods. Engine performance and emission analysis were performed 5% and 10% SPO blends with 0.50:4.75:4.75 and 1:4.50:4.50 (SPO: Ethanol: Diesel) ratio, respectively under 25%, 50%, 75% and 100% load conditions. 5% SPO blend shows better brake thermal efficiency with lower brake specific fuel consumption values than 10% blend. Furthermore, the emissions of HCs, (NO)x and smoke are slightly higher in SPO blends than diesel.

Transport and Interfacial Properties of Mixed Molten Carbonate/Hydroxide Electrolytes by Molecular Dynamics Simulations

We use molecular dynamics simulations based on a recently developed force field to obtain the viscosity, ionic conductivity, and liquid–vapor surface tension of molten alkali-metal carbonate–hydroxide mixtures over a range of cation and hydroxide compositions. Recent experimental and simulation studies have suggested that molten carbonates contain non-negligible amounts of hydroxide ions in the presence of water at low partial pressures of CO2. However, due to the high temperatures (≃600 °C or higher) required to melt pure alkali carbonates and their mixtures, there is a lack of experimental thermodynamic and transport data for these molten phases. Here, we deploy a recently parametrized force field for molten alkali carbonates and hydroxides [ J. Chem. Theory Comput. 2020, 16, 5736−5746, DOI: 10.1021/acs.jctc.0c00285] to simulate some physical properties using atomistic molecular dynamics. Our predictions show a consistent decrease in viscosity and an increase in ionic conductivity with increasing hydroxide fractions, whereas a higher Li+ mole fraction leads to an increase in both viscosity and ionic conductivity. The computed surface tension values exhibit an upward trend with higher asymmetry in cation and anion sizes. Structural analysis suggests that in the carbonate–hydroxide melts the smaller/harder ions, OH– and Li+, are more favored at the interface than the larger/softer ions, CO32– and K+/Na+. The current approach provides a systematic route to obtaining physicochemical properties of molten alkali carbonate and hydroxide mixtures over a large domain of chemical compositions and thus can steer future experimental research for molten carbonates and their applications.

Erosion dynamics of wet particle agglomerates

We study the erosion dynamics of wet particle agglomerates inside a simple shear flow of noncohesive granular materials by relying on the three-dimensional discrete-element simulations. The simulation model is discretized by assembling of wet and dry spherical particles. By systematically varying different parameters related to the shear flow of dry particles (the shear rate), the wet agglomerates (the amount of the binding liquid in the “pendular” state, the liquid viscosity, and the liquid–vapor surface tension), and the relative dry–wet density as well as the initial position of wet agglomerates, we measure the erosion of these agglomerates on their surface by quantifying the cumulative number of eroded particles. We show that the erosion rate increases proportionally to the inertial number and the height of the agglomerates decreases linearly with the liquid content and the liquid viscosity and decreases nonlinearly with the cohesion index (or liquid–vapor surface tension) for each value of the inertial number, whereas this rate is nearly independent to the relative dry–wet density with a low shear rate. It is worth noting that the normalized erosion rate by the shear rate collapses well on a master curve as a cutoff function of the erosion scaling parameter (combining the inertial number, the cohesion index, and the Stokes number), thus providing clear evidence for the unified description of the material and flow parameters on the erosion of wet agglomerates.

Nano-to-Microporous Networks via Inkjet Printing of ZnO Nanoparticles/Graphene Hybrid for Ultraviolet Photodetectors

Inkjet-printed photodetectors have gained enormous attention over the past decade. However, device performance is limited without postprocessing, such as annealing and UV exposure. In addition, it is difficult to manipulate the surface morphology of the printed film using an inkjet printer because of the limited options of low viscosity ink solutions. Here, we employ a concept involving the control of the inkjet-printed film morphology via modulation of cosolvent vapor pressure and surface tension for the creation of a high-performance ZnO-based photodetector on a flexible substrate. The solvent boiling point across different cosolvent systems is found to affect the film morphology, which results in not only distinct photoresponse time but also photodetectivity. ZnO-based photodetectors were printed using different solvents, which display a fast photoresponse in low-boiling point solvents because of the low carbon residue and larger photodetectivity in high-boiling point solvent systems due to the porous structure. The porous structure is obtained using both gas–liquid surface tension differences and solid–liquid surface differences, and the size of porosity is modulated from nanosize to microsize depending on the ratio between two solvents or two nanomaterials. Moreover, the conductive nature of graphene enhances the transport behavior of the photocarrier, which enables a high-performance photodetector with high photoresponsivity (7.5 × 102AW–1) and fast photoresponse (0.18 s) to be achieved without the use of high-boiling point solvents.

Sessile Liquid Features as Molds for Silicone Elastomers.

Liquid applied to a chemically patterned (wetting/nonwetting, lyophilic/lyophobic) substrate forms a 3-dimensional contoured surface, the shape of which depends on the volume of liquid applied and the shape of the three-phase contact lines of air (or other phase in contact), liquid, and the wetted pattern. The resulting binary contoured interface with air, which consists of flat unwetted regions of the substrate and the mean curvature liquid-vapor interfaces of the sessile structures, can be used as a mold for imprinting solid polymers by curing liquid resins in contact. The success, flexibility with regard to shape, and reproducibility of this molding process depend on numerous issues. These include the substrate surface chemistry, the liquid application method, properties of the liquid (vapor pressure, surface tension, viscosity, and permeability in the resin), the contact angles of the liquid on the patterned substrate, and the resin curing chemistry and conditions. We investigate the room temperature platinum(0)-catalyzed curing of the most commonly studied commercial silicone elastomer, Sylgard 184, using molds composed of sessile drops of liquids on circular wetting features (bare silicon wafer) patterned on covalently attached fluoroalkylsilyl monolayers. Liquids reported are water, glycerol, an ethylene glycol oligomer, and an ionic liquid. The vapor pressure of water and its permeability in dimethyl silicone were important (and problematic) issues that could be controlled by adjusting humidity. The ionic liquid N-ethyl-N'-methylimidazolium methanesulfonate poisoned/inhibited the curing chemistry and affected silicone cross-link density and the resulting feature shape, but its lack of vapor pressure was useful in studying flow coating as a scalable liquid application method. The ethylene glycol oligomer exhibited compatibility with (and diffusion into) the silicone. Glycerol proved to be the most well-behaved and controllable liquid studied and was used to demonstrate that condensation/evaporation can be used to adjust feature shape.