Macropore Space Research Articles

The saturated hydraulic conductivity of soils with n-modal pore size distributions

A model for water retention in bi-modal soils (soils having structural and matrix porosity only) is further developed into a model for tri-modal soils so that the effects of macro-pores can also be included. This model is based on the exponential (Boltzmann) water retention function. It is suggested that this could be extended to include any number, n, of modes of porosity as may exist due to the hierarchical nature of soil structure. This is then used in combination with Marshall's pore model for hydraulic conductivity to produce a model for the saturated hydraulic conductivity, K sat, of n-modal soils that appears as a very simple and elegant equation. This simple expression for K sat allows for scaling of the porosity and also the characteristic pore size. The model is illustrated with a Polish data set (a sub-set of the POLHYDRO database). Measured values of water retention, bulk density and K sat are used to estimate the parameters of the macro-pore term that explains the contribution of the soil macro-pores to K sat. The mean values and distributions of these macro-pore parameters are presented for the 42 Polish arable soils. However, the existence of macro-pores does not necessarily imply that they are connected and that they contribute to the hydraulic conductivity. It is found that in normal agricultural fields at harvest time, they do not contribute to the hydraulic conductivity. Examination of measured and predicted values of K sat showed that in the normal field conditions that were studied, water movement occurs almost entirely through the structural pores. However, it is predicted that flow through the macro-pore spaces occurs in freshly-tilled soil. The effects of compaction on K sat are easily modelled using the new approach. Different scenarios may readily be compared. A simple one in which the characteristic size of the structural pores decreases as the soil becomes more dense is found not to be consistent with the observations. We are left with the conclusion that the number of structural pores and not their size decreases as soil becomes more dense. It is concluded that the multi-modal nature of soil pore size distributions has a big influence on water retention and on saturated hydraulic conductivity. Taking the multi-modal pore size distribution into account leads not only to physically-based predictions of soil hydraulic properties, but also to improved understanding of soil structure and function.

Micromechanics of bone tissue-engineering scaffolds, based on resolution error-cleared computer tomography

Synchrotron radiation micro-computed tomography (SRμCT) revealed the microstructure of a CEL2 glass–ceramic scaffold with macropores of several hundred microns characteristic length, in terms of the voxel-by-voxel 3D distribution of the attenuation coefficients throughout the scanned space. The probability density function of all attenuation coefficients related to the macroporous space inside the scaffold gives access to the tomograph-specific machine error included in the SRμCT measurements (also referred to as instrumental resolution function). After Lorentz function-based clearing of the measured CT data from the systematic resolution error, the voxel-specific attenuation information of the voxels representing the solid skeleton is translated into the composition of the material inside one voxel, in terms of the nanoporosity embedded in a dense CEL2 glass–ceramic matrix. Based on voxel-invariant elastic properties of dense CEL2 glass–ceramic, continuum micromechanics allows for translation of the voxel-specific nanoporosity into voxel-specific elastic properties. They serve as input for Finite Element analyses of the scaffold structure. Young's modulus of a specific CT-scanned macroporous scaffold sample, predicted from a Finite Element simulation of a uniaxial compression test, agrees well with the experimental value obtained from an ultrasonic test on the same sample. This highlights the satisfactory predictive capabilities of the presented approach.

Modelling microscopic flow in woven fabric reinforcements and its application in dual-scale resin infusion modelling

Abstract A physical unit cell impregnation model is proposed for the micro-scale flow in plain woven reinforcements. The modelling results show a characteristic relationship between tow impregnation speed, the surrounding local macro-scale resin pressure and the tow saturation within the unit cell. This relationship has been formulated into a mathematical algorithm which can be directly incorporated into a continuum dual-scale model to predict the ‘sink’ term. The results using the dual-scale model show a sharp resin front in inter-tow-pore spaces and a partially saturated front region in intra-tow-pore spaces. This demonstrates that the impregnation of fibre tows lags behind the resin front in the macro pore spaces. The modelling results are in agreement with two reported experimental observations. It has been shown that the unsaturated region at the flow front could increase or have a fixed length under different circumstances. These differences are due to the variation in tow impregnation speed (or the time required for the tow to become fully impregnated), the weave architecture and the nesting and packing of plies. The modelling results have also demonstrated the drooping of the inlet pressure when flow is carried out under constant injection rates. The implementation of the algorithm into a dual-scale model shows coherence with a single-scale unsaturated model, but demonstrates an advantage in flexibility, precision and convenience in application.

Developing Risk Models of Cryptosporidium Transport in Soils from Vegetated, Tilted Soilbox Experiments

Transport of Cryptosporidium parvum through macroporous soils is poorly understood yet critical for assessing the risk of groundwater contamination. We developed a conceptual model of the physics of flow and transport in packed, tilted, and vegetated soilboxes during and immediately after a simulated rainfall event and applied it to 54 experiments implemented with different soils, slopes, and rainfall rates. Using a parsimonious inverse modeling procedure, we show that a significant amount of subsurface outflow from the soilboxes is due to macropore flow. The effective hydraulic properties of the macropore space were obtained by calibration of a simple two-domain flow and transport model that accounts for coupled flow in the matrix and in the macropores of the soils. Using linear mixed-effects analysis, macropore hydraulic properties and oocyst attenuation were shown to be associated with soil bulk density and rainfall rate. Macropore flow was shown to be responsible for bromide and C. parvum transport through the soil into the underlying pore space observed during the 4-h experiments. We confirmed this finding by conducting a pair of saturated soil column studies under homogeneously repacked conditions with no macropores in which no C. parvum transport was observed in the effluent. The linear mixed-effects and logistic regression models developed from the soilbox experiments provide a basis for estimating macropore hydraulic properties and the risk of C. parvum transport through shallow soils from bulk density, precipitation, and total shallow subsurface flow rate. The risk assessment is consistent with the reported occurrence of oocysts in springs or groundwater from fractured or karstic rocks protected only by shallow overlying soils.

Concentration Polarization and Nonequilibrium Electroosmotic Slip in Dense Multiparticle Systems

Electrical field-induced concentration polarization (CP) and CP-based nonequilibrium electroosmotic slip are studied in fixed beds of strong cation-exchange particles using confocal laser scanning microscopy (CLSM) and the macroscopic electroosmotic flow (EOF) dynamics. A key property of the investigated fixed beds is the coexistence of quasi-electroneutral macroporous regions between the micrometer-sized particles and the ion-permselective (here, cation-selective) intraparticle mesopores with a mean size of 10 nm. The application of an external electrical field to the particles induces depleted and enriched CP zones along their anodic and cathodic interfaces, respectively, by the local interplay of diffusive and electrokinetic transport. The intensity and dimension of the CP zones depend on the applied electrical field strength and the fluid-phase ionic strength. With increasing field strength a limiting current density through a particle is approached, meaning that charge transport locally through a particle becomes controlled by the dynamics in the adjoining extraparticle convective-diffusion boundary layer (depleted CP zone). In this regime a nonequilibrium electrical double layer can be induced electrokinetically in the depleted CP zone and intraparticle pore space, resulting in nonlinear EOF in the interparticle macropore space. The local CP dynamics analyzed by CLSM is successfully correlated with the onset of nonlinearity in the macroscopic EOF dynamics. We further demonstrate that multiparticle effects arising in fixed beds (random close packings) of ion-permselective particles modulate significantly the local pattern of CP and intensity of the nonequilibrium electroosmotic slip with respect to the undisturbed single-particle picture.

Steady water flow through unsaturated aggregated porous materials

The three-dimensional steady water flow through unsaturated aggregated porous materials composed of simple cubic open packed and tetrahedral close packed assemblies of uniform porous spheres is investigated with electric analogues and numerical computations. Water around the spheres is considered to be discontinuous with flow restricted through isolated annular water lenses held by surface tension forces around the contact points between spheres. It is found that the conductance of individual spheres depends only on the size of the water lenses and is independent of the radius of the sphere. For a simple cubic packing the conductance for small lens radii is given by Weber’s formula for flow from an electrified disc into an infinite medium. It follows that the bulk hydraulic conductivity of these assemblies of porous spheres is also independent of aggregate size over a range of water contents. This independence is also shown in measurements of hydraulic conductivity of aggregates of diatomaceous earth that show a convergence to a single relationship between conductivity and water content when there is no longer continuity of water in the macropore space. The effect of the three-dimensional flow through aggregates on solute leaching is demonstrated by considering the numerical results of the stream-tube pattern in a sphere.

Ground‐Based Forest Harvesting Effects on Soil Physical Properties and Douglas‐Fir Growth

Soil properties and forest productivity can be affected by heavy equipment used for harvest and site preparation but these impacts vary greatly with site conditions and operational practices. We assessed the effects of ground‐based logging on soil physical properties and subsequent Douglas‐fir [Pseudotsuga menziesii (Mirb) Franco] growth on a highly productive site receiving vegetation control in coastal Washington. We also tested the effectiveness of tillage in maintaining or enhancing site productivity. On average, about half of the area of ground‐based harvested plots was affected by vehicular traffic. Sixty‐three percent of the trees were planted on microsites with some degree of soil disturbance. Soil bulk density at the 0‐ to 30‐cm depth increased from 0.63 to 0.82 Mg m−3 in the most compacted portions of traffic lanes. The area‐weighted increase in soil bulk density in the 0‐ to 30‐cm depth was 27%. Soil strength in traffic lanes increased at all depths < 55 cm but never exceeded 1300 kPa. Tillage to the 60‐cm depth returned the soil to its initial strength condition. Volumetric soil water content in compacted traffic lanes was greater than that in noncompacted soil. Total soil porosity decreased 10 to 13% with compaction, while available water holding capacity increased. In compacted soil, macropore space was reduced 40 to 52%. The study revealed no detrimental effects on tree height and diameter from soil compaction at age 4. At stand age 3, a tree volume index was actually greater for trees planted on traffic lanes than for those on non‐disturbed soil.

Concentration Polarization-based Nonlinear Electrokinetics in Porous Media: Induced-Charge Electroosmosis

We have investigated induced-charge electroosmotic flow in a fixed bed of ion-permselective glass beads by quantitative confocal laser scanning microscopy. Externally applied electrical fields induce concentration polarization (CP) in the porous medium due to coupled mass and charge transport normal to the charge-selective interfaces. These data reveal the generation of a nonequilibrium electrical double layer in the depleted CP zones and the adjoining anodic hemispheres of the (cation-selective) glass beads above a critical field strength. This initiates CP-based induced-charge electroosmosis along curved interfaces of the quasi-electroneutral macropore space between glass beads. Caused by mutual interference of resulting nonlinear flow with (flow-inducing) space charge regions, an electrohydrodynamic instability can appear locally and realize turbulent flow behavior at low Reynolds numbers. It is characterized by a local destruction of the CP zones and concomitant removal of diffusion-limited mass transfer. More efficient pore-scale lateral mixing also improves macroscopic transport, which is reflected in the significantly reduced axial dispersion of a passive tracer.

Airflow dispersion in unsaturated soil

Dispersion data is abundant for water flow in the saturated zone but is lacking for airflow in unsaturated soil. However, for remediation processes such as soil vapour extraction, characterization of airflow dispersion is necessary for improved modelling and prediction capabilities. Accordingly, gas-phase tracer experiments were conducted in five soils ranging from uniform sand to clay at air-dried and wetted conditions. The disturbed soils were placed in one-dimensional stainless steel columns, with sulfur hexafluoride used as the inert tracer. The tested interstitial velocities were typical of those present in the vicinity of a soil vapour extraction well, while wetting varied according to the water-holding capacity of the soils. Results gave dispersivities that varied between 0.42 and 2.6 cm, which are typical of values in the literature. In air-dried soils, dispersion was found to increase with the pore size variability of the soil. For wetted soils, particle shape was an important factor at low water contents, while at high water contents, the proportion of macroporous space filled with water was important. The relative importance of diffusion decreased with increasing interstitial velocity and water content and was, in general, found to be minor compared to mechanical mixing across all conditions studied.

Consistency of Energy and Material Balances for Bidisperse Particles in Fixed-Bed Adsorption and Related Applications

This paper develops material and energy balances for fixed-bed adsorption of a multicomponent nonideal gas in bidisperse particles by defining enthalpies from thermodynamic paths chosen such that the reference states are an ideal gas for the fluid-phase and adsorbate-free solid for the adsorbent. The balances are developed in detail for a three-domain system: nanoporous domains, macropore space, and interstitial fluid. It is also shown that material and energy balances may be easily developed from the same enthalpy definitions for use in batch, stirred tank, or moving-bed adsorption systems. Material balances are developed first and are subsequently used to simplify the energy balances such that terms involving reference states cancel out of the equations. Simplifying assumptions are applied to the final equation set to obtain equations that resemble, but are somewhat different from, frequently used forms. A major difference, resulting from the use of thermodynamic paths, is often found in the treatment of heat capacities and heats of adsorption.

Change of soil hydrological properties of fens as a result of soil development

AbstractThis study deals with the change and evaluation of hydrological properties of peat soils (Histosols) in the course of soil development. Ash content, volumetric water content, and dry bulk density, unsaturated hydraulic conductivity, water retention function, and wetting properties were measured for 84 fen sites in 19 fen regions of North‐Eastern Germany. Soil development resulted in porosity decrease. On the contrary, the macropore space and the capillary rise increased. With the start of consolidation processes and the development of segregation structure, a'noticeable reduction of the macropores and unsaturated hydraulic conductivity were observed. In course of soil development and decreasing of aggregate size, these processes reversed. Both parameters increased from segregation structure horizon to earthyfied fen and weak moorshyfied fen horizon, until they partly exceeded the starting values of pedogenetic almost unchanged fen in strongly moorshyfied stadium. Differences in wetting properties of peat could not be explained by the changes of peat properties in the course of soil development.

Three‐dimensional visualization and quantification of soil macroporosity and water flow patterns using computed tomography

Abstract. Efforts to assess the role of soil structure in soil and water processes have been limited by data based on two dimensional geometry or disturbed samples. Computed tomography (CT) is a non‐invasive technique that allows for the three dimensional, non‐destructive examination of heterogeneous materials. X‐ray CT scanning was used to acquire serial images of contrasting soil types that could be used to render, and hence, quantify important soil constituents in three dimensions. By repeating the scanning techniques after an infiltration period, it was also possible to examine the nature of water movement in real time. Macropore architecture between different soil types was highly variable and generally appeared as unconnected pore segments at the resolution examined. As expected, water flow characteristics were similar to macropore structure, with wetting patterns resembling both Darcian and preferential flow identified in different soils. From this, it was found that in the sandy clay soil c. 90% of macropore space was active following infiltration, compared with c. 50% for the sandy loam soil. In addition, it was possible to visualize the three dimensional distribution of stones and other mineral material. The results illustrated the benefits of computed tomography over more conventional analyses of soil structure, and its potential as a tool for examining dynamic soil processes. However, problems associated with accessibility, resolution and artefacts still provide limitations of the technique.

Description and classification of soil structure using distance transform data

SummaryClassification of soil based on structure is useful for conveying information about physical properties and soil processes. The distance transform is an image analysis technique suitable for quantifying soil structure. An analysis of distance transform data, in the form of cumulative area distribution curves for previously published images of soil structures of various types, is presented. The images were used to derive a quantitative classification of structure using maximum distance of solid from a macropore (Dmax, measured), the distance from macropore space containing 50% of the solid area (k, derived by fitting a sigmoidal function to the cumulative area distribution curve), the total interface length between pore and solid per area of sample (IA, measured), the porosity or the proportion of pores per area of sample (PA, measured) and the pore distribution characteristic (n, derived by fitting a sigmoidal function to the cumulative area distribution curve) which is related to the number, continuity and distribution of pores. The influence of image resolution was investigated, and within limits found to be fairly small. The final classification of soil structure was based on the hypothesized relations between the descriptors and structure‐forming processes.

Characterization of Macroscopic Structural Disorder in Porous Media Using Mercury Porosimetry

A new structural model for the interpretation of mercury porosimetry data for samples having a bimodal pore size distribution is presented. The morphology of the macropore network of a porous solid has been studied using a combination of mercury porosimetry and NMR techniques. The model consists of a lattice-based network of pore bodies and pore necks. The model enables assessment of the spatial geometric distribution of the directly accessible macropore network of a porous material having a bimodal pore size distribution. A methodology that can be used to determine the spatial distribution of the critical pore neck diameters controlling access to the macroporous void space has also been described. This method was used to provide a map of the local critical macropore neck distribution over macroscopic length scales for an alumina tablet that is not possible with other techniques.

Effect of various cultivation methods on the structure and hydraulic properties of a soil in a semi-arid climate

In the absence of irrigation, the cultivation of cereals in semi-arid zones provides a poor yield. To study the consequences of fallow tillage on the soil structure and hydraulic properties, we conducted an experiment from 1991/1992 to 1997/1998 using various techniques (chisel or disc plough, early or late tillage) on a calcisol of the high plateaus of eastern Algeria. In the middle of the tilled horizon (between 8 and 25 cm depth), we measured the near-saturated hydraulic conductivity at four soil water tensions (or potentials) using multidisc infiltrometers, and quantified the morphology of the macropore space using image analysis. Results indicated that tillage increased soil conductivity mainly at low water potential (0.06 and 0.3 kPa). This increase was more significant with the chisel ( 1.84×10 −5 m s −1 at 0.06 kPa) than with the disc plough (1.25×10 −5 m s −1) , but was attenuated during the crop cycle. Significant differences appeared between tillage treatments for surface macroporosity (5.1% for chisel, 1.1% for disc), pore-space morphology (thin cracks separating compact aggregates under disc, loose fine soil assemblage separating smaller aggregates under chisel) and porosity distribution pattern (equivalent diameter of the macropores varied from 1 mm under disc to more than 2–3 mm under chisel). Analyses of porosity distributions indicate a possible relationship between the structure of the surface horizon and soil hydraulic properties. Durum wheat grain yield varied from 1.08 to 2.85 t ha −1 during the 7-year trial. Grain yield under shallow tillage was significantly higher than under disc plough treatment during wet years. Tillage-date varied across seasons and early tillage effect was more apparent when the fallow season was less rainy.

Effects of skidding on forest soil infiltration in west-central Alberta

Soil compaction during forest harvesting generally reduces macropore space, which reduces infiltration and increases the potential for surface erosion and waterlogging. Hydrological effects of 3, 7 and 12 skidding cycles and their persistence were evaluated for 3 yr at 14 sites, which represented a range of soil texture and compaction conditions in the foothills and boreal forests of Alberta. Saturated hydraulic conductivity (HC) was determined using a constant head method on soil cores collected from 5- and 10-cm depths; unconfined infiltration rate of the surface soil (IR) was measured in situ with tension infiltrometers at near saturation. A significant (P < 0.05) increase in bulk density during skidding caused a significant reduction in both HC and IR after the first three cycles at eight sites where soil water potential at the time of skidding was higher than −15 kPa; the decrease at the other sites was not significant. Additional traffic, up to 12 cycles, did not cause a further significant decrease in HC or IR. The infiltration rate of soil compacted by three skidding cycles showed a recovery trend. However, in more intensively trafficked soils, compaction effects on infiltration remained significant for at least 3 yr, which was possibly attributed to heavy snowpacks preventing soil freezing at lower depths. Key words: Saturated hydraulic conductivity, unconfined infiltration rate, tension infiltrometers, skidders, boreal forest, Alberta

The spatial distribution of soil hyphae in structured spruce-forest soils

The external mycelium forms the major part of the absorbing surface of mycorrhized tree roots. Because the macro pore space of acid forest soils is selectively depleted of mobile nutrient cations, it is ecologically important, whether soil hyphae grow into the soil aggregates or not. Seedlings of Norway Spruce (Picea abies (L.) Karst.) with defined mycorrhiza were grown in unsterilized soil cores taken from the A and B-horizon of a limed and an unlimed cambisol on triassic sandstone in the Northern Black Forest, Germany. Water-tension treatments were 10, 30, 160 and 900 hPa. On ground and polished vertical cuts stained with acridine orange, we identified and measured the location of hyphae and characterized their micropedological environment using an image analyzing system. Mean length density varied between 17 m/cm3 and 100 m/cm3 and was independent of aeration parameters. The percentage of hyphae completely embedded in the soil matrix varied between 30% and 8% and decreased significantly with increasing CO2 concentration in the soil air. Of the hyphae in the soil matrix, 70% were located in a 50 μm shell around the macro pores. Pair correlation functions show, that the majority of soil hyphae occur in clusters with diameters below 100 μm. Between 60% and 80% of randomly chosen circles with 250 μm diameters were completely devoid of hyphae. The inefficient opening up of the intra-aggregate space by soil hyphae is explained by the very slow oxygen diffusion between air-filled macro pores and the intra-aggregate space and mechanical restrictions for hyphae growth.

Autologous bone marrow stromal cells loaded onto porous hydroxyapatite ceramic accelerate bone repair in critical-size defects of sheep long bones.

The ability of marrow-derived osteoprogenitor cells to promote repair of critical-size tibial gaps upon autologous transplantation on a hydroxyapatite ceramic (HAC) carrier was tested in a sheep model. Conditions for in vitro expansion of sheep bone marrow stromal cells (BMSC) were established and the osteogenic potential of the expanded cells was validated. Ectopic implantation of sheep BMSC in immunocompromised mice led to extensive bone formation. When used to repair tibial gaps in sheep, cell-loaded implants (n = 2) conducted a far more extensive bone formation than did cell-free HAC cylinders (n = 2) over a 2-month period. In cell-loaded implants, bone formation was found to occur both within the internal macropore space and around the HAC cylinder while in control cell-free implants, bone formation was limited mostly to the outer surface and was not observed in most of the inner pores. As tested in an indentation assay, the stiffness of the complex HAC-bone material was found to be higher in cell-loaded implants compared to controls. Our pilot study on a limited number of large-sized animals suggests that the use of autologous BMSC in conjunction with HAC-based carriers results in faster bone repair compared to HAC alone. Potentially this combination could be used clinically in the treatment of extensive long bone defects.

Hydrology and erodibility of the soils and saprolite cover of the Swaziland Middleveld

The weathering mantle of the Middleveld of Swaziland consists of thick soil-saprolite complexes. The isovolumetric chemical weathering of the saprolites has led to mass losses of more than 50%. Compared with saprolites from quartz-diorite and granodiorite, those from diorite have higher portions of easily weatherable plagioclases and amphiboles and 20–30% greater total pore space. The macro pore space reaches a maximum (4.6–7.0%) in the central saprolite zone, corresponding to saturated hydraulic conductivities of 6.02–11.81 × 10 −7 m/s. Similar to the saprolites, the overlying ferrallitic soils show total pore volumes ranging from 39 to 52%. Compared to the soils, the available water capacity of the underlying saprolites is two to four times higher and the saturated hydraulic conductivity is about two times higher due to the high portion of medium pores which amount to 70% of total pore space. In the areas affected by sheet erosion, most of the soil cover is denuded and the underlying saprolites essentially determine the site properties. The hydrological properties of the saprolites are therefore of great importance with respect to erosion during wet periods and plant growth during drought periods. The low structural stability of the saprolites, indicated by shear strength values < 5 kPa, results from a silty texture, absence of organic matter, and low contents of Fe- and Al-oxides. As a result, saprolites are highly susceptible to erosion and represent an essential precondition for the development and rapid expansion of deep incising erosion gullies in areas with magmatic rocks. In contrast, the clay-rich ferrallitic soils developed from saprolite are comparatively stable, indicated by shear strength values ranging from 7 to 12 kPa. The inherent stabilizing properties of the soil are altered by overgrazing and unwise land use leading to infiltration capacities below 65 cm/day and high overland flow potentials at low rainfall intensities.

Chemical dilation and the dual porosity of humified bog peat

Hydrochloric acid and calcium and sodium chloride salt solutions were passed through humified bog peat cores of 1 m length to determine values for the material properties governing solute transport: hydraulic conductivity, effective porosity, and dispersivity. Chloride passed through the bulk peat cores about twice as fast as predicted from chloride breakthrough curves measured by selective-ion probes inserted into the peat 30 cm downgradient from the source. With continued experimentation, chloride dispersion increased. Chloride also passed through the peat up to 12 times slower than predicted by conservative advection. The retardation of the chloride fronts, the faster bulk transport, and the asymmetry of the breakthrough curves all indicate that humified bog peat is a dual porosity medium. Solutes both move preferentially through active macropore spaces and diffuse into dead pore space in the peat matrix. Neither the effective porosity nor the dispersivity of the peat could be precisely determined from the breakthrough curves because the hydraulic conductivity and discharge rates through the peat increased as much as five times when the chloride tracer solutions passed through the cores. In initial experiments, the shapes of the breakthrough curves were generally consistent with dispersivities of the order of centimeters, similar to that observed for granular soils at the same length scale (meters), whereas they `flattened' considerably during later experiments, suggesting dispersivities in the range of tens of centimeters. We hypothesize that the interaction of the chloride tracer solutions with organic-acid functional groups may cause macropores to dilate and micropores to close off.