CHAPTER 4 RESULTS AND DISCUSSION
4.3 C OMPARISON OF SIMULATION AND EXPERIMENT
Figure 4-24 shows that simulation of interdigitated electrode (323) compares to experimental photos, in which the applied voltage is 100V AC, dielectric thickness (nitride)3000Αo , channel height 20 m
μ
, and volume of droplet 5nl. The volumes of droplets are 4.2nl for interdigitated electrode (565) in Fig. 4-25 and 4.5nl for square electrode in Fig.4-26. The other parameters are the same as Fig. 4-24. There are a little different between experimental photos and simulation frames, it results from particles on electrodes, hydrolysis, electrodes of rough surface in experimental devices but smooth surface in simulation and etc. The devices of rough surface is caused by uneven of Au deposition, photolithography develop, dielectric layer deposition and coating Teflon layer. Because the pressure difference of droplet can’t be measured in the experiments, the larger pressure difference can make droplet move faster. From the experimental photos, the droplet moving for interdigitated electrodes (2323) and interdigitated electrodes (5656) are faster than square electrodes. Because the CCD camera is 30 frames per second, it is found that the mean velocity of droplet for interdigitated electrode (2323) was 11.36 mm/s, whereas the corresponding prediction was 13.291 mm/s. For 5656 arrangement, the mean experimental and numerical velocities were 11.07 and 11.542 mm/s, respectively. As to square electrode, they were 10.49 and 9.614 mm/s, separately. It proves indirectly that the trends of simulation are accuracy. In spite of a little discrepancy between experiment and simulation, from the comparison of Fig. 4-21, Fig. 4-22 and Fig. 4-23, it is confirmed that the results of simulation have the relative accuracy.Fig. 4-1 Illustration of Table 4-3
Fig. 4-2 Illustration of velocity and pressure for droplet (case 1) 1 2 3
Fig. 4-3 Pressure and velocity vs. time for case 1 (triangle: pressure of head droplet; square: pressure of tail droplet; circle: velocity of droplet head; diamond: velocity of tail droplet)
Ⅰ Ⅱ Ⅲ
1 2 3
Fig. 4-4 Illustration of velocity and pressure for droplet (case 3) 1 2 3
Fig. 4-5 Pressure and velocity vs. time for case 3 (triangle: pressure of head droplet; square: pressure of tail droplet; circle: velocity of droplet head; diamond: velocity of tail droplet)
1 2 3
Ⅱ Ⅲ
Ⅰ
Channel height (35
μ m
) Length of black line (mm)Presure difference (pa)
Case-16 0.2513 192
Case-14 0.4032 453
Case-05 0.4765 600
Fig. 4-7 The contact length of droplet occupying adjacent electrode
Channel height (35
μ m
) Case-16Case-14
Case-05
Channel height (35
μ m
) Length of black line (mm)Presure
difference (pa)
Case-08 0.3089 240
Case-10 0.3718 286
Case-05 0.4765 600
Fig. 4-9 The contact length of droplet to next electrode for coae-05, 08 and 10 (Table 4-2)
Fig. 4-10 Using tension to illustrate contact curve into adjacent electrode
Fig. 4-11 Pressure difference increases with area of electrode
Electrode square Electrode 232 Electrode 565
step 1 step 2
step 3 step 4
step 5 step 6
step 7 step 8
step 9 step10
Fig. 4-13 Simulations of cutting for droplet beginning position in middle electrode (channel hright 70
μ m
)step 1 step 2
step 3 step 4
step 5 step 6
step 7 step 8
Square electrode Interdigitated electrode (2323)
Fig. 4-15 Comparison of cutting for interdigitated electrode and square electrode at channel height 70
μ m
Fig. 4-16 Pressure distribution of interdigitated electrode (232) at channel height 70
μ m
Drag forces
step 1 step 2
step 3 step 4
step 5 step 6
step 7 step 8
step 9 step10
Fig. 4-17 Photos of creating (interdigitated electrode 2323)
step 1 step 2
step 3 step 4
step 5 step 6
step 7 step 8
step 1 step 2
step 3 step 4
step 5 step 6
step 7 step 8
step 9 step 10
Fig. 4-19 Photos of creating (interdigitated electrode 5656)
step 1 step 2
step 3 step 4
step 5 step 6
step 7 step 8
Fig. 4-21 square electrodes
Fig. 4-22 Electrode-Electrode capacitor Electrode Electrode
L A
H
0.5mm
0.5mm
Fig. 4-23 the total capacitor of interdigitated electrodes
Electrode 1 Electrode 2
L1 L2
L3
L5 L4 L6
L7
L8 L9
t=0~0.033 s t=0~0.033 s
t=0.033~0.066 s t=0.033~0.066 s
t=0.066~0.099 s t=0.066~0.099 s
Fig. 4-24 Comparison between simulation flames and experimental photos (interdigitated electrode 2323, channel height 20
μ m
)t=0~0.033 s t=0~0.033 s
t=0.033~0.066 s t=0.033~0.066 s
t=0.066~0.099 s t=0.066~0.099 s
Fig. 4-25 Comparison between simulation flames and experimental photos (interdigitated electrode 5656)
t=0~0.033 s t=0~0.033 s
t=0.033~0.066 s t=0.033~0.066 s
t=0.066~0.099 s t=0.066~0.099 s
Fig. 4-26 Comparison between simulation flames and experimental photos (square electrode)
CHAPTER 5
Conclusions and Future works
5.1 Conclusions
In this study, the square electrode which the length of electrode 2mm, channel gap 70
μ m
,applied voltage 40V and volume of droplet 0.3 lμ
,is done firstly. Then based on the experimental results above, the parameters of simulation are found out. However, according to numerical results, we design interdigitated electrodes to create nano-liter. There are 16 cases which are presented in Table 4-3 and Table 4-6, and a comparison is made between them. Based on results of simulation, the design of interdigitated electrodes (2323) can cause larger pressure difference to move the droplet.So we make the devices of square electrodes (0.5mm x 0.5mm), interdigitated electrodes (2323) (0.48mm x 0.5mm) and interdigitated electrodes (5656), to prove the results of simulation. However, the interdigitated electrode (2323) has the largest velocity, 13.219 mm/s in simulation and 11.36 mm/s in experiment for droplet moving. It manifests the designs of interdigitated electrodes are significance.
From the anticipation of CFD-RC+ and results of experiment, it concludes that the more area of electrode is touched by droplet, the larger pressure difference is generated. Therefore, in the experiment, it considers a way to
cases, design of interdigitated electrode 2323 (W=80
μ m
) can generate the largest pressure difference at the same channel height. In the experiments, to create a nano-liter droplet for interdigitated electrodes 2323 (W=80μ m
) is easier than interdigitated electrodes 5656 (W=25μ m
) and square electrodes.Because the design of interdigitated electrodes (2323) has larger pressure difference which can move and cut droplet easily. However, the volume of creating droplet is from 2.9nl to 8.5nl in experiments. Therefore, for droplet moving at channel height 20
μ m
, the experimental photos are fit by numerical frames, as shown Fig 4-24, 4-25 and 4-26. It is confirmed that the results of simulation have the relative accuracy.Finally, we also make a table 5-1 to discuss which electrodes is the best to create nano-liter. According to the experimental operation and numerical simulation, the interdigitated electrode 2323 (W=80
μ m
) is more proper design to create a nona-liter droplet surrounding air.Table 5-1 Comparison of three different electrodes interdigitated
electrode (2323)
interdigitated electrode (5656)
Square electrode Pressure difference
for moving, cutting and creating
1 2 3
hydrolysis 2 1 3
Channel height (
μ m
) 20Applied voltage (V) 100 AC (offset 50V) 80 AC (offset 40V)
Electrode shape Symmetry
Volume of creating droplet (nl)
2.9~8.5
5.2 Future works
There are some improvements for the experiments, such as adding a reservoir, droplet surrounding silicon oil, 2-D control electrodes arrays, designing a new electrode to abbreviate creating process and so on. Because the model of EWOD simulation is established, we can invent more novel electrode to forecast and to create pico-liter droplet by commercial software CFD-ACE+. Otherwise, it maybe simulate other solutions or droplet in different environments, such as organic solvents, isotonic solutions, blood, DI water in silicon oil, particles in droplet and so on. For experiments, the nano-liter droplet is created, but the liquid is DI water. In future work, the other liquids, such as isotonic solutions, created by EWOD devices, can be employed in DNA-chip, protein-chip or other microsystems. Nevertheless, there are many subjects about EWOD can be discussed furthermore.
References
1. Shih-Kang Fan, “Digital Microfluidics Cross-Reference EWOD Actuation:
Principle, Device and System”, Unversity of California Los Angeles, Degree doctor of philosophy, 2003.
2. Shih-Chyn Lin, “Study of WEOD-based Actuation for Digital Microfluidic System”, National Central University, Degree of master, June 2004.
3. T.A. Mcmahon and J.T. Bonner, On Size and Life, Scuentific American Books, New York, 1983.
4. Lippman G, Relation Entre Les , “Phenomens electriues et capillaries”, Ann. Chim. Phys, 1875: 494-59.
5. Michael G. Pollacka and Richard B. Fairb, “ Electrowetting-based actuation of liquid droplets for microfluidic applications”, Applied Physical Letters Volume 77, Number 11, 2000.
6. Junghoon Lee, Hyejin Moon, Jesse Fowler, Thomas Schoellhammer and Chang-Jin Kim, “Electrowetting and electrowetting-on-dielectric for microscal liquid handling”, Sensors and Actuators A, 95, pp. 259-268, 2002.
7. Neil Fortner and Benjamin Shapiro, “Equilibrium and Dynamic Behavior of Micro Flows Under Electrically Induced Surface Tension Actuation Forces”, Aerospace Engineering, University of Maryland
8. H. J. J. Verheijen, “Reversible Electrowetting and Trapping of Charge:
Electro-wetting-Based Actuation for Digital Microfluidic Circuits”, Journal of Micro-electromechanical Systems, Vol. 12, NO.1. Feb 2003.
10. Sung Kwon Cho and Chang-Jin Kim, “Particle Separation and Concentration Control for Digital Microfluidic Systems”, pp. 686–689, Kyoto, Japan, Jan 2003.
11. Vijay Srinivasan, Vamsee K.Pamula and Richard B. Fair, “Droplet-based Microfluidic Lab-on-a-chip for Glucose Detection”, Analytica Chimica Acta, 507, pp.145-150, 2004.
12. H. Ren, R.B. Fair and M.G. Pollack, “ Automated on-chip droplet dispensing with volume control by electro-wetting actuation and capacitance metering”, Sensors and Actuators, B 98, pp.319–327, 2004.
13. Debalina Chatterjee, Boonta Hetayothin, Aaron R. Wheeler, Daniel J.
King and Robin L. Garrell, “Droplet-based microfluidics with nonaqueous solvents and solutions”, The Royal Society of Chemistry 2006 Lab Chip, 6, pp. 199–206, 2006.
14. Yi-Liang Lin, “Creating Nano-Liter Droplets Based on Digital Microfluidic System”, National Cheng Kung University, Degree of master, June 2006
15. Guo-Hua Lin, “Simulation of Creating Nano-Liter Droplets Based on Digital Microfluidic Device”, National Chiao Tung University, Degree of master, June 2006.
16. Frank Gindele, Frank Gaul and Thomas Kolling, “Optical systems based on electrowetting”, MEMS MOEMS and Micromachining, Proc. Of SPIE Vol. 5455, 2004.
17. S. K. Cho, H. Moon, J. Fowler, S.-K. Fan and C.-J. Kim, “Splitting a Liquid Droplet for Electrowetting-Based Microfluidics”, Int. Mechanical Engineering Congress and Exposition, IMECE2001/MEMS-2383, New
York, Nov. 2001
18. Van Doormaal, J. P. and Raithby, G. D., “Enhancements of The SIMPLE Method for Predicting Incompressible Fluid Flows”, Numerical Heat Transfer, Vol. 7, Numerical Heat Transfer, pp.147-163, Apr.-June. 1984.
19. Kothe, D. B., Rider, W. J., Mosso, S. J., Brock, J. S. and Hochstein, J. I.,
“Volume Tracking of Interfaces Having Surface Tension in Two and Three Dimensions”, Aerospace Sciences Meeting and Exhibit, 34th, 1996.
20. Brackbill, J. U., Kothe, D. B. and Zemach, C. “A Continuum Method for Modeling Surface Tension”, Journal Computational Physics, Vol. 100, pp.
335-354, Issue 2, 1998.
21. C. W. Hirt and B. D. Nichols, "Volume of Fluid (VOF) Method for the Dynamics of Free Boundaries", Journal of Computational Physics, Vol. 39, pp.201-225, 1981.
22. W. J. Rider, D. B. Kothe, S. J. Mosso, J. H. Cerrutti and J. I. Hochstein,
"Accurate solution algorithms for incompressible multiphase fluid flows,"
AIAA Paper, 95-0699, 1995.
23. Noh, W. F. and Woodward, P. R., "SLIC Simple Line Interface Method)."
In A.I. van de Vooren and P.J. Zandbergen, editors, Lecture Notes in Physics 59, pp. 330-340 1976.
24. D.L. Youngs, Num. Met., “Fluid Dyn”, Academic Press, New York, 1982, pp. 273–285.
25. W.J. Rider, D.B. Kothe, J. ,”Comput. Phys”, 141 (1998) 112–152.
26. D. Gueyffier, J. Li, A. Nadim, R. Scardovelli, S. Zaleski, J. ,“Comput.Phys”, 152 (1999) 423–456.