Proceedings of FEDSM09
ASME Fluids Engineering Division Summer Meeting
Aug 02, 2009 - Aug 06, 2009, Vail, Colorado, USA
FEDSM2009-78059
MOLECULAR TAGGING TECHNIQUES FOR MICRO-FLOW AND MICRO-SCALE HEAT
TRANSFER STUDIES
Hu, Hui
Department of Aerospace Engineering
Iowa State University, Ames, IA 50011
Email: huhui@iastate.edu
ABSTRACT
We report recent progresses made in development of novel
molecule-based flow diagnostic techniques, named as
Molecular Tagging techniques, to achieve simultaneous
measurements of multiple important flow variables (such as
flow velocity and temperature) for micro-flows and micro-scale
heat transfer studies. Instead of using tiny particles, speciallydesigned phosphorescent molecules, which can be turned into
long-lasting glowing molecules upon excitation by photons of
appropriate wavelength, are used as tracers for both velocity
and temperature measurements. A pulsed laser is used to “tag”
the tracer molecules in the regions of interest, and the
movements of the tagged molecules are imaged at two
successive times within the photoluminescence lifetime of the
tracer molecules. The measured Lagrangian displacement of
the tagged molecules between the two image acquisitions
provides the estimate of the fluid velocity vector. The
simultaneous temperature measurement is achieved by taking
advantage of the temperature dependence of phosphorescence
lifetime, which is estimated from the intensity ratio of the
tagged molecules in the two images. The implementation and
application of the MTV&T technique are demonstrated by
conducting
simultaneous
velocity
and
temperature
measurements to qunatify the transient behavior of
electroosmotic flow (EOF) inside a microchannel and to reveal
the unsteady heat transfer, mass transfer and phase changing
process inside micro-sized water droplets pertinent to wind
turbine icing phenomena.
INTRODUCTION
The recent surge of microfluidic systems devices such as
such as micro-total-analysis systems (μ-TAS) and micro-heat
exchangers has created a need for the development of precision
micro-scale measurement techniques to monitor both
Koochesfahani, Manoochehr
Department of Mechanical Engineering
Michigan State University, East Lansing, MI 48824
Email: koochesf@egr.msu.edu
momentum and thermal transport. Presently, the most
commonly used tool for in-situ imaging of microflows is microParticle Imaging Velocimetry technique (μ-PIV) [1-2]. μ-PIV is
a particle-based technique that derives fluid velocity by
observing the motion of tracer particles seeded in the flow.
Several issues or implications may arise from the fact that
micro-PIV does not directly measure fluid motion but, rather
inferred it from the motion of tracer particles. μ-PIV
measurements may become “intrusive” when the size of tracer
particles is comparable with that of the measurement domain in
micro-scale thermal-fluid studies. The applications of particlebased flow diagnostic techniques can also be hampered by the
potential interaction of tracer particles with walls [3-4] or by
complications arising from the electrothermal, electrophoretic
and dielectrophorectic forces that act on tracer particles but do
not originate from working fluids [5-7]. For example, when μPIV is applied to an electroosmotic flow, the derived velocity is
actually the resultant velocity of electrophoresis of the charged
tracer particles and the electroosmosis of the bulk neutral fluid.
It is usually quite difficult and troublesome, if not impossible,
to precisely decouple electrophoresis velocity and
electroosmotic velocity, especially for cases with significant
temperature variations in electroosmotic flows due to Joule
heating. The implications of using particle-based flow
diagnostic techniques would become much more complicated
for micro-scale heat transfer studies, where the density of work
fluid usually changes with temperature while the density of
tracer particles usually does not. Using molecules as tracers in
such applications is expected to significantly mitigate, and
perhaps even eliminate, these issues.
Compared with particle-based techniques, molecule-based
methods are non-perturbative, and molecular tracers can be
seeded in-situ with considerably less interruption to fluid flows.
1
Copyright © 2009 by ASME
Several molecule-based techniques have been developed for
“in-channel” velocity measurement in microflows [8-14]. The
microscopic caged-dye imaging technique is one of the most
commonly used molecule-based velocimetries for microfluidic
studies [8-10]. Caged dyes are fluorescent dyes that have been
made non-fluorescent by binding chemical groups. The
“caging” groups can be removed by exposing the caged dye to
ultraviolet (UV) light, thus restoring it to its uncaged
(fluorescent) state. In a typical caged-dye imaging experiment,
a focused, spatially localized UV laser pulse is used to uncage
the dye in the regions of interest, thereby marking the fluid
flow. The flow velocity can be derived from the motion of the
uncaged fluorescent dye. However, it should be noted that the
all the water-soluble caged dyes currently available are
electrically charged when uncaged [13]. Therefore, in
electrokinetically-driven flows, the motion of the “charged tag”
can be monitored directly, but the motion of the neutral
working fluid cannot. The measurement results must be
corrected for the electrophorestic motion of the charged dye in
order to derive the electroosmotic velocity of the neutral
working fluid. In addition, the caged-dye imaging technique
may be ineffective at measuring polymeric microfluidics
because the dye can be adsorbed onto the polymeric walls,
which can dramatically affect the behavior of microflows. This
is especially true for electrokinetically driven flows [14]. The
progressive uncaging release of charged dye molecules can also
vary ion concentrations in working fluids, thereby, changing
the flow physics due to the diagnostics in electrokineticallydriven flows [15].
Several microscopic thermometry techniques have also
been developed recently for measuring “in-channel” fluid
temperatures in microfluidics. While NMR [16], Raman
spectroscopy [17], on-chip interferometric backscatter detecting
[18] and thermochromic liquid-crystal [19] techniques are
oftentimes used, the most popular procedure is the microscopic
Laser Induced Fluorescence (µ-LIF) technique [20-22]. µ-LIF
involves seeding working fluid with temperature-sensitive
fluorescent dyes (Rhodamine B is widely used [20-22]) and
then making fluid temperature measurements by detecting LIF
emission via microscopic imaging. Two-color µ-LIF technique
has been developed recently to study thermal transport at the
microscale for microfluidic systems in order to minimize the
effects of variations in the intensity of illumination source on
temperature measurements for better measurement accuracy
[23-24]. It should be noted that the temperature sensitivity of
most commonly-used fluorescence dyes is relatively low in
general. For example, the temperature sensitivity of Rhodamine
B, while highest among the fluorescent dyes, is about 2% per
degree Celsius [20-26] (i.e., the fluorescence intensity
decreases about 2% for every 1oC increase in temperature).
However, for many micro-scale thermal fluid studies, the
temperature differences in the measurement domain are usually
quite small (i.e. only a few degree Celsius [20-24]), it would be
quite challenging for achieving accurate measurements of the
small temperature differences due to the low temperature
sensitivity of fluorescent thermometry techniques. It would be
highly desirable to develop novel thermometry techniques that
can provide much higher temperature sensitivity compared to
that currently offered by fluorescent thermometry techniques
for accurately measuring the small temperature differences for
micro-scale heat transfer studies.
It should also be noted that all of the aforementioned
microscopic flow diagnostic techniques can measure only either
flow velocity or fluid temperature for micro-scale thermal-fluid
studies. None of those techniques is really capable of
simultaneously measuring flow velocity and fluid temperature.
For many challenging micro-scale thermal fluid problems such
as joule heating in electrokinetically-droven microfluidics,
simultaneous determination of flow velocity and fluid
temperature is needed in order to identify underlying physics to
improve our understanding about the micro-scale thermal fluid
phenomena. In the present study, we reported the recent
progress made in the development of novel molecule-based
flow diagnostic techniques, named as Molecular Tagging
techniques, for achieving simultaneous measurements of
multiple important flow variables (such as flow velocity and
temperature) to elucidate underlying physics to improve our
understanding about complicated micro-physical phenomena.
The work reported here can be considered as the
“microscopic version” of the Molecular Tagging Velocimetry
and Thermometry (MTV&T) technique developed by Hu &
Koochesfahani [27], which is capable of achieving
simultaneous measurements of flow velocity and temperature
distributions in “macro-flows”. Figure 1 shows an example of
typical MTV&T measurements. This example is taken from a
study of the effect of buoyancy force on the wake instability of
a heated cylinder in a water channel [27]. The measurement
region is tagged by multiple pulsed laser beams in a grid
pattern. The figure shows both the initially tagged regions and
their subsequent evolution after a time delay of 5.0ms, together
with the resultant simultaneous velocity and temperature
distributions derived from the image pair. Because the
instantaneous velocity and temperature distributions are
measured simultaneously, ensemble-averaged velocity and
temperature fields as well as the turbulent thermal flux vectors,
i.e., the correlation between the velocity and temperature
fluctuations, can also be derived from the measurements.
In the sections that follow, the technical basis of the
Molecular Tagging Velocimetry and Thermometry techniques
will be described briefly along with the related properties of the
phosphorescent tracer used for the Molecular Tagging
measurements. The implementation and applications of the
molecular tagging techniques will be demonstrated by
conducting simultaneous “in-channel” velocity and temperature
measurements to qunatify the transient behavior of an
electroosmotic flow (EOF) inside a microchannel and to reveal
the unsteady heat transfer and phase changing processes within
micro-sized, icing water droplets relevant wind turbine icing
phenomena.
2
Copyright © 2009 by ASME
a
c
b
heated
cylinder
0.03 m/s
O
d
Temperature ( C)
heated
cylinder
0.02 m . C /s
heated
cylinder turbulent thermal flux
O
e
0.03 m/s
2
2
Sqrt(u'T' +v'T' )
26.50
26.40
26.30
26.20
26.10
26.00
25.90
25.80
25.70
25.60
25.50
25.40
25.30
25.20
25.10
25.00
24.90
24.80
24.70
24.60
24.50
o
Temperature ( C)
25.80
25.70
25.60
25.50
25.40
25.30
25.20
25.10
25.00
24.90
24.80
24.70
24.60
24.50
24.40
24.30
24.20
0.20
0.19
0.18
0.17
0.16
0.15
0.14
0.13
0.12
0.11
0.10
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
Fig. 1: A typical MTV&T image pair and the resultant result (a) grid image at 0.5 ms after the laser excitation pulse; (b) 5.0 ms later;
(c) the simultaneous velocity and temperature fields derived from the images (a) and (b); (d) Ensemble-averaged velocity and
temperature fields; and (e) turbulent thermal flux vectors [27].
MOLECULAR TAGGING TECHNIQUES
A. MOLECULAR TAGGING VELOCIMETRY
Molecular Tagging Velocimetry (MTV) technique is a
“time-of-flight” method, which can be thought as the molecular
counterpart of the Particle Imaging Velocimetry (PIV)
technique for flow velocity field measurements. Compared with
PIV, MTV offers advantages in situations in which the use of
seed particles is either not desirable or may lead to
complications. For MTV measurements, specially-designed
tracer molecules (water soluble phosphorescent complex 1BrNp⋅Gβ-CD⋅ROH for the present study) are premixed in
working fluid (water for the present study) and a pulsed laser is
used to “tag” them in the regions of interest. Upon pulsed laser
excitation, the tagged tracer molecules emit long-lived
phosphorescence, becoming “glowing” marks that move with
the working fluid. Unlike that in PIV measurements which
usually require two pulses of laser illumination for the
acquisition of each PIV image pair, the phosphorescence
emission of the tagged molecules will be interrogated at two
successive times after the same laser excitation pulse in MTV
measurements. The Lagrangian displacement vectors of the
tagged molecules over the prescribed time delay between the
two interrogations provide estimate of flow velocity vectors.
Various advances in MTV technique in terms of available
molecular tracers, methods of tagging, detection/imaging and
data processing can be found in several review articles [28-32],
in addition to a special issue of Measurement Science and
Technology on this topic [33].
In the original work of Gendrich et al. [29], for each laser
pulse the MTV image pairs were acquired by a pair of aligned
image detectors viewing the same region in the flow. In the
present study, the two detectors are replaced by a single
intensified CCD camera (PCO DiCam-Pro) operating in dualframe mode, which allows the acquisition of two images of the
tagged regions with a programmable time delay between them.
The displacement of the tagged regions can be determined by a
direct digital spatial correlation technique [34]. Similar as the
procedure used for PIV image processing, a small window,
referred to as the source window, is selected from a tagged
region in the earlier image, and it is spatially correlated with a
larger roam window in the second image. A well-defined
correlation peak occurs at the location corresponding to the
displacement of the tagged region by the flow; the displacement
peak is located to sub-pixel accuracy using a multi-dimensional
polynomial fit [34].
3
Copyright © 2009 by ASME
For flow velocity measurement, MTV utilizes only the
information about the spatial distribution of the
photoluminescence of the tagged molecules within the regions
of interest to determine the displacement of the tagged tracer
molecules, therefore, the flow velocity. As described in the
following section, monitoring the phosphorescence intensity
decay rate (i.e. emission lifetime) of the tagged tracer
molecules within the tagged regions can provide information of
the fluid temperature distributions within those regions
simultaneously along with flow velocity information.
B. MOLECULAR TAGGING THERMOMETRY
It is well known that both fluorescence and
phosphorescence
are
molecular
photoluminescence
phenomena. Compared with fluorescence, which typically has a
lifetime on the of order nanoseconds, phosphorescence can last
as long as microseconds, even minutes. Since emission
intensity is a function of the temperature for some substances,
both fluorescence and phosphorescence of tracer molecules
may be used for temperature measurements. As described
above, Laser-induced fluorescence (LIF) techniques have been
widely used for flow temperature measurements [20-25].
Laser-induced phosphorescence (LIP) techniques have also
been suggested recently to conduct fluid temperature
measurements
[35-37].
Compared
with
fluorescent
thermometry techniques, the relatively long lifetime of LIP
could be used to prevent interference from scattered/reflected
light and any fluorescence from other substances (such as from
solid surfaces) that are present in the measurement area, by
simply putting a small time delay between the laser excitation
pulse and the starting time for phosphorescence image
acquisitions. Furthermore, LIP was found to be much more
sensitive to temperature variation compared with LIF [26-27,
35-37], which is favorable for the accurate measurements of
small temperature differences for micro-scale thermal fluid
transfer studies.
determined by the sum of all the deactivation rates:
τ −1 = k r + k nr ,
where kr and knr are the radiative and nonradiative rate constants, respectively. According to
photoluminescence kinetics [39], these rate constants are, in
general, temperature-dependant. The temperature dependence
of the phosphorescence lifetime is the basis of the present
lifetime-based MTT technique.
It should be noted that the absorption coefficient, ε , and
quantum yield, Φp, are also temperature dependent in general in
addition to phosphorescence lifetime,τ, resulting in a
temperature-dependent phosphorescence signal (S). Thus, in
principle, the collected phosphorescence signal (S) may be used
to measure fluid temperature if the incident laser intensity and
the concentration of the phosphorescent dye remain constant
(or are known) in the region of interest. It should be noted that
the collected phosphorescence signal (S) is also the function of
incident laser intensity (Ii) and the concentration of the
phosphorescent dye (C). Therefore, the spatial and temporal
variations of the incident laser intensity and the non-uniformity
of the phosphorescent dye (e.g. due to photobleaching) in the
region of interest would have to be corrected separately in order
to derive quantitative temperature data from the acquired
phosphorescence images. In practice, however, it is very
difficult, if not impossible, to ensure a non-varying incident
laser intensity distribution, especially for unsteady thermal
phenomena with a varying index of refraction. This may cause
significant error in the temperature measurements. To
overcome this problem, a lifetime-based thermometry [37] was
developed to eliminate the effects of incident laser intensity and
concentration of phosphorescent dye on temperature
measurements.
According to quantum theory [38], the intensity of a firstorder photoluminescence process (either fluorescence or
phosphorescence) decays exponentially. As described in
references [26-27] , for a diluted solution and unsaturated laser
excitation, the collected phosphorescence signal (S) by using a
gated imaging detector with integration starting at a delay time
to after the laser pulse and a gate period of δt can be given by
(
S = AI i Cε Φ p 1 − e
−δ t / τ
)e
− to / τ
Fig. 2: Timing chart of lifetime-based MTT technique
(1)
where A is a parameter representing the detection
collection efficiency, Ii is the local incident laser intensity, C is
the concentration of the phosphorescent dye (the tagged
molecular tracer), ε is the absorption coefficient, and Φp is the
phosphorescence quantum efficiency. The emission lifetime τ
refers to the time at which the intensity drops to 37% (i.e. 1/e)
of the initial intensity. For an excited state, the deactivation
process may involve both radiative and non-radiactive
pathways. The lifetime of the photoluminescence process, τ, is
The lifetime-based thermometry works as follows: As
illustrated in Fig. 2, laser-induced phosphorescence emission is
interrogated at two successive times after the same laser
excitation pulse. The first image is detected at the time t = t o
after laser excitation for a gate period δ t to accumulate the
phosphorescence intensity S1 , while the second image is
detected at the time t = t o + Δt for the same gate period to
accumulate the phosphorescence intensity S 2 . It is easily
4
Copyright © 2009 by ASME
shown [27, 37], using Equation (1), that the ratio of these two
phosphorescence signals (R) is given by
− Δt / τ
R = S 2 / S1 = e
(2) .
In other words, the intensity ratio of the two successive
phosphorescence images (R) is only a funtion of the
phosphorescence lifetime τ, and the time delay Δt between the
image pair, which is a controllable parameter. This ratiometric
approach eliminates the effects of any temporal and spatial
variations in the incident laser intensity and non-uniformity of
the dye concentration (e.g. due to bleaching). For a given
molecular tracer and fixed Δt value, Equation (2) defines a
unique relation between phosphorescence intensity ratio (R)
and fluid temperature T, which can be used for thermometry.
For a given molecular tracer, such as phosphorescent triplex
1-BrNp⋅Mβ-CD⋅ROH used in the present, and fixed Δt value,
Equation (2) can be used to calculate the phosphorescence
lifetime of the tagged molecules on a pixel-by-pixel basis,
which would resulting in a distribution of the phosphorescence
lifetime over a two-dimensional domain. Therefore, with a
calibration profile of phosphorescence lifetime vs. temperature
as the one shown in Fig. 3, a two-dimensional temperature
distribution can be derived from a phosphorescence image pair
acquired after the same excitation laser pulse.
7
TRACERS
The phosphorescent molecular tracer used in the present
study is phosphorescent triplex (1-BrNp⋅Mβ-CD⋅ROH). The
phosphorescent triplex (1-BrNp⋅Mβ-CD⋅ROH) is actually the
mixture compound of three different chemicals, which are
lumophore (indicated collectively by 1-BrNp), maltosyl-βcyclodextrin (indicated collectively by Mβ-CD) and alcohols
(indicated collectively by ROH). Further information about the
chemical and photoluminescence properties of the
phosphorescent triplex is available at [32, 40-41]. In the present
study, we used a concentration of 2×10−4 M for Mβ-CD, a
saturated (approximately 1×10−5 M) solution of 1-BrNp and a
concentration of 0.06 M for the alcohol (ROH), as suggested by
Gendrich et al. [29].
Upon the pulsed excitation of a UV laser (quadrupled
wavelength of Nd:YAG laser at 266nm for the present study),
the phosphorescence lifetime of the phosphorescent triplex (1BrNp⋅Mβ-CD⋅ROH) molecules in an aqueous solution change
significantly with temperature. Figure 3 shows the measured
phosphorescence lifetimes of 1-BrNp⋅Mβ-CD⋅ROH molecules
as a function of temperature. The data shown in Fig. 3 were
obtained though a calibration experiment. To acquire the
calibration data, the aqueous solution of 1-BrNp•Mβ-CD•ROH
was first heated to a pre-determined temperature level. After a
thermal steady state was established, the phosphorescence
images were acquired at two successive times after the same
laser excitation pulse by using a dual-frame CCD camera. The
phosphorescence lifetime at the pre-determined temperature
level was derived based on the Equation (2). The process was
repeated for different temperatures. Further information about
the calibration experimental setup and calibration procedure
can be found in [27, 37].
It can be seen clearly that phosphorescence lifetime of 1BrNp⋅Mβ-CD⋅ROH molecules varies significantly with
increasing temperature, decreasing from about 7.2ms to 0.4ms
as the temperature changes from 2.0oC to 50.0oC. The relative
temperature sensitivity of the phosphorescence lifetime is about
Phosphorescence Lifetime (ms)
C. PHOSPHORESCENT MOLECULAR
USED IN THE PRESENT STUDY
5.0% per ºC at 20ºC to 20.0% per ºC degree at 50ºC, which is
much higher than those of commonly-used fluorescent dyes.
For comparison, the temperature sensitivity of Rhodamine B
for LIF measurements is usually about 2.0% per degree Celsius
[20-26].
Curve fit
Experimental data
6
5
4
3
2
1
0
0
5
10
15
20
25
30
35
40
45
50
o
Temperature ( C)
Fig. 3: Variation of phosphorescence lifetime vs. temperature
To implement the lifetime-based thermometry technique
described above, only one laser pulse is required to excite or
“tag” the tracer molecules for each instantaneous temperature
field measurement. The two successive acquisitions of the
photoluminescence image of the excited or tagged tracer
molecules can be achieved using a dual-frame intensified CCD
camera. Compared to the two-color LIF thermometry
techniques [23-24] which usually require two CCD cameras
with proper optical filters to acquire two fluorescent images
simultaneously for each instantaneous temperature field
measurement, the present lifetime-based MTT technique is
easier to implement and can significantly reduce the burden on
the instrumentation and experimental setup. Forthermore, since
fluorescence emission is short lived with the emission lifetime
on the order of nanoseconds, LIF images are usually acquired
when the incident laser illumination is still on, therefore, LIF
images are vulnerable to the contaminations of
scattered/reflected light and any fluorescence from other
substances (such as from solid surfaces for surface water
droplet measurements). For the lifetime-based MTT technique
describe at here, as indicated schematically in Fig. 2, the small
time delay between the illumination laser pulse and the
phosphorescence image acquisition can effectively eliminate all
the effects of scattered/reflected light and any fluorescence
from other substances that are present in the measurement
region.
5
Copyright © 2009 by ASME
In summary, the Molecular Tagging Velocimetry and
Thermometry
technique
achieve
the
simultaneous
measurements of velocity and temperature in fluid flows by
using a pulsed laser to “tag” the specially-designed tracer
molecules in the regions of interest. The tagged molecules can
be turned into long-lived “glowing” tracers upon the pulsed
laser excitation. The movements of the tagged molecules are
interrogated at two successive times within the lifetime of the
photoluminescence. The measured Lagrangian displacement of
the tagged tracer molecules between the two interrogations
provides the estimate of the fluid velocity vector. The
simultaneous temperature measurement is achieved by taking
advantage of the temperature dependence of the
phosphorescence lifetime of the tagged tracer molecules, which
is estimated from the intensity ratio of the phosphorescence
image pairs.
It should be noted that the MTV&T technique, like most
measurement techniques, does not give information of fluid
flow at a ‘point’. Rather, it provides the spatially-averaged flow
velocity and temperature of a molecularly tagged region.
Similar to PIV, the effective spatial resolution of the
measurement is given by the sum of the source window size
and the measured Lagrangian displacement. Clearly, obtaining
spatially-resolved data for small-scale flow structures would
require tagging regions, and selecting interrogation windows,
consistent with the scales to be resolved. While the best spatial
resolution that can be achieved is set by the diffraction
limitations of optics used to generate the tagging pattern and
the resolution characteristics of image detection, the selection
of the source (interrogation) window often involves a choice
between the spatial resolutions of the measurement versus the
accuracy of the instantaneous measurement [27]. The temporal
resolution of the measurements is set by the time delay Δt
between the phosphorescence image pair. The choice of this
time delay influences the accuracy of the velocity data (larger
Δt leads to larger Lagrangian displacement of tagged tracer
molecules) and the temperature estimation through Equation
(2). Further discussions about the effects of these factors on the
flow velocity and temperature measurement accuracy by using
Molecular tagging techniques are avilable at [27, 34, 37].
APPLICATION EXAMPLES OF THE MOLECULAR
TAGGING TECHNIQUES
In this section, two application examples were presented to
demonstrate the feasibility and implementation of the molecular
tagging techniques described above to study complex and
challenging micro-scale thermal fluid problems. Molecular
tagging velocimetry and thermometry techniques were first
applied to achieve simultaneous measurements of “in-channel”
flow velocity and temperature distributions to qunatify the
transient behaviour of electroosmotic flow (EOF) inside a
microchannel upon the “switch-on” of an applied electric field.
The measurement results of the lifetime-based molecular MTT
technique to reveal the unsteady heat transfer, mass transfer and
phase changing process within convectively-cooled, micro-
sized water droplets will also be presented to elucidate
underlying physics to improve our understanding about
important micro-physical processes pertinent to wind turbine
icing phenomena.
A. QUALIFICATION OF AN ELECTROOSMOTIC
FLOW (EOF) IN TRANSIENT STATE
Fluid transport through microchannels plays a significant
role in a great number of emerging technologies such as micropower generation, chemical separation, cell analysis, and
biomedical diagnostics [42-43]. A considerable amount of
pressure difference may be required to drive fluid through a
channel of tens of micron meters in size by using conventional
pressure-driven technology. An alternative and efficient way of
moving fluid within microchannels is through electroosmosis.
Electroosmosis is the bulk movement of liquid relative to a
stationary charged surface due to externally applied electrical
field, which was first observed and reported about two
centuries ago [44]. Most solid substance will acquire a relative
electric charge (negative charge for fused-silica material such
as glass) when in contact with an aqueous electrolytic solution,
which, in turn, influences the charge distribution in the
solution. Ions of opposite charge (counter-ions) to that of
surface are attracted to the surface, and ions of the same charge
(co-ions) are repelled from the surface as illustrated in Fig. 4.
The net effect is the formation of a region close to the charged
surface called electrical double layer (EDL) in which there is
the an excess of counter-ions distributed in a diffusion manner.
The charge distribution in the solution, therefore, falls from its
maximum near the wall (which is usually termed as zeta
potential) to a zero charge in the fluid core. The thickness of the
EDL is characterized by Debye length, which is the wallnormal distance over which the net charge has decreased to 1/e
(37%) of the surface charge [45]. The thickness of EDL (i.e.
Debye length) can range from angstroms to nanometers
depending on the electrolyte solution. When an electric field is
applied parallel to the charged surface, the positively charged
cations and solvent molecules strongly absorbed at the wall will
remain stationary. However, the mobile cations in the EDL near
the surface wall will migrate toward the cathode due to the
excess charge in the layers. Through the action of viscous
forces, the core fluid will be pulled towards the cathode as well.
The resulting electroosmotic flow velocity, ueof, is given by the
well-known Smoluchowski equation ueof =εrεoζV/(µL) [46],
where μ is the liquid viscosity, εo the permittivity of vacuum, εr
the relative permittivity, ζ the zeta potential, V the applied
voltage, and L the length over which the voltage is applied.
(a). Ion distribution
(b). electric charge distribution (c). electroosmotic flow
Fig. 4: Schematic of electroosmotic flow
6
Copyright © 2009 by ASME
Temperature
o
( C)
Wall
40
150
3.0
2.5
Electroosmotic Velocity
100
35
50
Velocity
0
Temperature
-50
Velocity (mm/s)
Wall
300µm
Spatial Location (μm)
(a). 0.5 ms after
laser pulse
2.0
1.5
30
1.0
Temperature
0.5
25
0
-100
-0.5
Turn on electric field
-150
(b). 5 ms later
0
Temperature 30
o
( C)
1
2
3
4
5
32
34
36
38
40
20
-1.0
0
5
10
15
20
25
30
35
Time (seconds)
Velocity(mm/s)
(c). Instantaneous velocity
and temperature profiles
(d). dynamic response of “in-channel”
electroosmotic velocity and fluid temperature
Fig.5: Molecular tagging measurements in a transient electroosmotic flow [52].
Joule heating is the inherent by-product of the electric
work in electroosmotic flows. The heat is generated by ohmic
resistance of the electrolyte solution due to the passing
electrical current. From the microscopic viewpoint, the frequent
collision of migrated ions and solvent molecules convert some
of the kinetic energy done by the electric field into the heat.
This scenario is similar to the electrons moving through metal
atoms. This internal heat source not only elevates the absolute
fluid temperature but also generates temperature gradients in
the microchannels [20-22], the flow behavior is therefore
strongly affected. The effects of Joule heating can compromise
the performances of microfluidics or “lab-on-a-chip” devices
by increasing dispersion in electrokinetic separation and
inducing temperature sensitive chemical reactions [47-49].
Joule heating can also cause local liquid boiling in
microfluidics, sometimes even to the point of destroying
microchips [50]. As a consequence, Joule heating and microscale heat transfer in electrokinetically-driven microfluidics has
attracted much attention in recent years.
The inherent nature of the coupling of Joule heating to
electroosmotic flows requires simultaneous information on
electroosmotic velocity and fluid temperature in order to
elucidate the underlying physics and thus further our
understanding about Joule heating and micro-scale heat transfer
process. Although several advanced flow diagnostic techniques,
which include μ-PIV [7], microscopic caged-fluorescent-dye
imaging [13, 14], photobleach imaging[11-12], and µ-LIF [2022], have been developed for in-situ measurements in
microflows, none of these techniques is capable of
simultaneously measuring “in-channel” flow velocity and fluid
temperature in microflows. It should also be noted that,
although microscopic caged-dye imaging technique has been
used to measure “in-channel” electroosmotic velocity in
polymeric microfluidics [13, 14], the adoption of the caged dye
onto polymeric walls has been found to cause significant
measurement errors (up to 50% for PDMS microchips [14]).
While the μ-LIF technique with Rhodamine B as the
temperature-sensitive dye has been used to measure “inchannel” fluid temperature in polymeric microfluidics [20-22],
the absorption of Rhodamine B molecules onto the walls of the
polymeric microchannels may cause ambiguities to the
quantitative temperature measurements, and modify the surface
charge density of the polymeric microchannels [51]. A recent
study [52] has shown that the phosphorescent tracer molecules
of 1-BrNp⋅ Mβ-CD⋅ ROH used in the present study are
almost neutral in electric charge, which is highly desirable for
achieving flow velocity and temperature measurement in
electroosmotic flows. The phosphorescent tracer molecules of
1-BrNp⋅ Mβ-CD⋅ ROH were also found to be compatible
with polymeric material such as PDMS. Therefore, the
molecular tagging techniques described above can be used to
simultaneously measure “in-channel” electroosmotic flow
velocity and fluid temperature in polymeric microfluidics.
By using the molecular tagging velocimetry and
thermometry techniques described above, Lum et al. [53] have
recently achieved simultaneous measurements of “in-channel”
electroosmotic velocity and fluid temperature in an
electroosmotic flow inside a microchannel. Figure 5 shows
example results of the measurements in a transient
electroosmotic flow. The figure includes the initial position of
the tracer molecules in the electroosmotic flow tagged with a
focused laser beam and their subsequent position after a time
7
Copyright © 2009 by ASME
delay of 5.0 ms. The simultaneous electroosmotic velocity and
fluid temperature profiles across the 300 µm channel derived
from the image pair, as well as the dynamic responses of the
“in-channel” electroosmotic velocity and fluid temperature
before and after “switch-on” of the electric field (260V/cm), are
also shown in the figure. Further details about the
measurements in the electroosmotic flow to study the effects of
Joule heating are available at Ref. 52.
B. QUALIFICATION
OF
UNSTEADY
HEAT
TRANSFER, MASS TRANSFER AND PHASE
CHANGING PROCESS WITHIN MICRO-SIZED
WATER DROPLETS
Wind energy is one of the cleanest renewable power
sources in the world today. U.S. Department of Energy has
challenged the nation to produce 20% of its total power from
wind by 2030. It has been found that the majority of wind
energy potential available in U.S. is in the northern states such
as North Dakota, Kansas, South Dakota, Montana, Nebraska,
Wyoming, Minnesota, and Iowa, where wind turbines are
subjected to the problems caused by cold climate conditions.
Wind turbine icing represents the most significant threat to the
integrity of wind turbines in cold weather. It has been found
that wind turbine icing would cause a variety of problems to the
safe and efficient operations of wind turbines. Ice accretion on
turbine blades was found to reduce the aerodynamic efficiency
of wind turbines considerably, which results in wind turbine
power production reduction. It has also been found that the
operation of a wind turbine with an imbalance caused by ice
accretion would experience an increase in the loads imposed on
all turbine components, which would shorten the lifetime for
wind turbine components. Uncontrolled shedding of large ice
chunks from turbine blades was also found to be of special
danger to service personnel as well as nearby residents,
particularly when the wind power plant site borders public
roads, housing, power lines, and shipping routes. In addition,
Icing was found to affect tower structures by increasing
stresses, due to increased loads from ice accretion. This would
lead to structural failures, especially when coupled to strong
wind loads. Ice accretion was also found to affect the reliability
of anemometers, thereby, leading to inaccurate wind speed
measurements and resulting in resource estimation errors.
spatially-and-temporally-resolved temperature distributions
within micro-sized, convectively-cooled water droplets to
quantify the unsteady heat transfer, mass transfer and phase
changing process within the small water droplets to improve
our understanding about the underlying physics pertinent to
wind turbine icing phenomena for wind turbine icing
mitigation.
Figure 6 shows the schematic of the experimental setup
used to implement the lifetime-based MTT technique to
quantify unsteady heat transfer and phase changing process
within small icing water droplets. A syringe was used to
generate micro-sized water droplets (about 800μm in radius and
500μm in height) to impinge on a test plate to simulate the
processes of small water droplets impinging onto a wind
turbine blade. The temperature of the test plate, which was
monitored by using a thermocouple, was kept constant at a preselected low temperature level by using a Water Bath
Circulator (Neslab RTE-211). The small water droplets with
initial temperature of 20.5 oC (room temperature) would be
convectively cooled after they impinged onto the cold test
plate. Phase changing process would occur inside the small
water droplets when the temperature of the test plate was below
frozen. A laser sheet (~200μm in thickness) from a pulsed
Nd:YAG at a quadrupled wavelength of 266 nm was used to
tag the premixed 1-BrNp⋅Mβ-CD⋅ROH molecules along the
middle plane of the small water droplets. A 12-bit gated
intensified CCD camera (PCO DiCam-Pro, Cooke Corporation)
with a fast decay phosphor (P46) was used to capture the
phosphorescence emission. A 10X microscopic objective
(Mitsutoyo infinity-corrected, NA= 0.28, depth of field =
3.5μm) was mounted in the front of the camera. The camera
was operated in the dual-frame mode, where two full frame
images of phosphorescence were acquired in a quick succession
after the same laser excitation pulse. The camera and the
pulsed Nd:YAG lasers were connected to a workstation via a
digital delay generator (BNC 555 Digital Delay-Pulse
Generator), which controlled the timing of the laser
illumination and the image acquisition.
Advancing the technology for safe and efficient wind
turbine operation in atmospheric icing conditions requires a
better understanding of the important micro-physical processes
pertinent to wind turbine icing phenomena. In order to
elucidate underlying physics, advanced experimental
techniques capable of providing accurate measurements to
quantify important ice formation and accreting process, such as
the unsteady heat transfer and phase changing processes inside
small icing water droplets, are highly desirable. Recently, by
using the lifetime-based MTT technique described above, an
experimental study was conducted to achieve simultaneous
measurements of droplet size (in terms of volume, height,
contact area and contact angle of the water droplet) and
Fig. 6: Experimental setup for MTT measurements
8
Copyright © 2009 by ASME
Temperature
(oC)
600
8.0
Y (μm)
.0
8.0
101
.40.02
0
16.0
8.0
.0
8.0
200
0
Aluminum test plate
10
0
0.
.20
14
16
.0
.0
.0
12
18
400
10.0
~ 500μm
2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0
(c).
8.0
b)
a).
Aluminum test plate
Test plate (T= 2.0 oC)
-800
-600
-400
-200
0
200
400
600
80
X (μm)
Fig. 7: A typical MTT measurement: a).The 1st phosphorescence image; b).The 2nd phosphorescence image; c). The temperature
distribution derived from the image pair
Based on a time sequence of the measured transient
temperature distributions within the water droplet as the one
shown in Fig. 7, the unsteady heat transfer process within the
convectively-cooled water droplets was revealed quantitatively.
Fig. 8 shows the spatially-averaged temperature of the water
droplet as a function of the time after it impinged on the cold
test plate, which was calculated based on the time sequence of
measured instantaneous temperature distributions. The
characteristics of the unsteady heat transfer within the water
droplet in the course of convectively cooling process were
revealed quantitatively from the evolution of the spatiallyaveraged temperature of the water droplet. Since initial
temperature of the water droplet (20.5 oC) was significantly
higher than that of the cold test plate (TW = 2.0 oC), the
temperature of the water droplet was found to decrease rapidly
after it impinged on the test plate. The temperature
measurement results given in Fig. 8 also revealed that a thermal
steady state would be reached at about 140 seconds later after
the water droplet was impinged on the cold test plate. The
spatially-averaged temperature of the water droplet would not
decrease anymore when the thermal steady state was reached.
The spatially-averaged temperature of the water droplet was
found to be about 6.0 oC after the thermal steady state was
reached.
10
8
o
Temperature ( C)
Figure 7 shows a typical pair of acquired phosphorescence
images for MTT measurements and the instantaneous
temperature distribution inside the water droplet derived from
the phosphorescence image pair. The image pair was taken at
60.0s later after the water droplet (initial temperature 20.5 oC)
impinged on the cold test plate (Tw =2.0 oC). The first image
(Fig. 7a) was acquired at 0.5 ms after the laser excitation pulse
and the second image (Fig. 7b) at 3.5 ms after the same laser
pulse with the same exposure time of 1.5 ms for the two image
acquisitions. Since the time delays between the laser excitation
pulse and the phosphorescence image acquisitions can
eliminate scattered/reflected light and any fluorescence from
other substances (such as from solid surface) in the
measurement region effectively, the phosphorescence images of
the water droplet are quite “clean” even though no optical filter
was used for the phosphorescence image acquisition. As
described above, Equation (2) can be used to calculate the
phosphorescence lifetime of the tagged molecules on a pixelby-pixel basis, which resulting in a distribution of the
phosphorescence lifetime over a two-dimensional domain. With
the calibration profile of phosphorescence lifetime vs.
temperature as shown in Fig. 3, a two-dimensional,
instantaneous temperature distribution within the water droplet
can be derived from the phosphorescence image pair, which
was shown in Fig. 7(c).
6
4
Curve fitting
experimental data
2
0
60
80
100
120
140
160
Time after the droplet impinged onto the test plate (s)
Fig. 8: Spatially-averaged temperature vs. time
In additional to the transient temperature distribution
measurements within the small water droplet, the size (in terms
of volume, height, contact area and contact angle) of the water
droplet on the test plate can be also determined simultaneously
from the acquired phosphorescence images with a predetermined scale ratio between the image plane and objective
plane. Therefore, the characteristics of unsteady mass process
of the water droplet on the test plate due to evaporation can also
be quantified from the MTT measurement results.
Figure 9 shows schematically the definitions of water
droplet height, H, contact radius, R, contact angle, θ, and
volume, V. As shown in shown in Fig. 4, the edge of the water
droplet can be easily detected based on the significant intensity
changes in the acquired phosphorescence images. Thus, the
contact radius, R, and droplet height, H, can be measured
directly form the acquired phosphorescence images with a predetermined scale ratio between the image plane and objective
plane. The contact angle, θ, and volume, V, were determined
by using the following equations based on the fact that a
surface droplet is normally a sphere cap.
9
Copyright © 2009 by ASME
Water
droplet
R
Solid
surface
Fig. 9: Schematic of a surface water droplet
H
θ = 2 tan −1 ( )
R
πR 3 (1 − cos θ ) 2 (2 + cos θ )
3 sin 3 θ
V/Vo , H/Ho
V =
(6)
(7)
1.2
70
1.0
65
0.8
60
0.6
55
Droplet volume, V
Droplet heigth, H
Contact angle, θ
0.4
It is well known that there are two modes for the
evaporation of a liquid droplet on a solid surface [54-56]; one
consists of a constant contact angle with diminishing contact
area and the other one of a constant contact area with
diminishing contact angle. The phosphorescence image pair
given in the Figure 7 visualized clearly that the contact angle of
the water droplet on the aluminum test plate was less than 90o.
The radius of the contact area of the water droplet on the test
plate was found be almost constant during the experiments,
which indicates that the evaporation process of the water
droplet followed the constant contact-area mode. Figure 10
shows the measured droplet size (in the terms of volume and
50
0.2
0
60
Contact angle, θ (deg.)
V
H
height and contact angle) of the water droplet as a function of
time after it impinged on the test plate. It is can be seen clearly
that the droplet height, volume and contact angle of the water
droplet decreased linearly with the time due the evaporation.
The results were found to agree with the findings of Birdi et al.
[56], who suggested that, for a liquid drops on a low-surfacetension solid, the evaporation rate would be linear and follows
the constant contact-area mode if the initial contact angle of the
droplet is less than 90o.
45
80
100
120
140
40
160
Time (second)
Fig. 10: Evolution of volume, height and contact angle of
the water droplet vs. time.
~500μm Test plate, T=-2.5OC
a. t =1.0s
Test plate, T=-2.5OC
b. t =5.0s
Test plate, T=-2.5OC
c. t =10.0s
Liquid Water Solid Ice Test plate, T=-2.5OC
Test plate, T=-2.5OC
d. t =15.0s
e. t =20.0s
Test plate, T=-2.5OC
f. t =30.0s
Fig. 11. The evolution of the phase changing process within a small icing water droplet.
10
Copyright © 2009 by ASME
When the temperature of the test plate was adjusted to
below frozen temperature, water droplets on the test plate was
found to be frozen and turned to ice crystals. Fig. 11 shows the
time sequence of the acquired phosphorescence images of a
water droplet when it impinged onto the test plate below frozen
temperature (TW = -2.5°C). The transient behavior of the phase
changing process within the small icing water droplet was
revealed clearly from the acquired phosphorescence images. In
the images, the “brighter” region in the upper portion of the
droplet represents liquid phase - water; while the “darker”
region at the bottom indicates solid phase - ice. It can be seen
clearly that the water droplet was round, as a cap of a sphere at
the beginning. As the time goes by, the interface between the
liquid phase water and solid phase ice was found to rise upward
continuously, as it is expected. As a result, the droplet was
found to grow upward with more and more liquid phase water
turning into solid phase ice. Eventually, the spherical-capshaped water droplet was found to turn into be a puddle-shaped
ice crystal.
70
60
Experimental data
Exponential curve fit
Time (s)
50
40
30
20
10
-6
-5
-4
-3
-2
-1
0
0
Surface Temperature of Test Plate ( C)
Fig. 12: The required frozen time vs. the test plate temperature
The required frozen time, which is defined as the time
interval between the moment when a water droplet impinged on
the cold test plate and the moment when the water droplet was
turned into an ice crystal completely, can be determined based
on the time sequence of the acquired phosphoresce images.
Fig. 12 shows the variations of the required frozen time of the
water droplets with the surface temperature of the test plate
changed from -1.0 °C to -5.0 °C. As it is expected, the required
frozen time for the water droplets (initial temperature at
20.5°C) turning into ice crystal was found to strongly depend
on the temperature of the test plate. The required frozen time
was found to decrease exponentially with the decreasing
surface temperature of the test plate.
Based on the measurement results as those shown in Fig. 7
to Fig. 12, important micro-physical phenomena pertinent to ice
formation and accretion process as water droplets impinging on
wind turbine blades were revealed quantitatively. Such
measurements are highly desirable to improve our
understanding about the important micro-physical processes
pertinent to wind turbine icing phenomena in order to explore
effective and robust anti-/de-icing strategies tailored for wind
turbine icing mitigation to ensure safer and more efficient
operation of wind turbines in cold weather. Further information
and analysis about using the lifetime-based MTT technique to
quantify the unsteady heat transfer, mass transfer and phase
changing process within micro-sized, icing water droplets are
available at reference [57 – 59].
CONCLUSIONS
In the present study, recent progress made in development
of novel molecule-based flow diagnostic techniques, named as
Molecular Tagging Velocimetry (MTV) and lifetime-based
Molecular Tagging Thermometry (MTT) techniques, for the
simultaneous measurements of multiple important flow
variables (such as flow velocity and temperature) for microscale thermal flow studies were presented. MTV technique can
be thought as the molecular counterpart of the PIV technique
for flow velocity field measurements. Compared with PIV,
MTV offers advantages in situations in which the use of seed
particles is either not desirable or may lead to complications.
For Molecular tagging measurements, specially-designed
tracer molecules (water soluble phosphorescent complex 1BrNp⋅Gβ-CD⋅ROH for the present study) are premixed in
working fluid (water for the present study) and a pulsed laser is
used to “tag” them in the regions of interest. Upon pulsed laser
excitation, the tagged tracer molecules emit long-lived
phosphorescence, becoming “glowing” marks that move with
the working fluid. The phosphorescence emission of the tagged
molecules will be interrogated at two successive times after the
same laser excitation pulse. The Lagrangian displacement
vectors of the tagged molecules over the prescribed time delay
between the two interrogations provide estimate of flow
velocity vectors. The simultaneous temperature measurement is
achieved by taking advantage of the temperature dependence of
phosphorescence lifetime, which is estimated from the intensity
ratio of the tagged molecules in the two images.
Two application examples were presented to demonstrate
the feasibility and implementation of the molecular tagging
techniques to study complex and challenging micro-scale
thermal flow phenomena. Molecular Tagging Velocimetry and
Thermometry techniques were applied to achieve simultaneous
measurements of “in-channel” flow velocity and temperature
distributions to qunatify the transient behaviour of
electroosmotic flow (EOF) inside a microchannel upon the
“switch-on” of an applied electric field. Lifetime-based
molecular tagging thermometry (MTT) technique was used to
achieve simultaneous measurements of droplet size (in terms of
volume, height, contact area and the contact angle of the water
droplet) and temporally-and-spatially-resolved temperature
distributions within micro-sized water droplets to reveal the
unsteady heat transfer, mass transfer and phase changing
process within micro-sized, water droplets pertinent to wind
turbine icing phenomena. Such measurements are highly
desirable to improve our understanding about the important
micro-physical processes.
11
Copyright © 2009 by ASME
ACKNOWLEDGMENTS
The authors want to thank Dr. Chee Lum of Michigan
State University and Dr. Zheyan Jin of Iowa State University
for their help in conducting the experiments. The support of
National Science Foundation under award number of CHE0209898, DMR-9809688 and CTS-0545918 is gratefully
acknowledged.
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