Molecules always diffuse from areas of high concentration to areas of low concentration. The difference between the concentration of a substance in one area compared to another area is the concentration gradient. For example, placing ink on the surface of water establishes a concentration gradient in which the surface of the water has a high concentration of ink, and the rest of the water has a low concentration. As the ink diffuses, it moves from the area of high concentration to the area of low concentration, eventually resulting in a solution with equal concentrations of ink.
Science EncyclopediaScience & Philosophy: Dependency - The Intellectual Roots Of Dependency Thinking to Dirac equationDiffusion - Concentration Gradients, Diffusion In Cells, Water Diffusion: Osmosis - The importance of diffusion