Electrodiffusiophoresis of Spherical Hydrophobic Colloids

The present study investigates the controlled electrokinetic motion of spherical colloids under the combined influence of an applied electric field and an electrolyte concentration gradient. The primary goal is to demonstrate how a concentration gradient impacts particle electrophoresis. When a concentration gradient exists in the bulk electrolyte─whether introduced intentionally or not─it drives particle motion through a synergistic effect involving both conventional electrophoresis and an additional mechanism known as diffusiophoresis. In this study, the concentration gradient is aligned to either reinforce or oppose the applied electric field. Besides, the particle is assumed to be charged and hydrophobic. We derived an analytical expression for the electrodiffusiophoretic mobility of such particles within the Debye-Hückel electrostatic limit. We further deduced numerical results for the electrodiffusiophoretic mobility considering the impact of the ion steric effect. The deduced numerical results are validated using both the derived analytical expression for electrodiffusiophoretic mobility under the low charge limit as well as existing experimental data for particle motion driven by either an electric field or an electrolyte concentration gradient. We observed that parameter R, which defines the ratio between the applied electric field strength and the concentration gradient strength, is crucial. It plays a vital role in determining both the magnitude and the propulsion direction of the particle's mobility. Furthermore, the propulsion direction of the particle can be precisely controlled by adjusting other key parameters, including the choice of electrolytes (and their bulk concentration), hydrodynamic slippage, and the surface charge density of the particle.