Thermal diffusion may refer to:

• A thermal force on a gas due to a temperature gradient, also called thermal diffusion or Thermal transpiration.
  • It is used to drive a gas pump with no moving parts called a Knudsen pump.
  • It is the currently accepted theory for the rotation of the Crookes radiometer.
• Diffusion in a temperature gradient, also called thermodiffusion or thermophoresis.
  • It can be used as an obsolete method of making enriched uranium (see enriched uranium § thermal diffusion).
  • Thermal diffusion can be used to measure fluid flow, including perfusion and rCBF at one location over time.
• Brownian motion (at a constant non-zero absolute temperature).

See also:

• Molecular diffusion
• Heat conduction (i.e., diffusion of heat)