Motivation and Background
Over the past 15 years, integrated electrokinetic microsystems have been developed with a variety of functionalities including sample pretreatment, mixing, and separation. Microfabrication technology has enabled the application of electrokinetics as a method of performing chemical analyses and achieving liquid pumping in electronically- controlled microchip systems with no moving parts. These systems are a primary component of so-called micro total analysis systems (mTAS) which aim to integrate multiple chemical analysis functions onto microfabricated chip systems.
Introduction to electrokinetic flow
Electrokinesis involves the interaction of solid surfaces, ionic solutions, and electric fields. Electric fields can be used to generate bulk fluid motion (electroosmosis) and to separate charged species (electrophoresis). Electrophoresis is the simple drift of ions caused by an applied electric field. Electroosmosis describes the motion of electrolyte liquids with respect to a fixed wall that results when an electric field is applied parallel to the surface. Electrokinetic flows are in general a subclass of electrohydrodynamic (EHD) flows, and are distinguishable in that they involve liquid electrolyte solutions and the presence of electrical double layers. Below is a schematic of an electric double layer.
Figure 1. Most solid surfaces acquire a surface electric charge when brought into contact with an electrolyte (liquid). The most common mechanism in microfluidic EK systems is the deprotonation of surface groups on the surface of materials like silica, glass, acrylic, and polyester. This spontaneously- generated surface charge attracts nearby ions of opposite charge and repels ions of like charge. The interface is called an electrical double layer (EDL) and is typically 1 to 20 nm thick.
(Mouseover figure to begin movie)
Figure 2. Electroosmosis in thick and thin EDL systems. The region of excess charge formed by the counterions shielding the wall’s electric field can be used to impart a force on the bulk fluid through ion drag. Above are two examples of electroosmosis. On the left, is the start-up of an electroosmotic flow in a nanoscale channel where the EDL is on the order of channel height (e.g., and 100 nm channel). On the right, is a movie of the start-up dynamics of a thin EDL system (e.g., a 50 um channel). The system on the right quickly (within say microseconds) reaches a nearly-uniform “plug” flow.
On-chip Electrophoretic Separations
On-chip electroosmotic pumping is easily incorporated into electrophoretic separations and “laboratories on a chip” offer distinct advantages over the traditional, free standing capillary systems; these include reduced reagent use, tight control of geometry, the ability to network and control multiple channels on chip, and the possibility of massively parallel analytical process on a single chip.
(Mouseover figure to begin movie)
Figure 3. Movie of the electrokinetic injection and separation of two simple fluorophores (bodipy and fluorescein) in a microfabricated capillary electrophoresis system. The channels shown are 50 mm wide and 20 mm deep. The fluorescence images are ~20 ms exposures and consecutive images are separated by 250 ms.
Introductory Electrokinetics References from Our Group
- Devasenathipathy, S. and Santiago, J.G., "Electrokinetic Flow Diagnostics," in press, Micro- and Nano-Scale Diagnostic Techniques, Breuer, K.S. (ed.), Springer Verlag: New York, 2002.
Sharp, K.V., Adrian, R.J., Santiago, J.G., and Molho, J.I., "Liquid Flows in Microchannels," CRC Handbook of MEMS, M. Gad-el-Hak (ed.), CRC Press, New York, pp. 6-1 to 6-38, 2001.
- Hunter, R.J., Zeta Potential in Colloid Science; Academic Press: London, 1981.
Levich, V.G., Physicochemical Hydrodynamics; Prentice-Hall, Inc.: New Jersey, 1962.
Probstein, R.F., Physicochemical Hydrodynamics: An Introduction; John Wiley & Sons: New York, 1994.
Russel, W.B., Saville, D.A., and Schowalter W.R., Colloidal Dispersions; Cambridge University Press: London, 1999.