January 26, 2009Soumya K Srivastava
| Soumya K Srivastava - Dielectrophoretic manipulation of particles in a modified microfluidic H filter with multi-insulating blocks | | | | This article discusses COMSOL modeling and experiments to achieve separation of microparticles in a microfluidic H filter. Transportation of particles was by electro-osmosis and electrophoresis. Manipulation of particles was achieved by DEP. PDMS was used to fabricate the insulating blocks which served two purposes: 1) Enhanced DEP force generated 2) Controllability of the motion of particles were increased. There were two different input conditions: 1) DC field 2) combined AC and DC field. Here they have demonstrated the use of single power supply which can produce both AC and DC electric filed. DC voltage helped in transport of particles, whereas combined AC and DC helped for controlled manipulation of particles.
Channel length was 3.5 mm and depth was 60 microns. They chose 3 different sizes of microspheres depicting yeast cells and red/white cells. Particle motion was modeled and various forces involved (electrophoretic force, EO force, DEP force) were discussed. Boundary conditions for solving Laplace equation and Navier Stokes equation along with equations which can be seen in COMSOL are highlighted and discussed.
They reported threshold voltages for 10 and 15 micron particles to be 120 and 85V. For voltages above threshold, the particles were transported to the upper channel. For studying size dependence separation of particles, they chose two different sizes at a time. At 85V and 120V they achieved separation between 5 and 15 microns and 10 and 15 micron particles. By using combined DC and AC field, they could reduce the threshold voltage required for separation significantly.
Review of each sections:
Introduction:
1. Well presented introduction with good references
2. Discussed particle manipulation and separation techniques like FFF, optical tweezers, FACS, EP, and DEP
3. DEP based techniques classified into conventional DEP, traveling wave DEP, moving DEP and iDEP
4. Commercially available microfluidic H filter fails to separate large sized particles such as cells
Materials and Methods:
1. Fabrication by standard photolithography techniques
2. Bonding of glass slide and PDMS by oxygen plasma treatment
a. Hydrophobicity and surface charges of PDMS channel walls changes with time.
b. Bonded channel left overnight to obtain stable operation conditions
i. Decreases zeta potential and particle EP is stronger than the fluid EOF during operation
3. Concentration of particles in DI water 107/ml.
4. Only one representative particle was selected for plotting particle trajectory using Image processing software (ImagePro plus)
a. How did they select only one particle from so many particles?
b. How did they track the movement?
c. Did they tag just that particle?
d. Should we adopt the procedure in our lab?
Modeling of particle’s motion:
1. They assumed negligible joule heating, but there length of channel was 3.5 mm and voltage they applied was around 120V => 345 V/cm
a. Will joule heating be significant?
2. Well discussed equations
3. Boundary condition table well represented
a. What will be the BC for inlet in the Navier Stokes equation
4. How did they estimate the thickness of EDL and hydraulic diameter of microfluidic channel?
5. What does grid-independent study mean?
Results and Discussion:
Zeta potential of PDMS at room temperature= -23.5 mV
PBS density, viscosity and permittivity has to be known for our research
1. Effect of applied voltage:
a. By adjusting the voltage, particles are moved to lower or upper reservoir. This happens once the threshold voltage is reached.
b. They did not provide an voltage threshold for 5 micron particles
i. No evidence of experimentation on 5 micron particles
ii. Trajectory experiments only for 10 and 15 micron particles
2. Size-dependent separation of particles:
a. They tried to separate only two different sized particle even after choosing 3 different sizes for their experiment
b. Achieved separation at 85V for 15-5micron and 15-10 micron particle sizes. (243 V/cm)
3. Reduction of threshold voltage:
To reduce the applied threshold voltage, combined AC and DC power supply was used.
Reduced the threshold voltage and electric field strength was only 530V/cm in their device when AC and DC combined were used. In DC mode it was 580V/cm
Electro-osmotic flow:
1. μ-PIV results for experiment and simulation had certain discrepancy
Tracer particles DEP was not considered in simulation
μ-PIV measurement was not done for the combined AC and DC mode due to limitation of frame speed of CCD camera
Conclusions:
1. Article lacks a conclusion section.
2. Some sections like how did they choose a single particle and tracked its trajectory is not clear from the discussion
3. More information about COMSOL modeling would have been better
4. Connecting AC and DC module together for their experiments not clear
5. Overall, it is a good article with strong theory section.
I will accept the article with modifications
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