diffusion coefficient
Quick Summary
The diffusion coefficient, also known as diffusivity or diffusion constant, quantifies the rate at which a substance diffuses from an area of high concentration to an area of low concentration. This process is driven by the random motion of molecules.12
The diffusion coefficient is represented by the symbol D in Fick's law of diffusion, which states that the flow of a diffusing substance is proportional to the concentration gradient: j = -D(dc/dx). The SI units for the diffusion coefficient are square meters per second (m²/s).23
In gases, diffusion coefficients are typically in the range of 10⁻⁵ to 10⁻⁴ m²/s, while in liquids, they are much slower, ranging from 10⁻¹⁰ to 10⁻⁹ m²/s. Diffusion coefficients are inversely proportional to total pressure and increase with temperature.3
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