关于分子介质中三重态正子素(O-Ps)寿命 τ3的探讨

本文对o-Ps的寿命τ_3进行了探讨,认为介质的体积因素和介电性质是影响τ_3的主要因素,但也不应忽视介质分子对o-Ps的化学猝灭作用和正电子云团中活性自由基对o-Ps的顺磁猝灭作用的影响。提出了估计τ_3的半经验式。通过τ_3计算值与实验值的比较,作出某些有关介质的化学性质和正电子对介质的辐射效应等方面的预言.     

Temperature dependence of o-Ps lifetime in some porous media. Deviations from ETE model

Temperature dependence of ortho-positronium lifetime in some porous materials was investigated. Above the room temperature the extended Tao–Eldrup model works rather well, however at low temperatures one observes in all samples the lifetimes evidently longer than expected.     

On the relationship between o-Ps lifetime in liquids and their surface tension

It is shown that the Nakanishi-Jean formulae for o-Ps lifetime vs. radius of spherical cavity gives the empirical relationship of Tao which links the o-Ps annihilation rate with macroscopic surface tension.     

Temperature dependence of positron lifetime in ordered porous silica (SBA-3)

The temperature dependence of positron lifetime in uniform mesopores was analyzed. We used SBA-3 as the sample material, which possesses an ordered porous structure with uniform cylindrical mesopores. The positron lifetime corresponding to the annihilation in the mesopores increased gradually with a decrease in temperature down to 100 K, and its relative intensity also increased concomitantly. This result was attributed to the lower probability of the escape of the ortho-positronium (o-Ps) from the mesopores into the intergrain space at lower temperatures. An anomalous and sudden increase in the lifetime was observed at around 100 K; this result was in agreement with an increase in the positron lifetime reported in a previous study. It was revealed that the increase in the lifetime is very steep in cases of uniform mesopores, suggesting that the temperature dependence is influenced by the pore size.     

Phase change in porous media

We review notable advances published in the year 1999 in the area of phase change and phase growth in porous media. Phase equilibria thermodynamics, particularly in micropores, and growth kinetics, emphasizing the pore-network structure, are highlighted. Advances reported include the effects of confinement in phase transitions in micropores, and of the pore microstructure in the growth and dissolution of gas and liquid phases with applications ranging from capillary condensation to drying to gas evolution to condensation.     

Electrokinetic coupling in unsaturated porous media

We consider a charged porous material that is saturated by two fluid phases that are immiscible and continuous on the scale of a representative elementary volume. The wetting phase for the grains is water and the nonwetting phase is assumed to be an electrically insulating viscous fluid. We use a volume-averaging approach to derive the linear constitutive equations for the electrical current density as well as the seepage velocities of the wetting and nonwetting phases on the scale of a representative elementary volume. These macroscopic constitutive equations are obtained by volume-averaging Ampère's law together with the Nernst-Planck equation and the Stokes equations. The material properties entering the macroscopic constitutive equations are explicitly described as functions of the saturation of the water phase, the electrical formation factor, and parameters that describe the capillary pressure function, the relative permeability functions, and the variation of electrical conductivity with saturation. New equations are derived for the streaming potential and electro-osmosis coupling coefficients. A primary drainage and imbibition experiment is simulated numerically to demonstrate that the relative streaming potential coupling coefficient depends not only on the water saturation, but also on the material properties of the sample, as well as the saturation history. We also compare the predicted streaming potential coupling coefficients with experimental data from four dolomite core samples. Measurements on these samples include electrical conductivity, capillary pressure, the streaming potential coupling coefficient at various levels of saturation, and the permeability at saturation of the rock samples. We found very good agreement between these experimental data and the model predictions.     

Permeability of complex porous media

Absract  Hydrodynamic permeability of membrane composed of a set of porous spherical particles with rigid impenetrable cores is calculated. The cell method proposed by Happel and Brenner is used in calculations. All known boundary conditions on the cell surface, such as the Happel, Kuwabara, Kvashnin, and Cunningham (Mehta-Morse) models, are considered. The flow of liquid is described by the Brinkman equations. The problem of flow around a single spherical particle covered with porous layer by the uniform flow of viscous incompressible liquid is solved. Theoretical and empirical results are compared. Different limiting cases for which the derived formulas lead to results known from published literature are considered. Original Russian Text ? S.I. Vasin, A.N. Filippov, 2009, published in Kolloidnyi Zhurnal, 2009, Vol. 71, No. 1, pp. 32–46.     

Non-isothermal mass transport in porous media

Propagation of acoustic waves in non-isothermal dense media produces radiation-pressures whose nature is analyzed here. The case of a flux of thermal — rather than acoustic — energy can be analogously treated, and a molecular theory of thermal diffusion is accordingly formulated. p]The special case of thermal diffusion occurring within the liquid-filled channels of a thin partition — “thermodialysis” — is quantitatively analyzed within the frame of reference of the radiation-pressure approach. p]Some apparatuses developed for the experimental study of thermodialysis are described. The results obtained are discussed and shown to be in accord with the theoretical expectations.     

Foam flow through porous media

There are two parts to the interaction of foam with porous media. How the foam interacts with the surface and the flow within the substrate, which is the focus of this review. Flow-through porous media has been investigated experimentally with the main focus in literature being on enhanced oil recovery and remediation. Recently, investigation of the flow of foam through a deformable substrate for dishwashing application has led to the development of mathematical models. It has been proposed that foam flow through pore channels is similar to the behaviour observed within microchannels. Meaning that to investigate the effects these properties have on foam flow it is best to observe them within a model channel then build up to a 3D structure of interlinking channels to resemble porous media. In this review, it is highlighted that a large amount of work is needed in understanding the interaction of foam and/or liquid within porous networks. Methods that can be applied to better represent foam and liquid flow in porous media are discussed within this review, including both using microchannels to simulate individual pores and using these systems to build up to a 3D structure of interlinking pores. In addition, more advanced imaging techniques to observe the flow through porous materials are discussed, including computed tomography scanning nuclear magnetic resentence and confocal microscopy. There is still more work required to fully understand the flow within porous media, including observing the affect of dead-end pores, closed loops and rough channel walls have on the flow.     

Knowledge-based reconstruction of random porous media

An evolutionary optimization technique is used to reconstruct digitized material models of 300(3) nm3 size for mesoporous two-phase systems. The models are adapted to the two-point probability (TPP) and to a volume-based pore-size distribution (PSD) which were derived from SANS and adsorption experiments and which carry statistical information about morphology and topology of the pore system. To avoid extreme update-costs, the bulk of mutations are assessed by means of a suitable approximation of the PSD; it is demonstrated that a sporadic insertion of the PSD suffices to drive the algorithm towards satisfactory models in acceptable time. Our approach is knowledge-based in the sense that (i) the mutations are restricted to expedient exchanges of phase-voxels by a heuristic rule, and (ii) the sporadic calculation of the PSD from the current state of the model, in essence, provides an efficient self-control for the evolutionary process. We applied the method to reconstruct periodic models of the xerogel Gelsil 200. Such reconstructs of real mesoporous solids could be utilized, for instance, to verify theories of adsorption and capillary condensation.     

Tetrahydrofuran hydrate decomposition characteristics in porous media

Many tetrahydrofuran (THF) hydrate properties are similar to those of gas hydrates. In the present work THF hydrate dissociation in four types of porous media is studied. THF solution was cooled to 275.15 K with formation of the hydrate under ambient pressure, and then it dissociated under ambient conditions. THF hydrate dissociation experiments in each porous medium were conducted three times. Magnetic resonance imaging (MRI) was used to obtain images. Decomposition time, THF hydrate saturation and MRI mean intensity (MI) were measured and analyzed. The experimental results showed that the hydrate decomposition time in BZ-4 and BZ-3 was similar and longer than that in BZ-02. In each dissociation process, the hydrate decomposition time of the second and third cycles was shorter than that of the first cycle in BZ-4, BZ-3, and BZ-02. The relationship between THF hydrate saturation and time is almost linear.     

Vapor and liquid equilibria in porous media

The alteration of the vapor–liquid equilibrium (VLE) of volatile organic mixtures by placing porous media at the liquid–vapor interface was studied. Kelvin, assuming ideal behavior of fluids, first introduced the vapor pressure of liquid over a meniscus as a function of its surface tension and the radius of the curvature. A thermodynamic model (SS_mod model) predicting the VLE of non-ideal organic mixtures in porous media was developed as a function of pore sizes. The model was used to predict the VLE of two aqueous alcohol solutions, ethanol–water and propanol–water, and two binary solutions, methanol–isopropanol and ethanol–n-octane. Experiments were conducted using sintered metal and fritted glass plates as porous media, and the results were compared with the model predictions. Using the actual diameter of the porous media, the model prediction showed good agreement with the experimental results.     

Evaluation of bacterial detachment rates in porous media

The ability of published biomass detachment rate expressions to describe experimental data obtained from porous media reactors usingPseudomonas aeruginosa grown aerobically on glucose was evaluated. A first-order rate expression on attached biomass concentration best reflected effluent substrate concentration for combined data sets. Detachment rate coefficientk _d1 was dependent on initial substrate concentration. Simulation of porous media reactor experiments indicated that responses using higher influent substrate concentrations possessed greater sensitivity to variations ink _d1. Simulations of field bioremediation systems suggest the use of accurate biofilm development kinetics is important in the prediction of well bore biofouling.     

On the transport of emulsions in porous media

Emulsions appear in many subsurface applications including bioremediation, surfactant-enhanced remediation, and enhanced oil-recovery. Modeling emulsion transport in porous media is particularly challenging because the rheological and physical properties of emulsions are different from averages of the components. Current modeling approaches are based on filtration theories, which are not suited to adequately address the pore-scale permeability fluctuations and reduction of absolute permeability that are often encountered during emulsion transport. In this communication, we introduce a continuous time random walk based alternative approach that captures these unique features of emulsion transport. Calculations based on the proposed approach resulted in excellent match with experimental observations of emulsion breakthrough from the literature. Specifically, the new approach explains the slow late-time tailing behavior that could not be fitted using the standard approach. The theory presented in this paper also provides an important stepping stone toward a generalized self-consistent modeling of multiphase flow.     

Force-driven migration of particles in ordered porous media

Brownian dynamics simulation has been employed to study the behavior of force-driven particle migration in different ordered porous media comprised of periodically interconnected spherical cavities, representing inverted colloidal crystals. The effects of the imposed field strength and direction on the particle mobility and direction are investigated. The simulation results find that in a weak or intermediate field, the mobility normalized by the value in free solvent behaves in a similar way as the normalized diffusivity when the porosity is varied. Under a strong field, the normalized mobility can increase or decrease with the field strength, depending on the field direction relative to the cavity arrangement. If the imposed field is not aligned with any unobstructed pathway, the mobility tensor may become anisotropic and prolonged particle entrapment may also take place.     

Universal electro-osmosis formulae for porous media

Approximate analytical formulae valid for any porous media with elongated pores are derived for the electro-osmotic coefficient alpha and for the average ionic concentration n . A macroscopic Debye-Hückel length kappa (-)(-1) based on n is introduced. Simultaneously, the electro-osmotic coefficient alpha is systematically calculated for various media, zeta potentials and electrolyte concentrations by solving the local equations. Numerical results show that kappa (-)(-1) and alpha follow universal curves valid whatever the porous medium; these curves can be approximated by the analytical formulae previously derived. These formulae can be used to provide a priori estimates of the electro-osmotic coefficient.     

Competitive positron and positronium trapping in porous media

Positron annihilation acquired acceptance for structural investigation of solids but results in porous media—where positron lifetime spectroscopy (LT) reveals substantial Ps formation—were ambiguous. Data on zeolites lead to the conclusion that Ps trapping in competing “extended free volume” sites, inhomogeneous regions and grain boundaries occurs. Furthermore, positron trapping must also be considered. Systematic errors due to incomplete time range selection are discussed, significance and importance of corrections for 3γ/2γ counting efficiency differences are shown in practice.     

Magnetic resonance in porous media: recent progress

Recent years have seen significant progress in the NMR study of porous media from natural and industrial sources and of cultural significance such as paintings. This paper provides a brief outline of the recent technical development of NMR in this area. These advances are relevant for broad NMR applications in material characterization.     

Bacterial interactions and transport in unsaturated porous media

The convection-dispersion transport model, which can well define solute transport, has been introduced to describe bacterial transport. Due to different interaction natures within the porous media, bacterial transport in the subsurface, especially in the vadose zone is a complex scenario. When transported in the vadose zone, bacteria may be captured on the media surface, at the air–water interface, or at the media–air–water three-phase interface depending upon the predominant interactions of concerned bacteria within the pore system. In this study, transport of Echerichia coli, Pseudomonas fluorescens and Bacillus subtilis in silica sand under water unsaturated conditions was investigated using column experiments. Bacterial interactions within the system were characterized based on bacterial and media surface thermodynamic properties, which were determined independently by means of contact angle measurements. These calculated interactions provided solid evidence of the bacterial retention mechanisms in the pore system, which served as the bases for suitable assumptions of bacterial transport modeling. The micro-scale interaction investigations helped eliminate uncertainties arising with bacterial transport modeling.