MEASURING METHOD OF ELECTROOSMOTICFLOW VELOCITY AND ELECTRIC FIELD
DISTRIBUTIONS USING MICRO-PIV
K. Tatsumi, K. Fukuda, Y. Katsumoto and K. Nakabe
Department of Mechanical Engineering and Science, Kyoto University, JAPANABSTRACT
A μPIV measurement using two kinds of tracer particles with different electricsurface properties is proposed to obtain the distributions of electroosmotic flow velocity and electric field intensity in this study. Discussion is made on the validity ofthe present method through the use of two types of channels, namely, straight and Ubend channels. The results were compared with the experiment using fluorescentdye and also with numerical simulation, and a good agreement was achieved.
KEYWORDS: μPIV measurement, Electroosmotic flow, Electric field
INTRODUCTION
Measurements of electroosmotic flow velocity and electric fields in microchannels are important and challenging issues in terms of developing micro fluidicdevices. Many researches focusing on the measurements of the electroosmotic flow
have been reported using the florescent dye [1], intensity of the electric current [2]and μPIV [3, 4]. These methods, however, are mostly limited to geometrically simple straight channel flows.
The problem incurred in μPIV measurement when an electric field is applied tothe channel is that the electrophoretic force is exerted on the tracer particles, and,
therefore, the particle behaviours will no longer represent the fluid motion. To tackle
this problem, two kinds of particles with different electric surface properties wereused as the tracer particles in the present study. If the apparent velocity of the particles can be regarded as a superposition of the components induced by hydraulic andelectrophoresis forces, then, the electric and flow fields can be obtained by analyzing the velocities of these particles. A calibration experiment was first conductedwith μPIV measurement and flow visualization using fluorescent dye in a straight
micro-channel to obtain the correlation functions between the apparent electric field
intensity and particles velocities. μPIV measurement was, then, carried out for thetarget channel to simultaneously measure the electroosmotic flow and electric fields
by applying the same two kinds of tracer particles and the correlation functions.
EXPERIMENTAL
Calibration experiment was conducted using a straight channel made of PDMS
(polydimethylsiloxane). The channel dimension is shown in Fig. 1. DC electric field
was applying to the channel by platinum electrodes inserted in each reservoir. The
working fluid was a buffer fluid of pH=6.86 (Nacalai tesque Co. 37220-35). Measurements were conducted in the area located at z/H=0.5 and at the streamwise centerof the channel.
Twelfth International Conference on Miniaturized Systems for Chemistry and Life Sciences
October 12 - 16, 2008, San Diego, California, USA
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0

uep [μm/s]

-100
-200

L.S.M lines-300

uepA (Particle A)
uepB (Particle B)

-400
-500

Fig. 1 Schematic of straight-type-channel.

05

Ea [V/mm]

10Fig. 2 Correlation between uepA, uepB and Ea.

The capsule-type fluorescent tracer particles employed in the experiment weremade of polystyrene with carboxylate-layer (Molecular Probes Co. F8819: referred
to as particle A) and melamine (Sigma-Aldrich Co. 90518: particle B) respectively.
The nominal diameters of both particles were 1μm. The particles were excited by alight of wave length 530nm, and the fluorescent images of the particles were recorded by a high-speed camera. μPIV analysis was applied to these images to calculate the velocity field. In addition to this, electroosmotic flow velocity, ueof, was obtained by measuring the velocity of the interface between the fluids with/without
fluorescent dye (Rhodamine B) injected into the channel using a high-speed camera.
Figure 2 shows the correlation between the electrophoretic velocity, uep, of eachparticle and the intensity of the electric field, Ea. uep was obtained by subtracting ueof
from the apparent velocities, uA and uB, which were obtained respectively from themeasurements using fluorescent dye and μPIV. From this relation, a correlation
function of uep and ueof against uA and uB were derived respectively.

ueof [μm/s]

RESULTS AND DISCUSSION
Two types of channels were applied to evaluate the present method. One was astraight channel made of SU-8 (MicroChem Co.). The other was a U-bend channel
as shown in Fig. 3. In the U-bend channel,due to the existence of the singular point
generated at the corners, two-dimensional
profiles of not only the velocity field but also
the electric field are expected to be generated.
In the straight channel case, the channel
shape, size and measuring location wereidentical to those of the calibration experi- Fig. 3 Schematic of U-bend-type-channel.
ment. The velocity distributions of the two
600
tracer particles were first obtained by μPIV
500
μPIV
measurement, and, then, the correlation funcFluorescent dye
400
tions obtained from the calibration experi300
ment was applied to calculate the electroosL.S.M lines
200
motic flow velocity, ueof. The results are100
shown in Fig. 4. The abscissa, Ea2, is the intensity of the electric field obtained from the005Ea2 [V/mm] 10voltage applied to the electrodes. In addition
Fig. 4 Relation between ueof and Ea2.
to this, ueof obtained by an experiment using
Twelfth International Conference on Miniaturized Systems for Chemistry and Life Sciences
October 12 - 16, 2008, San Diego, California, USA698

y/W

1.5
fluorescent dye is also plotted in the figure.
2.0
Both results show an excellent agreement
1.6
1.0
under all Ea2 conditions.
1.2
In the U-bend channel case, two0.8
0.5
dimensional numerical simulation of the elec0.4
tric fields of the fluid and solid parts were00.0
conducted using a commercial code ANSYS
0.0
0.5
1.0
1.5
2.0
E /Ex/W
Multiphysics v11.0 (Cybernet Systems Co.).
(a) Experiment
The results were compared with those obtained from the experiment.
Figure 5 shows the contour distributionsof electric field intensity, Ea, obtained in the‘measurement area’ illustrated by the squarein Fig. 3. Here, Ea is normalized by Eam,which is the averaged value of Ea within thearea. In Fig. 5 (a), Ea/Eam takes a maximum
peak at the inner corner of the bend section,(b) Simulationand is minimum at the outer corner. In the Fig. 5 Electric field intensity distributions.
450
1.5
numerical result shown in Fig. 5 (b), a similar
360
tendency is observed indicating the validity
1.0of the present method for measuring the two270
dimensional distributions of the electric field.
1800.5Figure 6 shows the distributions of the90
absolute value of the electroosmotic flow ve0.00locity, |ueof|, obtained from the experiment.
0.0
0.5
1.0
1.5
2.0 |u | [μm/s]
x/W|ueof| takes maximum and minimum values atthe locations of the inner and outer corners Fig. 6 Flow velocity distributions (Exp.).
respectively. The electroosmotic flow velocity in the area adjacent to the outer corner became relatively small due to the flow stagnation and also smaller electric field
intensity appearing at the outer corner compared with that at the inner corner.
amy/Waeof

CONCLUSIONSThe present results indicated that the two-dimensional distributions of the electroosmotic flow velocity and electric field intensity in straight and U-bend microchannels can be obtained by the present μPIV measurements with two types of tracer
particles of different electric surface properties.
ACKNOWLEDGEMENTS
This research was partially supported by the Japanese Ministry of Education,
Culture, Sports, Science and Technology.
REFERENCES
[1]
[2]
[3]
[4]

D. Sinton and D. Li, Colloids Surfaces A, 222, p. 273-, (2003).
B.J. Kirby and E.F. Hasselbrink, Electrophoresis, 25, p. 187- (2004).
M.H. Oddy and J.G. Stantiago, J. Colloid and Interface Science, 269, p. 192-, (2004).
Y. Sato and K. Hishida, Fluid Dynamics Research, 38, p. 787-, (2006).

Twelfth International Conference on Miniaturized Systems for Chemistry and Life Sciences
October 12 - 16, 2008, San Diego, California, USA699