Therefore, it is difficult to obtain homogeneous materials composed of two or more kinds of matter in natural environments, owing to the distinction in density between molecules and the difference in the gravitational force that different kinds of molecules experience. If ideal uniform materials are to be prepared, the key is to decrease the difference in
gravitational force experienced by different kinds of molecules. It is also difficult to obtain gradient materials suitable for practical application in natural environments, because the gravity difference is not large enough to separate the molecules.