diffusion

Net spontaneous and random movement of molecules or particles in a fluid (gas or liquid) from a region in which they are at a high concentration to a region of lower concentration, until a uniform concentration is achieved throughout. The difference in concentration between two such regions is called the concentration gradient. No mechanical mixing or stirring is involved. For instance, a drop of ink added to water will diffuse down the concentration gradient until evenly mixed.

Diffusion occurs because particles in a liquid or gas are free to move according to the kinetic theory of matter. The molecules move randomly, but there is more chance that they will move out of the ink drop than into it, so the molecules diffuse until their colour becomes evenly distributed throughout. Diffusion occurs more rapidly across a higher concentration gradient and at higher temperatures.

Diffusion is quite different from the movement of molecules when a fluid is flowing. In this case movement is not random; all molecules are moving together and in the same direction.