Microporous Domains Research Articles

Determination of Preferential Flow Model Parameters

Solute transport models that include a preferential flow component require many input parameters. There are well established procedures to determine micropore parameters, but procedures to determine macropore parameters are not well established. The objective of this paper was to evaluate methods to independently measure macropore parameters. The test model used was MACRO, a transient‐state, two‐flow domain model. The key macropore parameters in the model are saturated and boundary hydraulic conductivities (Ks and Kb), the absolute value of the boundary head between macropore and micropore domains (hb), the exponent (n*) of the relation between K (variables are defined in the appendix) and water content (θ), the macropore fraction (θsma), and the half spacing (d) between equivalent parallel fractures. As an example this study used soils in the Des Moines lobe (Mollisols with textures ranging from sandy loam to silty clay). Data used to calculate model parameters included wet‐end K‐θ‐h and K(h), and results from image analysis. For the MACRO model, the parameters fit the equations best when hb was assumed to be 30 mm. For the measured data with assumed hb = 30 mm, n* had a median of 2.1 and a range from 0 to 5.2, median Kb was 15 mm h−1 with a range from 1 to 100 mm h−1, and the median Ks was 122 mm h−1 with a range from 7 to 741 mm h−1. The calculated d ranged from 1 to 847 mm, and θsma ranged from 0.001 to 0.053 m3 m−3. Depending on the data available, the various techniques can be used to determine input parameters for preferential flow models.

SW—Soil and Water: Two-component Transfer Function Modelling of Flow through Macroporous Soil

The macropore and micropore domains of a soil system were considered to be hydraulically distinct. Drainage and solute transport through the soil were then characterized by mixed probability distributions. The components of the mixed probability distributions were used to represent the distinct processes of drainage and solute transport in the macropore and micropore domains. A goodness-of-fit test showed that the lognormal distribution was the best theoretical distribution for solute transport in each of the two domains, while the two-parameter gamma distribution was best for drainage in each of the domains. A validation test showed that the mixed probability distribution representation of the transfer function adequately incorporated dual porosity in the transfer function model for drainage and solute transport through macroporous soil.

SWBCM: a soil water balance capacity model for environmental applications in the UK

The paper presents a daily-time step, multi-horizon capacity model of soil-water balance (SWBCM—Soil Water Balance Capacity Model) suitable for ecological and environmental applications investigating the spatial and temporal variability of soil water content determined by changes in soil hydraulic conductivity, soil water storage capacity and the pathways of water movement through the soil and across soil types. SWBCM simulates soil water content at horizon level and encompasses limits on the amount of drainage from one horizon to the next to allow the formation of temporary perched water tables, lateral drainage, matric potential and surface runoff. The model incorporates a dynamic sub-model of grass growth (SWARD—Dowle, K., Armstrong, A.C., 1990. A model for investment appraisal of grassland drainage schemes on farms in the UK. Agric. Water Manage. 18, 101–120; Armstrong, A.C., Castel, D.A., Tyson, K.C., 1995. SWARD: a model of grass growth and the economic utilisation of grassland. In: Pereira L.S., van den Broek B.J., Kabat P., Allen R.G. (Eds.), Crop-Water Simulation Models in Practice. Wageningen Press, Wageningen, The Netherlands, pp. 189–197). SWBCM’s predictive ability is tested across a range of soil types under permanent grass in the UK and outputs are compared with predictions made by MACRO (Jarvis, N.J., 1994. The MACRO Model Version 3.1. Technical Description and Sample Simulations. Swedish University of Agricultural Sciences, Department of Soil Sciences, Reports and Dissertations 19, Uppsala, Sweden, 51 pp.), a mechanistic solute transport model which incorporates a physically-based preferential flow model in which total soil porosity is divided into two flow domains (macro-pores and micro-pores), each characterised by a flow rate; soil water flow in the micro-pore domain is modelled using Richards’ equation. In the modelling experiment, SWBCM simulations have been shown to provide good approximations of point-scale experimental data under a range of soil, climate and drainage management conditions in the UK. SWBCM simulations are close to those developed by the mechanistic MACRO model, suggesting that the capacity model can be applied to describe the water balance of multi-horizon UK soil profiles. The modelling approach used is considered to be applicable to the wide range of soil lower boundary conditions, ranging from free-draining to impermeable, occurring in the field and to simulate transient perched water tables that commonly occur in most temperate, high latitude countries such as the UK.

Modelling of hillslope runoff processes

The present study is aimed at modelling hillslope flows with emphasis on subsurface stormflows that involve macropores. The physical processes connected with the runoff process on a hillslope are identified. The components which are considered in modelling the hillslope flow are the nature of flows in the macropore and micropore domains, the spatial and temporal characteristics of the macropore network, the interaction between the domains, and the initiation of flow in the macropores. Both Horton and Dunne's variable source area generation mechanisms are explicitly incorporated in the model. The dominant physical processes governing hillslope runoff are conceptualized in terms of parameters which are derived from the physical properties of the soil, the nature of macropores, and hillslope geometry. The conceptualization of the model is then used to examine infiltration and runoff production. This helps to compute the development of the groundwater table, runoff hydrograph, and soil moisture profile.

Dispersion and transport of gas-phase contaminants in dry porous media: effect of heterogeneity and gas velocity

The purpose of these experiments was to study the effects of physical heterogeneity and velocity on transport of gas-phase contaminants in dry porous media. Experiments were conducted at gas velocities ranging from 6 to 200 cm min −1 to examine the contributions of longitudinal molecular diffusion, hydrodynamic dispersion, and rate-limited diffusive mass transfer to solute spreading. Methane was used as a nonreactive tracer, and trichloroethene, benzene and toluene were used as reactive tracers. Total dispersion of methane during transport through a homogeneous porous medium was the sum of longitudinal molecular diffusion and hydrodynamic dispersion. The latter was the major process contributing to total dispersion at gas velocities greater than 40 cm min −1. The contribution of longitudinal molecular diffusion was negligible at gas velocities greater than 150 cm min −1. Transport of tracers in the heterogeneous (macroporous) medium exhibited preferential flow and tailing at gas velocities greater than about 100 cm min −1 as a result of rate-limited mass transfer between macropore and micropore domains. The spreading associated with rate-limited mass transfer between macropore and micropore domains was the main contributor to total dispersion at gas velocities greater than 120 cm min −1. The transport of the reactive tracers was successfully predicted using data obtained for the nonreactive tracer.

The tipping bucket equations as a model for macropore flow

The tipping bucket model, a computationally efficient scheme for simulating soil water transport which requires few input parameters, implicitly assumes that all water flow occurs through macropores. The model is limited in its elimination of both micropore flow and macropore flow with unsaturated micropores and its reliance on a fixed time step, normally taken to be daily. The reformulation of the tipping bucket model as a set of differential equations for macropore water and solute transport removes the above limitations. Previous work on simulation of water transport through oxisols using the Richards equation showed that an accurate fit to measured water contents could be obtained only by making the probably false assumption that the saturated water content is only 70% of the total porosity. Combining the tipping bucket equation for macropore water transport with the Richards equation for micropore water transport produces an equally good fit to measured water contents under the more reasonable assumption that saturated water content is equal to total porosity. A critical assumption in the production of a good fit is that the saturated water content is scale-invariant, i.e. the macropore and micropore domains have the same saturated water content. The tipping bucket equations introduce two parameters that are difficult to estimate, the macropore drainage parameter and the mass transfer coefficient for drainage from macropores into micropores. However, for the particular data set studied in this paper, the above parameters are largely irrelevant as long as macropore water does not drain downwards through macropores faster than it drains into micropores, i.e. as long as the macropore and micropore domains are not decoupled.

Molecular accessibility in interpenetrating organometallic polymer networks (IOPNs): An ESR evaluation

Interpenetrating organometallic polymer networks can be seen as a new type of supported metal catalysts. They are prepared by the generation of a cross‐linked organometallic copolymer (see figure) inside the macro‐ and microporous domains of organic and inorganic supports. Here, ESR analysis demonstrates that the microporous domains of such materials are accessible to molecules of substantial size. magnified image

Rearrangement of the porous structure of macroporous copolymers

Abstract Changes in the porous structure of the strongly crosslinked macroporous copolymers glycidyl methacrylate-ethylene dimethacrylate and styrene-divinylbenzene were studied. Via interaction with organic solvents (dioxane, methanol) changes are brought about in the specific surface areas and pore volumes, which are interpreted in terms of changes in the microporous and macroporous domains of the porous structure of the polymers. According to mercury porosimetry, there are also changes in pore distribution. The phenomenon has also been observed in microphotographs of polymer sections obtained by electron microscopy and in the sorption of solutes.

Interpenetrating Organometallic Polymer Networks (IOPN's): Novel Advanced Materials

ABSTRACTA cross-linked copolymer of vinylferrocene and dimethyl-acrylamide is generated inside the less dense microporous domains of a cross-linked polydimethylacrylamide matrix. I.S.E.C. and E.S.R. data reveal that the obtained composite is in fact an I.P.N.(Interpenetrating Polymer Network) material.

Domain structure in MP (mesophase pitch)-based fibres

The microstructure of a number of commercially available (Union Carbide, Thornel, or Amoco) mesophase pitch-based (MP) carbon fibres was investigated using light, scanning and transmission electron microscopy. Size, shape, and distribution of structure were examined in longitudinal and transverse sections. Because microtome sectioning methods produced considerable structural damage, a technique was developed for preparing ion-milled transverse sections for TEM. A domain structure is very common, perhaps ubiquitous, in MP fibres. Domains are long, thin needle-like or ribbon-like structural components of fibres that extend up to 100 μm parallel to the fibre axis and may be elongate in transverse section, the longer axis being 0.5 to 3 μm while the shorter axis is typically 0.2 μm. The fabric of domains is identified from textures in transverse polished and fractured sections. Many MP fibres examined have domains arranged in an oriented-core texture. This texture consists of polar zones where layering of domains is chaotic separated by an equatorial zone of layering continuous across each filament. PAC-man morphology can develop in some oriented-core filaments by radial failure in the flat equatorial regions. Not all domains are of the same kind. In dense domains, graphene layers are densely stacked with comparatively little void space, while microporous domains consist of thin walls of graphene sheets enclosing a high proportion of void space. Graphene layers within dense domains are oriented predominantly parallel both to the long axis of the domain in transverse section and to the fibre axis. Graphene layers in microporous domains have no preferred orientation. Dense domains represent former mesophase, while microporous domains represent a phase inferred to have been capable of forming at least some mesophase with generation of volatile substances. In some MP fibres, ordering in dense domains is turbostratic. In others, true graphite with three-dimensional crystal structure occurs, not as a separate domain type, but as a component of the domains described. All domains appear to originate at the time of pitch maturation (mesophase forming) and are considerably modified during spinning. Domains and their distribution influence mechanical properties. Factors such as the state of ordering within domains, volume fraction and distribution of microporous material and presence of folded and kinked layering and of flaws will modify processes like initiation and propagation of fracture and affect properties such as elastic modulus.

Aryl-Bridged Polysilsesquioxanes - New Microporous Materials.

ABSTRACTThe first representatives of a new family of microporous, aryl-bridged polysilsesquioxanes have been prepared by sol-gel processing of bis-1,4-(triethoxysilyl)benzene la, bis-4,4′- (triethoxysilyl)biphenyl 2a, bis-4,4′-(triethoxysilyl)terphenyl 3a, and bis-9,10-triethoxysilyl anthracene 4a. The bis(trichlorosilyl) analogs of la and 2a (lb and 2b, respectively) were also examined. The materials produced by hydrolysis and condensation of the monomers provide an opportunity to fully condense to a network with rigid-rod organic spacers interspaced at regular intervals in the silicate-like framework. The xerogels produced upon subsesquent processing of the gels have extremely high surface areas (256–1100 m2/g; BET) with porosities confined to the micropore domain (< 200 nm). Solid state (CP MAS) 13C and 29Si NMR were used to evaluate the extent of hydrolysis and degree of condensation in the xerogels. The porosity and thermal stability of the aryl bridged polysilsesquioxanes may lead to applications as chromatographic absorbents. The transparent materials may also have optical applications arising from both the gels′ high refractive indices and the covalent incorporation of ultraviolet chromophores.