Particle-localized AC and DC manipulation and electrokinetics

Colloidal particles suspended in water respond to direct (DC) or alternating current (AC) fields in a variety of ways, including directional motion along or across the field direction, field-gradient dependent response and induced particle–particle interaction. We review here some of these effects and their applications in new techniques for particle manipulation and assembly, making of novel biomaterials and designing of new self-propelling microdevices. The coupling of the counterionic layer mobility, fluid flows and the resulting particle motion are the basis not only of the classic electrophoretic effects, but also of the recent developments in AC electrohydrodynamics and induced charge electrophoresis of asymmetric particles. We also discuss how dielectrophoresis (particle interaction with external AC field gradients), could be used to manipulate and assemble objects on any size scale. We discuss the interactions leading to the assembly of such structures, ways to simulate the dynamics of the process and the effect of particle size and conductivity on the type of structure obtained. Finally, we demonstrate how an additional level of complexity can be engineered to turn miniature semiconductor diodes into prototypes of self-propelling micromachines and micropumps. The diodes suspended in water propel themselves electro-osmotically when a uniform alternating electric field is applied across the container. Semiconductor diodes embedded in channel walls could serve as distributed microfluidic pumps and mixers powered by a global external field.