XIX Sympozjum Srodowiskowe PTZE - materialy.pdf

XIX Sympozjum PTZE, Worliny 2009
This force part vanishes in the absence of a net charge occuring in the particle or in the case of
an alternating field whose time average is zero. The additional force terms arise from the
interaction of dielectric polarization components induced in the particle by the electric field
with spatial inhomogeneities in that field.
These additional dielectric force terms only vanish if the field is spatially homogeneous that is
when ∇E = 0. Pohl [10] was first one who recognize and explore the use of dielectric forces
for the manipulation of different particles, particularly living cells, and he named the
movement of particles induced by themby term dielectrophoresis (DEP). DEP is the electric
analog of the other phenomenon named magnetophoresis, the familiar force that collects
metal particles at magnet poles (because magnetic monopoles do not exist, there is no
magnetic analog of electrophoresis). Although Pohl identified DEP with the real part of the
second term of (1), the expressions dielectrophoresis and DEP have since broadened to mean
particle translation resulting from all force components embodied in Eq. (1) including
quadrupole Q and higher order phenomena as well as traveling wave effects arising from
translation of the electric field distribution with time.
DEP enables controlling by excitation voltage trapping, focusing, translation, fractionation
and characterization of particulate mineral, chemical, and biological segregation within a fluid
suspending medium. Because the dielectric properties of these particles depend on both its
geometric shape, structure and composition, dielectrophoretic forces allow investigation a
much richer set of particle properties than electrophoresis. DEP is particularly well suited to
applications and analysis at the small scales of microfluidic devices and chips, is open to to
integration by inexpensive fabrication methods, is easily and directly interfaced to
conventional electronics, and can reduce or eliminate the need for complex and expensive. On
a larger, preparative scale, DEP methods are applicable to the purification, enrichment, and
characterization of a wide range of environmental, biological and clinical components and
significant progress has been made in developing technologies in these areas.
In the frequency domain, the induced particle dipole moment is given by [4]
m E (2)
3
( ω) = 4 πε mr fCM
( ω)
where ω is the angular frequency of the applied field, r the particle radius, and fCM the
polarization factor (Clausius–Mossotti factor) defined as
* here, ε p and *
m
respectively.
* *
ε p − ε m
fCM
( ε p , εm)
= (3)
* *
ε p + 2ε
m
ε are the complex permittivities of the particle and its suspending medium,
*
p
p p j σ
ε = ε − (4)
ε
ε = ε − (5)
ε
* m
m m j σ
However, by utilizing the fact that the mixed partial derivatives of the field with respect to
space and time must obey the Swartz relationships for the field to remain continuous, we
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