Biomedical Engineering Reference
In-Depth Information
particularly reliable method when using samples that have not been heavily preconditioned.
On the other hand, EOF pumping—having a nearly lat velocity proile (except just at the walls,
where the no-slip condition still applies)—is not as sensitive to Taylor dispersion as pressure-
driven low, so it is a preferred method for separation techniques in which dispersion adds noise.
3.3.3 Dielectrophoresis
here is an entirely diferent electrokinetic efect that can be used to great advantage in micro-
luidics. It is called
dielectrophoresis
, a phenomenon discovered by Herbert Pohl in 1951. As its
name suggests, it relies on the diference between the dielectric constant of a particle and the
luid in which it is surrounded for its motive force.
Dielectrophoresis is the motion of uncharged particles as a result of polarization induced by
nonuniform electric ields. At a microscopic level, it is caused by induced dipole moments at the
interface between the particle and its medium. A particle that is more polarizable than the sur-
rounding medium is attracted toward a region of increasing ield strength, a scenario known as
positive dielectrophoresis
(
Figure 3.7a
). Conversely, when the solvent is of higher polarizability
than the particle, then the particle is repelled from the high-ield regions because the dipoles of the
luid are the ones that move the particle toward the electrical ield minima—this situation is known
as
negative dielectrophoresis
(
Figure 3.7b
). he dielectrophoretic (DEP) force depends on the par-
ticle size, shape and internal structure, and on the magnitude and degree of nonuniformity of the
applied electric ield. If the lossy dielectric particle is much smaller than the ield nonuniformities,
the dipole approximation to the DEP force for a spherical particle is as shown in
Equation 3.35
:
ε
−
+
ε
p
m
3
2
F
=
2
πε
R
Re
(
ω
)
×
E r
( )
(
3.35
)
m
ε
2
ε
p
m
(from J. Voldman et al., “Holding forces for single-particle dielectrophoretic traps,”
Biophys. J.
80
, 531-541, (2001).)
where:
ε
m
= permittivity of the medium surrounding the sphere
R
= radius of the particle
ω = radian frequency of the applied ield
r
= radial vector describing the spatial coordinates
E
= complex applied electrical ield
ε
m
and
ε
p
= complex permittivities of the medium and the particle, respectively.
a
b
Positive
dielectrophoresis
Negative
dielectrophoresis
+
(-)
δ-
(δ+)
δ+
(δ-)
-
(+)
+
(-)
-
(+)
δ+
(δ-)
δ-
(δ+)
Direction of
particle
movement
Direction of
particle
movement
FIGURE 3.7
Positive.and.negative.dielectrophoresis..(From.Nicole.Pamme,.“Continuous.low.sepa-
rations.in.microluidic.devices,”.
Lab Chip
.7,.1644-1659,.2007..Reproduced.with.permission.from.
The.Royal.Society.of.Chemistry.)
