AC ELECTROKINETICS - A REVIEW OF FORCES IN MICROELECTRODE STRUCTURES

Ac electrokinetics is concerned with the study of the movement and beh aviour of particles in suspension when they are subjected to ac electr ical fields. The development of new microfabricated electrode structur es has meant that particles down to the size of macromolecules have be en manipulated, but on this scale forces other than electrokinetic aff ect particles behaviour. The high electrical fields, which are require d to produce sufficient force to move a particle, result in heat dissi pation in the medium. This in turn produces thermal gradients, which m ay give rise to fluid motion through buoyancy, and electrothermal forc es. in this paper, the frequency dependency and magnitude of electroth ermally induced fluid flow are discussed. A new type of fluid flow is identified for low frequencies (up to 500 kHz). Our preliminary observ ations indicate that it has its origin in the action of a tangential e lectrical field on the diffuse double layer of the microfabricated ele ctrodes. The effects of Brownian motion, diffusion and the buoyancy fo rce are discussed in the context of the controlled manipulation of sub -micrometre particles. The orders of magnitude of the various forces e xperienced by a sub-micrometre latex particle in a model electrode str ucture are calculated. The results are compared with experiment and th e relative influence of each type of force on the overall behaviour of particles is described.