Biomedical Engineering Reference
In-Depth Information
Although electrorotation presents some similarities with DEP, it also bears some
fundamental differences in the sense that the direction of the ac electric field is not
fixed but is continuously moving.
The full calculation based on (10.29) shows that the direction of the torque
depends on the electrical characteristics of the particle and the medium but not on
the frequency: there is no equivalent to the crossover frequency (no change of sign
in the rotation direction). However, the amplitude of this rotation is not monotonic
and, depending on its sign, exhibits a maximum or a minimum precisely at the
crossover frequency.
Electrororation is successfully used as an analytical tool to probe the dielectric
properties of particles or cells. By varying the frequency, dielectrophoretic spectros-
copy can be performed [47, 46]. It can also be useful in complement of DEP cages
to stabilize them or to get more information out of these trapping measurements.
An extension of this calculation is the application of a similar sequence to elec-
trodes aligned on a surface. Imagine this setup as the unfolding of the electrorota-
tion setup. Instead of having a rotating electric field we now deal with a traveling
wave of electric field [51]. This configuration therefore implies the successive ad-
dressing of many electrodes linearly arranged with phase shifted signals.
Traveling waves have been effectively used for the pumping of liquids [53] and
to induce the motion of particles or cells [47, 52]. As with electrorotation, high
velocity switching between the electrodes leads to the asynchronous motion of the
particles. This strategy can then been coupled with microfluidics and should lead
to a directed motion in a channel. However, practically, a DEP force directly acting
on the particles often comes into play and, to avoid sticking of the particles on the
electrodes, care should be taken to work in negative DEP conditions.
10.2.6 Instabilities
10.2.6.1 Pearl Chaining
We have seen how the external electric field can induce a dipole in a particle.
The interaction between this dipole and the electric field then drives the particle
toward the high field region (for positive DEP). The story however does not stop
here. If there is another particle in the vicinity of the first one, the electric field
experienced by this second object is not the externally applied field but a field
modified by the presence of the first particle (of course, the field experienced by
the first particle is also modified by the presence of the second one). This leads
to an arrangement of the particles in “pearl chains” where they touch each other
and where this chain follows the fields lines (the whole assembly acts like a large
dipole) (Figure 10.18) [34, 54].
This effect effectively increases the trapping efficiency [55]. The critical value
of the electric field necessary to observe pearl chaining has been computed by Pohl
[33]. The calculation leads to:
ε ε
ε ε
πε
+
2
kT
p
l
E
»
(10.30)
crit
3
-
2
a
p
l
l
A detailed review on pearl-chaining effects can be found in [34].
