Mass Transfer

• Self-diffusion

  • diffusion in a pure substance
  • consider tagging molecules and watching how they migrate

• Diffusion equations

  • Combine mass balance with Fick’s law
  • Take as boundary condition a point concentration at the origin

• Solution

• Second moment

RMS displacement increases as t1/2

Compare to ballistic r ~ t

For given configuration, each molecule represents a point of high concentration (fluctuation)

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