When hard spheres are positioned next to the wall of a box, a portion of the excluded volume lays outside of the box and leaves more residual space for the rest of the hard spheres. This is entropically favorable as the entropy of a system increases with the increase of accessible microscopic states in phase space for degrees of freedom. As a result, hard spheres statistically prefer to locate in the regions adjacent to the wall. Accordingly, in a binary mixture of hard spheres with two different radii, the larger spheres, relatively, prefer to occupy the regions next to the wall . This entropic force that attracts hard spheres to the wall and is arisen from the depletion zone is called depletion force.