Electrokinetics
What are Electrokinetics?
From an historical perspective, the study of electrostatics or electrokinetics has been around for some time. However, it is only recently (since the early 80’s) that the theory has been more fully developed due to better technologies, e.g., atomic force microscopy, available for the characterization of these phenomena.
First observed in 1809 by F.F. Reus, electrokinetic effects have come to the forefront of research in the past twenty years beginning which experiments at Stanford University in the early eighties. Initially (19th Century) experiments with electrokinetics involved naturally occurring filtration such as sand beds and the like, i.e., sedimentation. Today, electrokinetics in the specific form of electrophoresis is being regularly studied and used by researchers in the biological sciences and chemistry. Some examples of these processes can be found in the areas of micro-electromechanical systems (MEMS) and nanoscale separation. Unlike macroscopic flow theory, electrokinetics provides an understanding of the mechanism of fluid transport at microscopic levels.
The generic term electrokinetics alludes to two aspects of this physical phenomenon: electrical charge and motion. This phenomenon results in the movement of particles by exploited the electrostatic properties created at the interface between the fluid medium and the microchannel wall. At the interface between a solid and a liquid there occurs a charge difference due to the redistribution of positive and negative ions. The motion is accomplished when the mobile portion of the electric double layer experiences a shear force. This phenomenon is only observed in microchannels.
In general, there are two types of forces of interest in electrokinetics: stochastic and deterministic. The stochastic or random forces are represented by thermal or Brownian motion. This influences the creation of the diffuse double layer. This force is outside the boundaries of experimental control other than changing the viscosity of the liquid. The deterministic forces include viscous and electrical forces.
Essential to this discussion of electrokinetics is microfluidics. Unlike traditional hydrodynamic theory, microfluidics does not ignore surface phenomena such as chemical and physical properties. The surface to volume ratio of microchannels is large and this has ramifications for transport phenomena. Unlike macroscopic flow theory that can neglect surface effects, these surface effects form the core of studies and research in microfluidics. Thus, microfluidics involves the investigation of electrostatic and thermodynamic effects at the interface between the solid and liquid surfaces.
Practically, electrokinetics is further broken down into further categories: electrophoresis and electroosmosis. Electrophoresis is a process whereby charged (non-zero net charge) particles are moved through the application of an electric field. In the biological sciences electrophoresis can be used to manipulate, separate, and characterize proteins. Electroosmosis is a process used to move ions in solution by the application of an electric field. The use of this last principle results in bulk fluid flow and devices commonly called electrokinetic pumps.
Electrokinetics is a vast and ever-expanding area of current research.
