Environmental Engineering Reference
In-Depth Information
gradient, together with the particle volume. The direction of the force depends on
the Clausius Mossotti factor, which is a measure of the polarizability of the particle
in the suspending medium and importantly varies with the frequency of the applied
potential (as will be shown in the theoretical part). The particles are either attracted
(positive DEP) to the region of maximum field intensity or repelled (negative DEP)
from it, depending on the effective particle polarizability relative to the media. The
particle “chaining” force is described as a result of the attraction of the induced
dipoles within the particles. The chaining force acting between particles of the same
type is always positive and attractive. AC electric fields can be used to manipulate
many types of colloidal particles in different media by simply adjusting AC electric
field parameters (magnitude, frequency, wave shape, wave symmetry, and phase)
[
6
-
8
].
Due to its ability to manipulate particles based solely on their dielectric proper-
ties and size, DEP is used for a wide variety of applications. DEP methods can be
used in many forms (electrorotation, traveling wave DEP, negative and positive
DEP) to manipulate and more generally, control the position, orientation,
and velocity of micro- and nanometer scale particles, including carbon nanotubes
and biological particles such as viruses, DNA, bacteria, and cells of various
kinds [
7
,
12
].
DEP-based separation methods usually employ fluid flow as a force competing
with DEP in order to achieve the separation of two or more populations of particles.
By using this method it is possible to sort particles according to size (because the
DEP force depends on the volume of the particle, while drag depends on the radius
of the particle) and dielectric properties. The DEP force depends (amongst other
things) on the particle volume, therefore high electric field gradients are required to
move nanometric particles. Because of electrohydrodynamic (EHD) effects, the
high intensity electric fields give rise to fluid displacement and may cause particle
movement through the viscous drag force. On the other hand, employing of
nonuniform, AC electric fields avoids the problem of electrolysis that is caused
when using DC electrophoresis for similar tasks.
The fundamental aspects of nanoparticle dynamics in fluid flows are also of great
interest for numerous applications in medicine, biotechnology, and pharmaceutical
research, therefore the manipulation of colloidal particles and fluids in
microsystems by using electrical forces has many existing and potential applica-
tions. The ability to manipulate particles is of particular importance for micro- and
nanofluidics, where the particles can be physical or biological objects manipulated
with different methods (electrokinetic or magnetic forces, ultrasound, or optical
tweezers) [
7
,
11
,
13
].
Particle dynamics in fluid flows has been the focus of attention for a long time, in
various contexts and at different scales, therefore the manipulation of nanoparticles
in microsystems by using DEP forces has many existing and potential applications,
presenting the advantages of voltage-based control and dominance over other
forces. For most applications involving micrometer and submicrometer particles,
the forces that tend to dominate in microdevices are viscous forces and electrical
forces; in the range above a few millimeters, the electrical forces are rather
