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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Spherical Spontaneous Capillary-Wave Resonance on Optically Trapped Aerosol Droplet Takuya Endo, Kyohei Ishikawa, Mao Fukuyama, Masaru Uraoka, Shoji Ishizaka, and Akihide Hibara J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03784 • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 23, 2018

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The Journal of Physical Chemistry

Spherical Spontaneous Capillary-Wave Resonance on Optically Trapped Aerosol Droplet Takuya Endo†‡, Kyohei Ishikawa‡, Mao Fukuyama†, Masaru Uraoka§, Shoji Ishizaka§, Akihide Hibara†‡ †

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan

‡

Department of Chemistry, Graduate School of Science, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguroku, Tokyo 152-8551, Japan

§

Department of Chemistry, Graduate School of Science, Hiroshima University, Kagamiyama, Higashi-hiroshima 739-8256, Japan * Phone: +81-22-217-5616. Fax: +81-22-217-5616. E-mail: [email protected] ABSTRACT: We report a contactless surface tension measurement method of micrometer-sized aerosol droplets. In this method, we assume spherical spontaneous resonance of a thermally induced capillary wave. First, an aerosol droplet with a radius ranging from 4.7 to 12.4 µm is trapped by means of a simple single-beam optical-trapping configuration, and the frequency shift power spectrum of the light passing the droplet is measured. The spectrum in each case exhibits several peaks in a frequency range of several tens to several hundred kilohertz. The peak frequencies agree well with theoretical ones predicted by the spherical resonant modes. After validating the abovementioned assumption, we measure the surface tension of aerosol droplets containing sodium dodecyl sulfate, and we successfully obtain the surface tension value. The present method utilizes just two phenomena, that is, the droplet-surface light scattering and spontaneous resonance of the capillary wave. These can be easily observed in aerosol droplets, and they can be utilized to gain scientific insights. The present method based on the nature of droplets can be used in various applications in aerosol science.

In addition to droplets in artificial systems, droplets in natural systems have also been studied. In recent years, the interfacial mechanism for aerosol chemistry and cloud droplet formation has been considered an essential problem in geochemistry.13-14 The study of droplet surface tension has also attracted attention because it is one of the most influential factors in the mechanism underlying cloud formation process, via the so-called Kelvin effect. In particular, organic compounds in atmospheric environments can be dissolved in aerosols, and they sometimes exhibit surface activity; thus, organic aerosols have attracted considerable attention in recent years. In this context, in addition to static surface tension, Nozière et al.15 emphasized the importance of the adsorption dynamics of surface-active compounds in the droplets. However, despite such interest, experimental verification of the mechanism based on direct aerosol tension measurements has not been established due to lack of a suitable measurement method.

Introduction Micrometer-sized liquid droplets and their interface behaviors have attracted considerable research attention in terms of both theory and application, and in this context, microfluidic bioanalysis systems have seen rapid developments in recent years. Furthermore1-3, characteristic droplet experiments such as magnetic self-assembly by ferrofluid instability4 and ultrasonically induced assembly of the water-rich phase in aqueous media5 have been demonstrated based on interface-related principles. Recently, inkjet soft-material printing has been utilized in various electronic and bio-devices, wherein jetted-droplet properties often determine the printing precision.6-7 Interfacial tension (or surface tension) is one of the important parameters characterizing liquid interfaces. In microfluidic devices, some image-based tension measurements utilizing static pressure,8-9 droplet10 deformation, and capillary-introduction11-12 have been reported These image-based methods are simple and useful, but they are not suitable for liquid interface characterization, especially in microchamber devices.

From the viewpoint of surface tension measurements, the conventional hanging-drop method, wherein surface deformation due to gravity and surface tension is ana-

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lyzed, is still the most common approach. In this context, Nozière et al.15 used a 0.8-mm-diameter hanging drop at the edge of a needle to examine the effect of organic compounds on aerosol formation and other behaviors. They reported that for smaller droplets, the drop shape is not significantly affected by the surface tension. This is a common limitation of hanging-drop-type surface tension measurements. Even with a constant surface tension, γ, capillary pressure, ∆, increases with decrease in the radius of curvature, , as ∆ 2γ/. In the case of smaller droplets, capillary pressure dominantly determines the droplet shape over gravity and other forces. In this regard, recently, Grassian et al. reported the surface tension analysis of sub-micrometer-sized sessile drops on a substrate by use of an atomic force microscope.16-17 This method is very attractive because smaller drops can be analyzed; however, the substrate potentially affects the measurement. In this context, isolated droplet analysis is obviously more preferable.

mally induced capillary wave having random wavenumbers and small amplitudes (typically a few Ångströms). If capillary waves spontaneously resonate on a droplet and the resonant modes can be measured optically, the method can provide effective measurements of the droplet. In this context, the quasi-elastic light scattering (QELS) method has been successfully used to measure Doppler shift during laser light scattering by capillary waves.23-26 Via QELS, the spontaneous resonance of thermally induced capillary waves has been demonstrated on liquid surfaces with spatial confinement in a microchannel27 and 2D-confined apertures. In these experiments, capillarywave modes satisfying the resonant conditions continue oscillating, and those under non-resonant conditions soon disappear. Therefore, the characteristic peaks corresponding to the resonant modes appear in the QELS power spectrum. Subsequently, surface tension has been successfully measured from the peak frequencies. The principle is promising for optical surface tension measurements; however, the occurrence of spontaneous resonance on a droplet has not thus far been proven.

Another approach for surface tension analysis of an isolated droplet involves the measurement of its boundary oscillation. Lamb18 derived the relationship between surface tension, density, radius, and frequency of the spherical oscillation modes. The derived equation indicates that the surface tension of the droplet can be obtained from the characteristic oscillation frequency measurement when its density and radius are known. In this context, there are certain droplet tension measurements based on imaging of the oscillation during morphological relaxation after forced deformation. Kowalewski et al. demonstrated the characteristic frequency by imaging the droplet deformation after its launching from an ejector. Further, Sakai et al.19 modified the method by inducing an external field to trigger the deformation, and both these approaches utilized the strobe flushing method. However, they are not suitable for aerosol observation because they require repetitive droplet ejection.

Figure 1 illustrates our conception to analyze the droplet surface tension. In the study, we first assumed that resonance of a capillary wave occurs on a spherical liquid surface, and subsequently, optically trapped aerosol droplets were measured by the QELS method. Based on the resonant-mode analysis, we successfully measured the surface tension of droplets containing dissolved salt and surfactants. Here, we also discuss the advantages and limitations of the approach in the context of aerosol science.

The optical trapping of aerosol droplets appears to be most favorable for measurements. Recently, Reid et al.20-22 reported on the impulse response of a 6-µm-radius droplet induced after coalescence of two optically trapped droplets. They measured the coalescence-induced deformation via an optical reflection measurement. In the study, they obtained a deformation of several microns to several tens of nanometers. Their method was utilized to demonstrate surface tension measurements of a small droplet with a radius as small as 6 µm. However, the method suffers from two obvious problems: 1) The coalescence that is required to induce the deformation may change the aerosol properties. 2) The applicable radius (of around 6 µm) may possibly approach the fundamental limit,20 while aerosol scientists are sometimes interested in aerosols with radii of 1 µm or smaller.

Figure 1 Conception of contactless aerosol surface tension measurement utilizing light scattering at the surface and spontaneous resonance of thermally induced capillary waves.

Against this backdrop, in this study, we introduce an approach similar to the abovementioned deformationbased methods, but one that is also essentially different from them. A free liquid surface can experience a ther-

Principle


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The Journal of Physical Chemistry Quasi-elastic light scattering method. The interface between a gas/liquid is perturbed by thermal fluctuations with an amplitude of a few Ångströms. In the absence of spatial confinement, thermally induced capillary waves with random wavenumbers exist at the interface. For a certain wavelength , the corresponding wavenumber is defined as 2π⁄.

Spherical resonance mode. When the liquid interface has a microscale boundary such as those in a microchannel and is confined in a cylindrical chamber, the capillary waves interfere spontaneously, and several resonant modes can be observed. Each resonant mode has its characteristic frequencies related to surface tension. Thus, by measuring the optical frequency shifts due to the resonant modes, the surface tension can be obtained.

The optical configuration around the optically trapped aerosol droplet is illustrated in Figure 2. When divergent laser light with a frequency of F and wavenumber of K is incident on the liquid surface, the beam irradiates the liquid surface (travelling from liquid to gas) over a range of incident angles. For simplicity, we select three angles, , , and (θ1 > θ2 > θ3) for explanation.

A spherical droplet does not have wall boundaries, but a periodic boundary condition may be applied to consider the resonant modes. Lamb18 derived the characteristic resonant frequency, , as

!"

,

(1)

where #, $, ρ, and & represent the droplet surface tension, radius of the sphere, density, and an integer representing oscillation mode & 2, 3, 4, ⋯, respectively. These spontaneously resonating modes are reflected in the QELS power spectrum of an optically trapped aerosol droplet.

For light scattering along the normal direction, the momentum conservation, expressed as sin , is maintained for each incident angle. During scattering by the capillary wave (with wavenumber ), light frequency is shifted as . Therefore, the frequency shifts vary depending on the angle. Consequently, the optical beats between the non-scattered transmitted and scattered light signals can be considered as the convolution of the frequency shifts, and a broad peak is obtained in the resulting spectrum.

Experimental Chemicals. Ammonium sulfate (Wako Pure Chemicals Co. Ltd) and sodium dodecylsulfate (SDS, Tokyo Chemical Industry Co. Ltd) were purchased and used without further purification. For sample preparation, ultrapure water (Merck Millipore, Milli-Q Integral 3) was used. Apparatus. The apparatus used for generation and trapping of aerosol droplets was similar to previously reported ones.28-29 Briefly, droplets of the sample aqueous solution were generated by means of an ultrasonic nebulizer. The droplets were fed into a sample chamber set on an inverted microscope. The chamber had a gas flow inlet and outlet flow and a cylindrical detection region with an inner diameter of 40 mm. The top and bottom surfaces of the detection region were covered with glass slides. In order to maintain the relative humidity, the inner wall of the chamber was covered with glass wool moistened with pure water. A single micrometer-sized droplet was trapped by a focused single-mode stabilized laser beam with a wavelength of 532 nm. The laser beam was focused by an objective lens with a magnification of 40 (NA = 0.60) or 60 (NA = 0.70). In our study, aerosol droplets with radii ranging from 4.7 µm to 12.4 µm were trapped and analyzed. The divergent laser beam after passing through the trapped droplet was collected by a lens (f = 70 mm) and fed into an avalanche photodiode for QELS measurement. The output current of the photodiode was amplified by a low-noise amplifier and read out by a spectrum analyzer. A CCD camera was installed in the microscope for imaging the droplet. Furthermore, a polychromator with an EM-CCD was used for Raman scattering measurements. From the Raman spectra, the concentration of ammoni-

Figure 2. Principle of quasi-elastic light scattering method. The optical configuration around the upper surface (with light traveling from the liquid side to the gas side) is illustrated.


The Journal of Physical Chemistry um sulfate was calculated. In addition, whispering-gallery modes in the –OH stretching mode were used to precisely determine the radius of the droplet as required.28, 30-31

production, or by collision and coalescence between droplets in flight, the trapped droplets exhibited varying diameters. Based on Raman scattering measurements, the radii and contents of the trapped droplets were determined and summarized (Supporting Information, Figure S1 and Table S1). Here, we note that in the aerosol optical trap, the trapping force “lifts” the droplet while gravity works in the opposite direction. The balance between the trapping force and gravity should be maintained to obtain stable measurements. In the case of smaller aerosol traps, the laser intensity must be correspondingly set to smaller values. This intensity dependence limits the applicable size, and the applicability of the method is discussed in the final part of this section.

Operation procedure. First, the air flow and the ultrasonic nebulizer were turned on, and numbers of aerosol droplets having radius variety were introduced into the chamber. The laser power was set between 2 to 50 mW. After a wait of several seconds to several tens of seconds, the trapping of a droplet was observed with the CCD camera. Subsequently, the air flow was switched off to prevent further introduction of the aerosol droplets, and to allow the remaining droplet to fall down. Thus, isolation of the trapped single droplet was achieved. Keeping in mind that the droplet radius may change in order to achieve equilibrium with the relative humidity in the chamber, we recorded the observation to confirm the radius stability. Subsequently, a droplet micrograph was acquired to measure the droplet radius. When the whispering gallery mode of –OH Raman scattering was measured, the observation path was switched to the polychromator.

Figure 4 shows the QELS power spectra for an aerosol droplet with a radius of 12.4 µm. The red points and blue line correspond to the experimental data and the spectrum fitted with multi-Lorentzian functions, respectively. The arrows and numbers represent the fitted frequency and the resonant mode numbers obtained by comparing the fitted and theoretical ones (as per Eq. (1)).

Next, the QELS spectrum was measured, wherein the measurement frequency window was set from 0 to 1000 kHz. A single spectrum was obtained every 3 ms and stored in a computer. Five hundred continuously obtained spectra were averaged for further analysis.

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Figure 3. Experimental setup for trapping aerosol droplet and measuring its surface tension. Figure 4. Quasi-elastic light scattering (QELS) spectrum of a 12.4-µm-radius aerosol droplet. The red points denote the experimental data and the blue line indicates the spectrum fitted with multi-Lorentzian functions. The arrows indicate the fitted frequencies that correspond to the resonant modes of the droplet.

Results and discussion Proof of spherical resonance. The spontaneous resonance of thermally induced capillary waves is expected to assist QELS surface tension measurements. However, in the case of a spherical drop, the occurrence of spontaneous resonance has not been proven thus far. In our study, first, we verified the validity and applicability of the optical-trap QELS method. By means of the ultrasonic nebulizer, droplets of 0.500 M ammonium sulfate solution were produced and introduced into the chamber. When a droplet was trapped, it appeared in the bright-field view. Because of heterogeneity of the droplet


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measurement method20 is based on optical trapping, but a special optical trapping configuration is used. In this approach, in order to induce droplet surface deformation, two droplets are trapped at multiple trapping points in a beam generated by a spatial light modulator. Although the method is elegant, the setup is fairly complicated.

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On the other hand, as shown in Figure 3, our optical system utilizes the single-beam configuration. We only added a lens, mirror, and detector to the setup. Because of the simplicity of the setup, the method can be utilized not only by spectroscopy experts, but also by nonspecialist researchers in various fields, including laboratory geochemistry.

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Effect of organic compounds. Next, in order to demonstrate the efficacy of the method when considering the effect of organic compounds present in the droplet on the surface tension, we measured droplets containing surfactants. In this experiment, 5 mM and 10 mM SDS solutions including 500 mM ammonium sulfate were nebulized. Subsequently, droplets with radii of 5.2, 5.5, 8.0, 9.0, and 11.0 µm (5 mM SDS) and those with radii of 5.2, 7.1, and 8.0 µm (10 mM SDS) were analyzed by means of the QELS method.

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Figure 5. Relationship between radius, resonant mode number, and resonant frequency. The broken lines correspond to the theoretical prediction obtained with Eq. (1) by use of γ = 72.5 mN/m32 and ρ = 1.03 kg/m3.33 The error bars denote the uncertainty of the spectral fitting.

Figure 6 shows the QELS spectrum of the 5.5-µm-radius droplet (corresponding to the 5 mM SDS solution). Upon fitting the multi-Lorentzian functions, we obtained 3 significant peaks (204, 392, and 808 kHz) along with a weak one (613 kHz). As regards the poor resolution of the l = 4 mode and the missing l = 6 mode, we note here that although the peak intensity variation may include resonant mode information, the underlying reason for this variation could not be clarified. Here, we focus on the most important parameter, the peak frequency.

We also measured droplets with radii between 4.7 µm and 12.1 µm. The obtained QELS spectra were fitted with multi-component Lorentz functions, and we observed the following characteristics: 1) a reduced number of resonant modes appeared in smaller aerosol droplets (relative to larger ones) because of the reduced light intensity available for the optical trapping force balance, and 2) ghost peaks appeared in the low-frequency region, which may be assigned to signals from the gas–liquid interfaces on the chamber windows. These peaks were also observed when no droplet was trapped. After identifying the ghost peaks manually and neglecting them, we obtained certain characteristic peaks assigned to each trapped droplet. Figure 5 depicts the peak frequency as a function of the aerosol radius. The broken lines indicate the theoretical prediction corresponding to Eq. (1) for & 2, 3, 4, ⋯. As can be clearly observed in the figure, there is a close agreement between the experimental and theoretical values. This result indicates that spontaneous resonance does occur on the aerosol droplets as expected and that the resonance can be measured optically. Thus, our approach demonstrates spontaneous resonance on aerosol droplets for the first time. In principle, the linewidth of the peak also accounts for the viscosity effect, but line broadening due to multi-angle optical setup prevents the possibility of viscosity analysis. The quantification based on numerical simulation will be discussed elsewhere.

The inset of Figure 6 shows the dependence of the fitted frequencies on parameter l. Upon fitting the data with Eq. (1), we obtained the surface tension as 32.0 ± 1.0 mN/m. Similarly, the fitted peaks of the droplets were analyzed, and surface tension values between 26.1 to 29.6 mN/m (with deviation ranging from 1.0 to 2.8 mN/m for 5 mM SDS) and those between 26.1 to 32.6 mN/m (with deviation ranging from 1.1 to 2.3 mN/m for 10 mM SDS) were obtained (Supporting Information, S2). The resulting spectra (Figures S2 and S3) and the summary of the fitted parameters (Tables S2 and S3) are also presented. Upon SDS addition, the surface tension of water (without other salts) decreases to around 40 mN/m, and with a salt concentration of 10−1 M (for e.g., 0.5 M NaCl), the surface tension decreases to around 30 mN/m. In the present case, the initial solution contained 500 mM ammonium sulfate and 5 or 10 mM SDS. The actual composition was not controlled, but the SDS concentration is expected to deviate in the surface tension plateau region beyond the critical micellar concentration (CMC). Thus, the experimental surface tension value of the aqueous solution of SDS with ammonium sulfate (ranging from 26.1 to 32.6 mN/m) appeared reasonably accurate.

Simplicity of optical configuration. Here, we draw attention to the simplicity of our optical setup. From the initial application of optical traps,34-35 the vertical singlebeam configuration has been the popular choice of trapping configurations. The conventional surface tension


The Journal of Physical Chemistry Thus, we were able to perform measurements reflecting the effect of organic compounds on the surface tension of a single aerosol droplet.

Chandrasekhar estimated the smallest radius of the spherical mode (l = 2) of water as 2.3 × 10−6 cm (23 nm). Meanwhile, one of the most interesting topics in aerosol science is cloud formation, and in this context, the direct tension measurement of droplets with a 1-µm-radius or less is desirable. Thus, the spontaneous resonance principle has great potential as a breakthrough technique in this context.

Accuracy and precision. As shown in Figure 5, the obtained frequencies agreed closely with the theoretical ones, with no significant systematic errors being observed, which indicates the accuracy of the QELS method. Next, we considered the precision of the obtained results based on Figure 6. By fitting Eq. (1) to the experimental value for various γ values, we obtained an error of ±1.0 mN/m (Δγ⁄γ 3.1%). Similarly, we fitted the data for droplets with diameters between 5.2 µm and 11 µm (Supporting Information, S2. See Tables S2 and S3). The errors were in the range from 3.9 to 10.3%, and the average error was obtained as 1.7 mN/m (6.0%). Here, only the uncertainty due to the peak frequency was considered; however, we note that the droplet radius and density deviation also potentially affect the uncertainty. By considering the ranges of the radius and density deviation (Supporting Information, S3), we concluded that the frequency deviation dominates the precision. Frequency (kHz)

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On the other hand, in the present experimental setup, a weaker light intensity is applied to trap smaller droplets, which leads to a corresponding reduction in signal intensity. The l = 2 peak of the 4.7-µm-radius droplet has a signal-to-noise ratio (SNR) of 13, and the SNR of the l = 2 peak depends on the size as SNR = 0.51a1.95. When the limit is set to SNR = 2, we can detect droplets as small as 2.0 µm. The trapping approach appears to be the bottleneck in the present stage. In order to overcome the problem, other trapping approaches such as multi-beam trapping37-38 are required. In the multi-beam trapping approach, a balance of radiation forces along two or more directions needs to be maintained in order to trap an aerosol droplet. This allows for the possibility of increase in the light intensity, and thus, a higher SNR can be expected for smaller droplets. However, certain technical barriers need to be overcome before practical application. For example, with an ordinary optical setup for the multi-beam trap, it is difficult to maintain radiation pressure balance. One possible approach to simplify the configuration is the utilization of state-of-the-art micro-optics technologies such as lithographically aligned optics techniques.39-41 The requirements for this type of approach and its technological challenges will be discussed elsewhere.

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In conclusion, we successfully demonstrated contactless QELS measurements of the surface tension of micrometer-sized aerosol droplets. Importantly, we proved the existence of spherical spontaneous resonance of thermally induced capillary waves for the first time. Further, we measured a droplet with a radius as small as 4.7 µm, which is comparable to or smaller than those measured by conventional remote surface tension measurement methods utilizing forced oscillation. Furthermore, we successfully evaluated the effects of organic compounds on the droplet tension, which forms one of the areas of greatest interest in aerosol science. As mentioned in the introductory section, the dynamic behaviors of droplets have attracted significant attention, and our method can potentially be applied to their continuous monitoring.

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Figure 6. Quasi-elastic light scattering (QELS) spectrum of 5.5-µm-radius droplet containing sodium dodecyl sulfate (SDS) and ammonium sulfate. The blue line indicates the fitting results obtained with multi-Lorentzian functions. (Inset) Equation (1) is fitted to the obtained frequencies to obtain a surface tension of 32.0 ± 1.0 mN/m.

Limit to droplet size. From the perspective of the size of the droplet, we successfully measured a 4.7-µm-radius droplet, although only two peaks were successfully fitted to the corresponding QELS spectrum. The present measurement radii are comparable to or smaller than those measured by contactless methods thus far utilized, including induced-deformation methods.19-20, 36

The simplicity of our optical setup is noteworthy; we only added certain optical components to the classical single-beam optical trapping system. Therefore, the method can potentially easily be used by even non-expert researchers. We believe that our method will significantly

Our measurement method utilizes only spontaneous surface deformation and does not need induced deformation. This feature can in principle allow the measurement of droplets with very small radii. In this regard,


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The Journal of Physical Chemistry 9. Liu, J.; Li, H.; Lin, J.-M., Measurements of Surface Tension of Organic Solvents Using a Simple Microfabricated Chip. Anal. Chem. 2006, 79, 371-377. 10. Cabral, J. T.; Hudson, S. D., Microfluidic Approach for Rapid Multicomponent Interfacial Tensiometry. Lab Chip 2006, 6, 427-436. 11. Li, L. X.; Kazoe, Y.; Mawatari, K.; Sugii, Y.; Kitamori, T., Viscosity and Wetting Property of Water Confined in Extended Nanospace Simultaneously Measured from Highly-Pressurized Meniscus Motion. J Phys Chem Lett 2012, 3, 2447-2452. 12. van Honschoten, J. W.; Brunets, N.; Tas, N. R., Capillarity at the Nanoscale. Chem. Soc. Rev. 2010, 39, 1096-1114. 13. Farmer, D. K.; Cappa, C. D.; Kreidenweis, S. M., Atmospheric Processes and Their Controlling Influence on Cloud Condensation Nuclei Activity. Chem. Rev. 2015, 115, 41994217. 14. Ruehl, C. R.; Davies, J. F.; Wilson, K. R., An Interfacial Mechanism for Cloud Droplet Formation on Organic Aerosols. Science 2016, 351, 1447-1450. 15. Nozière, B.; Baduel, C.; Jaffrezo, J.-L., The Dynamic Surface Tension of Atmospheric Aerosol Surfactants Reveals New Aspects of Cloud Activation. Nature Communications 2014, 5, 3335. 16. Lee, H. D.; Estillore, A. D.; Morris, H. S.; Ray, K. K.; Alejandro, A.; Grassian, V. H.; Tivanski, A. V., Direct Surface Tension Measurements of Individual Sub-Micrometer Particles Using Atomic Force Microscopy. J. Phys. Chem. A 2017, 121, 82968305. 17. Morris, H. S.; Grassian, V. H.; Tivanski, A. V., Humidity-Dependent Surface Tension Measurements of Individual Inorganic and Organic Submicrometre Liquid Particles. Chemical Science 2015, 6, 3242-3247. 18. Lamb, H.; Caflisch, R., Hydrodynamics. 6th Ed. ed.; Cambridge University Press: 1993; p 475. 19. Ishikawa, T.; Sakai, K., Dynamic Surface Tension Measurement with Temporal Resolution on Microsecond Scale. Applied Physics Express 2014, 7, 077301. 20. Bzdek, B. R.; Power, R. M.; Simpson, S. H.; Reid, J. P.; Royall, C. P., Precise, Contactless Measurements of the Surface Tension of Picolitre Aerosol Droplets. Chemical Science 2016, 7, 274-285. 21. Bzdek, B. R.; Collard, L.; Sprittles, J. E.; Hudson, A. J.; Reid, J. P., Dynamic Measurements and Simulations of Airborne Picolitre-Droplet Coalescence in Holographic Optical Tweezers. The Journal of Chemical Physics 2016, 145, 054502. 22. Boyer, H. C.; Bzdek, B. R.; Reid, J. P.; Dutcher, C. S., Statistical Thermodynamic Model for Surface Tension of Organic and Inorganic Aqueous Mixtures. J. Phys. Chem. A 2017, 121, 198-205. 23. Langevin, D., Light Scattering by Liquid Surfaces and Complementary Techniques; Taylor & Francis, 1992. 24. Su, B.; Abid, J.-P.; Fermín, D. J.; Girault, H. H.; Hoffmannová, H.; Krtil, P.; Samec, Z., Reversible VoltageInduced Assembly of Au Nanoparticles at Liquid|Liquid Interfaces. J. Am. Chem. Soc. 2004, 126, 915-919. 25. Hibara, A.; Nonaka, M.; Tokeshi, M.; Kitamori, T., Spectroscopic Analysis of Liquid/Liquid Interfaces in Multiphase Microflows. J. Am. Chem. Soc. 2003, 125, 14954-14955. 26. Tanaka, R.; Nomoto, T.; Toyota, T.; Kitahata, H.; Fujinami, M., Delayed Response of Interfacial Tension in Propagating Chemical Waves of the Belousov–Zhabotinsky Reaction without Stirring. J. Phys. Chem. B 2013, 117, 13893-13898.

contribute to the field of aerosol laboratory experiments, particularly to the long-standing discussion of cloud nucleation.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Properties of trapped droplets; QELS spectrum for aqueous droplets; Potential errors(PDF)

AUTHOR INFORMATION Corresponding Author * Phone: +81-22-217-5616. Fax: +81-22-217-5616. E-mail: [email protected]

Author Contributions All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT We appreciate Ms. Yuki Chikasue and Mr. Yuta Tanaka of Hiroshima University for their experimental assistance. This work was partially supported by JSPS KAKENHI Grant Number 15H03825 and 18H03912, by JSPS-RFBR Japan-Russia Research Cooperative Program, and by the Research Program of the “Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials” in the “Network Joint Research Center for Materials and Devices.” We would like to thank Editage (www.editage.jp) for English language editing.

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