In a pure liquid, molecules touch one another but change neighbors frequently. External forces cause the liquid to change shape by viscous flow. Thermal agitation causes molecules to undergo self-diffusion. The two phenomena--viscous flow and self-diffusion--often result from a single rate-limiting process: molecules change neighbors. This simple picture is amply confirmed by the Stokes-Einstein relation, which links the viscosity and self-diffusivity for many liquids over wide ranges of temperature.
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