1. 1.3 Introduction to dielectrophoresis
Since the object of this project has been to understand and apply dielectrophoresis, I
find it natural to include a brief introduction to this physical phenomenon and its various
applications. The term dielectrophoresis is due to Pohl (1951, 1978) ([7], [8]). It is de¯ned
as the motion of dielectric particles in the presence of an inhomogeneous ac-electric field.
Pohl undoubtedly derived the word from the related term electrophoresis, which is used
to describe the motion of electrically charged particles. Unlike electrophoresis, dielec-
trophoresis (DEP) does not require particles to be charged in order for them to interact
with the electric field, and the DEP-force is insensitive to the field polarity. Basically, a
particle can interact with an inhomogeneous ac-field in two ways: either it is attracted
towards areas of higher field magnitudes (this we call positive DEP) or it is repelled by
such areas (negative DEP). The properties of the particle and the field frequency deter-
mine the outcome. Important contributions to the theoretical understanding of DEP were
among others given by Kallio (1978) [9] and Jones and Kallio (1979) [10], who unified the
theories of Pohl and Kuz'min to give the presently widely used formula for the DEP-force.
The manipulation of dielectric particles using ac-fields is not restricted to translational
displacements. Arnold and Zimmermann (1982) [11] were the first scientists to rotate a
single cell using a rotating ac-field. The rotational spectrum obtained from this technique,
known as electrorotation or ROT, has later proven very useful in the investigation of
the interior dielectric properties of biological particles. Other manipulation techniques
include the 3D field-cages, where a three-dimensional electrode configuration generates a
local three-dimensional field minimum in which cells can be trapped and confined, as well
as travelling wave DEP [12], where a travelling electric field moves the cells in a conveyer
belt-like fashion.
At the time of its discovery, the field of application of DEP was limited by the lack of
high frequency power generators and particularly by the lack of reliable micro-electrode
1.4. THESIS OUTLINE 3
fabrication techniques. With the emergence of micro-technology, it became possible to
manufacture electrodes on the micro-scale allowing generation of the large gradients in
field magnitude, which are necessary for the DEP-force to reach a magnitude of practical
relevance. The advantages of DEP as a cell manipulating tool became obvious. Of these,
Pethig [13] lists the ability of DEP to both attract (positive DEP) and repel (negative
DEP) particles and the few requirements to instrumentation. The fact that the DEP
handling method requires no prior tagging of the cells in order to manipulate and even
distinguish noninvasively between them should also be mentioned. This latter property
has inspired many researchers to use DEP as a cell sorting tool. A common approach
to sorting [1] is to use positive DEP to hold on to the cells of interest, while the rest is
°ushed away and then release the cells for further handling. To avoid the discrete steps
of which this batch-wise sorting method consists, e®ort has been put into developing a
continuous cell sorting techniques. Markx [14] has thus reported the use of negative DEP
to push °owing cells into di®erent regions of a parabolic °ow profile hence achieving a
spatial dispersion re°ecting the dielectric properties of the cells. A similar technique, in
which cells travelling in one laminar °ow are guided into another by means of negative
DEP, has been reported by Doh [15] and, as mentioned, by Seger [6].
1.4 Thesis outline
The chapters of this thesis re°ect, to some extent, the time spent on the respective sub-
jects they treat. However, though e®ort and time devoted to unfruitful experimental
approaches, has been documented, their lack of results inevitably introduces some dispro-
portion in retrospect. The thesis is outlined as follows:
² Chapter 2. The aspects of electrodynamics of which we shall make use are presented.
2. ² Chapter 3. Relevant equations governing microfluidcs are outlined and the solutions
to two important flow problems are derived.
² Chapter 4. The theory is applied to three electro-fluidic devices in order to solve
the dielectrophoretic force field. A mathematical model for cell sorting is presented,
and its predictions are accounted for. Minor calculations on cell dielectric properties
and the t
![1.3 Introduction to dielectrophoresis
Since the object of this project has been to understand and apply dielectrophoresis, I
find it natural to include a brief introduction to this physical phenomenon and its various
applications. The term dielectrophoresis is due to Pohl (1951, 1978) ([7], [8]). It is de¯ned
as the motion of dielectric particles in the presence of an inhomogeneous ac-electric field.
Pohl undoubtedly derived the word from the related term electrophoresis, which is used
to describe the motion of electrically charged particles. Unlike electrophoresis, dielec-
trophoresis (DEP) does not require particles to be charged in order for them to interact
with the electric field, and the DEP-force is insensitive to the field polarity. Basically, a
particle can interact with an inhomogeneous ac-field in two ways: either it is attracted
towards areas of higher field magnitudes (this we call positive DEP) or it is repelled by
such areas (negative DEP). The properties of the particle and the field frequency deter-
mine the outcome. Important contributions to the theoretical understanding of DEP were
among others given by Kallio (1978) [9] and Jones and Kallio (1979) [10], who unified the
theories of Pohl and Kuz'min to give the presently widely used formula for the DEP-force.
The manipulation of dielectric particles using ac-fields is not restricted to translational
displacements. Arnold and Zimmermann (1982) [11] were the first scientists to rotate a
single cell using a rotating ac-field. The rotational spectrum obtained from this technique,
known as electrorotation or ROT, has later proven very useful in the investigation of
the interior dielectric properties of biological particles. Other manipulation techniques
include the 3D field-cages, where a three-dimensional electrode configuration generates a
local three-dimensional field minimum in which cells can be trapped and confined, as well
as travelling wave DEP [12], where a travelling electric field moves the cells in a conveyer
belt-like fashion.
At the time of its discovery, the field of application of DEP was limited by the lack of
high frequency power generators and particularly by the lack of reliable micro-electrode
1.4. THESIS OUTLINE 3
fabrication techniques. With the emergence of micro-technology, it became possible to
manufacture electrodes on the micro-scale allowing generation of the large gradients in
field magnitude, which are necessary for the DEP-force to reach a magnitude of practical
relevance. The advantages of DEP as a cell manipulating tool became obvious. Of these,
Pethig [13] lists the ability of DEP to both attract (positive DEP) and repel (negative
DEP) particles and the few requirements to instrumentation. The fact that the DEP
handling method requires no prior tagging of the cells in order to manipulate and even
distinguish noninvasively between them should also be mentioned. This latter property
has inspired many researchers to use DEP as a cell sorting tool. A common approach
to sorting [1] is to use positive DEP to hold on to the cells of interest, while the rest is
°ushed away and then release the cells for further handling. To avoid the discrete steps
of which this batch-wise sorting method consists, e®ort has been put into developing a
continuous cell sorting techniques. Markx [14] has thus reported the use of negative DEP
to push °owing cells into di®erent regions of a parabolic °ow profile hence achieving a
spatial dispersion re°ecting the dielectric properties of the cells. A similar technique, in
which cells travelling in one laminar °ow are guided into another by means of negative
DEP, has been reported by Doh [15] and, as mentioned, by Seger [6].
1.4 Thesis outline
The chapters of this thesis re°ect, to some extent, the time spent on the respective sub-
jects they treat. However, though e®ort and time devoted to unfruitful experimental
approaches, has been documented, their lack of results inevitably introduces some dispro-
portion in retrospect. The thesis is outlined as follows:
² Chapter 2. The aspects of electrodynamics of which we shall make use are presented.](https://image.slidesharecdn.com/1-130109042227-phpapp01/85/1-1-320.jpg)
