Analysis of the response of suspended colloidal soft particles to a constant electric field

Abstract

A network model, originally designed for an electrokinetic study of soft particle suspensions, has been used for an in depth analysis of the physical behavior of these systems under the action of an externally applied DC electric field. The versatility of the network simulation method used, makes it possible to readily obtain information not only about the electrophoretic mobility, but also about any physical variable of interest at all points around the suspended particle: electric potential, ion concentrations, fluid velocity. The field-induced polarization of the double layer is described in terms of the dependence of these and other derived variables (volume charge density, electric field components, ion flux components) with the distance to the membrane-solution interface. In contrast with colloidal suspensions of hard particles, which basically depend on just two parameters (the reciprocal Debye length multiplied by the particle radius, kappa*a, and the zeta potential, Z), soft particle suspensions require a wider parameter set. Firstly, there are two characteristic diffusion lengths in the system (one inside the membrane and the other in the solution), and two geometrical lengths (the core radius a and the membrane thickness (b-a)). Furthermore, there is the fixed charge density inside the membrane (and possibly a surface charge density over the core) that cannot be represented by a Z potential. Finally, the parameter that characterizes the interaction between the fluid and the permeable membrane gamma, strongly influences the behavior of the system. Dependences with all these parameters (except the geometrical ones) are included in this study.