Article Cite This: J. Phys. Chem. B XXXX, XXX, XXX-XXX
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Initial Collision Process of Two Miscible Droplets Kazuma Anahara and Jun-ya Kohno* Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan ABSTRACT: Dynamic properties of the metastable interface between two miscible solutions are investigated by the collision of two droplets. A clear interface is observed between the two colliding droplets. The interface moves in the colliding droplet toward the side where the original droplet has a lower surface tension. The interface is set to the middle of the colliding droplet by controlling the surface tension of the droplets to observe the chemical reactions at the droplet interface by cavity-enhanced Raman spectroscopy. This study provides a foundation for further research on the initial process of the chemical reactions of two miscible solutions. of a piezo droplet nozzle.15,16 Simpson and co-workers have applied the collision of liquid droplets for the investigation of the rapid reaction kinetics triggered by mixing two solutions.17−19 They merged two streams of droplets and observed the amount of the reactant and product species by Raman spectroscopy. They determined the rate constant of the complex-formation reaction of Fe2+ and 1,10-phenanthroline by using this method.18 To confirm the applicability of the method to rapid chemical reactions, they examined the mixing time following the coalescence of the droplets by observation of the neutralization reaction of HSO4− with OH−. The mixing time ranged from 0.2 to 4 ms depending on the collision angle, velocity, and the sizes of the droplets.19 So far, we have been studying the dynamics of molecules with the use of droplets: gas-phase isolation of biomolecules from a droplet20−24 and morphological and chemical dynamics induced by the collision of two droplets.15,25−27 We have developed a novel droplet Raman spectroscopy method to obtain the cavity-enhanced Raman spectra.28 We measured the rate constant of the rate-determining step of a coloring reaction of phenolphthalein.26 The coloring reaction showed an induction period of ∼400 μs, which was the mixing time of the colliding droplet in the experimental conditions. We have also measured the cavity-enhanced droplet Raman spectra of alcohol solutions on a several-millisecond time scale from the generation of the droplet.29 A decrease and subsequent increase of the alcohol concentration at the droplet surface was observed in the set of measurements, which showed that the surface region was observed by our measurement. To measure the chemical reaction more rapidly, it is necessary to observe the dynamic liquid−liquid interface. Actually, we have succeeded in observing the reaction dynamics at 10−40 μs from the droplet collision of a dansyl chloride solution and isopropyl amine by a fluorescence enhancement.27 In the present paper, we analyzed the spatial and temporal profiles of the miscible liquid−liquid interface which emerged
1. INTRODUCTION Reactions in solutions are of importance in a wide range of processes. Most of the synthetic and biological reactions proceed in solution. Usually, the reactions are triggered by mixing two miscible solutions, each of which contains a different reactant species. When we discuss a reaction which proceeds more rapidly than the mixing process, the initial process of the reaction includes the diffusion of the reactants across the interface of the two solutions. Hence, it is important to elucidate the reaction dynamics of molecules at the liquid− liquid interface. So far, the dynamic properties of molecules at an immiscible liquid−liquid interface have been investigated extensively: The structure of the liquid−liquid interface has been studied by spectroscopic methods, such as sum frequency generation spectroscopy, because the sum frequency light emerges only at the interface.1−3 Sum-frequency generation spectroscopy has also been applied to studies on reactions at the liquid−liquid interface. For example, photoinduced electron transfer reactions at the liquid−liquid interface have been investigated kinetically.1,4 Electrochemical analysis in combination with the scanning probe method has also been applied to the studies on the reaction at the liquid−liquid interface.5−8 The liquid−liquid interface provides a unique reaction field and has practically been used to synthesize inorganic particles,9 polymers10,11 and so on.12 Most of the studies on the liquid−liquid interface have been performed for immiscible liquids to obtain a stable interface. However, studies on a miscible liquid−liquid interface are indispensable to elucidate the initial reaction processes of the chemical reaction triggered by mixing two miscible solutions, because most reaction processes are performed by mixing two miscible solutions. So far, the dissolution dynamics of miscible liquids has been investigated in a capillary13 or on a plate with reticulated channels,14 which have suggested the importance of barodiffusion for the dissolution of the two solutions. A spatially and temporally reproducible generation of the liquid−liquid interface is essential to conduct investigations on the miscible interface. In that sense, liquid droplets facilitate the studies on the dynamics at the liquid−liquid interface, because small liquid droplets can be generated reproducibly by the use © XXXX American Chemical Society
Received: August 25, 2017 Revised: September 27, 2017 Published: September 28, 2017 A
DOI: 10.1021/acs.jpcb.7b08526 J. Phys. Chem. B XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry B
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 microscope. 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, DMK41BU02) and spectra (Watec, W-01MAB2) individually, so that the Raman image and the corresponding spectrum were simultaneously recorded for each laser shot.
in the course of the droplet collision. The profiles of the liquid−liquid interface were revealed to be highly dependent on the surface tension of the liquids. Upon collision of two droplets, the droplet with less surface tension spreads on the other.
2. EXPERIMENTAL SECTION A detailed description of the apparatus employed in this study has been reported previously.28 Here, we briefly describe the apparatus and the experimental procedures employed in the present study. The droplet-collision apparatus was built around a microscope, which observed the collision of two droplets of ∼70 μm in diameter. The droplets were produced by a set of piezo-driven nozzles (Microdrop, MD-K-130), which were triggered independently by electric pulses supplied from a pulse generator (Stanford, DG645). A light-emitting diode (LED) was used as a strobe light to image the droplet collision. The LED was mounted under the collision region and illuminated the colliding droplets from beneath, which resulted in shadow images of the droplets. The duration of the LED pulse was set to 1 μs, which was the time resolution of the image measurement. The pulse generator used to trigger droplet generation was also synchronized with LED pulses with a variable delay. A series of droplet-collision images was recorded by changing the LED timing with respect to the droplet generation. The observation time window was up to ∼3 ms,26 because the droplet had small jitters in direction and velocity. Linear electrodynamic trap technique would help to elongate the observation time window. 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. Collision velocity, Weber number, and dimensionless impact parameter of the collision were calculated from the transformed images at different times before the collision in the same way as that in the previous reports.15,25,26 In the present study, the impact parameter was set close to zero. The colliding droplets then exhibited a cylindrical symmetry aligned along the dropletto-droplet axis throughout the collision process. A pulsed laser and a CCD spectrometer were used to obtain the Raman spectra of the mixing region of the colliding droplets. We employed the second harmonic of a Q-switched Nd:YAG laser (Rayture Systems, GAIA-I) for the Raman spectroscopy. The colliding droplet was irradiated with the laser beam focused through an objective lens (Mitsutoyo, M Plan Apo NIR 20×). 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 to the grazing position of the mixing region of the colliding droplets. Raman scattered light was collected by the objective lens, passed through a long-pass filter to remove the Rayleigh scattering, and was divided into two components using a half mirror with a 70% reflectance. The transmitted light was focused onto the CCD of a camera for observation of the images of the Raman scattered light, and the reflected light was guided to a CCD spectrometer, constructed in-house, to analyze and record the corresponding Raman spectra. Our spectrometer included a reflective concave grating (Shimadzu, P1200−01) which has a resolution sufficient to resolve the Raman bands of HSO4− (980 cm−1) and SO42− (1050 cm−1). A slit, the grating, and a CCD camera were mounted in a blackcoated box following the specifications in regard to the grating. Light of a certain wavelength entered the detector CCD plane
3. RESULTS 3.1. Time Evolution of Miscible Liquid/Liquid Interface. Figure 1 shows a collision sequence of water and 1 M
Figure 1. Collision sequence of water and 1 M Na2SO4 aqueous solution droplets with sizes of 76.2 μm (left) and 76.6 μm (right). The images are shown in a center-of-mass frame with the collision velocity parallel to the horizontal axis of the images.
Na2SO4 aqueous solution droplets with sizes of 76.2 μm (left) and 76.6 μm (right), respectively. The collision velocity, the Weber number, and the dimensionless impact parameter are 3.3 m/s, 12, and 0.055, respectively, for which the outcome of the collisions is predicted to be coalescence.15,30 As shown in Figure 1, a mixing region is generated as a disk between the two droplets after 10 μs from the collision, and the disk grows into an oblate shape (30 μs). The merged droplet then undergoes an oblate-prolate-oblate oscillation. In the present analysis, the collision velocity was adjusted horizontally to the image, and hence the droplet−droplet axis was horizontal in the image. Note that the longitudinally or horizontally elongated shapes in the image correspond to an oblate or a prolate shape, respectively, because the colliding droplets have cylindrical symmetry when the impact parameter is zero. Figure 2 shows the collision of droplets of 1 M Na2SO4 and ethanol aqueous solutions, where the elapsed times from the droplet collision (denoted as collision time) were set to 80 and 200 μs. The ethanol concentrations were 0, 0.8, and 1.6% for panels a, b, and c, respectively. A clear interface was observed in each image, which moved toward the ethanol-solution side with an increase in the collision time. The interface was observed only for the prolate-shaped droplets. By comparison of panels a−c, however, one can see that the velocity of the interface movement increased with the increase in the ethanol concentration. Note that the two solutions were both dilute aqueous solutions and hence were miscible, which means that the observed interface was metastable. The metastable interface, however, survived throughout the present measurements performed up to 300 μs from the collision. The interface position was evaluated by the ratio, X, of the edge-to-interface to an edge-to-edge lengths of the droplet, as represented in Figure 3a. The edge-to-interface length was measured from the Na2SO4 edge. Since the diameters of B
DOI: 10.1021/acs.jpcb.7b08526 J. Phys. Chem. B XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry B
not observable for the oblate-shaped droplets. However, it is highly likely that the oblate-shaped droplets have interfaces, because the re-emerged prolate-shaped droplets still showed interfaces. From Figure 3b, one can see that (1) the X value increased with an increase in the collision time as well as the ethanol concentration, and (2) the X value showed plateaus around the collision times of 80 and 200 μs. The X values increased before and after the plateau time with an increase in the collision time. The velocities of the interface movement were evaluated in the following manner: The values of X at the collision times of 80, 200, and 290 μs (denoted as X80, X200, and X290, respectively) are represented by the average of the X values at collision times of 60−100, 180−220, and 280−300 μs. The average interface velocities at the collision times of 0−80, 80− 200, and 200−290 μs were then calculated from the time evolution of the averaged X as X80/80, (X200 − X80)/(200−80), and (X290 − X200)/(290−200), respectively. The average velocities are shown in Figure 4 as a function of the ethanol
Figure 2. Snapshots of the colliding droplets of 1 M Na2SO4 and ethanol aqueous solutions at collision times of 80 and 200 μs. The ethanol concentrations are 0, 0.8, and 1.6% for panels a, b, and c, respectively.
Figure 4. Average interface velocities at the collision times of 0−80 (red), 80−210 (blue), and 210−290 (green) μs as a function of the ethanol concentration.
concentration. The average interface velocity of the collision times of 0−80 was larger than those of 80−210 and 210−290 μs. In addition, the average interface velocities of the collision times of 0−80, 80−200, and 200−290 μs increased, leveled off, and decreased, respectively, with an increase in the ethanol concentration. 3.2. Raman Spectroscopy of Colliding Droplets. Figure 5 shows the spectra and corresponding images of the Raman scattered light emitted from colliding droplets of 1 M NH3 and (0.5 M H2SO4 + ethanol) aqueous solutions. The collision time was 270 μs, when the colliding droplet was oblate. Figure 5 shows the Raman spectra obtained from the droplets with ethanol concentrations of 1.1 and 2.0%, and the corresponding images of the Raman scattered light. Note that the image of the Raman scattered light mainly originated from the OH band of water, which appeared at the Raman shift of ∼3400 cm−1 (not shown in the Raman spectra). The Raman spectra consisted of HSO4− and SO42− bands, which appeared at 980 and 1050 cm−1, respectively. The observation of SO42− indicated that the neutralization reaction between HSO4− and NH3 proceeded at the collision interface of the droplets. Since the Ramanscattering cross section of HSO4− is 5.7% larger than that of SO42−,31 the mole fraction of SO42−, f SO42−, was estimated from the Raman intensities as
Figure 3. (a) Definition of a parameter, X, which represents the interface position of the colliding droplets. (b) The parameter, X, as a function of the collision time for the droplet of 1 M Na2SO4 and that of the ethanol aqueous solution, where the ethanol concentration ranges from 0% to 2.4%.
Na2SO4 and ethanol droplets were almost the same and the interface moved to the ethanol side of the colliding droplet, the X value equaled 0.5 at the collision time of zero and increased with an increase in the collision time. Figure 3b shows the ratio, X, as a function of the collision time for the droplet of 1 M Na2SO4 and that of ethanol aqueous solutions with different ethanol concentrations. The X-value data were lacking at the collision times around 150 and 260 μs, because an interface was C
DOI: 10.1021/acs.jpcb.7b08526 J. Phys. Chem. B XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry B
Figure 5. Spectra and corresponding images of Raman scattered light emitted from colliding droplets of 1 M NH3 and (0.5 M H2SO4 + x M ethanol) aqueous solutions, where the ethanol concentrations are 1.1 and 2.0%. The green ellipsoids in the images represent the laser irradiation regions.
fSO 2− = aISO4 2−/(aISO4 2− + IHSO4−) 4
Figure 7. Mole fraction of SO42− as a function of the collision time for the droplet collision of 1 M NaOH and 0.5 M H2SO4 aqueous solutions.
collision of 1 M NaOH and 0.5 M H2SO4 aqueous solutions. The Raman-intensity ratio had a constant low value until a certain induction period, ti, and increased rapidly with an increase in the collision time. The time evolution of f SO42− represents the mixing of the two droplets, which is analyzed by
(1)
where ISO42− and IHSO42− represents the integrated intensities of the Raman band of SO42− and HSO4−, respectively, and a = 1.057. Figure 6 shows the mole fraction of SO42− as a function of the average interface position, X, obtained from the droplet
fSO 2− = ctcol < t i
(2)
fSO 2− = c + (1 − c)(1 − e−k(tcol − t i))tcol > t i
(3)
4
4
where tcol and ti show the collision time and the induction period of mixing, respectively, and c and k are parameters representing the abundance of background SO42− and the mixing rate, respectively. The fitting result is shown as a red curve in Figure 7, which gives the parameters, ti, c, and k to be 2.2 × 102 μs, 0.14, and 2.4 × 104 s−1, respectively.
4. DISCUSSION 4.1. Quadruple Oscillation of Colliding Droplets. The colliding droplet merged with a dumping oblate−prolate− oblate oscillation, as is shown in Figure 1. Note that the longitudinally and horizontally elongated shapes in the images correspond to oblate and prolate shapes, respectively, because the colliding droplets have cylindrical symmetry along with the collision velocity (horizontal axis of the image) when the impact parameter is zero. Since the Weber number range and the dimensionless impact parameter were 5−10 and ce: the H2SO4 droplet spreads on the NH3 droplet, in the course of the collision. Under these three conditions, the Raman spectra are likely to be the following: (1) c < ce: neither SO42− nor HSO4− are observed, (2) c = ce: the reaction product, SO42− is abundant, and (3) c > ce: both SO42− and HSO4− are observed. The result that no Raman signals were observed for the droplets with X less than 0.48 (Figure 6) corresponds to case (1). Namely, the NH3 droplet covered the H2SO4 droplet at the mixing region of the droplet, so that no sulfate/bisulfate located at the peripheral of the mixing region of the colliding droplet. However, the Raman-intensity ratio had a large value around X = 0.5 and decreased with an increase in X (see Figure 6). This result is explained by the interface is included in the Raman scattering area for the colliding droplet with X = 0.5, and the interface moved farther with an increase in X, which resulted in the decrease of the observed SO42− mole fraction. Figure 6 also shows that the Raman-intensity ratio at X ∼ 0.5 increased with an increase in the collision time. This result is explained by a diffusive chemical reaction at the interface. At the small collision time, the diffusively mixing region is so thin that the Raman scattering region also includes the unmixed region. The thickness of the Raman scattering region was roughly estimated from the Raman scattering ratio to be 1 μm, because (1) the Raman-intensity ratio at a collision time of 40 μs was almost half of that at 190 μs, (2) the Raman-intensity ratio was unity at the collision time of 190 μs, and (3) the diffusion area was calculated to be 0.6 and 1.3 μm at collision times of 40 and 190 μs, respectively, according to the onedimensional diffusion model shown in Section 4.1.
5. CONCLUSIONS We report a metastable interface and its movement, which results from the collision of two miscible droplets. By modifying the surface tension of the droplet, we observe the chemical reaction at the interface by Raman spectroscopy. This study provides a technical basis to facilitate spectroscopic investigations on the chemical reactions at the interface between two miscible solutions.
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DOI: 10.1021/acs.jpcb.7b08526 J. Phys. Chem. B XXXX, XXX, XXX−XXX
Article
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DOI: 10.1021/acs.jpcb.7b08526 J. Phys. Chem. B XXXX, XXX, XXX−XXX
