Daniel J. Laser

Design Division, Department of Mechanical Engineering,Stanford University

Design Portfolio
Electrokinetics and Electroosmotic Flow
The Electric Double Layer and Electrokinetic Effects
At the interface between a glass surface and a liquid such as water, silanol molecules (SiOH) on the glass surface react with free hydroxyl ions (OH-) in the water, forming Si(OH)2-and leaving the glass surface negatively charged.  Free H+ ions in the water are attracted by the negatively-charged surface and accumulate near it.  As a result, although the interiors of both the glass and the liquid remain electrically neutral, an electrical potential gradient arises in the vicinity of the interface.  The region containing this electrical potential gradient is called the electric double layer, illustrated at left.  Similar electrochemical reactions occur at most liquid-solid interfaces as well as at other phase interfaces (solid-gas, etc.).  The electric double layer is the basis for a category of phenomena known as electrokinetic effects.
Electroosmotic Flow in a Fused Silica Capillary
Filling a fused silica capillary with water results in the formation of an electric double layer at the liquid-solid interface.  External application of an electrical potential along the length of the capillary results in motion toward the cathode of some of the H+ ions in the liquid phase of the electric double layer.  Viscous interactions between these ions and the remainder of the fluid in the capillary results in net fluid flow.  This phenomena, known as electroosmosis, can be used as the basis for a micropump.
R. J. Hunter, Zeta Potential in Colloid Science, Academic Press, Inc., San Diego (1981).
R. F. Probstein, Physicochemical Hydrodynamics, Wiley, New York, 1994.