What is Electrokinetics?

(Contributor: Pedro Arce, Chair Chem. Engineering, Tenessee Tech

The term “Electrokinetics” refers to the motion of small particles in fluids induced by an electrical field. Four basic phenomena constitute the domain of electrokinetics: Electrophoresis, electroosmosis, sedimentation potential and streaming potential. Electrophoresis is the motion of charged particles by an applied electrical field, electroosmosis is the flow induced by a charged surface when a fluid moves over it, sedimentation potential appears when charged particles move relative to a charged surface. This potential opposes the mechanical transfer of charge leading to ion diffusion and (to a lesser extent) electroosmosis. The transfer of charge due to these two types of mechanisms is called leak current. At equilibrium conditions between the leak current and the streaming current, one can measured the streaming potential. The sizes of particles involved in electrokinetic phenomena vary but they are generally small ranging from a few fraction of a micron (ions) up to a few microns (bacteria, pollen, macromolecules).

Electrophoresis is the most widely known member of the electrokinetics family. Recently however, electrokinetic remediation (ER), a novel technique to remove contaminates from poluted soils and also microfluidics have attracted the attention of scientists and engineers. ER is applied in different designs but the “lasagna” with both vertical and horizontal modes is a typical one with high cleaning efficiency. The fact that an electrical field is present may lead to heat effects known as the Joule phenomena. This, in turn, may yield buoyancy effects inside the domain. As a result, hydrodynamic (pressure driven and buoyancy) as well as electro-assisted transport (i.e., electrosmosis) may be significant in ER. The influence of these factors is currently being investigated.

Microfluidics has been made possible by major advances in the microelectronic industry. Etching techniques are now available to draw different channel patterns on a surface. This, in addition to the availability of microelectrodes and novel microscopic flow visualization techniques have paved the way to the development of separation techniques for protein and DNA in microchips. Microfluidics is also a relevant technique to processing material for the computer industry. The basics principles of electrokinetics mainly electrophoresis and electrosmosis play a key role in the understanding of the flow patterns in microfluidics.