Diffusion is a spontaneous process, which can be considered as the outcome of a random
walk. The mechanism of diffusion is the Brownian motion, whereby a molecule makes a random
walk about a central location since by kinetic theory the mean velocity of a pixel is zero
when is not subject to any external forces. Due to collisions with neighbouring molecules the
motion of the particle is characterized by a mean free path which tends to limit the pixel. But
since there is no potential field acting to restore a pixel to its original position, it is free to
move through the fluid, in which it is located.