Biomedical Engineering Reference
In-Depth Information
Figure 10.18
Interaction between particles. Each particle (in this case they are more polarizable
than the medium) deflects the field lines and thus the effective electric field for the other particle.
Decreasing the concentration of particles in the solution is not an efficient way
to avoid pearl chaining: As the high electric field region (close to the electrodes) col-
lect the particles, their local concentration increases considerably in these area to
reach values for which pearl chaining is observed even with extremely dilute initial
solutions.
10.2.6.2 Electrohydrodynamic Instabilities
Most of the time, positive DEP brings hydrodynamic instabilities that can be strong
enough to impair an efficient trapping or to modify the position of trapping. As a
matter of fact, it has been often observed that the position of stable trapping is not
at the point of maximal field but slightly away from it: on the electrodes themselves
or on their edges. These effects can be qualitatively understood by taking into ac-
count field induced flows in the solution.
Most of the time thermal gradients in the solution are the strongest contribu-
tion to these effects [53]. The dissipated heat is given by Q~
s
E
2
. Thus, the same
field nonuniformities that give rise to the DEP effect, causes local thermal gradients
that in turn, are the source of convective flows. In negative DEP regime, these flows
actually stabilize the trapping. However, they tend to destabilize positive DEP trap-
ping. Temperature rise has other consequences such as modifying the conductivities
or the permittivities, which are both function of temperature (see [56] for a detailed
review).
In addition to these thermal effects, the electric field directly interacts with the
double layer present at the surface of the electrodes. Effects similar to electro-os-
mosis will manifest themselves at time scales larger than the time necessary to es-
tablish this double layer. Thus, for sufficiently small frequencies (that depend on the
characteristics of the medium but that can be roughly estimated 10-500 kHz), this
effect, sometimes complicated by charge injection [57], manifests itself with a much
higher intensity than the DEP itself. Low-frequency observations where the trap-
ping site is at the center of the electrodes or precisely on their edges can be explained
by a combination of flows and DEP.
