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SOUTHERN TAIWAN UNIVERSITY
Multi-step dielectrophoresis
for separation of particles
Student: Bui Tuan Anh (裴俊英)
Professor: Yi – Chu Hsu
Class: Nano-MEMS
SOUTHERN TAIWAN UNIVERSITY
CONCEPT
1. Introduction
2. Theory of method
3. Experiments and calculations
4. Results and dicussion
5. Conclusion
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1. Introduction
Separation of micro particles with different
properties is an issue of great importance.
One of most
common
methods is
flow cytometry
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This paper proposed a separation method
based on principle of Dielectrophoresis (DEP).
-
+
+++ --
(-)
+ +
+ ++
+
(+)
Positive DEP
-- +
+
++++
(+)
-
+ ++-
--
(-)
Negative DEP
Particles in micro-flow channel will be trapped and
released in a number of steps. Each step will improve
resolution.
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2. Theory of method
2.1. DEP mobility
If the medium is moving with velocity uf, then
the total velocity utot of particles is:
utot  u f  uDEP
uDEP is the velocity of a particle induced by DEP.
uDEP  DEP  E 2
DEP is the DEP mobility.
 E 2 is the gradient of the squared electrical field.
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A spherical particle with radius r, in the
medium with viscosity  , the DEP
2
mobility is:
r
DEP 
18
R( )
R ( )
is the real part of the complex
effective polarisability  of the particle.
2 f ( p   m )  i ( p   m )
  3 m
2 f ( p  2 m )  i ( p  2 m )

is conductivity and

is permittivity
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2.2. Multi-step DEP trapping
The trap-release step incluse 3 phase:
Particles X
L0
a)
b)
Particles Y
Flow
Electrode array
AC
Particles X
L0
Particles Y
c)
Flow
Electrode array
AC
Phase a:
Particles
are focused in
the middle
of channel
Phase b:
Trap
Phase c:
Release
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Electric field
+
-
The particles are
trapped and released
from electrode.
If the difference in mobility bteween the
particles to small, using one trap and release
step will not be enough to achieve compete
separation. Besides that, it’s difficult to trap
the particles with only single pair of
electrodes.
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Therefore, an array of interdigitated
electrodes will be utilized.
Particles X
Particles Y
L0
Electrode array (connected to AC)
Flow
After several steps the separation is
complete.
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To quantify the level of fractionation in this
method, a DEP resolution factor RDEP between
two particle populations A and B is define as:
3d
RDEP 
w A  wB
d is the distance between the two centers of each
particle populations
w is the distance between the particles most far
apart within each population.(wide of population)
Two particle populations
separated if RDEP=1.5.
are
completely
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The resolution will continue to increase as a
direct function of the number of steps.
Hence, the DEP resolution RDEP can be
expressed as:
RDEP  C R N DEP
Where the CR is a constant value
reflecting the separation increase in each
step. And NDEP is a number of steps.
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3. Experiments and calculations
Experimental conditions:
Particles: Polystyrene micro beads
Flow channel: micro-flow channel
Width>>L 0
Height=L 0
Number of eletrode: 15 electrodes /1 step
Velocity of flow is 50.
Voltage phase angle between two adjacent
electrodes is 180° ( ).
Width of one electrode and distance between
two adjacent electrode is 0.5 L0.
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To icrease the trapping efficiency, and to extend
the separation range, superpositioned AC field
are utilized.
This means that the arrays of interdigitated
electrodes are used both at top and bottom of
the channel.
Then the velocity of the particles in the
trapping phase is calculated:
utot  u f  u pDEP  unDEP
where upDEP ,unDEP are the velocity induced by
positive DEP and negative DEP, respectively.
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In the release phase, both arrays of
electrodes push the particles to the
middle of the channel, then velocity
become:
utot  u f  unDEP
In this research, the radius of
polystyrene beads is set to 0.05 L0.
Then, the maximum DEP mobility of
polystyrene particle is 1.2x10-18m4/V2s
at low frequencies(<5kHz).
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DEP mobility [m4/V2s]
x10-18
uDEP  DEP  E 2
1.5
0.5
utot  u f  uDEP
0
-0.5
-1
-1.5
102
104
106
108
1010
Frequency (Hz)
At low frequency (<5kHz), the positive DEP
motion will be increased. And at high
frequency it could be reduced. For negative
DEP, a suitable frequency would be 5MHz.
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4. Results and discussion
A difference in size will have largest influence on DEP
2
mobility   r R( )
DEP
18
Separate particles with a
Resolution
2.5
2
1.5
1
0.5
0
0
5% size difference, only 2
step are used to obtain a
5%
complete
separation
(RDEP>1.5). For 2% size
2%
difference, we need 4
step, while 8 would be
1%
required
to
separate
1
2
3
4
particles
with
1%
Number of steps
difference in size.
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2.5
2
Resolution
Resolution
2.5
0.5%
1.5
1
0.5
0
15
20
Number of steps
25
2
0.2%
1.5
1
0.5
0
180
200
220
Number of steps
When size difference is 0.2%, number of steps
we need is about 200 steps.
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Differences in Conductivity have a lower impact
on DEP mobility than differences in size.
Resolution
1.6
1.4
1.2
42%
32%
1
19%
0.8
0.6
0.4
0.2
0
0
1
2
3
Number of steps
4
If
differences
in
conductivity is reduced
to 18%, no separation at
all is achiveved in the
first trapping phase, and
then
any
separation
would be impossible.
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Resolution
2
1.5
203%
83%
1
62%
0.5
0
0
1
2
3
Number of steps
4
Also with a differences in permittivity
of 62%, it is a limitation of separation.
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5. Conclusions
-The separation method based on repetitive DEP
trapping and release in a flow system.
-The resolution RDEP is a funtion of the number of
steps.
-Calculations for some model particles showed
that it should be to obtain a complete separation
for a 0.2% size difference after about 200 steps.
-The limiting value of diffrences in conductivity is
18% that separations can not performed.
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The power of multi-step DEP could be of
great interest, not only for fractionation
of particles, but also for measuring
changes in surface conductivity.
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Thanks for your attention.