Diffusion coefficient

A parameter that quantifies how quickly particles spread through a medium by diffusion. Its value depends on the particles, the medium, and conditions such as temperature.

Fick's laws of diffusion: Equations relating diffusive flux to concentration gradients and the time evolution of concentration. Fick's laws use the diffusion coefficient to quantify how concentration gradients drive transport.
Diffusion: The net movement of particles from regions of higher concentration toward regions of lower concentration through random motion. The coefficient quantifies the rate of this transport process.
Pulsed-field gradient NMR: An NMR method that encodes molecular displacement with magnetic-field gradients to measure diffusion. It estimates molecular diffusion coefficients from signal attenuation during controlled observation times.
Thermal diffusivity: A material property describing how quickly temperature differences spread by heat conduction. It measures heat spreading, not particle spreading, despite the shared term diffusivity.
Brownian motion: Random motion of particles suspended in a fluid, caused by collisions with surrounding molecules. Random microscopic steps produce the macroscopic spreading measured by a diffusion coefficient.
Concentration gradient: The spatial rate and direction of change in a substance's concentration. In Fick's first law, the gradient combines with the coefficient to determine diffusive flux.
Fluorescence correlation spectroscopy: A technique that analyzes fluorescence fluctuations to infer molecular concentrations and dynamics. The timescale of molecules crossing a small observation volume reveals their diffusion coefficient.
Kinematic viscosity: A fluid property equal to dynamic viscosity divided by density, describing momentum diffusion. It quantifies momentum transport rather than molecular or solute transport.
Mean squared displacement: The average squared distance particles move from their starting positions over a specified time. Its growth over time provides a direct route to estimating a diffusion coefficient.
Temperature: A physical quantity measuring thermal state and, microscopically, average particle kinetic energy. Higher thermal energy generally increases molecular diffusion in fluids.

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