The isopycnic focusing has for a longtime been considered as a phenomenon occurring exclusively in a fluid which behaves as a continuum with regard to the focused species. It has been shown recently, both theoretically and experimentally, that such an assumption is not necessary and the focusing can appear in a complex fluid composed of bidisperse suspension of the colloidal particles of different but commensurable sizes or in a polydisperse particulate suspension as well. Under such conditions, the question arose about the nature of the driving forces acting on a discrete microscopic scale and generating the macroscopically observed focusing phenomena. In this work, the theoretical analysis is developed to determine the general conditions under which the focusing phenomenon can emerge and to specify the effective driving forces susceptible to contribute to the resulting focusing force. The accompanying experiments were carried out in order to check the particular conclusions following from the theory. On the basis of the general theoretical approach, a new phenomenon, the osmotic pressure gradient focusing (OPGF) is predicted theoretically.