Electro-Osmotic Flow

Reuss first observed the phenomenon of electro-osmotic flow through clay particles in +1809; the technique has been used for drying closely packed particles of sand, soils and other minerals. The process has also been used to force a liquid flow through silica capillaries for electrophoretic and chromatographic purposes. The inside surface of a silica capillary is covered with silanol groups which, at a pH above 3 will be ionized and, consequently, the inner surface of the silica tube will become negatively charged. If there are any cations in a solution that is in contact with the silica wall, then these cations will migrate to the surface and form a double-layer. If a electric potential is now applied with appropriate electrodes across the length of the tube, the cations of the double layer will be made to migrate towards the cathode dragging the associated liquid with them (including the anions). This will result in a flow of liquid through the tube which has been termed the electro-osmotic flow. This liquid flow is separate and independent of electrophoretic migration. Employing the water-silica combination the flow is relatively large but not absolutely large as producing flow velocities of only a few mm/sec.