southern taiwan university multi-step dielectrophoresis for separation of particles student: bui...
DESCRIPTION
SOUTHERN TAIWAN UNIVERSITY 1. Introduction Separation of micro particles with different properties is an issue of great importance. One of most common methods is flow cytometryTRANSCRIPT
SOUTHERN TAIWAN UNIVERSITY
Multi-step dielectrophoresis for separation of particles
Student: Bui Tuan Anh ( 裴俊英 )Professor: Yi – Chu HsuClass: Nano-MEMS
SOUTHERN TAIWAN UNIVERSITY
CONCEPT1. Introduction2. Theory of method3. Experiments and calculations4. Results and dicussion5. Conclusion
SOUTHERN TAIWAN UNIVERSITY
1. IntroductionSeparation of micro particles with different properties is an issue of great importance.
One of most common methods is flow cytometry
SOUTHERN TAIWAN UNIVERSITY
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.
SOUTHERN TAIWAN UNIVERSITY
2. Theory of method2.1. DEP mobilityIf the medium is moving with velocity uf, then the total velocity utot of particles is:
uDEP is the velocity of a particle induced by DEP.
is the DEP mobility. is the gradient of the squared electrical field.
2DEP DEPu E
tot f DEPu u u
DEP2E
SOUTHERN TAIWAN UNIVERSITY
A spherical particle with radius r, in the medium with viscosity , the DEP mobility is: is the real part of the complex effective polarisability of the particle.
is conductivity and is permittivity
2
( )18DEPrR
( )R
2 ( ) ( )3
2 ( 2 ) ( 2 )p m p m
mp m p m
f if i
SOUTHERN TAIWAN UNIVERSITY
2.2. Multi-step DEP trappingThe trap-release step incluse 3 phase:
Particles X
Particles YFlow
AC
Particles X Particles Y
Flow
AC
a) b)
c)
L0
L0
Electrode array
Electrode array
Phase a: Particles are focused in the middle of channelPhase b: Trap Phase c: Release
SOUTHERN TAIWAN UNIVERSITY
+ -
Electric fieldThe 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.
SOUTHERN TAIWAN UNIVERSITY
Electrode array (connected to AC)
L0
Particles X Particles Y
Flow
After several steps the separation is complete.
Therefore, an array of interdigitated electrodes will be utilized.
SOUTHERN TAIWAN UNIVERSITY
To quantify the level of fractionation in this method, a DEP resolution factor RDEP between two particle populations A and B is define as:
3DEP
A B
dR
w w
d is the distance between the two centers of each particle populationsw is the distance between the particles most far apart within each population.(wide of population)
Two particle populations are completely separated if RDEP=1.5.
SOUTHERN TAIWAN UNIVERSITY
The resolution will continue to increase as a direct function of the number of steps. Hence, the DEP resolution RDEP can be expressed as:
DEP R DEPR C N
Where the CR is a constant value reflecting the separation increase in each step. And NDEP is a number of steps.
SOUTHERN TAIWAN UNIVERSITY
3. Experiments and calculationsExperimental conditions:Particles: Polystyrene micro beadsFlow channel: micro-flow channel
Number of eletrode: 15 electrodes /1 stepVelocity 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.
Width>>L0
Height=L0
SOUTHERN TAIWAN UNIVERSITY
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:
where upDEP ,unDEP are the velocity induced by positive DEP and negative DEP, respectively.
tot f pDEP nDEPu u u u
SOUTHERN TAIWAN UNIVERSITY
In the release phase, both arrays of electrodes push the particles to the middle of the channel, then velocity become:
tot f nDEPu u u
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).
SOUTHERN TAIWAN UNIVERSITY
Frequency (Hz)
1.5
0.5
-0.5
-1
0
-1.5
102 104 106 108 1010DEP
mob
ility
[m4 /V
2 s] x10-18
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.
2DEP DEPu E
tot f DEPu u u
SOUTHERN TAIWAN UNIVERSITY
4. Results and discussionA difference in size will have largest influence on DEP mobility
00
1
0.5
1
2
1.5
2.5
2 3 4
5%
2%
1%
Res
olut
ion
Number of steps
2
( )18DEPrR
Separate particles with a 5% size difference, only 2 step are used to obtain a complete separation (RDEP>1.5). For 2% size difference, we need 4 step, while 8 would be required to separate particles with 1% difference in size.
SOUTHERN TAIWAN UNIVERSITY
When size difference is 0.2%, number of steps we need is about 200 steps.
1800
1
0.5
2
1.5
2.5
200 220
0.2%
Res
olut
ion
Number of steps15
0
1
0.5
2
1.5
2.5
20 25
0.5%
Res
olut
ion
Number of steps
SOUTHERN TAIWAN UNIVERSITY
Differences in Conductivity have a lower impact on DEP mobility than differences in size.
Res
olut
ion
Number of steps
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
00 1 2 3 4
42% 32%
19%
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.
SOUTHERN TAIWAN UNIVERSITY
Also with a differences in permittivity of 62%, it is a limitation of separation.
00
1
0.5
1
1.5
2 3 4
203% 83%
62%R
esol
utio
n
Number of steps
2
SOUTHERN TAIWAN UNIVERSITY
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.
SOUTHERN TAIWAN UNIVERSITY
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.
SOUTHERN TAIWAN UNIVERSITY
Thanks for your attention.