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The higher the diffusivity (of one substance with respect to another), the faster they diffuse into each other. Typically, a compound's diffusion coefficient is ~10,000× as great in air as in water. Carbon dioxide in air has a diffusion coefficient of 16 mm 2 /s, and in water its diffusion coefficient is 0.0016 mm 2 /s. [1] [2]
Fick's first law relates the diffusive flux to the gradient of the concentration. It postulates that the flux goes from regions of high concentration to regions of low concentration, with a magnitude that is proportional to the concentration gradient (spatial derivative), or in simplistic terms the concept that a solute will move from a region of high concentration to a region of low ...
Apart from the basic soil composition, which is constant at one location, soil thermal properties are strongly influenced by the soil volumetric water content, volume fraction of solids and volume fraction of air. Air is a poor thermal conductor and reduces the effectiveness of the solid and liquid phases to conduct heat.
Mass transfer coefficients can be estimated from many different theoretical equations, correlations, and analogies that are functions of material properties, intensive properties and flow regime (laminar or turbulent flow). Selection of the most applicable model is dependent on the materials and the system, or environment, being studied.
The first modern theoretical models for soil consolidation were proposed in the 1920s by Terzaghi and Fillunger, according to two substantially different approaches. [1] The former was based on diffusion equations in eulerian notation, whereas the latter considered the local Newton’s law for both liquid and solid phases, in which main variables, such as partial pressure, porosity, local ...
In pharmaceutical sciences, tortuosity is used in relation to diffusion-controlled release from solid dosage forms. Insoluble matrix formers, such as ethyl cellulose , certain vinyl polymers, starch acetate and others control the permeation of the drug from the preparation and into the surrounding liquid.
The advection-like term of the Soil Moisture Velocity Equation is particularly useful for calculating the advance of wetting fronts for a liquid invading an unsaturated porous medium under the combined action of gravity and capillarity because it is convertible to an ordinary differential equation by neglecting the diffusion-like term.
Poroelasticity is a field in materials science and mechanics that studies the interaction between fluid flow, pressure and bulk solid deformation within a linear porous medium and it is an extension of elasticity and porous medium flow (diffusion equation). [1] The deformation of the medium influences the flow of the fluid and vice versa.