Electroosmotic flow

Electroosmotic flow (or Electro-osmotic flow, often abbreviated EOF) is the motion of liquid induced by an applied potential across a capillary tube or microchannel. Electroosmotic flow is an essential component in chemical separation techniques, notably capillary electrophoresis. Electroosmotic flow can occur in natural unfiltered water, as well as buffered solutions.

Cause

The cause of electroosmotic flow is an electrical double layer that forms at the stationary/solution interface. In capillary electrophoresis, the narrow channels are made up of silica, and silanol groups form the inner surface of the capillary column. These silanol groups are ionized above pH3. Thus, the inner surface of the channel is negatively charged. In solutions containing ions, the cations will migrate to the negatively charged wall. This forms the electric double layer. When an electrical potential is applied to the column, with an anode at one end of the column and a cathode at another, the cations will migrate towards the cathode. Since these cations are solvated and clustered at the walls of the channel, they drag the rest of the solution with them, even the anions. This results in an electroosmotic flow, not to be confused with the electrophoretic migration.

History

Electroosmotic flow was first reported in 1809 by F.F. Reuss in the Proceedings of the Imperial Society of Naturalists of Moscow. He showed that water could be made to flow through a plug of clay by applying an electric voltage. Clay is composed of closely packed particles of sand and other minerals, and water flows through the narrow spaces between particles just as it would through a narrow glass tube. Any combination of an electrolyte (a fluid containing dissolved ions) and insulating solid would generate electro-osmotic flow, though for water/silica (that is what glass or sand is, chemically) the effect is particularly large. Even so, flow speeds are typically only a few millimeters per second.

An early application of electroosmotic flow was in drying or decontaminating soil. There has been a great deal of interest and research in electroosmotic flow in the last decade, since it was realized that it provides a very efficient way to generate fluid flows in microfluidic devices, including electroosmotic pumps that can generate flow rates as large as a few milliliters per minute, and pressures as large as hundreds of atmospheres. Another reason for the increased interest in electroosmotic flow is its effect on capillary electrophoresis, where the flow tends to counteract the electric field used to drag the DNA molecule.

References

Skoog, et al. Principles of Instrumental Analysis.

Barz, D.P.J., Ehrhard. P., Model and verification of electrokinetic flow and transport in a micro-electrophoresis device, Lab Chip, 2005, 5, 949 - 958.