Nano-organized structures in a field: Equilibrium and steady-state gradient focusing in multicomponent colloidal systems

Focusing behaviour appearing in multicomponent colloidal systems exposed to the action of a field can be considered as a special case of the formation of the organized structures on a nanosize scale. The emergence of such structures was observed under the thermodynamic equilibrium conditions as well as at a steady state characterized by an entropy production due to an energy flux. It has already been proven that a complex liquid forming a primary concentration gradient should not necessarily behave as a continuum with respect to the focused species but the focusing effect can appear as well. Two theoretical models predicting equilibrium or steady-state gradient focusing under various conditions were published recently. The first model is based on a macroscopic dynamic mechanism and the other on a microscopic kinetic mechanism. Although seemingly complementary, they bring along some paradoxes. The dynamic model describes correctly the focusing of the large species in a continuum or pseudocontinuum gradient. Nevertheless, its application to the description of the focusing emerging in a bidisperse or multicomponent mixture of the colloidal particles of commensurable sizes does not seem to be physically adequate. The kinetic model provides a coherent physical image of the focusing in such a bidisperse or multicomponent mixture but, on the other hand, it gives rise to the mentioned paradoxes. In this study, both models were compared with the former theoretical approaches dealing with equilibrium or steady states established in multicomponent and/or concentrated colloidal systems interacting with a field. Moreover, computer simulation was carried out to elucidate the consequences of the mentioned paradoxes and to discuss the domain of the prevailing contribution of the macroscopic and microscopic mechanisms to the resulting focusing phenomenon.