Vapor Pressure Difference Research Articles

Morphology control of bi-component polymer nanofibers produced by gas jet process

This study investigates morphology control of polymer nanofibers produced from solutions of two immiscible polymers in mutually miscible solvents by gas jet fiber (GJF) method. The morphology is controlled by selecting the solvents with a range of vapor pressure and solubility parameter to govern respectively the evaporation rates and the polymer-solvent affinity. Bi-component nanofibers with core–shell, bi-lobal, and interpenetrating network (IPN) morphology are produced by evaporation-induced phase separation of the polymers during spinning of homogeneous solutions of hydrophobic polyvinyl acetate (PVAc) and hydrophilic polyvinyl pyrrolidone (PVP) in two mutually miscible solvents. The tendency of phase-separation is inferred from the values of polymer-solvent mixture affinity. The results indicate that solvent pairs with closely matched vapor pressure yield IPN morphology, while solvents with large differences in vapor pressure produce core–shell morphology. Bi-lobal morphology is produced for solvent pairs with moderate differences in vapor pressure. In each case, nanofibers of smaller diameter are obtained when solvent pairs of lower vapor pressure are used.

Flux Enhancement in Membrane Distillation Using Nanofiber Membranes

Membrane distillation (MD) is an emerging separation technology, whose largest application potential lies in the desalination of highly concentrated solutions, which are out of the scope of reverse osmosis. Despite many attractive features, this technology is still awaiting large industrial application. The main reason is the lack of commercially available membranes with fluxes comparable to reverse osmosis. MD is a thermal separation process driven by a partial vapour pressure difference. Flux, distillate purity, and thermal efficiency are always in conflict, all three being strictly connected with pore size, membrane hydrophobicity, and thickness. The world has not seen the ideal membrane yet, but nanofibers may offer a solution to these contradictory requirements. Membranes of electrospun PVDF were tested under various conditions on a direct contact (DCMD) unit, in order to determine the optimum conditions for maximum flux. In addition, their performance was compared to commonly available PTFE, PE, and PES membranes. It was confirmed that thinner membranes have higher fluxes and a lower distillate purity and also higher energy losses via conduction across the membrane. As both mass and heat transfer are connected, it is best to develop new membranes with a target application in mind, for the specific membrane module and operational conditions.

Analysis on design parameters of water-lubricated journal bearings under hydrodynamic lubrication

The article aims to investigate the relationship between eccentricity ratio and the Sommerfeld number based on hydrodynamic water-lubricated journal bearings. Considering the differences between water and oil, especially the difference in vapor pressure, the effect of cavitation on pressure distribution of water film is analyzed based on three boundary conditions and a cavitation model by computational fluid dynamics (CFD). The result shows that the three boundary conditions are all not suitable to replace cavitation model in numerical analysis of water-lubricated journal bearings. Then numerical analysis of journal bearings with different dimensions is undertaken with the cavitation model under different working conditions. Based on the analysis, a reference is produced for designing water-lubricated journal bearings under hydrodynamic lubrication. At last, the reference is verified by experiments.

Stomatal responses to alterations of soil and air humidity in grapevines

In 1985, during 31 diurnal cycles the assimilation and transpiration of unstressed (predawn Ψw -2 to -3 bar) and cyclically stressed (predawn Ψw -2 to -8 bar) Riesling and Silvaner vines were recorded: In unstressed vines, the average stomatal conductance at daily maximum assimilation (g at A max ) was lower in Riesling than in Silvaner ( - 29 % ). The higher water use efficiency (assimilation to transpiration ratio, A/E at A max ) of Riesling was mainly due to its lower transpiration. At reduced water supply, g (at A max ) was reduced in both varieties causing a decline of A and E (at A max ); especially in Riesling the A/E ratio ( at A max ) was increased due to water stress. g (at A max ) and A max were always positively correlated. In Silvaner, water stress led to a steeper slope of the regression line which indicates an increase of adaptation to drought; in Riesling, water stress caused a higher correlation coefficient. This closer correlation at water stress, the high A/E ratio (at A max ) and the steep slope of the regression line suggest that Riesling, especially under stress conditions, has a more precise functioning of stomatal action and herewith a higher adaptability to drought conditions than Silvaner. In unstressed vines, the A/E ratio (daily average values) declined at increasing leaf-air water vapor pressure difference (∆ w ), Riesling exhibiting generally higher A/E values than Silvaner. In 1986, Riesling leaves were subjected to increasing ∆ w (from 13 to 18 mbar · bar -1 ) at constant light and temperature: At high leaf water potentials ( -4 to - 6 bar), alterations of ∆ w led to simultanous decreases of g, A and E. Related to stomatal conductance, transpiration increased in a linear, assimilation in a curvilinear manner. The optimal stomatal conductance was given where the marginal transpiration rate equaled the marginal assimilation rate; the A/E ratio and the correlation coefficient g (at A max )-A max were found tobe highest in that range of stomatal conductance.

Two-dimensional bismuth-rich nanosheets through the evaporative thinning of Se-doped Bi2Te3

High bulk conductance obscures the behavior of surface states in the prototypical topological insulators Bi2Te3 and Bi2Se3. However, ternary phases of Bi2Te3−ySey with balanced donor and acceptor levels may lead to large bulk resistivity, allowing for the observation of the surface states. Additionally, the contribution of the bulk conductance may be further suppressed by nanostructuring, increasing the surface-to-volume ratio. Herein we report the synthesis of a ternary tetradymite newly confined to two dimensions. Ultra-thin large-area stable nanosheets were fabricated via evaporative thinning of a Bi2Te2.9Se0.1 original phase. Owing to vapor pressure differences, a compositional shift to a final Bi-rich phase is observed. The Se/Te ratio of the nanosheet increases tenfold, due to the higher stability of the Bi–Se bonds. Hexagonal crystal symmetry is maintained despite dramatic changes in thickness and stoichiometry. Given that small variations in stoichiometry of this ternary system can incur large changes in carrier concentration and switch majority carrier type, the large compositional shifts found in this case imply that compositional analysis of similar CVD and PVD grown materials is critical to correctly interpret topological insulator performance. Further, the characterization techniques deployed, including STEM-EDS and ToF-SIMS, serve as a case study in determining such compositional shifts in two-dimensional form.

Cryogenic molecular separation system for radioactive (11)C ion acceleration.

A (11)C molecular production/separation system (CMPS) has been developed as part of an isotope separation on line system for simultaneous positron emission tomography imaging and heavy-ion cancer therapy using radioactive (11)C ion beams. In the ISOL system, (11)CH4 molecules will be produced by proton irradiation and separated from residual air impurities and impurities produced during the irradiation. The CMPS includes two cryogenic traps to separate specific molecules selectively from impurities by using vapor pressure differences among the molecular species. To investigate the fundamental performance of the CMPS, we performed separation experiments with non-radioactive (12)CH4 gases, which can simulate the chemical characteristics of (11)CH4 gases. We investigated the separation of CH4 molecules from impurities, which will be present as residual gases and are expected to be difficult to separate because the vapor pressure of air molecules is close to that of CH4. We determined the collection/separation efficiencies of the CMPS for various amounts of air impurities and found desirable operating conditions for the CMPS to be used as a molecular separation device in our ISOL system.

Modelling heat and mass transfer in a membrane-based air-to-air enthalpy exchanger

The diffusion of total energy recovery systems could lead to a significant reduction in the energy demand for building air-conditioning. With these devices, sensible heat and humidity can be recovered in winter from the exhaust airstream, while, in summer, the incoming air stream can be cooled and dehumidified by transferring the excess heat and moisture to the exhaust air stream. Membrane based enthalpy exchangers are composed by different channels separated by semi-permeable membranes. The membrane allows moisture transfer under vapour pressure difference, or water concentration difference, between the two sides and, at the same time, it is ideally impermeable to air and other contaminants present in exhaust air. Heat transfer between the airstreams occurs through the membrane due to the temperature gradient. The aim of this work is to develop a detailed model of the coupled heat and mass transfer mechanisms through the membrane between the two airstreams. After a review of the most relevant models published in the scientific literature, the governing equations are presented and some simplifying assumptions are analysed and discussed. As a result, a steady-state, two-dimensional finite difference numerical model is setup. The developed model is able to predict temperature and humidity evolution inside the channels. Sensible and latent heat transfer rate, as well as moisture transfer rate, are determined. A sensitive analysis is conducted in order to determine the more influential parameters on the thermal and vapour transfer.

Corrigendum: Does increased air humidity affect stomatal morphology and functioning in hybrid aspen?

Received 28 August 2015. Accepted 28 August 2015. A. Niglas, M. Alber, K. Suur, P. Kupper, and A. Sellin. Institute of Ecology and Earth Sciences, University of Tartu, Lai 40, 51005 Tartu, Estonia. A.K. Jasinska. Institute of Ecology and Earth Sciences, University of Tartu, Lai 40, 51005 Tartu, Estonia; Institute of Dendrology, Polish Academy of Sciences, Parkowa 5, 62-035 Kornik, Poland. Corresponding author: Aigar Niglas (e-mail: aigar.niglas@ut.ee). Fig. 3. Stomatal conductance (gS) versus leaf-to-air vapour pressure difference (VPDL) in leaves showing increasing response (IR) of stomatal conductance with rising VPDL. The IR data were divided into seven groups by VPDL; the data points represent means of the groups. 709

The Wegener-Bergeron-Findeisen process – Its discovery and vital importance for weather and climate

The Wegener-Bergeron-Findeisen process refers to the rapid growth of ice crystals at the expense of surrounding cloud droplets, which frequently occurs in atmospheric mixed-phase clouds. The process is a result of the difference in saturation vapor pressures with respect to liquid and ice, and may in some circumstances lead to abrupt and complete cloud glaciation at temperatures between −40 °C and 0 °C in the Earth’s atmosphere. The process is named after three eminent scientists who were active in the first half of the 20 th century, among them being German meteorologist Walter Findeisen (1909–1945). In his classical paper published in 1938, Findeisen described the contemporary understanding of the Wegener-Bergeron-Findeisen process and other key cloud microphysical processes. Here, we compare the understanding of aforementioned processes at the time with that of the present, and find that they are remarkably similar. We also discuss how the Wegener-Bergeron-Findeisen process is implemented in state-of-the-art numerical models of the atmosphere, and highlight its importance for both weather and climate.

Mechanical alloying and thermal analysis of Ta–Ti alloys

Tantalum–titanium alloys have widespread potential in biomedical applications due to their superior biocompatibility, favorable mechanical properties, high corrosion resistance, and ability to exhibit shape memory behavior. However, this system is plagued by processing difficulties due to significant differences in melting temperatures, specific weights, and vapor pressures of Ta and Ti. In the present study, mechanical alloying (MA) using high‐energy ball milling of Ti–xTa (where x = 50, 60, 70, and 85 wt%) was investigated. The alloyed powders were characterized by X-ray diffraction, electron microscopy (SEM and TEM), and differential scanning calorimetry. It was established that α-Ti (hcp) gradually dissolves into α-Ta (bcc), with the alloyed particles becoming chemically homogeneous as a bcc structure. This structure corresponds to a meta-stable phase and should decompose to yield two solid solutions, Ti-rich hcp and Ta-rich bcc. To overcome this thermodynamic preference, MA-generated Ta–Ti bcc solid solution powders possess relatively high internal strain energy.

Attributes of Lake Okanagan evaporation and development of a mass transfer model for water management purposes

Understanding the water budget of the valley lakes in the water-stressed Okanagan region of British Columbia is important for allocating resources to maximize social well-being, environmental quality and the economy. However, high uncertainty in existing estimates of lake evaporation prevents sound water resource decision making. To address this uncertainty, buoy- and shore-based eddy covariance and meteorological instrumentation were deployed on the largest of the valley lakes, Lake Okanagan, for approximately 3 years. The objectives were to address the uncertainty in existing Lake Okanagan evaporation estimates by describing seasonal cycles and annual rates and the meteorological attributes controlling evaporation, and developing an accurate and useful model suitable for water managers and policy makers. Results indicate that two sites on Lake Okanagan experienced average annual evaporation of 725 and 835 mm over the study period. The difference can be attributed to spatial differences in surface water temperatures, vapour pressure gradients and atmospheric stability across the lake. Good relationships were found between evaporation rates measured with the eddy covariance systems and meteorological conditions at the offshore buoys, specifically between wind speed and the surface–atmospheric vapour pressure differences. From these relationships, a mass transfer model was developed. Accounting for the seasonal cycle in atmospheric stability increased the accuracy of monthly and annual evaporation estimates from this mass transfer model, but it remains inappropriate to predict daily or hourly evaporation. The study period included years that were climatically typical, so the evaporation observations could represent values close to the long-term mean, but this is unknown. The findings of this study highlight that long-term observations of the atmosphere consistently conditioned to the lake surface are needed for water managers and decision makers to have sound data and information on lake evaporation.

Comparison and analysis of bare soil evaporation models combined with ASTER data in Heihe River Basin

Based on ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) remote sensing data, bare soil evaporation was estimated with the Penman-Monteith model, the Priestley-Taylor model, and the aerodynamics model. Evaporation estimated by each of the three models was compared with actual evaporation, and error sources of the three models were analyzed. The mean absolute relative error was 9% for the Penman-Monteith model, 14% for the Priestley-Taylor model, and 32% for the aerodynamics model; the Penman-Monteith model was the best of these three models for estimating bare soil evaporation. The error source of the Penman-Monteith model is the neglect of the advection estimation. The error source of the Priestley-Taylor model is the simplification of the component of aerodynamics as 0.72 times the net radiation. The error source of the aerodynamics model is the difference of vapor pressure and neglect of the radiometric component. The spatial distribution of bare soil evaporation is evident, and its main factors are soil water content and elevation.

Does increased air humidity affect stomatal morphology and functioning in hybrid aspen?

The study investigated the effects of exposure to increased relative air humidity (RH) on stomatal morphology and sensitivity to stomata closure inducing stimulus (low RH) in hybrid aspen (Populus tremula L. × P. tremuloides Michx.) coppice growing in field conditions. Artificially elevated RH reduced air vapour pressure deficit by 5%–10% and altered stomatal sensitivity; trees grown under high RH exhibited stronger stomatal response to decreasing air humidity. We found no difference in mean stomatal pore length between treatments and a small decline in stomatal density under humidification. The lack of correlation between stomatal sensitivity and morphological traits suggests that stomatal sensitivity was unaffected by stomatal morphology. In light of rising atmospheric humidity predicted for high latitudes, strict stomatal control over water loss might be beneficial for trees if drought events become more frequent in the future. However, our experiment revealed that about two-thirds of the leaf-to-air vapour pressure difference (VPDL) response curves demonstrated the opposite pattern, i.e., stomatal opening in response to increasing VPDL. Strict stomatal regulation is probably not beneficial to fast-growing aspen coppice under low RH, as this trait may restrict their carbon gain and growth rate.

Ubiquity of ice nucleation in lichen — possible atmospheric implications

Ice nucleation has previously been described in only a few lichens from a single location. Here we greatly extend this work and suggest that in lichens ice nucleation is a water harvesting adaption. Fifty-seven lichen samples from a variety of widespread locations were tested for ice nucleation by differential scanning calorimetry (DSC). Samples initiated freezing in the range -5.1° to -20°C and the median freezing temperature was -7.2°C. The vapour pressure difference between ice and water is significant at this temperature, and so ice grows at the expense of water (Bergeron—Findeisen process). Therefore, the ability to form ice at these temperatures provides a useful water-harvesting mechanism for lichens. Ice nucleation appears to be ubiquitous in lichens and is more likely to be associated with the mycobiont and may influence atmospheric processes.

Channeled coolant plate: A new method to enhance freshwater production from an air gap membrane distillation (AGMD) desalination unit

Air gap membrane distillation (AGMD) desalination process incorporates phase change and transport of vapor from saline water through a hydrophobic membrane and an air gap due to difference in vapor pressure across the membrane and a coolant plate. In this study, an attempt was made to observe the effect of a channeled coolant plate instead of a flat plate for distillation of saline water. Since the AGMD process has limited distillate production due to the resistance offered by the air gap, a specially designed coolant plate was used to manipulate the mass transfer by enhancing the heat transfer during condensation. For the same equivalent air gap, the freshwater production enhanced maximum up to 50% compared to a flat coolant plate for a coolant temperature of 25°C. A one dimensional heat transfer model is developed and validated using the experimental data. The results obtained from the model matched well with experiments and it was used to predict freshwater production from various designs of finned coolant plates.

Review of membrane distillation process for water purification

Membrane distillation (MD) is a non-isothermal separation process driven on the vapor pressure difference, induced by the temperature difference across the hydrophobic membrane. This paper offers the review of the potentability of MD process for purification application and water desalination. It covers the basic fundamental of MD process, MD modules, membrane materials, heat and mass transfer phenomena, operating parameters, and performance of MD process. It also covers the review of MD processes driven by renewable energy sources and current innovations in the process. The recent research results in these different areas are presented and discussed. The multi-effect MD process is found to be a new generation MD process and attractive research area in the wastewater treatment and purification application for the commercial approach.

Effects of cumulus clouds on microclimate and shoot-level photosynthetic gas exchange in Picea engelmannii and Abies lasiocarpa at treeline, Medicine Bow Mountains, Wyoming, USA

Here we describe the dynamic effects of cumulus clouds on microclimate and shoot-level photosynthetic gas exchange in saplings of two treeline conifer species—Picea engelmannii and Abies lasiocarpa. Measurements were made during both clear-sky and partly cloudy conditions (∼10–70% cumulus cloud cover) throughout the 2012 growing season within an alpine-treeline ecotone. Cumulus clouds generated dynamic fluctuations in photosynthetically active radiation (PAR), higher maximum PAR (>2500μmolm−2s−1), 2–4 fold increases in diffuse PAR, reduced daily mean and cumulative PAR, lower needle temperatures, and reduced leaf-to-air vapor pressure differences relative to clear-sky conditions. Onset of cloud-shade corresponded with declines in photosynthesis, needle temperatures, and evapotranspiration, which were proportional to cloud duration and opacity. Despite increased diffuse light and greater sunlight intensity during cloud-gaps, photosynthesis was never higher on partly cloudy days compared to clear days in either species, during cloud-gaps or cloud-shade. However, reduced transpiration paired with photosynthesis comparable with clear-sky levels during cloud-gaps resulted in greater instantaneous water use efficiency relative to clear-sky measurements. There was no apparent photoinhibition of photosynthesis reflected in gas exchange measurements in response to abrupt and dramatic changes in PAR levels caused by cumulus clouds. We conclude that cumulus clouds reduce instantaneous and daily carbon gain, although lower needle temperatures and associated reductions in transpirational water loss may alleviate daily and seasonal water stress, thereby enhancing carbon gain over the growing season.

Temperature responses of mesophyll conductance differ greatly between species.

The temperature responses of mesophyll conductance (gm ) were investigated for nine species using carbon isotope techniques combining tunable diode laser spectroscopy and gas exchange measurements. Species included the evergreen trees Eucalyptus pauciflora and Quercus engelmannii; the tropical evergreen tree Lophostemon confertus; as well as the herbaceous species Nicotiana tabacum, Oryza sativa, Triticum aestivum, Gossypium hirsutum, Glycine max and Arabidopsis thaliana. Responses varied from a two- to threefold increase in mesophyll conductance between 15 and 40 °C observed for N. tabacum, G. hirsutum, G. max and E. pauciflora to almost no change in L. confertus and T. aestivum. To account for the different temperature responses between species, we suggest that there must be variation in both the activation energy for membrane permeability and the effective pathlength for liquid phase diffusion. Stomatal conductance was relatively independent of increases in leaf temperature and concomitant increases in leaf to air vapour pressure difference. Two exceptions were Eucalyptus and Gossypium, where stomatal conductance increased with temperature up to 35 °C despite increasing leaf to air vapour pressure. For a given species, temperature responses of stomatal and mesophyll conductance were independent of one another.

Recent Progress in Membrane Distillation

Membrane distillation (MD) is an emerging membrane separation technique which provides a competition for the conventional separation process such as reverse osmosis (RO) and thermal distillation. The MD process was first developed in the 1960s, but only recently garnered the interest from academics and industry due to the advancement of membrane fabrication technique. The MD is a thermal-driven process which has an ability to be integrated with renewable energy and/or waste heat. The driving force of the MD process is vapor pressure difference where the feed vapor is transported through the non-wetted hydrophobic porous membrane to the permeate regime where permeate will be collected via condensation. As such, the MD possesses a theoretical rejection rate of nearly 100%. This review addressed the recent progress of the MD process in terms of membrane fabrication, integration with renewable energy and/or other membrane separation process as well as applications of MD in various industries. This paper may serve as an update of the recent progress of MD which in some way, is able to help the researchers explore the new investigation field in MD for it to be commercially more viable.

Gadolinium Zirconate/YSZ Thermal Barrier Coatings: Plasma Spraying, Microstructure, and Thermal Cycling Behavior

Processing of Gd2Zr2O7 by atmospheric plasma spraying (APS) is challenging due to the difference in vapor pressure between gadolinia and zirconia. Gadolinia is volatilized to a greater extent than zirconia and the coating composition unfavorably deviates from the initial stoichiometry. Aiming at stoichiometric coatings, APS experiments were performed with a TriplexPro™ plasma torch at different current levels. Particle diagnostics proved to be an effective tool for the detection of potential degrees of evaporation via particle temperature measurements at these varied current levels. Optimized spray parameters for Gd2Zr2O7 in terms of porosity and stoichiometry were used to produce double‐layer TBCs with an underlying yttria‐stabilized zirconia (7YSZ) layer. For comparison, double layers were also deposited with relatively high torch currents during Gd2Zr2O7 deposition, which led to a considerable amount of evaporation and relatively low porosities. These coatings were tested in thermal cycling rigs at 1400°C surface temperature. Double layers manufactured with optimized Gd2Zr2O7 spray parameters revealed very good thermal cycling performance in comparison to standard 7YSZ coatings, whereas the others showed early failures. Furthermore, different failure modes were observed; coatings with long lifetime failed due to TGO growth, while the coatings displaying early failures spalled through crack propagation in the upper part of the 7YSZ layer.