Motion of a Coarse-Grained Polymer Chain in a Porous Medium: Effect of Porosity and External Field.
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Conventional gas diffusion measurements in coarse‐textured and aggregated porous media are severely limited due to hydrostatically induced variations in water content and air‐filled porosity. Motivated by the need to measure gas diffusion in coarse‐textured plant growth media designed for use in microgravity (e.g., aboard the International Space Station), our objectives were (i) to develop and test an automated diffusion measurement system on earth with water content adjustment capability and that minimizes hydrostatic effects, and (ii) to model characteristics of gas diffusion in partially saturated aggregated porous media. The horizontally oriented O 2 diffusion cell design for reducing the gravitational effect was based on a thin profile rectangular cell. Continuous measurement of O 2 in sealed dual‐chamber diffusion cells provided concentration data for fitting diffusion coefficients where water content was controlled volumetrically using a porous membrane with an imposed pressure for incremental addition or removal of water. Gas diffusion was modeled as a function of air‐filled porosity in millimeter‐sized aggregated particles exhibiting a substantial difference between internal and external aggregate pore sizes. For this case, the internal aggregate porosity contribution to diffusion compared with external aggregate pore space was minor as described by a dual‐porosity diffusion model. The measurement approach described can be used in other coarse‐textured and structured porous media.
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Three kinds of porous media (isotropic, perpendicular anisotropic and parallel anisotropic porous media) with the same porosity, different pore size distributions and fractal spectral dimensions were reconstructed by random growth method. It was aimed to theoretically study the impact of microscopic pore structure on water vapor diffusion process in porous media. The results show that pore size distribution can only denote the static characteristics of porous media but cannot effectively reflect the dynamic transport characteristics of porous media. Fractal spectral dimension can effectively analyze and reflect pores connectivity and moisture dynamic transport properties of porous media from the microscopic perspective. The pores connectivity and water vapor diffusion performance in pores of porous media get better with the increase of fractal spectral dimension of porous media. Fractal spectral dimension of parallel anisotropic porous media is more than that of perpendicular anisotropic porous media. Fractal spectral dimension of isotropic porous media is between parallel anisotropic porous media and perpendicular anisotropic porous media. Other macroscopic parameters such as equilibrium diffusion coefficient of water vapor, water vapor concentration variation at right boundary in equilibrium, the time when water vapor diffusion process reaches a stable state also can characterize the pores connectivity and water vapor diffusion properties of porous media.
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The porosity is a macro-variable which can represent partly micro-structures of granular materials. Based on the continuum model of granular materials, an evolution formula for the porosity with local average volumetric strain of granular assembly is deduced. Provided the change of grain volume is uniform, and the formula is allied with pore water state equation, the relationship among the pore water pressures and the porosity, the volumetric module of grain and the volumetric strain of grain for saturated granular materials is presented. The results obtained can be applied to the numerical simulation of fluid-solid coupling for statured granular materials or to the multi-scale analysis of fluid-solid seepage for porous materials.
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