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Pulsed 2-Dimensional Raman Spectroscopy on Microdroplets
By Helge Moritz and Gustav Schweiger*
In this study, we used a technique based on pulsed Raman spectroscopy to determine the average concentration in the liquid
phase and concentration distribution in the gas phase surrounding desorbing linear arrays of binary microdroplets. For this study,
monodisperse microdroplets consisting of acetylene dissolved in acetone were generated by a modified vibrating orifice aerosol
generator (VOAG). The 2-dimensional concentration fields of acetylene desorbed from the droplets were determined with a
spatial resolutionof about 40 mmboth alongthe direction ofthe droplet chain andperpendicular to it. Thedetection limitsfor the
gas and liquid phase concentrations were about 100 ppm and 1 mmol/l, respectively.
1
Introduction
uniform droplets having controllable droplet size, spacing, and
velocity.Thistechniquehasbeenusedforhigh-precisionoptical
studies,i.e.,Devarakondaetal.[12],RayandNandakumar[13],
Lin et al. [14], Eversole et al. [15], and Tzeng et al. [16].
Newly developed techniques allow the simultaneous
measurement of the gas and liquid phase as shown by Vehring
et al. [17]. They studied fast transport processes, such as
For a detailed experimental investigation of mass transfer in
aerosols or sprays, it is essential to determine the concentra-
tion of the individual chemical components in the vapor and
the liquid phase. Because of the small mass of the particles and
the transient nature of many processes, optical measurement
methods are recommendable for in situ studies of micro-
particle systems. Raman spectroscopy on optically levitated
microdroplets were first analyzed by Thurn and Kiefer [1].
The characterization of laboratory-generated microparticles
by Raman spectroscopy up to 1990 was reviewed by Schweiger
absorption of SO
microdroplets generated with a VOAG. Profiles of gaseous
CO near the droplets could be measured with a spatial
2 2
in and desorption of CO from aqueous
2
resolution of 50 mm.
For a detailed experimental investigation of heat and mass
transport, the determination of the gas composition as close to
the surface of the particles is desirable. However, due to slight
random changes in the droplet position, no measurements can
be made closer to the surface of the droplet than approxi-
mately 100 mm. To overcome this limitation of the spatial
resolution, we employed pulsed 2-dimensional Raman spec-
troscopy in this investigation.
[
2]. Raman studies of gas/liquid and gas/solid aerosol chemical
reactions were reported by Davies [3] and Tang and Fung [4].
The intensity of scattered light from microparticles depends
linearly on the local field, the scattering volume, and the
molecular concentration as long as stimulated and nonlinear
processes have not to be considered. However, Raman
scattering on microparticles depends in a complicated way on
the position of the scattering molecule within the particle, the
size, and index of refraction. Despite the complicated relation
betweentheangularRamanscatteringof individualmolecules
and their position in the particle, the angular averaged total
Ramanintensityforhomogeneousparticlesissimplyafunction
of the particle volume, Schweiger [5], and Vehring and
Schweiger [6]. If MDRs (morphology-dependent resonances)
are excited, Raman scattering from the volume covered by the
resonant mode is enhanced. The mode volume is more or less
close to the surface, depending on the mode order. For
inhomogeneous particles the ratio between Raman scattering
on- and off-resonancemay thereforebe affectedbyconcentra-
tiongradientsintheparticle.Thiswasshownexperimentallyby
Moritz and Schweiger [7], and Lin and Campillo [8] and
analyzed theoretically by Lange and Schweiger [9].
Various theoretical models exist to describe heat and mass
transfer processes from microdroplets. Some authors [18,19]
2
use the ªd -lawº [20] or related models. Most of the models
were developed for isolated single particles. These models are
of limited benefit for the investigation of transport processes
in droplet chains. The interdroplet separation distance is so
small that droplet-droplet interactions become significant.
Based on a point source model (PSM), Ray and Davis [18]
have taken these effects into account and developed simple
algebraic expressions for steady-state evaporation of finite as
well as uniformly distributed infinite droplet arrays. Ume-
mura [21] has reviewed the theoretical models available for
predicting the degree of interaction in quasi-steady-state
processes.
The degree of interactions in a transfer process (momen-
tum, heat or mass transfer) can be defined in terms of an
interaction parameter which is the ratio between the rate of a
transfer process from a droplet in an array to that from a single
isolateddroplet. For mass transferthe interaction parameter is
Measurements on highly monodisperse droplet chains are
possible with the vibrating orifice aerosol generator (VOAG)
developed by Berglund and Liu [10], which has been stabilized
by Lin et al. [11]. The VOAG uses a piezoelectric (PZT) crystal
to oscillate an orifice, breaking a liquid jet into a chain of
.
ꢀ
ˆ m_ array m_ iso
[
*] Dr.-Ing. Dipl.-Phys. H. Moritz, Prof. Dr. techn. G. Schweiger, Ruhr-
Universität Bochum, Maschinenbau, Laseranwendungstechnik und
Meûsysteme, Geb. IB 2/126, 44780 Bochum, Germany; e-mail: schwei-
ger@lat2.lat.ruhr-uni-bochum.de
For linear droplet arrays generated by a VOAG the
interaction parameter depends only on the nondimensional
spacing between the droplets which is defined as e = l/a, where
Chem. Eng. Technol. 23 (2000) 4,
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a is the size and l the center-center spacing between successive 2.2 Gated Detection
droplets. Annamalai and Ryan [22] have reviewed the
experimental and theoretical advances made in the study of
interaction effects on droplet dynamics, heat and mass
transfer, ignition and combustion of droplet arrays. Silverman
and Dunn-Rankin [23] have studied the effect of interdroplet
separation distance on vaporization and combustion rates of
methanol and hexane droplets. Devarakonda [24] and
Devarakonda and Ray [25] have studied the evaporation of
single (ethanol + methanol) and multicomponent (etha-
nol + methanol, dibutyl pthalate + freon) droplets. From
the observed evaporation rates, they found that the inter-
droplet interactions are significant (h ~ 0.3) for e < 7.0and are
negligible for e > 20.
To increase the spatial resolution, an image intensifier
PCO, Model IRO) was mounted in front of the CCD camera
Fig. 1) and gated synchronously with the droplet generator as
(
(
described above. A gate pulse length of 1 ms was used for
appropriate space resolution. The delay time (0±25 ms)
between the gate pulse and the trigger pulse from the droplet
generator was varied in steps of 1 ms. This made it possible to
shift the observation region relative to the droplet position. A
shift of the delay time by 1 ms corresponded to a shift of the
observation region by 24 mm relative to the droplet position.
The gas composition could be analyzed between the droplets
at different locations by this technique.
2
Experimental
2
.1 Experimental Setup
A schematic of the experimental setup is shown in Fig. 1.
The components of the experimental setup were described
earlier in detail [17]. A linear stream of monodisperse
microdroplets is generated by a vibrating orifice aerosol
generator [10], VOAG, which has been stabilized according to
1/3
Lin et al. [11]. The droplet size is given by d = (6Q/pf) , where
f is the frequency of the square wave from a synthesizer
(HP 3325 B) applying to the PZT crystal with an amplitude of
1
0 V. The volumetric rate, Q, is monitored by continuously
weighting the liquid reservoir (Sartorius, MC1, LC 3200D). It
is kept constant by pressuring the liquid from a 50 l ballast gas
tank. With a backing pressure of 1.5 bar, an orifice diameter of
Figure 1. Sketch of the experimental setup.
The total observation time at each step was 10 s. Typical
results are shown in Fig. 2, which shows a sequence of Raman
spectra. Spectra with high intensities indicate the presence of a
droplet. Between the droplets only the acetylene signal of the
2
0 mm and a frequency of 43.9 kHz one gets droplets with a
diameter of 62 mm and a droplet spacing of about 362 mm.
The droplets consist of acetylene dissolved in acetone with a
concentration of 0.7 mol/l. These droplets are injected with an
initial velocity of 10 m/s in an air atmosphere of 1 bar. The
droplets were observed 5 mm below the generator exit. This
corresponds to a droplet residence time of 0.39 ms.
±1
gas phase (1973 cm ) is visible. The vibrational Raman line of
±1
nitrogen of the surrounding air is visible at 2331 cm . From
these measurements the concentration of the evaporated
acetylene can be determined.
+
The droplet chain wasilluminated bya cwAr laser (Spectra
Physics, 2030) operating in single mode (l = 514.5 nm,
P = 5 W). Plasma emission from laser was filtered by a
combination of prism and field stop. The polarization of the
laser beam is set to be perpendicular to the scattering plane. A
3
5 mm focal length achromat focussed the laser beam to a
waist diameter of about 30 mm onto the droplet chain. The
light collecting optics was arranged under 90ꢀ to the droplet
chain and the direction of laser beam. A high-quality, low-f-
number objective imaged the scattered light onto the entrance
slit (600 mm) of a double monochromator (Spex 1403). This
instrument was equipped with two 600 groves/mm gratings
and a holographic band-pass filter (POC, RHE 514.1D) to
reject Mie-scattering. Images and spectra from the CCD
camera (Wright Instr. 1) were processed with a PC-based
imaging software (Wright Instr., AT1) and software devel-
oped in-house.
Figure 2. Raman spectra of the acetone/acetylene-droplet chain at various axial
positions. Spectra with high intensities indicate the presence of a droplet.
±1
Between the droplets only acetylene signal of the gas phase (1973 cm ) is
±1
visible. At 2331 cm lies the line of the air nitrogen.
3
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3
Results and Discussion
volume (vertical band over the full height of the image). It is
much more intense than the signal of the gas-phase acetylene.
Therefore, the nitrogen signal was used as an internal
reference to determine the concentration of the acetylene
vapor concentration as a function of distance perpendicular to
the droplet chain axis.
3
.1 2-D Spectroscopy
The concept of 2-D Raman spectroscopy [17,26] is
illustrated in Fig. 3. As mentioned earlier, a laser beam is
passed perpendicular to the droplet chain. The laser beam is
imaged onto the entrance slit of the spectrograph in such a way
that the laser axis is parallel to the slit height. With this optical
setup, the diameter of the laser beam and the height of the
entrance slit define the observation volume. The CCD image
recorded in the exit plain of the spectrograph contains spectral
and local information. In the exit plain of the spectrograph the
light from the observation volume is dispersed spectro-
scopically in the horizontal direction, whereas the spatial
information is preserved in the vertical direction. The
horizontal spectral information allows the identification of
the chemical components in the observation volume. The
vertical direction gives information about the spatial distribu-
tion of a species in the observation volume, i.e., the direction of
the laser beam. The acetylene concentration perpendicular to
the droplet chain axis was determined by reading out the CCD
camera without binning. This is shown in Fig. 3. The
horizontal direction is the direction of spectroscopic disper-
sion.
Figure 4. Grey scale representation of a Raman image at a position between the
droplets. The horizontal direction is the direction of spectroscopic dispersion as
indicated with the spectrum below. The vertical direction is parallel to the
direction of the laser beam and gives information about the spatial distribution
of the species in the scattering volume.
3
.2 Data Analysis
In the horizontal direction, the data corresponding to the
-dimensional images were evaluated line by line. Each line
2
represents a spectrum at a specific position in the droplet
chain. The spatial resolution is defined by the height of the
pixel of the CCD camera and the magnification of the imaging
system (5.3 mm in this case). The spatial resolution perpendi-
cular to the laser beam is defined by the diameter of the laser
beam and was approximately 37 mm. For a quantitative
evaluation of the obtained spectra, the line intensities of
single-component spectra (acetylene (g), acetylene (l),
acetone and nitrogen) were fit to the measurements. Normal-
ization of the acetylene signal to the nitrogen signal leads to
the absolute concentration distribution of the acetylene in the
gas phase at a specific position between the droplets.
Figure 3. Sketch of the 2-dimensional spectroscopy.
A grey scale representation of a Raman image at a specific
position between the droplets is given in Fig. 4. The length of
the vertical side of the image defines the length of the
scattering volume. Each line of the image corresponds to the
concentration distribution along the laser beam of a specific
chemical component in the observation volume at one specific
position. The Raman signal of the liquid phase (dissolved
acetylene and acetone) appears only as a relatively small band
in the center of the image. This band corresponds to the region
where the droplets crossed the laser beam. Fig. 4 shows that
the nitrogen signal is present throughout the scattering
3.3 Data Interpretation
Measurements were made at 23 equidistant axial positions
in steps along the direction of the droplet chain. The spatial
resolution for each step was increased by the particle
movement to about 20 mm. The evaluation procedure
described above was used to determine the gas-phase
Chem. Eng. Technol. 23 (2000) 4,
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concentrations at each position with a spatial resolution of
intensity is proportional to the number of molecules in the
observation volume. If part of the observation volume is
occupied by the liquid droplet, the number of gas molecules in
the scattering volume is reduced and a lower intensity is
measured although the concentration of the gas phase has not
changed. Fig. 6, therefore, represents the mole fraction of
liquid acetone and acetylene, respectively, within the scatter-
ing volume.
»
40 mm. From this, the 2-dimensional concentration distribu-
tion shown in Fig. 5 were constructed. The position of the
droplets is indicated by the grey ellipse. White areas in the
vicinity of the droplets indicate the region where concentra-
tion measurements were impossible (droplet influence zone).
In these regions, no gas concentrations could be measured,
due to fluctuations in the droplet position. The droplet
propagation in this representation is from top to bottom.
The direction of the laser beam is from left to right. The
apparent high acetylene concentration at the right side of the
droplets of Fig. 5 is caused by distortion of the laser beam by
the droplets.
Figure 5. A grey scale representation of the 2-dimensional concentration field of
the released acetylene between the droplets in an acetone/acetylene-droplet
chain. The presence of the droplets is indicated by the white areas where no gas
exists. The droplet propagation in this representation is from top to the bottom.
The direction of the exciting laser beam is from left to right side.
Figure 6. The concentrations of the particle phase in arbitary units vs. the
distances from the droplet chain and in direction of the droplet propagation: the
concentrations of the released acetylene (left) and the acetone (right).
It can be seen that the concentration of gaseous acetylene
decreases with increasing distance from the droplet chain axis,
as expected. Furthermore, the highest acetylene volume 4 Summary and Conclusions
concentration of approximately one per thousand is in the
wake behind the droplet. The acetylene concentration seems
to decrease near the droplet influence position. This is an
artifact caused by the fitting procedure and the contour plot
calculation.
It has been shown that the pulsed 2-dimensional Raman
spectroscopy can be applied for the determination of average
concentration in the liquid phase and concentration profiles in
the gas phase surrounding fast-moving linear streams of
microdroplets. This technique was applied on the binary
system acetylene dissolved in acetone. The 2-dimensional
concentration field of the desorbed acetylene could be
determined with a spatial resolution of approximately 40 mm
in direction of the droplet propagation and perpendicular to
the droplet chain. The time resolution was about 1 ms. The
detection limit of the pulsed 2-dimensional Raman spectros-
copy for gaseous components in these experiments was about
100 ppm. The detection limit for the determination of average
concentration in the liquid phase was about 1 mmol/l.
The pulsed 2-dimensional Raman spectroscopy yields not
only the gas phase concentration but also information on the
liquid phase. This can be used for testing the measurements
because only at the position of the particles the liquid phase
should be visible. In fact, Raman signals of the liquid acetylene
and acetone are only visible at the particle position. The data
shown in Fig. 6 represents the concentration of liquid of the
specific component in arbitrary units within the scattering
volume. The decrease in signal intensity at the boundary of the
droplet has the following reason. The Raman-scattered
3
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[
[
11] Lin, H.-B.; Eversole, J. D.; Campillo, A. J., Rev. Sci. Instrum. 61 (1990) pp.
Acknowledgment
1018±1023
12] Devarakonda, V.; Ray, A. K.; Kaiser, T.; Schweiger, G., Aerosol Sci.
Technol. 28 (1998) pp. 531±547.
The authors wish to thank Dr. Thomas Kaiser for his help
with the evaluation of the data and Dr. Reinhard Vehring for
fruitful and exciting discussions. Financial support was given
by the Deutsche Forschungsgemeinschaft (DFG) (Aktenzei-
chen: SCHW 184/19-1).
[
[
13] Ray, A. K.; Nandakumar, R., Appl. Opt. 34 (1995) pp. 7759±7770.
14] Lin, H. B.; Eversole, J. D.; Campillo, A. J., Opt. Commun. 77 (1990) pp.
407±410.
[
[
[
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K., J. Opt.Soc. B 10 (1993) pp. 1955±1968.
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273±275.
Received: July 1, 1999 [CET 1137]
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[20] Law, C. K.; Law, H. K., AIAA J. 20 (1982) pp. 522±527.
[21] Umemura, A. A., Prog. Energy Combust. Sci. 20 (1994) pp. 325±372.
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