The control and handling of fluids and fluid-based samples is central to the majority of applications in the areas of Micro Analysis systems and the Lab-on-a-chip. As a result, there is a great deal of research and industrial interest in developing specific technologies for this purpose: micropumps, micromixers, microstirrers, etc. One widely used technology in these systems is electrokinetics, the use of electric fields for the manipulation and control of fluids and particles. In DC electrokinetic systems, high voltages (typically ~1kV) are required for controlled manipulation and separation. The use of AC electric fields presents a range of different potential applications as well as the potential for better integration into microsystems. AC Electrokinetic devices for the handling of fluid require significantly lower voltages (~10V) and therefore a four order of magnitude reduction in power requirements. This paper presents devices based on AC electroosmosis and Electrothermal Electrohydrodynamics. The first mechanism involves the interaction of the Electrical Double Layer induced on electrodes by an applied potential and the electric field generated by the same potential. The second involves the interaction of an electric field with gradients in polarisability of the fluid produced by non-uniform heating. Several different designs are presented with applications in pumping, mixing and the general area of micro AC electric field microfluidic control. A specific example is presented: the use of the technique for the local modification of streamlines and deflection of fluids is presented and applications to analysis and sensing are discussed.
The precise control and manipulation of small masses of liquids is an important requirement in the lab-on-a-chip technology. Net fluid flows induced by ac potentials applied to arrays of co-planar interdigitated microelectrodes are reported. Two types of microelectrode structures have been studied: arrays of unequal width electrodes subjected to a single ac signal, and arrays of identical electrodes subjected to a travelling-wave potential. Experiments were performed using solutions of KCl in water of conductivities around 1mS/m placed on top of the electrodes. Fluorescent latex particles were used as tracers. In both microstructures, two fluid flow regimes have been observed: at small voltage amplitudes the fluid moves in a certain direction, and at higher voltage amplitudes the fluid flow is reversed. The fluid flow seems to be driven at the level of the electrodes in the two regimes. A theoretical model of ac electroosmosis is described. The model is based upon the Gouy-Chapman-Stern theory of the double layer. The theoretical results are in qualitative accordance with the experimental observations at low voltages.
In this paper we examine the motion and behavior of particles suspended in aqueous solutions subjected to non-uniform ac electric fields. The particles can move due to forces exerted over them, or due to the motion of the surrounding liquid. In the first case, we have dielectrophoresis, due the action of ac electric fields over polarizable particles. The high strength electric fields often used in separation systems can give rise to fluid motion, which in turn results in a viscous drag on the particle. The electric field generates heat, leading to volume forces in the liquid. Gradients in conductivity and permittivity give rise to electrothermal forces; gradients in mass density to buoyancy. In addition, non-uniform ac electric fields produce forces on the induced charges in the diffuse double layer on the electrodes. This gives a steady fluid motion known as ac electroosmosis. We also discuss the effects of Brownian motion in this context. We calculate the different forces and displacements and compare them for a simple system consisting of a saline solution subjected to a traveling wave electric field. This example provides scaling laws of a wider applicability.