Environmental Engineering Reference
In-Depth Information
major limitations for precise investigation of their unique physical and chemical
characteristics [
6
,
7
].
The traditional methods for nanoparticle manipulation (retention and separation)
have not been successful. In a typical particle-capture device, only a small fraction
is collected and the method is successful only when the nanoparticles are attached
to larger ones. Mechanical methods or other devices based on controlling the
particles towards a desired location are becoming of less value, while chemical
methods of sorting particles are usually slow and may contaminate the particles
under manipulation. Mechanical devices used for filtering the postcombustion
gases resulted in waste incinerators (cyclones, bag filters, sedimentation chambers)
are less effective at this scale because of the nanoparticles
low weight while
Corona electrostatic filters have high micrometric particle retention efficiencies
(93-99 %), yet most nanoparticles still remain undetected [
1
,
2
]. Optical methods
sometimes used in trapping nanoparticles have the significant disadvantage that
they produce heating of the fluid in which the particles are suspended. They also
require the use of specific optical equipment, which is often large and difficult to
integrate on a micro- or nano-analysis device. Methods such as transmission
electron microscopy (TEM) and size-exclusion chromatography were introduced
so far for identifying and separating nanoparticles. These methods, however, have
some inherent problems in the detection processes involving the degradation of
sample, irreversible adsorption, etc. [
3
,
4
].
The methods utilizing electric fields are emerging as most promising techniques
for nanoparticle manipulation that involves within microfluidic systems many
processes including patterning, focusing, sorting, trapping, handling, and separa-
tion, where electrical forces can act both on particles and on the suspending fluid.
The most promising technique for nanoparticle trapping—defined as electric field-
induced particle immobilization at certain regions in a device—and controlled
spatial separation is a method based on dielectrophoresis (DEP), phenomenon
that emerges upon application of an electric field (DC, or AC) to a suspension of
particles in a fluid medium [
6
,
8
,
9
]. The application of an electric field across a
suspension of colloidal particles leads to their polarization. The DEP force arises
when the particles
0
induced dipoles interact with a nonuniform electric field leading
to particle movement. Hence, the particle undergoes a DEP motion under the
resulting translational force, when the DEP force overcomes other competing
forces such as the buoyancy, thermal force, hydrodynamic force, and Brownian
motion. This movement was termed “dielectrophoresis” by Pohl [
10
,
11
], a com-
bination of the word for force, “phoresis” in Greek, and the word “dielectric.” This
force does not require the particle to be charged.
The strength of the DEP force depends strongly on the medium and particles
'
'
electrical properties, on the particles
shape and size, as well as on the frequency of
the electric field. Consequently, fields of a particular frequency can manipulate
particles with great selectivity. Since the relative dielectric polarization of the
nanoparticles depends on the driving frequency of the applied electric field, an
alternating (AC) electric field is usually applied to generate DEP forces of different
magnitudes and directions. The force depends on the magnitude of the field and the
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