Theory of dynamic equilibrium focusing generated by two physical fields in bidisperse suspension of colloidal particles

Abstract

The behaviour of a model bidisperse suspension of colloidal particles undergoing the action of two effective physical fields is studied theoretically. Each of the two fields interacts exclusively with one of two particle populations but this ideal model can easily be extended to consider the application of one or more fields interacting with polydisperse colloidal species in a more complex manner. The conditions under which an equilibrium focusing phenomenon can appear are defined and the microscopic scale forces contributing to the resulting focusing force are analyzed. Whenever the sizes of two particle populations are commensurable, the focused particles can no longer be considered as being suspended in a continuum and the position dependent volume forces, resulting from the macroscopic density gradient established due to the concentration distribution of one of the particle populations, are no longer applicable. On the other hand, the microscopic approach allows one to demonstrate the role of surface forces, which can be the sole position dependent forces under these conditions and explains the incoherences between the existing particular theories and experimental observations.