Microfluidics - Introduction to BioMEMS - page 117

Stern layer

Shear layer

Solid

Diffusive layer

q wall

q stern
q ζ

0 y

Approx. I D

FIGURE 3.6 The.role.of.surface.charges.in.electro-osmosis..(From.Dominik.P..J..Barz.and.Peter.Ehrhard,.
“Model. and. veriication. of. electrokinetic. low. and. transport. in. a. micro-electrophoresis. device,”. Lab
Chip 5,.949-958,.2005..Reproduced.with.permission.from.The.Royal.Society.of.Chemistry.)

Debye length ) is free to move by electrophoresis parallel to the channel surface. If a pair of elec-
trodes imposes an external ield on the channel, the counterion layer that sheaths the core of the
luid moves together. Because the interaction of the ions with their immediate surrounding sol-
vent molecules is strong, the ions drag the solution with them, so that the electrophoresis pulls
the entire column of water with it. he core of the channel is viscously entrained by the moving
solvent at the channel walls, so the luid moves along at the same speed except very close to the
walls where the speed is zero (a “ plug low proile ”), as opposed to a pressure-driven parabolic
low proile. his principle— electro-osmotic low (EOF)- based pumping—is ubiquitously used
in capillary electrophoresis systems (see Section 4.4.1).
EOF is found wherever electrophoresis is underway, and can be a signiicant interfering efect in
an electrophoretic separation. You can imagine the problem if you are trying to separate a charged
analyte and detect that band by running it past a detector, only to ind that the solution in which
the analyte is running is pumping itself rapidly in the opposite direction, in which case your ana-
lyte band can be made to go in the opposite direction than that intended. EOF can be suppressed
either by (a) working with a channel material that has a very low surface charge at the operat-
ing bufer temperature; (b) working at very high salt concentrations, which reduces the surface ζ
potential; or (c) adding a coating to the surface that suppresses the movement of the irst few tens of
nanometers of solution relative to the channel walls, like a covalently bound polymer layer.
EOF-based pumping in channels that are much wider than the electric double layer produces
good pumping rates, but little ability to pump against backpressure because low can return
along the midline of the channel in which the electric double layer does not penetrate. As the
channels become narrower, and enter the regime of nanoluidics, the efective pumping pres-
sures can increase very rapidly, at the expense of lower pumped volumes. In general, scaling
down the dimensions of a microchannel favors electrokinetic phenomena because, in narrower
channels, the Debye layer occupies a larger proportion of the channel and thus a larger propor-
tion of its volume participates in transport. Conversely, electrokinetic phenomena do not work
well on wide channels (i.e., scaling up does not work well).
EOF pumping (intentional or otherwise) is only as reliable as the surface potential. Anything
that changes the surface charge of the channel wall alters the local pumping driving force, so
the overall pump speed can change radically as something adsorbs to or desorbs from the chan-
nel walls. his is a particular problem with proteins and “raw” biological samples that con-
tain unknown proteins and other polymers. As a consequence, electrophoresis itself is not a