Diffusion is the action of molecules moving from an area of high concentration to an area of lower concentration. It is caused by kinetic energy.
When two non-identical gases or liquids are brought into proximity, the kinetic energy of the molecules causes them to intermingle at the edges. Over time, this intermingling results in a relatively even blend of the two.
Osmotic filtration works on the principle of diffusion. The osmotic filter allows small molecules, but not large molecules, to pass through. The result of the filtration process is a purer gas or liquid. When the substances on both sides of the filter are different, they create osmotic pressure, a critical component of membrane transport in living things.