Isothermal and non-isothermal water transport in porous membranes. II. The steady state

Thermoosmotic behaviour was studied in simple systems constituted of grossly porous hydrophobic membranes permeated by distilled water. Attention was focused on steady-state conditions, characterized by absence of net transmembrane volume flow, obtained equilibrating thermoosmotic pressure with an external counterpressure. Comparison of hydraulic and thermoosmotic fluxes with steady-state pressure gives insight in the peculiar thermofluidodynamics of volume flow in non-isothermal membrane channels. This investigation was extended to volume transport caused by combination of the two thermodynamic forces constituted of the temperature and pressure gradients, synergic or antagonistic across the membrane. The experimental findings can be fruitfully compared with theoretical predictions of the system's behaviour derived from different approaches. Results obtained with six different membrane types, under a wide range of experimental conditions, lend support to the “thermal radiation pressure theory” which attributes the various effects of matter transport produced by a temperature gradient to the transfer of momentum from drifting thermal excitations to atoms and molecules in the material crossed by heat flow.     

Temperature and concentration polarization in membrane distillation of aqueous salt solutions

In this paper we have studied water transport in membrane distillation using a flat PTFE membrane. Experiments have been carried out with water and aqueous solutions of NaCl as feed. The effects of temperature and concentration polarization on the reduction of vapour pressure differences across the membrane with regard to the vapour pressure differences corresponding to the bulk phases which are separated by the membrane, are evaluated. A coefficient which measures this reduction has been introduced. This coefficient and the temperature polarization coefficient coincide when water is used as feed, but they are more and more different when the salt concentration of feed increases.The measured flux results and the calculated polarization results are discussed for different temperatures, recirculation rates and solution concentrations.     

Functional evaluation of a liquid-molding two component combination resin

Functional parameters contributing to the recognizable good performance of a highly crosslinked olefinic polymer based on cyclopentadiene have been evaluated by a limited range of stress decay and creep recovery from a fixed deformation over constant time periods. The experimental design for these involved a calculated stress (load) at the initial sample deformation (displacement) for determination of changes in stress in bending mode and flex creep under applied load at three constant temperature segments of 6 h each with 20 min recovery time between segments. The results have identified a behavior which is typical of interfacial orientational effect in a two-component system. Thermal conductivity, heat capacity and density at the same temperatures were measured for determination of variations in thermal diffusivity as an indication of the competing effects of densification and thermal expansion for such a system.Static charging and polarization experiments were performed for evaluation of other desired uses of this material, in moving parts and in direct or induced voltage applications. The latter allows measurement of dynamic charge transport through the resin matrix. Calculations indicate generation of charge carriers from weak secondary bonding typical of polymer interlaces. The polarization itself consists of dipole orientation characterized by relatively high activation energy.The intrinsic mechanical properties for this material had been determined by the manufacturer, but the performance parameters reported in this study have been determined for the first time for exploration of further uses for this material.The author wishes to acknowledge the assistance provided by Ralph Pitman in the preparation of the samples for this work.     

Isothermal and non-isothermal water transport in porous membranes. I. The power balance

A quantitative study of water transport through porous, unselective membranes of various types is presented. Effects produced by hydraulic pressure are compared with those due to a transmembrane temperature gradient. p]The quantities directly determined for five types of porous partitions of different structure are: hydraulic permeability, thermoosmotic permeability, activation energies of both these transport processes and thermal pressure. Experiments have been systematically conducted at temperatures from +20°C to +60°C. From the experimental data, thermohydraulic conductivity, thermal conductivity, heat of transport, ratio of conductive to convective heat fluxes and thermodynamic efficiency of the transport process have been calculated. Each of these quantities is expressed in terms of specific physical properties of system's components. p]These findings provide deeper insight in the fundamental physico-chemical aspects of thermodialysis, and open at the same time promising perspectives of practical applications for this process of direct transformation of thermal into mechanical (and electrochemical) energy.     

Thermo-osmosis of sorbable gases in porous media : Part II. Cascades

The theoretical basis of mixture separation by thermo-osmosis has been developed for two different experimental arrangements. In the first of these two vessels are connected only by a membrane across which a temperature gradient is maintained. Expressions were obtained for the separation factor of binary mixtures in terms of heats of transport, for pressure and composition changes across the membrane, and for determining the heat of transport of each component. In the second arrangement the vessels are connected via the membrane and also by a capillary of appropriate geometry, so that in the steady state there is a constant circulation of each component of the mixture. Expressions have again been derived for the steady state separation factor, and pressure and composition differences for binary mixtures.     

Concentration polarization in gas permeation

The aim of this article is to show the importance of concentration polarization effects in the separation of gas–vapor mixtures using membranes. In the experimental part of this work, gas mixture measurements are conducted with a specially designed test cell. The experimental data are analyzed using a two-resistance model for the transport through the membrane, which is derived in the theoretical part of this work. The two resistances considered are the transport through the boundary layer on the feed side of the membrane and through the separation layer. For the transport through the separation layer an extended free volume model is derived. This model considers not only the feed side but also the influence of the permeate side on the separation properties of the membrane. The results of the measurements show the influence of concentration polarization effects and their dependencies on feed pressure, membrane thickness, and feed flow rate.     

A non-equilibrium thermodynamics approach to model mass and heat transport for water pervaporation through a zeolite membrane

Pervaporation through zeolite membranes involves local heat effects and combined heat and mass transport. The current state-of-the-art Maxwell–Stefan (M–S) models do not take these effects into account. In this study, transport equations for the coupled heat and mass transport through a zeolite membrane are derived from the framework of non-equilibrium thermodynamics (NET). Moreover, the assumption of equilibrium between the adjacent bulk phases at the feed and permeate sides of the zeolite layer is abandoned in favor of local equilibrium. The equations have been used to model pervaporation of water through a 2 m thick NaA type zeolite membrane, deposited on an asymmetric -alumina support, at a feed temperature of 348 K. Assuming a flux of 10 kg m⁻² h⁻¹(0.15 mol m⁻² s⁻¹), the transport through the zeolite layer, as well as the liquid feed side boundary layer and the support layers is modeled. The activity, fugacity, and temperature profiles are calculated with and without taking coupling effects and surfaces into account. The profiles show distinct differences between the two cases. Including the surface effects leads to discontinuities in the activity and temperature at the membrane interfaces. A significantly higher temperature drop of 1.3 K is calculated across the zeolite, compared to 0.4 K when surface and coupling effects are not accounted for. The calculated decrease in temperature over the zeolite layer is dominated by the surfaces. This could indicate that temperature polarization is, to a large extent, a surface effect. The heat flux induces an extra driving force for mass transport, reducing the activity difference over the membrane. A positive jump in activity is observed at the interfaces, revealing the mass transport across the interfaces is governed by the coupling with the heat flux. The support layers contribute significantly to the total mass transport resistance.     

Chemically induced phospholipid translocation across biological membranes

Chemical means of manipulating the distribution of lipids across biological membranes is of considerable interest for many biomedical applications as a characteristic lipid distribution is vital for numerous cellular functions. Here we employ atomic-scale molecular simulations to shed light on the ability of certain amphiphilic compounds to promote lipid translocation (flip-flops) across membranes. We show that chemically induced lipid flip-flops are most likely pore-mediated: the actual flip-flop event is a very fast process (time scales of tens of nanoseconds) once a transient water defect has been induced by the amphiphilic chemical (dimethylsulfoxide in this instance). Our findings are consistent with available experimental observations and further emphasize the importance of transient membrane defects for chemical control of lipid distribution across cell membranes.     

Electrochemistry of capillary systems with narrow pores. VI. Convection conductivity (theoretical considerations)

Generally, the electrical convection current and the electrical convection conductivity (Smoluchowski's surface conductivity) have to be taken into account to describe transport phenomena across membranes with narrow pores although the electrical charge distribution within the pores cannot be described as a Helmholtz electrical double layer. In collodion membranes, which have a comparatively low fixed ion concentration, the contribution of the convection conductivity to the electrical conductivity of the pore fluid may be neglected. This assumption was made tacitly in the analysis of our data obtained with this type of membrane.In this communication equations are derived which take the convection conductivity into account. They are in agreement with the phenomenological transport equations developed by Staverman on the basis of the thermodynamics of irreversible processes.The electrical convection conductivity can be considered to be the contribution of the fixed ion concentration to the electrical conductivity. It is argued that this contribution cannot be neglected in ion exchange membranes with a high fixed ion concentration and a high mechanical permeability. Neglecting the electrical convection conductivity in such systems could lead to considerable differences between experimental conductivity data and the theoretical predictions. An electrical conductivity term for the fixed ions is proposed which can be used as a correction factor in the equations in which the contribution of the electrical convection conductivity has been neglected. Suggestions are made for the measurement of the electrical convection conductivity in systems with narrow pores and high electrical conductivity (e.g. ion exchange resins). The consequences of the electrical convection conductivity in practical applications of ion-exchange resins are discussed (acceleration of the rates of ion exchange; improvement of the separation properties by the application of a direct electrical current flow).     

Permeation and thermal osmosis of water through cellulose acetate membranes

Isothermal permeation and thermal osmosis of pure water through cellulose acetate membranes have been studied. In all cases the results have been analyzed and compared with those of the literature. Two kinds of isothermal permeation experiments have been carried out, in order to find the influence of temperature and stirring rate on the phenomenon. The activation energy for permeation has been calculated.In the case of thermal osmosis two different kinds of experiments have been developed. In the first, thermoosmotic flows have been measured. In the second, evolution towards steady states has been obtained. In both cases the experiments have been performed by varying separately the temperature difference, the mean temperature and the stirring rate. The activation energy for thermal osmosis has been calculated. The influence of stirring rate on thermal osmosis has shown that “temperature polarization” cannot be avoided with the experimental setup, but its influence may be evaluated. A method is proposed to evaluate the differential thermoosmotic permeability. It is suggested that the results obtained from steady states are more reliable than those obtained from thermoosmotic flows.     

Electrochemistry of capillary systems with narrow pores. VI. Convection conductivity (theoretical considerations)

Generally, the electrical convection current and the electrical convection conductivity (Smoluchowski's surface conductivity) have to be taken into account to describe transport phenomena across membranes with narrow pores although the electrical charge distribution within the pores cannot be described as a Helmholtz electrical double layer. In collodion membranes, which have a comparatively low fixed ion concentration, the contribution of the convection conductivity to the electrical conductivity of the pore fluid may be neglected. This assumption was made tacitly in the analysis of our data obtained with this type of membrane.

In this communication equations are derived which take the convection conductivity into account. They are in agreement with the phenomenological transport equations developed by Staverman on the basis of the thermodynamics of irreversible processes.

The electrical convection conductivity can be considered to be the contribution of the fixed ion concentration to the electrical conductivity. It is argued that this contribution cannot be neglected in ion exchange membranes with a high fixed ion concentration and a high mechanical permeability. Neglecting the electrical convection conductivity in such systems could lead to considerable differences between experimental conductivity data and the theoretical predictions. An electrical conductivity term for the fixed ions is proposed which can be used as a correction factor in the equations in which the contribution of the electrical convection conductivity has been neglected. Suggestions are made for the measurement of the electrical convection conductivity in systems with narrow pores and high electrical conductivity (e.g. ion exchange resins). The consequences of the electrical convection conductivity in practical applications of ion-exchange resins are discussed (acceleration of the rates of ion exchange; improvement of the separation properties by the application of a direct electrical current flow).     

Numerical analysis of the thermal effect on electroosmotic flow and electrokinetic mass transport in microchannels

Joule heating is present in electrokinetically driven flow and mass transport in microfluidic systems. Nowadays, there is a trend of replacing costly glass-based microfluidic systems by the disposable, cheap polymer-based microfluidic systems. Due to poor thermal conductivity of polymer materials, the thermal management of the polymer-based microfluidic systems may become a problem. In this study, numerical analysis is presented for transient temperature development due to Joule heating and its effect on the electroosmotic flow (EOF) and mass species transport in microchannels. The proposed model includes the coupling Poisson-Boltzmann (P-B) equation, the modified Navier-Stokes (N-S) equations, the conjugate energy equation, and the mass species transport equation. The results show that the time development for both the electroosmotic flow field and the Joule heating induced temperature field are less than 1 s. The Joule heating induced temperature field is strongly dependent on channel size, electrolyte concentration, and applied electric field strength. The simulations reveal that the presence of the Joule heating can result in significantly different characteristics of the electroosmotic flow and electrokinetic mass transport in microchannels.     

Effect of testing conditions on the laser flash thermal diffusivity measurements of ceramics

Several experimental techniques either under steady state or transient heat transfer conditions, have been developed to evaluate thermal conductivity and thermal diffusivity of materials. However, testing difficulties resulting from specimen size, extended testing time and heat losses, have somewhat impaired the applicability of many of them. In this respect, the use of the laser flash technique for thermal diffusivity measurements, is a very convenient alternative, considering its basic modeling equation is independent of the temperature gradient as well as the heat flow, and in addition the heat losses can be analytically treated. Another important advantage of the technique is its rapid experimental execution. In this work, it is presented as an investigation concerning how the testing conditions such as specimen coating, laser power and pulse duration, base line adoption, heat losses correction methods, and specimen thickness, may affect the thermal diffusivity measurements of some ceramic materials using the laser flash technique.     

Modified cellulose acetate flat membranes for desalination

In this article, the radiation grafting of acrylamide on to cellulose acetate flat membranes using UV-irradiation on the initiator is described. The modified membranes thus obtained have been characterized by IR, DSC, and TGA. Their transport properties have been studied. The modified membranes exhibit higher salt rejection with slightly reduced water flux as compared with cellulose acetate membrane. The work is further extended to study the thermal stability of these modified membranes in a dry state. These modified membranes up to 330°C are stable.     

Design and development in self-reference differential scanning calorimetry

An intermediate range (50–1000°C) self-referencing differential scanning calorimeter (SR-DSC) has been built and its performance evaluated. The SR-DSC measures heat flow across a heat flow metal plate, and any changes to the heat flow caused by a thermal transition occurring in a centrally placed sample is monitored by a temperature difference across the plate. The criteria for high sensitivity are that the circular plate should be as thin as possible and have a low thermal conductivity. The best sensitivity conducive with robust behaviour was achieved with an inconel thermal plate of uniform thickness, 75 m, this gave reproducible results, and the enthalpy of the thermal transition was proportional to sample mass. Calorimeter sensitivity decreased with increasing temperature and a sloped baseline was observed. Both of these effects can be corrected mathematically. An example of the use of the SR-DSC in polymer characterisation was limited to a study of the physical ageing of PET.This revised version was published online in November 2005 with corrections to the Cover Date.Paper was read at the TAC2001 Conference in Liverpool.     

A numerical study of spatial structure during oscillatory combustion in closed vessels in microgravity

The existence and spatial development of gas-phase, thermokinetic oscillations under the influence of mass and thermal diffusion have been investigated by numerical methods in a 1-dimensional system. The conditions correspond to those that would be experienced under microgravity. The interest arises because there have been recent experimental investigations of oscillatory reactions, involving cool flames during butane oxidation, as part of the NASA, KC135 microgravity flight programme. The Sal'nikov, thermokinetic scheme, which is a two-variable model representing an intermediate chemical species and reactant temperature (taking the form P-->A-->B), forms the basis of the present work. In this model, thermal feedback occurs through the exothermicity of the second step and the non-linearity is derived from its temperature dependence. There are no known chemical examples that satisfy Sal'nikov's formal structure but Griffiths and co-workers conceived an experimental analogue under terrestrial conditions whereby a gaseous reactant was allowed to flow from an external reservoir into a closed, heated reactor at a controlled rate via a capillary tube which fed the reactant to the centre of the vessel. The exothermic reaction that occurred in the vessel satisfied the necessary conditions for the second step and the inflow, with no temperature dependence, represented a physical analogue to the first step of the Sal'nikov scheme. Thermokinetic oscillations were observed and the range of conditions for their existences was investigated. One of the experimental systems was the exothermic reaction between hydrogen and chlorine. To represent the Sal'nikov conditions hydrogen was fed slowly into the reactor, which already contained chlorine. We have exploited this chemical system and its experimental implementation in the present paper to investigate the behaviour when no convection or bulk gas motion occurs and when heat and mass transport is driven solely by diffusion. We study the response of alternative numerical approaches to the way in which the first step of the scheme is simulated. In the first, the precursor (P) is supplied at the same rate simultaneously throughout the cells representing the reactor. This is close to the concept of the Sal'nikov model. In the second method, a fixed rate of supply is applied at the inner boundary of the axisymmetric, 1-dimensional system. This is analogous to the experimental procedure. The numerical results show how oscillatory states can be sustained as a result of heat and mass transport by diffusion. The temporal and spatial evolution of reaction in a range of circumstances is discussed.     

Ionic liquid-based stable nanofluids containing gold nanoparticles

A one-phase and/or two-phase method were used to prepare the stable ionic liquid-based nanofluids containing same volume fraction but different sizes or surface states of gold nanoparticles (Au NPs) and their thermal conductivities were investigated in more detail. Five significant experiment parameters, i.e. temperature, dispersion condition, particle size and surface state, and viscosity of base liquid, were evaluated to supply experimental explanations for heat transport mechanisms. The conspicuously temperature-dependent and greatly enhanced thermal conductivity under high temperatures verify that Brownian motion should be one key effect factor in the heat transport processes of ionic liquid-based gold nanofluids. While the positive influences of proper aggregation and the optimized particle size on their thermal conductivity enhancements under some specific conditions demonstrate that clustering may be another critical effect factor in heat transport processes. Moreover, the remarkable difference of the thermal conductivity enhancements of the nanofluids containing Au NPs with different surface states could be attributed to the surface state which has a strong correlation with not only Brownian motion but also clustering. Whilst the close relationship between their thermal conductivity enhancements and the viscosity of base liquid further indicate Brownian motion must occupy the leading position among various influencing factors. Finally, a promisingly synergistic effect of Brownian motion and clustering based on experimental clues and theoretical analyses was first proposed, justifying different mechanisms are sure related. The results may shed lights on comprehensive understanding of heat transport mechanisms in nanofluids.     

Analytical study of Joule heating effects on electrokinetic transportation in capillary electrophoresis

Electric fields are often used to transport fluids (by electroosmosis) and separate charged samples (by electrophoresis) in microfluidic devices. However, there exists inevitable Joule heating when electric currents are passing through electrolyte solutions. Joule heating not only increases the fluid temperature, but also produces temperature gradients in cross-stream and axial directions. These temperature effects make fluid properties non-uniform, and hence alter the applied electric potential field and the flow field. The mass species transport is also influenced. In this paper we develop an analytical model to study Joule heating effects on the transport of heat, electricity, momentum and mass species in capillary-based electrophoresis. Close-form formulae are derived for the temperature, applied electrical potential, velocity, and pressure fields at steady state, and the transient concentration field as well. Also available are the compact formulae for the electric current and the volume flow rate through the capillary. It is shown that, due to the thermal end effect, sharp temperature drops appear close to capillary ends, where sharp rises of electric field are required to meet the current continuity. In order to satisfy the mass continuity, pressure gradients have to be induced along the capillary. The resultant curved fluid velocity profile and the increase of molecular diffusion both contribute to the dispersion of samples. However, Joule heating effects enhance the sample transport velocity, reducing the analysis time in capillary electrophoretic separations.     

Ion transport across model lipid membranes containing light-harvesting complex II: an effect of light

The effect of light on proton transport across lipid membranes of small unilamellar liposomes containing incorporated major light-harvesting pigment-protein complex of Photosystem II (LHCII) has been studied with the application of pH-sensitive dyes entrapped inside vesicles. Proton permeability coefficient for LHCII-modified membranes was found to be about twice as high as in the case of the control pure lipid vesicles. Illumination of the samples with light absorbed by the LHCII-bound photosynthetic pigments considerably affects the kinetics of proton transport: it increases the rate and decreases the steady-state level of proton gradient across the membranes. The effect was interpreted in terms of heat-induced conformational changes of LHCII molecular structures that affect proton buffering capacity of this protein. Both the control and the LHCII-modified lipid membranes have been found to be practically impermeable to Ca(++) ions, as demonstrated by fluorescence of liposome-entrapped calcium-sensitive probe calcium crimson. The slight differences in the proton transport across the LHCII-containing membranes under the presence of Ca(++) suggest calcium binding to this antenna protein.