I123

The Seventh JSME-KSME Thermal and Fluids Engineering Conference,Oct. 13-16, 2008, Sapporo, Japan

Measurements of Electroosmotic Flow and Electric Fieldin Micro-channels using Micro-PIV
Keisuke Fukuda, Kazuya Tatsumi, Yoichi Katsumoto & Kazuyoshi Nakabe
Department of Mechanical Engineering and Science, Kyoto University, Japan
Corresponding e-mail address: tatsumi@mbox.kudpc.kyoto-u.ac.jp

Keywords: Electroosmotic flow, Electric field, Micro-PIV, Two particles correlation
Abstract: The authors have proposed a method that can measure the electroosmotic flow velocity and electric field in
micro-channels using the μPIV measurement technique. Two kinds of tracer particles with different electrical surface
properties were used in the measurement. The correlation function between the electric field charged in the channel
and the velocities of these particles were firstly obtained from a calibration experiment. Measurements of theelectroosmotic flow and electric field in two types of micro-channels, i.e. straight and U-bend channels, were, then,carried out by applied this correlation function to the μPIV measurements. These results were compared with thoseobtained from the experiment using fluorescent dye and numerical simulation. In both comparisons, an excellent
agreement was achieved in the flow velocity and electric field distributions indicating the validity of the method.

Fig. 1: Schematic of the straight micro-channel.
0-100

uep [μm/s]

Measurements of the flow and electric fields inmicro-channel are important and challenging issues interms of developing micro-devices with fluid containing
components, such as μ-TAS and Lab-on-a-chip. μPIV isregarded as an effective measurement technique toobtain the flow field in such micro-channels (Meinhartet al. (1999)). Many problems, however, are incurred
when an electric field is applied to the channel,
particularly due to the electrophoretic force appearing in
the tracer particles (Sato & Hishida (2006)).
In the present study, a method, which can measure
the electroosmotic flow and electric field using the μPIV
measurement technique, is proposed. In thismeasurement, two kinds of particles with different
electrical surface properties were used as tracer particles.
If the apparent velocity of the particles can be regardedas a superposition of the components induced byhydraulic and electrophoresis forces (Yan et al. (2006)),
then, the electric and flow fields can be obtained byanalyzing the velocities of the two particles when therelation between the electric field and electrophoretic
velocities of the each particle is known.
A calibration experiment was first conducted using astraight channel made of PDMS (polydimethylsiloxane)
as shown in Fig. 1. Height and width of the channel were
50μm and 200μm respectively. DC electric field wasapplying 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 thestreamwise center of the channel.
The capsule-type fluorescent tracer particles
employed in the experiment were made of polystyrene
with carboxylate-layer (Molecular Probes Co. F8819:-200

L.S.M lines-300

uepA (Particle A)
uepB (Particle B)

-400
-500

05Ea [V/mm] 10Fig. 2: Correlation between uepA, uepB and Ea.

referred to as particle A) and melamine (Sigma-AldrichCo. 90518: referred to as particle B) respectively. The
nominal diameter of both particles was 1μm. The
particles were excited by a light of wave length 530nm,
and the fluorescent images of the particles wererecorded by a high-speed camera. PIV analysis wasapplied to these images to calculate the velocity field. In
addition to this, electroosmotic flow velocity, ueof, wasobtained by injecting fluorescent dye (Rhodamine B)
into the channel, and measuring the velocity of theinterface between the fluids with/without dye using ahigh-speed camera.
Figure 2 shows the correlation between theelectrophoretic velocity, uep, of each particle and theCopyright © 2008 by the Japan Society of Mechanical Engineers600μPIVFluorescent dye

400300

L.S.M lines

200

Fig. 4: Schematic of the U-bend channel.

005

Ea2 [V/mm] 10Fig. 3: Relation between ueof and Ea2.

intensity of the electric field, Ea. uep was obtained bysubtracting ueof from the apparent velocities, uA and uB,
which were obtained respectively from themeasurements using fluorescent dye and μPIV. Asshown in the figure, uepA and uepB linearly decreases as Eaincreases. From this relation, a correlation function of
uep and ueof against uA and uB were derived respectively.
Two types of channels have been investigated toevaluate the present method. One was a straight channel
made of a material SU-8 (MicroChem Co.), which
differs from that of the channel used in the calibration.
The channel shape, size and measuring location were set
to be identical to those of the calibration experiment.
The velocity distributions of the two tracer particles
were first obtained by μPIV measurement. The
correlation functions obtained from the aforementioned
calibration experiment was, then, applied to the results
to calculate the electroosmotic flow velocity, ueof. The
results are shown in Fig. 3 presented by the □ symbols.
The abscissa axis, Ea2, is the intensity of the electric fieldobtained from the DC voltage applied to the electrodes.
Addition to this, ueof obtained by conducting anexperiment using fluorescent dye is also plotted in thefigure presented by the ● symbols. Both results show anexcellent agreement under all Ea2 conditions, indicating
the validity of the present method.
The other experiment to evaluate the present method
was a U-bend channel as shown in Fig. 4. In this case,due to the existence of the singular point generated at thecorner, two-dimensional forms of not only the velocity
field but also the electric field are expected to beproduced. The results obtained from the experiment
applying the present method are compared with thoseobtained from a numerical simulation.
The height and width of the channel were 50μm and200μm respectively, and other dimensions of thechannel are described in the figure. The channel material
was PDMS and the electric field was produced in thesame way as in the above cases. Numerical simulation of
the electric field was conducted using a commercial
code ANSYS Multiphysics v11.0 (ANSYS Inc.).
Symmetric boundary condition was applied at thestreamwise center of the channel, and the electric fieldsof the fluid and solid parts were both solved.
Figure 5 shows the contour distributions of electricfield intensity, Ea, in the area illustrated by the square inFig. 4 (bend section). The results obtained respectively
from the experiment and simulation are shown in Figs.
(a) and (b). Here, Ea is normalized by Em, which is theaveraged value of Ea within the area.

1.5

2.0
1.6

1.0

1.2

y/W

100

0.8

0.5

0.4
0

0.0
0.0

0.5

1.0

1.5x/W

2.0

Ea/Eam(a) Experiment(b) Simulation
Fig. 5: Electric field intensity distributions.
450

1.5

360
1.0

270y/Wueof [μm/s]

5001800.5

90
0.0
0.0

0.5

1.0

1.5x/W

02.0 |u | [μm/s]
eofFig. 6: Flow velocity distributions (Exp.).

In Fig. 5 (a), Ea/Em takes a maximum peak at theinner corner of the bend section, and is minimum at theouter corner. In the numerical results shown in Fig. 5 (b),a similar tendency is observed indicating the validity ofthe present method in measuring the two-dimensional
distributions of the electric field.
Figure 6 shows the distributions of the absolute valueof the flow velocity, |ueof|, obtained from the experiment.
|ueof| takes maximum and minimum values at thelocations of the inner and outer corners respectively.
The reason for this is believed to be the relatively small
electric field intensity appearing at the outer corner
resulting in the reduction of the electroosmotic flow
velocity, and the flow stagnation produced in the areaadjacent to the outer corner.
Meinhart, C.D. Werely, S.T. & Santiago, J.G. 1999,
Experiments in Fluids, Vol. 27, pp. 414-419.
Sato, Y. & Hishida, K. 2006, Fluid dynamics Research,
Vol. 38, pp. 787-802.
Yan, D., Yang, C., Nguyen, N.T. & Huang, X. 2006,
Electrophoresis, Vol. 27, pp. 620-627.