diffusion -
(physics) the process in which there is movement of a substance from an area of high concentration of that substance to an area of lower concentration
natural action,
natural process,
activity,
action a process existing in or produced by nature (rather than by the intent of human beings); "the action of natural forces"; "volcanic activity"
natural philosophy,
physics the science of matter and energy and their interactions; "his favorite subject was physics"
osmosis (biology, chemistry) diffusion of molecules through a semipermeable membrane from a place of higher concentration to a place of lower concentration until the concentration on both sides is equal