13th Int. Symp on Appl. Laser Techniques to Fluid Mechanics, Lisbon, Portugal, June 26 – 29, 2006
Evanescent molecular tagging technique for electrokinetic effects
on velocity field in the vicinity of electrolyte-glass interface
Hiroki Fukumura1, Mitsuhisa Ichiyanagi2, Yohei Sato3
1: Dept. of System Design Engineering, Keio University, Japan, fukumura@mh.sd.keio.ac.jp
2: Dept. of System Design Engineering, Keio University, Japan, ichiyanagi@mh.sd.keio.ac.jp
3: Dept. of System Design Engineering, Keio University, Japan, yohei@ sd.keio.ac.jp
Keywords: Nanoscale, Evanescent wave, Caged fluorescent dye, Electroosmotic flow
The investigation of flow structure in the vicinity of
electrolyte-glass interface contributes to control accurately
the liquid flow in electrokinetic microfluidic devices,
because the electroosmotic flow is governed by the ions
which exsits in the vicinity of the interface. The present
study proposes a novel technique for investigation of
nanoscale flow structure in a microchannel. The technique
realizes the velocity measurement of the ion in the vicinity
of the interface by utilizing the caged fluorescent dye and
evanescent wave illumination of the glass wall.
Caged fluorescent dye is initially non-fluorescent because
a chemical group is attached to quench the fluorescence of
the dye. After irradiated by ultraviolet light, the chemical
group is cleaved and the dye restored the fluorescence,
known as an uncaging event. In order to excite the uncaged
dye in the vicinity of the interface selectively, evanescent
wave illumination is employed as shown in Fig. 1. The
velocity of the uncaged dye was obtained by tracing
fluorescence[1] from the dye.
Figure 2 shows the microchannel comprised of a
poly(dimethylsiloxane) (PDMS) and a 170 µm thickness
borosilicate cover glass. The DMNB-caged fluorescein
dextran and CMNB-caged fluorescein was prepared by
dissolving in carbonate buffer at pH 9.3. The experimental
setup is comprised of an inverted microscope and two types
of laser as illustrated in Fig. 3. A pulsed ultraviolet laser
beam was focused as a laser sheet into the microchannel
cross-section for the uncaging event. A 60× magnification
objective lens (NA = 1.45) was employed to generate
evanescent wave. Evanescent wave illumination can be
easily switched to epi-fluorescent illumination, i.e., mercury
lamp by changing the optical path in the microscope.
Fluorescence from the uncaged dye was captured by a CCD
camera through the objective lens.
A comparison between the fluid flow measured by
epi-fluorescent and evanescent wave illumination was
carried out. In the pressure-driven flow and the
electroosmotic flow (EOF), the calculated velocity profiles
are plotted in Fig. 4 and 5, respectively. In the
pressure-driven flow, the velocity obtained by evanescent
wave illumination is much decreasing than that by
epi-fluorescent illumination, while the almost same velocity
was obtained by the both illumination methods in the EOF.
Evanescent molecular tagging technique enables to detect
the ions in the vicinity of the interface and measure the ion
velocity successfully. This technique can be applicable to the
investigation of the flow structure on the order of nano
meters from the channel wall.
Techniques to Fluid Mechanics, Web-site, (2002).
Pulsed ultraviolet laser beam
Wall
Flow
direction
few hundreds nanometers
Z
X
Illuminated region
Fig. 1 Schematic concept of evanescent wave
illumination.
X
Y
Inlet
400 µm
PDMS
Outlet
400 µm
Z Y
170 µm
( b)
29 mm
50 µm
(a)
Borosilicate glass cover slip
Fig. 2 Schematic illustration of the microchannel in
(a) top view and (b) cross-sectional view.
Mirror
Lens
Microscope stage
Mirror
Microchannel
Y
Objective lens
Mercury lamp
Filter block
Filter block
Lens
Shutter
X
Attenuator
Mirror
Laser (355nm, 1W)
Beam expander
0.6 × TV lens
Cooled CCD camera
Laser (473nm, 30mW)
Fig. 3 Schematic illustration of experimental setup.
Dye velocity [ µm/s]
120
Epi-fluorescent illumination
Evanescent wave illumination
100
80
60
40
20
0
140 160 180 200 220 240 260
Y-direction position [µm]
Fig. 4 Calculated
pressure-driven flow.
velocity
profile
in
the
in
the
Dye velocity [ µm/s]
140
120
100
80
60
40
0
References
[1] Yamamoto et al., 11th Int. Symp. on Appl. Laser
Fig.
EOF
29.2
5
Epi-fluorescent illumination
Evanescent wave illumination
20
140 160 180 200 220 240 260
Y-direction position [µm]
Calculated
velocity
profile