Flow Structure in Continuous Flow Electrophoresis Chambers
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MATERIALS PROCESSING IN THE REDUCED GRAVITY ENVIRONMENT OF SPACE
217
Guy E. Rindone
FLOW STRUCTURE IN CONTINUOUS FLOW ELECTROPHORESIS CHAMBERS
J. A. DEIBER & D. A. SAVILLE Department of Chemical Engineering,
Princeton University,
Princeton, NJ 08544
ABSTRACT There are at least two ways that hydrodynamic processes can limit continuous flow electrophoresis. One arises from the sensitivity of the flow to small temperature gradients, especially at low flow rates and power levels. This sensitivity can be suppressed, at least in principle, by providing a carefully tailored, stabilizing temperature gradient in the cooling system that surrounds the flow channel. At higher power levels another limitation arises due to a restructuring of the main flow. This restructuring is caused by buoyancy, which is in turn affected by the electro-osmotic crossflow. Approximate solutions to appropriate partial differential equations have been computed by finite difference methods. One set of results is described here to illustrate the strong coupling between the structure of the main (axial) flow and the electro-osmotic flow.
INTRODUCTION In a continuous flow electrophoresis chamber, a lateral electric field is imposed across a vertical channel through which a buffer solution moves slowly under the influence of a pressure gradient. Figure 1, a schematic drawing of the chamber, shows the general arrangement. A sample stream is continuously injected into this flow and carried through the electrode region where particles in the sample stream migrate laterally at speeds corresponding to their respective electrophoretic mobilities. In a sample where some of the particles have one mobility and the remainder another, a complete separation will occur if the residence time is sufficient. However, with more complicated sample populations fractionation occurs. The various fractions are collected continuously at the outlet. Several factors related to the characteristics of the flow affect the quality of the fractionation. Here we describe how electroosmosis can alter the flow structure. Since the rigid boundaries of the chamber are in contact with an ionic solution, they are electrically charged. The zeta-potential of the surface is a measure of the amount of charge per unit area. Ions from electrolytes dissolved in water, the buffer solution, form a thin, mobile diffuse layer of charge adjacent to the surface. In most cases of interest this layer is only a few tens of nanometers thick, nevertheless the action of the imposed electric field exerts a body force on the fluid in the diffuse layer and causes the fluid to move. This motion is termed electro-osmosis. Other forces influence the flow and for an accurate picture all must be taken into account. For illustrative purposes imagine fluid between two large, parallel plates. If an electric field is imposed parallel to the plates, the electro-osmotic flow will also be parallel to the surfaces and the velocity profile appears flat, if we ignore the thin transition regions adjacent to the
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