Simultaneous Detection of Images and Raman Spectra of CollidingDroplets: Composition Analysis of Protrusions Emerging duringCollisions of Ethanol and Water DropletsTomoko 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 usingsimultaneous analysis of spectra and images of Raman-scattered light emitted byirradiation with a pulsed laser. This enabled spatially and temporally resolved Ramanspectra of the colliding droplets to be obtained. Colliding droplets of ethanol and waterproduce a characteristic protrusion from the contact point to the antipode of the waterdroplet in the course of interaction. From its Raman spectrum, the protrusion is seento be composed of water. This result supports our surface-tension release modelpreviously proposed to describe the mechanism of protrusion formation because theprotrusion is the result of positive interference of a capillary wave propagating over thesurface of the water droplet in this model.

1. INTRODUCTIONThe dynamics of droplet collisions is of great importance innature and have been investigated extensively across diversedisciplines.1,2 From a meteorological perspective, the collisionof water droplets is the key process in the mechanism ofraindrop formation.3,4 There is also strong industrial interest indroplet collisions of hydrocarbons or alcohols, the dynamics ofwhich are particularly relevant in applications such as spraycombustion in engines because the spray characteristics dependon droplet collision outcomes.5−7

To date, droplet-collision dynamics have been studiedthrough experimental observations of morphology using opticalmicroscopes and stroboscopic techniques8−17 along with thehelp of numerical calculations.18−27 Specifically, using piezo-driven vibrating-orifice aerosol generators, two opposingdroplet streams produced collisions that were analyzed indetail. In this experiment, a single stroboscopic image of aninstance displays the collision sequence because the imageincludes a series of droplets that are produced at fixed timeintervals. This technique enables a collection of many collisionsequences in a short time and is particularly suitable ininvestigating the dependence of the droplet collision outcomeon collision parameters such as the impact parameter and theWeber 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 collisionparameters also being reported.2

In contrast, molecular level studies on the droplets have beenperformed by spectroscopic methods. Using droplets improvesthe sensitivity in various light-emission spectroscopies, such asRaman and fluorescence spectroscopies, because the emittedlight is enhanced in intensity within the droplet owing topositive interference of Raman-scattered or fluorescent light

trapped within the droplet, which acts as an optical cavity with avery high quality factor.28 The intense electric field of lightfacilitates nonlinear optical effects, such as the stimulatedRaman effect, that further enhance the Raman intensity. Thisspectroscopic technique has been referred to by differentnames, such as morphology-dependent resonance, whispering-gallery mode, and cavity-enhanced droplet spectroscopy28

(CEDS); hereafter, we shall use the term CEDS. Thecharacteristic of droplet-enhanced scattering was investigatedusing spectra, theoretical calculations, and image observa-tions.29,30 CEDS was employed as an analytical tool toinvestigate the size and composition of the droplets.31 Sizedetermination in nanometer accuracy was reported using CEDSwith an analysis based on Mie scattering.32,33 CEDS can beapplied 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 moleculesin solution by isolating them from droplets in the gas phase byinfrared-laser ablation.35−38 Recently, our studies have focusedon the droplet itself. We developed a novel spectroscopicmethod, called scanning cavity-enhanced droplet spectroscopy(SCEDS), to investigate the dynamics of molecules insolution39 without using laser-ablation. SCEDS collects CEDSspectra by scanning the incident laser wavelength. This enablesa continuous Raman spectrum to be constructed as an envelopeof peak positions in the discrete Raman spectra originatingfrom the discrete cavity-enhancement condition with respect tothe wavelength.

Received: April 2, 2014Revised: May 6, 2014Published: May 7, 2014

Article

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© 2014 American Chemical Society 5781 dx.doi.org/10.1021/jp503285w | J. Phys. Chem. B 2014, 118, 5781−5786

We also have been studying the collision dynamics ofdroplets from the perspective of collisional reaction of droplets.In a previous paper,40 we described the characteristics ofprotrusions that appeared during collisions between ethanoland water droplets. The protrusion grows from the initialcontact point of the droplets toward its antipode in the waterdroplet. We proposed a mechanism for its formation as adeformation produced at the contact point that propagatestoward the antipode and positively interferes to result in theprotrusion. However, the model is based only on morpho-logical observations of the colliding droplet and neededmolecular-level experimental confirmation. We later reportsupport for this protrusion−formation mechanism, (1) thecollision-velocity dependence of the propagation velocity of thedeformation and (2) the local composition of the collidingdroplets determined from simultaneous observation of theRaman spectra and the corresponding images.

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

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

image measurement. The pulse generator used to triggerdroplet generation was also synchronized with LED pulses withvariable delay. A series of droplet-collision images was recordedby changing the LED timing with respect to droplet generation.The images in the series showed different droplets, but becauseof sufficiently small timing jitters in droplet generation theyshowed the collision dynamics of the droplets. The recordedimages were taken as laboratory-frame images, which weretransformed into a center-of-mass frame by extracting arectified part of the image, as described in our previouspaper.40 In the series of center-of-mass images, the collisionvelocity was set parallel to the horizontal axis of the extractedimage. Analysis of these images gave a collision velocity, Webernumber, and a dimensionless impact parameter.1 In the presentstudy, the impact parameter was set close to zero. The collidingdroplets then have cylindrical symmetry aligned along thedroplet-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 usingRaman spectroscopy to analyze the composition of thecoalescing droplets. We employed the second harmonic of aQ-switched Nd:YAG laser (Rayture Systems, GAIA-I) toinduce Raman scattering. The colliding droplets were irradiatedwith 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 theintensities from the laser beam. The size of the focal spot of thelaser was ∼15 μm, which was measured from an image takenunder irradiation by the laser onto a quartz plate at the focalregion. The laser power was set to ∼25 μJ pulse−1. The focalposition of the laser was adjusted so as to irradiate the desiredposition of the colliding droplets. Raman scattered light wascollected by the objective lens, passed through a long-pass filterto remove most of the Rayleigh scattering, and divided into twocomponents using a 50% half mirror. The transmitted light wasthen focused onto the CCD of a camera for images of theRaman-scattered light to be viewed, and reflected light wasguided to a CCD spectrometer, constructed in-house, toanalyze and record Raman spectra. Our spectrometer includeda reflective concave-brazed holographic grating (EdmundOptics, model 47563). A slit, grating, and CCD camera weremounted in a black-coated box following specifications inregard to the grating. Light of a certain wavelength enters thedetector CCD plane as a vertical line. The spectrum was thencalculated by summing up the intensities of the CCD imagealong the line. The wavelength was calibrated by introducinglight from a Ne lamp through an optical fiber, one of whoseedges was set to the focal plane of the apparatus. The spectralresolution of the spectrometer was ∼0.77 nm, which wasmeasured 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, W-01MAB2) individually. The Raman image and the correspond-ing spectrum were simultaneously recorded for each laser shot.

3. RESULTSFigure 2 shows a collision sequence of ethanol and waterdroplets of sizes 71 (left) and 78 μm (right), respectively, asreported in our previous study.40 Panels a and b representresults taken with long and short time windows, respectively.The collision velocity, the Weber number, and the dimension-less impact parameter are 2.9 m/s, 8.6, and 0.045,40 for whichthe outcome of collisions is predicted to be coalescence.8 As

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

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seen in Figure 2a, a disk-like contact region emerges betweenthe droplets and grows to an oblate shape, which we term themixed region in the present paper. Along with the growth of themixed region, a protrusion is produced from the contact pointof the droplet toward its antipode of the water droplet. In theshort time window (Figure 2b), in contrast, we found that asmall shoulder appears on the water-side circumference of thedisk at ∼2 μs after collision, which propagates over the water-droplet surface toward the antipode gaining in volume, asindicated by white arrows in Figure 2b. After the shoulderreaches the antipode of the water droplet, the protrusion growsin the direction from contact point to antipode. Thepropagation velocity of the shoulder was measured by tracingthe peak position of the shoulder over the elapsed time of thecollision. Figure 3a,b shows the propagation velocity of theshoulder as a function of the collision velocity of ethanol−waterand water−water droplets, respectively. The propagation

velocities are 6.0 to 6.5 m·s−1 regardless of the collisionvelocity 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

droplets, a water droplet, and an ethanol droplet. Figure 4a,bshows the protrusion and mixed region of the collidingdroplets, respectively. The colliding droplets were irradiatedwhen the protrusion from the coalesced droplet was longest.The Raman spectrum of the protrusion (Figure 4a) consists ofa 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 obtainedfrom droplets of water (Figure 4c) and ethanol (Figure 4d).The Raman spectra of the water and ethanol droplets bothshow a series of peaks at ∼3400 cm−1 and a peak at ∼2930cm−1. These Raman peaks are assignable to the stretchingvibration modes of the OH bond in water and the CH bonds inethanol. All spectra were narrower than the spontaneousRaman spectra of water and ethanol, being the characteristic ofthe stimulated Raman spectra.To obtain the composition of the protrusion and mixed

region of the colliding droplets, we recorded cavity-enhancedsingle-shot Raman spectra of droplets of ethanol−watersolution with different compositions. Figure 5 shows theRaman spectra of the ethanol/water droplets with ethanol molefraction ( f EtOH) of 0.045, 0.07, and 0.2. For droplets with small,moderate, and large f EtOH, the Raman spectra consist of onlythe OH bands, both OH and CH bands, and only CH band,respectively. The peak intensities of the OH and CH bands areobtained by integrating the Raman spectrum in the ranges3300−3500 and 2900−3000 cm−1, respectively. Figure 6 showsthe OH and CH band intensities as a function of the molefraction of ethanol in the droplet. There the OH and the CHbands appear together in the Raman spectra in f EtOH range0.047 to 0.09. The CH band disappears at f EtOH below 0.047.

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

Figure 3. Collision-velocity dependence of propagation velocities ofdeformations from surface-tension release propagating over the water-droplet surface. Data points are in red and blue; the green linerepresents velocity averages.

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

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4. DISCUSSION4.1. Surface-Tension-Release Model. In our previous

paper,40 we proposed a surface-tension-release (STR) mecha-nism (Figure 7) for the protrusion formation during theethanol−water droplet collision. In brief, the four-stagemechanism is as follows. (a) When ethanol and water dropletscollide, a mixed region appears at the contact point where the

surface tension is much smaller than that of water. (b) Throughits surface tension, the remaining water droplet pulls the mixedregion on the water side, which results in deformation at theinterface of the water and contact region (STR deformation).(c) The STR deformation propagates as a capillary wave towardthe antipode of the water droplet, gaining volume through thecohesive force due to surface tension. (d) The capillary wavereaches the antipode and interferes constructively to form theprotrusion. This model is supported by a previous work of Gaoet al.,8 who found that the STR deformation was producedfrom an unbalanced surface tension. Additional support hasbeen obtained by Ding et al., who investigated collisions ofdroplets with a solid surface and showed that a droplet smallerthan the original is produced following the propagation of acapillary wave on the droplet surface.41

We give later more evidence for the STR model thatconfirms that (1) the propagation of the surface wave is acapillary wave driven by surface tension of the water dropletand (2) the protrusion is the result of constructive interferenceof the propagating STR deformation.

4.2. Capillary-Wave Propagation. The STR modelincludes the capillary-wave propagation of the STR deforma-tion on the surface of the water droplet. The capillary-wavecharacter is verified by the result that the propagation velocity isindependent of collision velocity for the ethanol−waterdroplets, as shown later. The STR deformation propagatesfrom the contact point of the droplets to its antipode in thewater droplet. The deformation is considered to be driven bythe surface tension of the water droplet (capillary wave). In ourprevious paper, we verified this point by comparing thepropagation velocity with a theoretical value calculated with theassumption that the propagation follows the capillary oscillationof the water droplet.40 The calculation was performed byapplying the mode number of the oscillation of ∼5, which isconsistent with the observation that the wavelength of thecapillary wave is ∼1/5 of the great circle for the water droplet.We also reported that the protrusion forms from the antipodeof the contact point on the water droplet with the directionparallel to the droplet−droplet axis in any impact parameter,which also supports the STR model.40

Here, moreover, we provide additional evidence of thispropagation mechanism. The STR deformation may propagateusing either surface tension or inertial forces as restoring force.In this experiment, we confirmed that the inertial force isnegligible because the propagation velocity is independent ofthe inertial energy for the droplet system, which is determinedby the relative velocity of the colliding droplets (Figure 5).Thus, we can conclude that the deformation emerging at thecontact point of the ethanol−water droplets propagates as acapillary wave toward the antipode in the water droplet.

4.3. Characteristic of Cavity-Enhanced Raman Spec-tra. In the present study, spectra and corresponding images ofRaman-scattered light were taken at the same instance andposition during droplet collision. The protrusion forms withintimes of order of several microseconds to tens of microseconds,whereas the emission of the stimulated Raman scatteringproceeds within several nanoseconds.42 Therefore, the spectraand the images of the Raman-scattered light provideinstantaneous snapshots of the droplet collision. In contrast,the images shown in Figure 4a,b clearly show the positionalsources of the Raman scattered light as two bright spots in theimage. Thus, we can selectively record the signal of theprotrusion and that of the mixed region.

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

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

Figure 7. Protrusion formation model showing the collision sequenceof ethanol (left) and water (right) droplets. The thickness of the curverepresents the surface tension of the fluid.

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Each image of the positional source is in fact a longitudinalcross-section of the coalescing droplet displaying cylindricalsymmetry, which is confirmed by an analysis of the cavity-enhancement condition of the Raman spectrum obtained fromthe protrusion. Given the present experimental conditions, thecylindrical symmetry is aligned along the droplet−droplet axis.The Raman-scattered light emerges more intensely when theincident laser grazes the circular cross-section of the cylinder.The Raman scattering images show two bright spots appearingat the point of laser incidence and its diametrically oppositepoint, indicating that the cylinder-like coalescing droplets forma light cavity where the light in it leaks tangentially to thecircular cross sections. The simplest approximation of thecavity-enhancement condition in the cavity is given by

π λ=aN n (1)

where a, N, λ, and n represent the diameter of the cross-sectionof the protrusion, refractive index of water (1.333), the incidentlaser wavelength (532 nm), and the mode number (integer),respectively. A least-squares fitting of the experimentallyobtained peak positions to eq 1 is performed by employing aand n as fitting parameters. For the analysis, we selected a set ofpeaks appearing at a similar interval from the Raman spectrumin Figure 4a. A value of 30.04 ± 0.04 μm is obtained fordiameter a by the fitting. The CCD image gives independentlya diameter of 29 ± 2 μm, which agrees well with the fittedresult. This result indicates that the Raman-scattered light (1)originates from points on the surface of the dropletscorresponding to the bright spots of the image and (2)resonate transversely across the droplet, thus including thepoints corresponding to the two bright spots. As described, thecolliding droplet keeps their cylindrical symmetry around thedroplet−droplet axis throughout the collision process becausethe impact parameter is set to zero in the present experiment.Therefore, the composition is identical in any position withinthe circular cross-section of the colliding droplet. Namely, theRaman spectra of the bright spots can be regarded to representthe composition of the cross section transverse across thecolliding droplets.4.4. Composition of Protrusion and Mixed Region.

The Raman spectrum of the protrusion is composed of only theOH stretching band, which indicates that the protrusion mostlyconsists of water. Figure 6 shows only the OH band appearingfor f EtOH < 0.047 and only the CH band appearing for f EtOH >0.09. This result shows that f EtOH is <0.047 for the protrusion(>0.09 for the mixed region) because only the OH (CH) bandappears in the Raman spectrum of the protrusion (the mixedregion). The obtained composition agrees with a previousreport: Reid and coworkers applied CEDS for the determi-nation of the size and composition of ethanol−water droplets.31They reported that the OH and CH bands simultaneouslyappear in the Raman spectra for solutions with ethanol molefractions of 0.03 to 0.07 (8−19% by volume), which coincideswith 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 surfacewater molecules. Our composition evaluation follows theprediction, thus strongly supporting the validity of the STRmodel.

5. CONCLUSIONS

In summary, we have analyzed collision processes of water−water and ethanol−water droplets of ∼70 μm size. Theprotrusion formation process in the ethanol−water collisionwas observed and analyzed in detail using CEDS. An STRdeformation was found to propagate toward the antipode of thewater droplet as a capillary wave. The capillary waveconstructively interferes at this antipode to the initial contactpoint of the droplets to form the protrusion that is mostlycomposed of water. The result gives a basis to futureinvestigations into fast chemical-reaction dynamics of twosolutions using droplet collisions.

■ AUTHOR INFORMATION

Corresponding Author*Tel: +81-3-3986-0221. Fax: +81-3-5992-1029. E-mail: jun-ya.kohno@gakushuin.ac.jp.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

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

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