Microstructural and phase configurational effects determining water content: Dielectric relationships of aggregated porous media

Abstract

[1] Many porous media in which we determine water content are aggregated and characterized by a dual-porosity pore network, composed of interaggregate pores and intra-aggregate pores. This paper reports sample-scale permittivity measurements made in four stable aggregate media with dual porosity. Results indicate two distinct dielectric responses depending on whether the aggregates are surrounded by water or air. We relate transitions in the permittivity response to the water retention characteristic (WRC), showing that after the interaggregate pores have drained, the slope of the water contentpermittivity relationship is significantly reduced (permittivity values ranging from 5 to 7). The hydraulic critical water content (q hc ) is defined as the point where all the interaggregate pores are air filled and all the intra-aggregate pores are water saturated and is determined from the WRC. The dielectric critical water content (q dc ) is defined as the point where a slope change in the measured water content-permittivity relationship occurs. A two-step model is presented and designed to capture the physical characteristics of the permittivity response to drainage. Measurements of q hc and q dc reveal a separation in these two values. The difference is considered to be a function of the connectivity of the intra-aggregate pore network. A connectivity ratio is defined as 1-(q dc /q hc ), where values close to 0 indicate low connectivity between the intra-aggregate pores and values tending to 1 indicate a high level of connectivity. Results from this work indicate that the reduced permittivity response measured in the water content-permittivity relationship is due to microstructure and phase configuration and not to ''bound'' water.

Numerical analyses of effective dielectric constant of multiphase microporous media

“…3. There are four main directional growth probabilities D 1-4 and four diagonal directional growth probabilities D [5][6][7][8] . To obtain an isotropic structure in such systems, we have to set the directional growth probability ratio D 1-4 : D 5-8 =4, consistent with the equilibrium density distribution function for uniform media.…”

“…The random location of obstacles is the simplest one to construct an artificial porous medium when the microstructure details are negligible. 7,23,24 Tacher et al 25 presented a discrete reduced distance method to generate spherical/elliptical twophase granular porous media. Based on the work of Tacher et al, Pilotti developed a grain sedimentation algorithm.…”

“…[1][2][3][4][5] These heterogeneous systems composed of two or more phases show different conductive and dielectric properties depending on not only the electric property and volume fraction of each phase but also the geometrical shapes and arrangements of the inclusions. 6,7 Most popular theoretical models for the dielectric constant of porous media are based on networks of parallel and series modes [8][9][10][11] or introducing the morphology effects of a porous structure by some empirical parameters. [12][13][14][15][16][17][18][19][20][21] No analytical model has been found that deal successfully with a dielectric constant of natural porous media with random phase distributions and interactions up to now.…”

Mechanical, hydraulic, and dielectric characterisation of fine-grained soils during densification

“…However, the results emphasised that a universal approach of the dielectric behaviour dependent only on the water content does not reproduce the behaviour for all soils. The results further highlighted that not only water content influences the permittivity at 1 GHz, but also mineralogy [204], soil structure [285] and particularly soil suctions acting on the bound water phase [19,23,212]. Therefore, theoretical mixture approaches for cohesive soils should be more suitable to better reproduce the dielectric measurement for fine-grained soils.…”

“…Current research focuses on the systematic analysis and inclusion of EM waves interactions in soils in consideration of structural, mineralogical, and compositional aspects, as well as soil phase interactions [14,19,23,25,140,152,153,204,212]. Subsequently, two innovative theoretical approaches are introduced that couple the hydraulic and EM properties of soils in consideration of the relaxation behaviour of free water and bound water [19,23,25,48,140].…”