Artigo - Electroosmotic flow

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SIMTech technical reports (STR_V10_N3_10_MMP) Volume 10 Number 3 Jul-Sep 2009 
186 
Electroosmosis flow (EOF) mobility 
measurement in polymer micro channels 
Z. F. Wang, Y. X. Wu1, and K. Puttachat
 
1 School of Engineering, Ecole Polytechnique Federale de Lausanne, Switzerland 
Abstract – The most popular microfluidic applica-
tions are based on electrically moving separation in 
capillaries. Capillary electrophoresis (CE) is one of 
the fundamental methods for analysis using elec-
troosmotic flow (EOF) and the mobility of charged 
species (ions) by attraction or repulsion in an electric 
field. Microchannels fabricated from various polymer 
manufacturing processes result in various CE proper-
ties, especially the difference on EOF mobility, which 
determine the separation efficiency and other per-
formance of the microfluidic device. In this report, we 
introduce the method and experimental setup of EOF 
mobility measurement of microchannel. Some meas-
urement results of EOF mobility in polymer micro 
channels are also included. We prepare the test sam-
ples from various fabrication methods, and the EOF 
measurement results reveal the effects of polymer 
manufacturing process towards CE property. 
Keywords: Microfluidics, Electroosmosis flow, 
Polymer microfabrication,Separation 
1 BACKGROUND 
In the past decade, the field of microfluidics has 
been developed for most areas of interest in bio 
technology, medical science, and analytical chemistry. 
The most popular microfluidic applications are based 
on electrically moving separation in capillaries. Cap-
illary electrophoresis is one of the fundamental 
methods for analysis using electroosmotic flow and 
the mobility of charged species (ions) by attraction or 
repulsion in an electric field. Microchip capillary 
electrophoresis offers faster and higher resolution 
separation. 
Instead of using glass, recently polymers were 
often employed as the base materials of the microchip 
device. It shows that many polymers are extremely 
suitable for mass- production using low-cost fabrica-
tion methods such as molding, imprinting, and plasma 
etching or laser ablation. Furthermore, their flexibility 
for microfabrication is adequate for miniaturisation, 
and 3D-integration. 
The other characteristics such as the capability 
for surface modification, good optical transparency, 
and durability to acids also show several merit points 
for the analysis property of microfluidic chip. Elec-
troosmotic flow (EOF) is the driving wheel of elec-
trophoresis and the value of the EOF mobility, µEOF, is 
a crucial parameter for the electrophoretic experi-
ments. 
In polymer microfluidic devices, the various 
polymer substrate materials exhibit different EOF 
mobility values. A reliable measurement of parameter 
µEOF is highly desirable for designing the micro cap-
illary electrophoresis (µCE) based microfluidic de-
vices. Moreover, the EOF mobility may also be af-
fected by the manufacturing techniques applied in the 
device fabrication. For example, during various pat-
terning processes, e.g. hot embossing, injection 
molding, laser cutting and micro milling, the polymer 
substrate experiences different pressure (or force), 
temperature profile and machine tool (or mold). Re-
sulting channel surface may exhibit difference CE 
property and also vary in EOF reliability, which is 
equally important in configuring the CE-based mi-
crofluidic devices. 
2 OBJECTIVE 
This project is to set up the measurement tech-
nique for EOF mobility of polymer micro channels, 
and examine the effects from various polymer 
manufacturing processes. By measuring the mobility 
of electroosmosis flow, the separation efficiency of 
microfluidic device can be well modelled, which is 
essential in design of microfluidic devices for separa-
tion and diagnosis. 
3 METHODOLOGY 
3.1 EOF Mobility Measurement 
There are different ways to measure the EOF 
mobility. The most commonly used one is the Cur-
rent-Monitoring method [1], which is an indirect 
method for measuring the average electroosmotic 
velocity in microchannels. 
In general, the EOF mobility is known as [2]: 
η
ζεεμ r0EOF = (1) 
Here ε0, and εr are the permittivity of vacuum, 
and the dielectric constant respectively, ζ is the zeta 
potential of the capillary surface and η is viscosity of 
the solution. 
Z. F. Wang, Y. X. Wu, and K. Puttachat 
187 
A straight microchannel is filled with buffer so-
lution which has low ionic concentration. Both ends 
of the capillary connect with reservoirs. An electrical 
field is applied into the microchannel. High concen-
tration buffer is now loaded to reservoir at anode. By 
electroosmosis phenomenon, the ions migrate towards 
to cathode and gradually move to the other end of the 
reservoir. The total electrical conductivity of the mi-
crochannel increase due to the increasing ion con-
centration, and therefore the current being recorded 
between the two reservoirs will increase. 
The change of current across the two reservoirs is 
recorded through a data acquisition system as a func-
tion of time. When the higher concentration buffer is 
completely driven to cathode, the current will reach a 
plateau value. The EOF mobility can be then derived 
from current-time response curve. This should be 
noted that hydraulic pressure has no interfere in this 
system. 
By ignoring the nonlinear characteristic which 
usually exhibits at the beginning stage, the mobility 
can be derived as below [2]: 
tV
LL
E
v efft
EOF ==μ (2) 
Here Lt and Leff are the total and effective lengths 
of the microchannel between the two reservoirs, re-
spectively. V is the voltage which was applied across 
the distance of Lt, and t is the migration time of the 
marker of EOF travelling in the distance of Leff. In the 
current-monitoring method, t stands for the time the 
total current reaches plateau. It has been reported that 
the EOF mobility of different PMMA (Polymethyl 
Methacrylate) microfluidic chips were all about 1.1 x 
10−4 cm2V−1s−1 [1]. 
EOF mobility can also be measured using Laser 
Induced Fluorescence (LIF) detection. The channel 
design in LIF measurement has two crossed channels. 
First channel is a short channel which uses for sample 
injection. The second channel is long channel which 
use for sample separation and detection. 
At first step, the sample, which added fluorescent, 
is loaded to a reservoir of injected channel, and it is 
driven by electroosmosis flow under high voltage. 
When sample is passing the cross junction, the electric 
filed is switched to the separation channel. Only 
sample in crossed area is driven to the separation 
channel under influence of electric filed. At the almost 
end of separation channel is focused with laser light, 
493 nm in wave length, and a visible light sensor. 
Fluorescent was energised by laser and emitted a 
visible light. 
The EOF mobility can then be calculated ac-
cording to Equation (2). The t is the migration time 
that the sample plug travels from the channel cross 
junction to the detection point, which distance is Leff. 
In this technical report, we only include the meas-
urement results of current-voltage method. 
The value of EOF mobility is also varied through 
the change of buffer solution, e.g. pH and concentra-
tion. Some published measurement results on PMMA, 
silica and PDMS microchannels are summarised in 
Table 2 as reference. 
 
Table 2. EOF mobility measurement in literature [3-7]. 
Buffer solution/ 
microchannel substrate 
EOF mobility 
(cm2V-1s-1) 
NH4Ac / PMMA 1.4~1.6 x 10-4 
KCL / PMMA 1.0 x 10-4 
1 x TAE / silica 4.9 x 10-4 
1 x TBE / silica 3.1 x 10-4 
Phosphate/ PMMA 2.83 x 10-4 
BSA / PDMS 1.73 10-4 
 
3.2 Experimental Setup 
Polymer chips with two microchannel designs are 
fabricated for the EOF mobility test. One design is 
with the straight channel, which is for the cur-
rent-monitoring method. Another design comprises 
cross channels for the LIF measurement, which results 
are not included in this report. 
Two kinds of polymers are used as the substrate 
materials for fabricating microchannels. One is 1.5 
mm-thick PMMA sheet (supplied by DAMA Enter-
prise). Another is PDMS (Poly-dimethylsiloxane). 
The PMMA chips with microchannels are fabricated 
in two ways. One is hot embossing, which transfers 
the micro features from a mold to the PMMA sub-
strate by applying heat and pressure. We use SpecacTM 
thermal press as the process tool. The mold is 4-inch 
single crystal silicon wafer, where the micro features 
are fabricated through deep reactive ion etching 
(DRIE) process. 
Other than hot embossing, we also use CO2 laser 
to cut the microchannels directly. After the channel 
patterning process, we laminate the PMMA chips 
using another piece of PMMA sheet through thermal 
diffusion bonding, or simply through adhesive tape, to 
encapsulate the channels. 
The PDMS chips are made from casting out of 
SU-8 photoresist, which is shaped through typical 
photolithography process. We use SlygardTM 184 kit, 
which contains a PDMS elastomer and a PDMS cur-
ing agent, to form the PDMS prepolymer by mixing 
them in a ratio of 10:1, and degassing in vacuum oven. 
We then pour the prepolymer over the mold, which is 
the 4-inch silicon wafer being fabricated through 
DRIE process, and further degas the prepolymer in the 
vacuum oven for 30 min. 
After curing at 100ºC for 4 hours, the PDMS chip 
can be de-molded and ready for bonding process. We 
used three different covers to laminate the PDMS 
chips, e.g. PDMS sheet, glass plate, and adhesive tape. 
The tape we use is 3MTM8161, which has 25 µm ad-
hesion layer. The PDMS-glass and PDMS-PMDS 
bonding are all done at room temperature. 
Electroosmosis flow (EOF) mobility measurement in polymer micro channels 
188 
TBE (Tris-Borate-EDTA) buffer, which is 
commonly used in DNA electrophoresis applications, 
was used in our CE experiments due to its high buff-
ering capacity and low conductivity, especially its 
good electrical stability under high voltage [8], which 
is always required for generating electroosmosis flow. 
The high voltage source used in the CE meas-
urement is a LabSmithTM-HVS448/3000V high volt-
age sequencer. The current measurement is done 
using KeithleyTM 8800 source meter. A ZeissTM Ax-
iovert40-CFL inverted microscope is set up as the test 
station (shown in Fig. 1.) The chip holder is machined 
with polycarbonate for housing the polymer chip 
during the test. The holder consists of two parts. The 
bottom one is holding the chip and the upper one is 
fixing the CE electrodes, which is made of 0.5mm 
diameter platinum wire (99.9%, from Sigma-Aldrich) 
and plastic cover. 
 
 
(a) The ZeissTM Axiovert40-CFL inverted microscope as the 
test station 
 
 
(b) The polycarbonate chipholder and CE electrodes 
Fig. 1. Experimental setup of EOF mobility measurement. 
NI-USB 6259 16-bit data acquisition (DAQ) card 
is used to control the CE driving voltage and collect 
the current measurement data. We use LabVIEWTM 
8.5 to build the current-voltage EOF mobility meas-
urement platform, which user interface has been 
shown in Fig. 2. The configuration for LIF method is 
also included in this LabVIEWTM platform. 
 
Fig. 2. LabviewTM interface for current-voltage measure-
ment of EOF mobility. 
4 RESULTS & DISCUSSION 
The micrographs of PMMA chip fabricated 
through hot embossing and laser cutting are shown in 
Fig. 3. The two kinds of chip have the identical single 
straight channel design. The length and width of the 
micro channel are 50 mm and 100 µm, respectively. 
From Fig. 3 we can find out that the hot embossing 
process offers a rectangular cross-section of channel 
and a smooth inner channel surface. While the laser 
cutting process generates micro channel with V-shape 
cross-section, and the inner channel surface appears to 
be much rougher than the embossed samples. 
 
 
(a) Top view of hot embossing PMMA chip 
 
(a) Cross-section of hot embossing PMMA chip 
Z. F. Wang, Y. X. Wu, and K. Puttachat 
189 
 
(c) Top view of laser cutting PMMA chip 
 
(d) Cross-section of laser cutting PMMA chip 
Fig. 3. Micrograph of channel. 
The patterned PMMA chips are then laminated 
through thermal diffusion bonding. Figure 4 shows 
some results from hot-embossed PMMA chips under 
various measurement conditions. In this current- 
voltage measurement, Lt and Leff are all around 50 mm 
long. The voltage across the capillary (channel) is 
1,500 V. We found that the pre-treatment to the 
channel with NaOH before each use can significantly 
increase the EOF mobility and also make the meas-
urement results stable. From Fig. 4 we can also find 
out that when using the buffer with higher concentra-
tion, higher EOF mobility can be achieved. This may 
because that higher concentrated buffer generates 
more joule heat due to larger current, and leads to a 
lower viscosity [2], which raises the EOF velocity 
according to Eq. (1). 
We also compare the CE measurement results 
getting from the PMMA chips fabricated from hot 
embossing and laser cutting. As shown in Fig. 5, 
throughout the samples we measured, the laser cutting 
samples exhibit consistently lower EOF mobility 
comparing with the hot embossed samples. This dif-
ference may mainly contribute from the different 
inner channel surface property, such as roughness and 
wettability. It’s obvious that these two patterning 
processes offer very different CE property to PMMA 
microfluidic chips. 
 
Fig. 4. EOF mobility measurement for PMMA channel. 
 
 
Fig. 5. EOF mobility measurement of PMMA chips: hot 
embossing vs laser cutting; both chips were thermal bonded. 
We use the PDMS chips to examine the effects of 
lamination technique on the EOF mobility. As men-
tioned earlier, after fabricating the channels, the 
PDMS chips are bonded with another piece of PDMS, 
glass sheet, and adhesive tape, respectively. The 
channel design is the same as we use in PMMA chips. 
1,500 V EOF driving voltage is applied and 1 x TBE 
buffer is used. Figure 6 shows the measurement re-
sults from the PDMS chips with the 3 bonding covers. 
 
 
Fig. 6. EOF mobility of PDMS chip with various bonding 
techniques. 
The average EOF mobility for these 3 kinds of 
chip are 2.21 x 10-4 cm2V-1s-1, 1.68 x 10-4 cm2V-1s-1 
and 2.84 x 10-4 cm2V-1s-1, respectively. The results 
show the clear effects of bonding technique on CE 
property. The tape-bonded PDMS chips exhibit very 
much higher EOF velocity, and the PDMS-glass con-
figuration gives the lowest EOF mobility. 
 
 
Electroosmosis flow (EOF) mobility measurement in polymer micro channels 
190 
5 CONCLUSION 
Through the EOF mobility measurement of mi-
crochannels in PMMA and PDMS microfluidic de-
vices, we find out that the manufacturing process 
varies the results significantly. As the structuring 
process, laser cutting contributes much lower EOF 
mobility comparing with hot embossing. This may 
due to the poor inner channel surface finishing gen-
erated through laser process. We also examine the 
effects of various bonding techniques on CE property 
of PDMS chips. The results show that the channels 
covered by adhesive tape exhibit the highest EOF 
mobility, followed by the glass covered chips and 
PDMS covered ones, respectively. 
6 INDUSTRIAL SIGNIFICANCE 
Through this research, the effects of some poly-
mer manufacturing process on µCE property are dis-
covered throughthe EOF mobility measurement. The 
results are useful in building up the design for per-
formance library of polymer microfluidic devices 
manufacturing. The µCE property of the polymer 
micro channels would then be well controlled and the 
separation efficiency could be designed accordingly. 
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