Electroosmotic Flow

This is a process which gives rise to the flow of buffer through the channel. It can be quite significant,

Figure 2 Some typical channel arrangements. Reservoirs A and B start and terminate the separation channel. Reservoirs C and D permit a known amount of sample to be injected into the separation channel. The reservoirs E and F permit the addition of other reagents to the separation channel.

reaching linear velocities of around 5 cmmin-1 or greater. The rate of movement due to EOF is normally greater than the electrophoretic mobility, thus ensuring that all ionic (and uncharged) species pass the detector. However, unlike electrophoretic mobility, EOF will only occur in the presence of an electrical double layer at the surface of the channel. In Figure 3, an axial view of a channel etched in glass can be seen; the surface is covered in silanol groups.

When the pH of the buffer is above & pH 9, all the silanol groups are ionized. Cations from the buffer migrate towards the negative wall of the channel, and a double layer is formed. When a voltage is applied across the channel, these cations migrate towards the cathode, thereby inducing bulk flow. Electro-driven flow has a characteristically flat profile compared to the parabolic profile observed for pressure-driven systems. This significantly reduces the dispersion due to flow, and is considered to be a reason for the high efficiency separations possible. Another reason for the low dispersion observed is that the Reynolds numbers for liquids in such a system are very low, which results in limited dispersion. The electroosmotic mo bility (|EOF) is given in eqn [2] where y is the viscosity of the buffer, s is the dielectric constant of the buffer and £ is the zeta potential (charge on the capillary wall):

The EOF velocity can be calculated from eqn [3] which has striking similarities to eqn [1]. Here, the EOF velocity (v) is related to the electroosmotic mobility (pEOF), and the electric field gradient (E):

From this, it is apparent that the overall velocity of the ionic species is the algebraic sum of the migration velocity, and the EOF velocity. By summing the two velocity terms and subsequent rearrangement of the equation, the actual velocity (va) of an ionic species is given by eqn [4]:

Situations do arise, such as during the analysis of anions with high electrophoretic mobility, when the

Direction of EOF

+ + + + + + + + + + + 0' 0' p~ p~ 0" O" O' p' O' O'

Electrical double layer

Figure 3 The double layer formed in silica channel. The layers of cations which collect along the walls of the channel will migrate towards the cathode when a voltage is applied. This gives rise to the electroosmotic flow (EOF) with the characteristic flat flow profile.

direction of EOF needs to be reversed. This can be achieved by coating the walls of the channels with a cationic surfactant. This gives an apparently positive charge to the walls, so that anions (not cations) will form the double layer. Then, when the potential is applied, EOF will be in the opposite direction. Since the influence of the double layer is generally considered to extend less than 1 |im into the solution, overlap of the double layer should not be an issue for channels of greater than 5 |im minimum dimension. However, for channels of smaller dimension, the flat flow profile model may no longer be valid, and great care should be exercised in describing the flow.

To prevent EOF completely, the walls of the channel need to be rendered neutral. In silica channels, this ought to be achievable by coating the walls with a compound such as trimethylchlorosilane, to end-cap all terminal silanol groups. However, in practice, it is impossible to eliminate all EOF since residual surface charge remains. Since many microsystems are now being constructed from polymeric substrates, EOF normally does not occur to any appreciable extent. This is due to the absence of ionizable or charged surface groups. In this situation, EOF could be induced by coating the walls of the channel with a charged compound, such as cetyltrimethylam-monium bromide.

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Solar Panel Basics

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