Simultaneous Detection of Images and Raman Spectra of Colliding

May 7, 2014 - Processes involved between colliding droplets were investigated using simultaneous analysis of spectra and images of Raman-scattered lig...
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Simultaneous Detection of Images and Raman Spectra of Colliding Droplets: Composition Analysis of Protrusions Emerging during Collisions of Ethanol and Water Droplets Tomoko Suzuki and Jun-ya Kohno* Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan ABSTRACT: Processes involved between colliding droplets were investigated using simultaneous analysis of spectra and images of Raman-scattered light emitted by irradiation with a pulsed laser. This enabled spatially and temporally resolved Raman spectra of the colliding droplets to be obtained. Colliding droplets of ethanol and water produce a characteristic protrusion from the contact point to the antipode of the water droplet in the course of interaction. From its Raman spectrum, the protrusion is seen to be composed of water. This result supports our surface-tension release model previously proposed to describe the mechanism of protrusion formation because the protrusion is the result of positive interference of a capillary wave propagating over the surface of the water droplet in this model.

1. INTRODUCTION The dynamics of droplet collisions is of great importance in nature and have been investigated extensively across diverse disciplines.1,2 From a meteorological perspective, the collision of water droplets is the key process in the mechanism of raindrop formation.3,4 There is also strong industrial interest in droplet collisions of hydrocarbons or alcohols, the dynamics of which are particularly relevant in applications such as spray combustion in engines because the spray characteristics depend on droplet collision outcomes.5−7 To date, droplet-collision dynamics have been studied through experimental observations of morphology using optical microscopes and stroboscopic techniques8−17 along with the help of numerical calculations.18−27 Specifically, using piezodriven vibrating-orifice aerosol generators, two opposing droplet streams produced collisions that were analyzed in detail. In this experiment, a single stroboscopic image of an instance displays the collision sequence because the image includes a series of droplets that are produced at fixed time intervals. This technique enables a collection of many collision sequences in a short time and is particularly suitable in investigating the dependence of the droplet collision outcome on collision parameters such as the impact parameter and the Weber number. The droplet collision outcome is classified as (1) a bounce, (2) a coalescence, (3) a reflexive separation, or (4) stretching separation, with the accompanying collision parameters also being reported.2 In contrast, molecular level studies on the droplets have been performed by spectroscopic methods. Using droplets improves the sensitivity in various light-emission spectroscopies, such as Raman and fluorescence spectroscopies, because the emitted light is enhanced in intensity within the droplet owing to positive interference of Raman-scattered or fluorescent light © 2014 American Chemical Society

trapped within the droplet, which acts as an optical cavity with a very high quality factor.28 The intense electric field of light facilitates nonlinear optical effects, such as the stimulated Raman effect, that further enhance the Raman intensity. This spectroscopic technique has been referred to by different names, such as morphology-dependent resonance, whisperinggallery mode, and cavity-enhanced droplet spectroscopy28 (CEDS); hereafter, we shall use the term CEDS. The characteristic of droplet-enhanced scattering was investigated using spectra, theoretical calculations, and image observations.29,30 CEDS was employed as an analytical tool to investigate the size and composition of the droplets.31 Size determination in nanometer accuracy was reported using CEDS with an analysis based on Mie scattering.32,33 CEDS can be applied to droplets of not only spherical shape but also others, such as toroids, discs, spheroids, and cylinders.34 Droplets have been used in our dynamic studies of molecules in solution by isolating them from droplets in the gas phase by infrared-laser ablation.35−38 Recently, our studies have focused on the droplet itself. We developed a novel spectroscopic method, called scanning cavity-enhanced droplet spectroscopy (SCEDS), to investigate the dynamics of molecules in solution39 without using laser-ablation. SCEDS collects CEDS spectra by scanning the incident laser wavelength. This enables a continuous Raman spectrum to be constructed as an envelope of peak positions in the discrete Raman spectra originating from the discrete cavity-enhancement condition with respect to the wavelength. Received: April 2, 2014 Revised: May 6, 2014 Published: May 7, 2014 5781

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image measurement. The pulse generator used to trigger droplet generation was also synchronized with LED pulses with variable delay. A series of droplet-collision images was recorded by changing the LED timing with respect to droplet generation. The images in the series showed different droplets, but because of sufficiently small timing jitters in droplet generation they showed the collision dynamics of the droplets. The recorded images were taken as laboratory-frame images, which were transformed into a center-of-mass frame by extracting a rectified part of the image, as described in our previous paper.40 In the series of center-of-mass images, the collision velocity was set parallel to the horizontal axis of the extracted image. Analysis of these images gave a collision velocity, Weber number, and a dimensionless impact parameter.1 In the present study, the impact parameter was set close to zero. The colliding droplets then have cylindrical symmetry aligned along the droplet-to-droplet axis throughout the collision process. A pulsed laser and a CCD spectrometer were introduced to the droplet-collision apparatus to aid gathering data using Raman spectroscopy to analyze the composition of the coalescing droplets. We employed the second harmonic of a Q-switched Nd:YAG laser (Rayture Systems, GAIA-I) to induce Raman scattering. The colliding droplets were irradiated with the laser beam focused through the same objective lens (Mitsutoyo, M Plan Apo NIR 20×) as the image observation. The objective lens was used because it can sustain the intensities from the laser beam. The size of the focal spot of the laser was ∼15 μm, which was measured from an image taken under irradiation by the laser onto a quartz plate at the focal region. The laser power was set to ∼25 μJ pulse−1. The focal position of the laser was adjusted so as to irradiate the desired position of the colliding droplets. Raman scattered light was collected by the objective lens, passed through a long-pass filter to remove most of the Rayleigh scattering, and divided into two components using a 50% half mirror. The transmitted light was then focused onto the CCD of a camera for images of the Raman-scattered light to be viewed, and reflected light was guided to a CCD spectrometer, constructed in-house, to analyze and record Raman spectra. Our spectrometer included a reflective concave-brazed holographic grating (Edmund Optics, model 47563). A slit, grating, and CCD camera were mounted in a black-coated box following specifications in regard to the grating. Light of a certain wavelength enters the detector CCD plane as a vertical line. The spectrum was then calculated by summing up the intensities of the CCD image along the line. The wavelength was calibrated by introducing light from a Ne lamp through an optical fiber, one of whose edges was set to the focal plane of the apparatus. The spectral resolution of the spectrometer was ∼0.77 nm, which was measured from the width of the spectral line of Ne light. Electric pulses triggered the CCD camera to take images (Imaging Source, DMK-41BU02) and spectrum (Watec, W01MAB2) individually. The Raman image and the corresponding spectrum were simultaneously recorded for each laser shot.

We also have been studying the collision dynamics of droplets from the perspective of collisional reaction of droplets. In a previous paper,40 we described the characteristics of protrusions that appeared during collisions between ethanol and water droplets. The protrusion grows from the initial contact point of the droplets toward its antipode in the water droplet. We proposed a mechanism for its formation as a deformation produced at the contact point that propagates toward the antipode and positively interferes to result in the protrusion. However, the model is based only on morphological observations of the colliding droplet and needed molecular-level experimental confirmation. We later report support for this protrusion−formation mechanism, (1) the collision-velocity dependence of the propagation velocity of the deformation and (2) the local composition of the colliding droplets determined from simultaneous observation of the Raman spectra and the corresponding images.

2. EXPERIMENTAL SECTION A schematic of the apparatus used in this study is shown in Figure 1. With the droplet-collision apparatus from our

Figure 1. Schematic view of the droplet collision apparatus. Liquid droplets produced by opposing piezo-driven nozzles are subjected to collision. The colliding droplets are illuminated with a Nd:YAG laser beam. After removal of Rayleigh-scattered light, Raman scattered light is split into two beams, one for imaging and the other for spectrum analysis. The Raman image and corresponding spectrum are simultaneously recorded for each laser shot.

previous study,40 modifications were introduced to enable the analysis of local composition with the addition of a pulsed laser and a CCD spectrometer to simultaneously observe spectra and corresponding images of Raman-scattered light emitted from the colliding droplets. Here we describe the droplet-collision apparatus briefly and subsequently the modifications in detail. The apparatus was built around a microscope used to observe droplets tens of micrometers in size. From reservoirs of deionized or distilled water and commercially available ethanol, droplets were produced using a set of piezo-driven nozzles (Microdrop, MD-K-130), which were triggered independently by electric pulses supplied from a pulse generator. Droplet velocities were variable by changing the width and voltage of the electric pulse applied to the nozzle. A light-emitting diode (LED) was used as a strobe light to aid in imaging the droplet collision. The LED was mounted under the collision region and thus illuminated the colliding droplets from beneath. The objective lens of the microscope above the collision region focused the light shadow for imaging. Duration of the LED pulse was set to 1 μs, which was the time resolution of the

3. RESULTS Figure 2 shows a collision sequence of ethanol and water droplets of sizes 71 (left) and 78 μm (right), respectively, as reported in our previous study.40 Panels a and b represent results taken with long and short time windows, respectively. The collision velocity, the Weber number, and the dimensionless impact parameter are 2.9 m/s, 8.6, and 0.045,40 for which the outcome of collisions is predicted to be coalescence.8 As 5782

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velocities are 6.0 to 6.5 m·s−1 regardless of the collision velocity for either ethanol−water or water−water collisions. The colliding droplets finally coalesce to a single droplet. Figure 4 shows the spectra and corresponding images of the Raman-scattered light emitted from colliding ethanol−water

Figure 2. Collision sequence of ethanol (left) and water (right) droplets with diameter 71 (left) and 78 μm (right), respectively. Panels a and b show the same collision sequence but over longer and shorter time windows, respectively.

seen in Figure 2a, a disk-like contact region emerges between the droplets and grows to an oblate shape, which we term the mixed region in the present paper. Along with the growth of the mixed region, a protrusion is produced from the contact point of the droplet toward its antipode of the water droplet. In the short time window (Figure 2b), in contrast, we found that a small shoulder appears on the water-side circumference of the disk at ∼2 μs after collision, which propagates over the waterdroplet surface toward the antipode gaining in volume, as indicated by white arrows in Figure 2b. After the shoulder reaches the antipode of the water droplet, the protrusion grows in the direction from contact point to antipode. The propagation velocity of the shoulder was measured by tracing the peak position of the shoulder over the elapsed time of the collision. Figure 3a,b shows the propagation velocity of the shoulder as a function of the collision velocity of ethanol−water and water−water droplets, respectively. The propagation

Figure 4. Spectra and corresponding images of Raman-scattered light. Panels a and b show the spectra and images of the Raman-scattered light emitted from the colliding ethanol/water droplets at the protrusion and mixed region, respectively. Panels c and d similarly show spectra and images from water and ethanol, respectively. Marked peaks in panel a were used in the size analysis of the protrusion.

droplets, a water droplet, and an ethanol droplet. Figure 4a,b shows the protrusion and mixed region of the colliding droplets, respectively. The colliding droplets were irradiated when the protrusion from the coalesced droplet was longest. The Raman spectrum of the protrusion (Figure 4a) consists of a series of peaks appearing at a Raman shift of ∼3400 cm−1, whereas that of the mixed region consists of a single peak at ∼2930 cm−1. These features coincide with the spectra obtained from droplets of water (Figure 4c) and ethanol (Figure 4d). The Raman spectra of the water and ethanol droplets both show a series of peaks at ∼3400 cm−1 and a peak at ∼2930 cm−1. These Raman peaks are assignable to the stretching vibration modes of the OH bond in water and the CH bonds in ethanol. All spectra were narrower than the spontaneous Raman spectra of water and ethanol, being the characteristic of the stimulated Raman spectra. To obtain the composition of the protrusion and mixed region of the colliding droplets, we recorded cavity-enhanced single-shot Raman spectra of droplets of ethanol−water solution with different compositions. Figure 5 shows the Raman spectra of the ethanol/water droplets with ethanol mole fraction (f EtOH) of 0.045, 0.07, and 0.2. For droplets with small, moderate, and large f EtOH, the Raman spectra consist of only the OH bands, both OH and CH bands, and only CH band, respectively. The peak intensities of the OH and CH bands are obtained by integrating the Raman spectrum in the ranges 3300−3500 and 2900−3000 cm−1, respectively. Figure 6 shows the OH and CH band intensities as a function of the mole fraction of ethanol in the droplet. There the OH and the CH bands appear together in the Raman spectra in f EtOH range 0.047 to 0.09. The CH band disappears at f EtOH below 0.047.

Figure 3. Collision-velocity dependence of propagation velocities of deformations from surface-tension release propagating over the waterdroplet surface. Data points are in red and blue; the green line represents velocity averages. 5783

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surface tension is much smaller than that of water. (b) Through its surface tension, the remaining water droplet pulls the mixed region on the water side, which results in deformation at the interface of the water and contact region (STR deformation). (c) The STR deformation propagates as a capillary wave toward the antipode of the water droplet, gaining volume through the cohesive force due to surface tension. (d) The capillary wave reaches the antipode and interferes constructively to form the protrusion. This model is supported by a previous work of Gao et al.,8 who found that the STR deformation was produced from an unbalanced surface tension. Additional support has been obtained by Ding et al., who investigated collisions of droplets with a solid surface and showed that a droplet smaller than the original is produced following the propagation of a capillary wave on the droplet surface.41 We give later more evidence for the STR model that confirms that (1) the propagation of the surface wave is a capillary wave driven by surface tension of the water droplet and (2) the protrusion is the result of constructive interference of the propagating STR deformation. 4.2. Capillary-Wave Propagation. The STR model includes the capillary-wave propagation of the STR deformation on the surface of the water droplet. The capillary-wave character is verified by the result that the propagation velocity is independent of collision velocity for the ethanol−water droplets, as shown later. The STR deformation propagates from the contact point of the droplets to its antipode in the water droplet. The deformation is considered to be driven by the surface tension of the water droplet (capillary wave). In our previous paper, we verified this point by comparing the propagation velocity with a theoretical value calculated with the assumption that the propagation follows the capillary oscillation of the water droplet.40 The calculation was performed by applying the mode number of the oscillation of ∼5, which is consistent with the observation that the wavelength of the capillary wave is ∼1/5 of the great circle for the water droplet. We also reported that the protrusion forms from the antipode of the contact point on the water droplet with the direction parallel to the droplet−droplet axis in any impact parameter, which also supports the STR model.40 Here, moreover, we provide additional evidence of this propagation mechanism. The STR deformation may propagate using either surface tension or inertial forces as restoring force. In this experiment, we confirmed that the inertial force is negligible because the propagation velocity is independent of the inertial energy for the droplet system, which is determined by the relative velocity of the colliding droplets (Figure 5). Thus, we can conclude that the deformation emerging at the contact point of the ethanol−water droplets propagates as a capillary wave toward the antipode in the water droplet. 4.3. Characteristic of Cavity-Enhanced Raman Spectra. In the present study, spectra and corresponding images of Raman-scattered light were taken at the same instance and position during droplet collision. The protrusion forms within times of order of several microseconds to tens of microseconds, whereas the emission of the stimulated Raman scattering proceeds within several nanoseconds.42 Therefore, the spectra and the images of the Raman-scattered light provide instantaneous snapshots of the droplet collision. In contrast, the images shown in Figure 4a,b clearly show the positional sources of the Raman scattered light as two bright spots in the image. Thus, we can selectively record the signal of the protrusion and that of the mixed region.

Figure 5. Raman spectra of droplets of ethanol/water solution of various compositions. Panels a−c correspond to ethanol mole fractions of 0.045, 0.07, and 0.2, respectively.

Figure 6. Peak intensities of the OH and CH bands in Raman spectra of ethanol/water droplets as a function of ethanol mole fraction in the droplet.

4. DISCUSSION 4.1. Surface-Tension-Release Model. In our previous paper,40 we proposed a surface-tension-release (STR) mechanism (Figure 7) for the protrusion formation during the ethanol−water droplet collision. In brief, the four-stage mechanism is as follows. (a) When ethanol and water droplets collide, a mixed region appears at the contact point where the

Figure 7. Protrusion formation model showing the collision sequence of ethanol (left) and water (right) droplets. The thickness of the curve represents the surface tension of the fluid. 5784

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5. CONCLUSIONS In summary, we have analyzed collision processes of water− water and ethanol−water droplets of ∼70 μm size. The protrusion formation process in the ethanol−water collision was observed and analyzed in detail using CEDS. An STR deformation was found to propagate toward the antipode of the water droplet as a capillary wave. The capillary wave constructively interferes at this antipode to the initial contact point of the droplets to form the protrusion that is mostly composed of water. The result gives a basis to future investigations into fast chemical-reaction dynamics of two solutions using droplet collisions.

Each image of the positional source is in fact a longitudinal cross-section of the coalescing droplet displaying cylindrical symmetry, which is confirmed by an analysis of the cavityenhancement condition of the Raman spectrum obtained from the protrusion. Given the present experimental conditions, the cylindrical symmetry is aligned along the droplet−droplet axis. The Raman-scattered light emerges more intensely when the incident laser grazes the circular cross-section of the cylinder. The Raman scattering images show two bright spots appearing at the point of laser incidence and its diametrically opposite point, indicating that the cylinder-like coalescing droplets form a light cavity where the light in it leaks tangentially to the circular cross sections. The simplest approximation of the cavity-enhancement condition in the cavity is given by πaN = nλ

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AUTHOR INFORMATION

Corresponding Author

(1)

*Tel: +81-3-3986-0221. Fax: +81-3-5992-1029. E-mail: jun-ya. [email protected].

where a, N, λ, and n represent the diameter of the cross-section of the protrusion, refractive index of water (1.333), the incident laser wavelength (532 nm), and the mode number (integer), respectively. A least-squares fitting of the experimentally obtained peak positions to eq 1 is performed by employing a and n as fitting parameters. For the analysis, we selected a set of peaks appearing at a similar interval from the Raman spectrum in Figure 4a. A value of 30.04 ± 0.04 μm is obtained for diameter a by the fitting. The CCD image gives independently a diameter of 29 ± 2 μm, which agrees well with the fitted result. This result indicates that the Raman-scattered light (1) originates from points on the surface of the droplets corresponding to the bright spots of the image and (2) resonate transversely across the droplet, thus including the points corresponding to the two bright spots. As described, the colliding droplet keeps their cylindrical symmetry around the droplet−droplet axis throughout the collision process because the impact parameter is set to zero in the present experiment. Therefore, the composition is identical in any position within the circular cross-section of the colliding droplet. Namely, the Raman spectra of the bright spots can be regarded to represent the composition of the cross section transverse across the colliding droplets. 4.4. Composition of Protrusion and Mixed Region. The Raman spectrum of the protrusion is composed of only the OH stretching band, which indicates that the protrusion mostly consists of water. Figure 6 shows only the OH band appearing for f EtOH < 0.047 and only the CH band appearing for f EtOH > 0.09. This result shows that f EtOH is 0.09 for the mixed region) because only the OH (CH) band appears in the Raman spectrum of the protrusion (the mixed region). The obtained composition agrees with a previous report: Reid and coworkers applied CEDS for the determination of the size and composition of ethanol−water droplets.31 They reported that the OH and CH bands simultaneously appear in the Raman spectra for solutions with ethanol mole fractions of 0.03 to 0.07 (8−19% by volume), which coincides with our result. The STR model predicts that the protrusion consists of mostly water. According to the model described in Section 4.1, the protrusion is interpreted as a cohesive aggregate of surface water molecules. Our composition evaluation follows the prediction, thus strongly supporting the validity of the STR model.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the Special Cluster Research Project of Genesis Research Institute, Inc.

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REFERENCES

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