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Charge Effects on the Efflorescence in Single Levitated Droplets Gunter Hermann, Yan Zhang, Bernhard Wassermann, Henry Fischer, Marcel Quennet, and Eckart Ruehl J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b05760 • Publication Date (Web): 16 Aug 2017 Downloaded from http://pubs.acs.org on August 17, 2017

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The Journal of Physical Chemistry

Charge Effects on the Efflorescence in Single Levitated Droplets G. Hermann, Y. Zhang, B. Wassermann, H. Fischer, M. Quennet, and E. Rühl*

Physical Chemistry, Institute of Chemistry and Biochemistry, Freie Universität Berlin, Takustr. 3, 14195 Berlin, Germany

*Corresponding author: [email protected] Abstract The influence of electrical excess charges on the crystallization from supersaturated aqueous sodium chloride solutions is reported. This is accomplished by efflorescence studies on single levitated microdroplets using optical and electrodynamic levitation. Specifically, a strong increase in efflorescence humidity is observed as a function of the droplet's negative excess charge, ranging up to -2.1 pC, with a distinct threshold behavior, increasing the relative efflorescence humidity, at which spontaneous nucleation occurs, from 44% for the neutral microparticle to 60%. These findings are interpreted by using molecular dynamic simulations for determining plausible structural patterns located near the particle surface that could serve as suitable precursors for the formation of critical clusters overcoming the nucleation barrier. These results, facilitating heterogeneous nucleation in the case of negatively charged microparticles, are compared to recent work on charge-induced nucleation of neat supercooled water, where a distinctly different nucleation behavior as a function of droplet charge has been observed.

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Introduction Charge-induced nucleation of aqueous droplets is a process that is known since decades and was first quantitatively investigated by Wilson.1 This process is of interest to the atmospheric environment, since excess charges are known to occur due to a variety of different processes. Specifically, the occurrence of ionic complexes in the stratosphere has been reported and possible implications regarding growth mechanisms were discussed.2 The role of atmospheric ions regarding aerosol nucleation has been reviewed before3 and more recently the response of cosmic rays to aerosol and cloud formation has been evaluated.4 In the laboratory, various experimental approaches have been developed and applied for studying nucleation processes. This includes various types of aerosol chamber studies.5,

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Alternatively, single, trapped, levitated microparticles have been investigated.7,

9

8,

Furthermore, the nucleation properties of sea salt particles were communicated, where specifically phase transitions of NaCl aerosols were discussed.10 It is known that the saturated solution of NaCl is formed at a relative humidity of 74%, which corresponds to the deliquescence point. At a relative humidity below the deliquescence point isolated microparticles become supersaturated and remain liquid over long time periods unless the surrounding humidity is above the efflorescence point that is known to be at ~45% relative humidity.10 These phase changes have been systematically investigated for a variety of different salt solutions by using so-called humidograms.11 This implies that single trapped microparticles containing salts or sea spray can be dissolved and crystallized repeatedly.12 The solubility of NaCl in water has been investigated by molecular dynamics simulations using different force fields.13 The crucial interactions, such as ion-ion, ion-water, and waterwater interactions have been evaluated in ref.

13

. More recently, a study on the nucleation of

NaCl has been reported, in which the critical size is shown to be a concentration dependent property ranging between 6 and 31 molecules, which is derived from supersaturated solutions ranging between 12 to 8 mol/kg.14 Particular attention was devoted in this work to monitor the local ionic environment during nucleation by assigning the clusters either a wurtzite or rock-salt structure. Furthermore, studies on the crystallization of sodium chloride from aqueous solutions identified the occurrence of a dihydrate (NaCl·2 H2O).15-17 We have published in the past results on the homogeneous nucleation of trapped microparticles containing either supercooled water,18, constituents,20,

21

19

binary mixtures of atmospheric

well as salt solutions.22 Furthermore, particle charging and discharging

mechanisms were communicated before.23


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Excess charges possibly affecting the homogeneous nucleation of, e.g. water, have been considered before, but no clear changes in nucleation properties have been observed.19 More recently, more systematic studies were published, where specifically the role of surface charges affecting homogeneous nucleation was addressed.24 It was found that even in a wider range of surface charges than exists in the Earth's atmosphere, reaching up to 4 pC per droplet, no significant changes in nucleation behavior were observed. This was interpreted as evidence that such processes are of no importance for homogeneous nucleation rates of supercooled cloud droplets. This is the motivation for the present work, where we report on systematic measurements of crystallization times of microdroplets consisting of aqueous NaCl solutions while varying the number of negative excess charges and the relative humidity, which corresponds to the partial water vapor pressure in the surroundings of the droplet under study. These results are compared to previous work on the homogeneous nucleation of neat water in order to derive a mechanism of charge-induced nucleation from the experimental results. These findings are further supported by molecular dynamics simulations in order to gain a detailed understanding on the molecular level of the processes occurring in supersaturated microparticles containing highly concentrated sodium chloride solutions and negative excess charges.

Experimental details Single microparticles are prepared by a piezo nozzle (Microdrop). From there droplets of 30 m in diameter are ejected into a microparticle trap. Two different particle traps are used: (i) an electrodynamic trap for suspending charged particles at a given charge-to-mass ratio25 and (ii) an optical trap,26 which is not sensitive to the charge of the particles rather than their mass. Note that larger particles were also injected into the electrodynamic trap (70 m), which did not show any changes in crystallization time. Most studies communicated in this work were performed using the optical trap, since it can store neutral as well as charged droplets. This was due to the fact that the high voltage driver of the electrodynamic trap could only trap microparticles with negative charges q up to ca. -300 fC. Thus, the electrodynamic trap was mostly used to validate the results gathered in the optical trap at low charge values. The designs of both traps used in the current work are briefly described in the following. The setup for electrodynamic levitation has been described before.22, 27, 28 The trap consists of a top and bottom cap electrode and a central ring electrode. A dc-voltage ranging between 0 and 300 V is applied on the cap electrodes in order to stabilize the levitated droplet by compensating its gravitation force. Typically, this voltage was set to 0 V. The ring electrode is charged by an ac-voltage in the range up to 6 kV with an adjustable frequency in order to


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meet the conditions of stable suspension of the trapped particle, which is typically below 500 Hz. The trap is surrounded by a climate chamber which allows us to stabilize the humidity in the surroundings of the levitated droplets at ambient pressure and temperature. The humidity in this trap chamber is controlled by a combination of a water bath and the drying agent phosphorous pentoxide (P2O5). These compartments are connected to the climate chamber by two variable leak valves. This allows us to adjust the desired humidity in the trap. It can be stabilized over several hours within ±0.5%. The relative humidity (RH) is measured by a calibrated sensor (Dostmann, P570) with an accuracy of 1%. The optical levitation cell has a size of 2 cm × 2 cm × 7 cm.29 It confines single, levitated microdroplets. The walls of this trap are made of plexiglass. A cw-laser beam ( = 532 nm, Coherent, Verdi 5 W) is vertically focused (f = 80 mm) from the bottom into the levitation cell. Droplets with a size around 30 m are trapped using a laser beam power of up to 2.0 W. This allows us to trap single microdoplets for hours. The optical system is mounted on a microstep motor stage (PI) which permits to move the droplet, once it is trapped by the laser beam. We used a CCD camera (Motion Pro X4, Imaging Solutions) that is mounted perpendicular to the laser beam in order to view, if a droplet is trapped. It also serves for monitoring the particle size by taking Mie scattering patterns around 90º scattering angle, as is outlined below. The humidity in this levitation chamber is controlled by a moisture exchanger (Perma Pure METMSeries), which is connected to a gas flow system. This device uses a Nafion membrane. The flow of dry nitrogen is split into two parts. The first one is moisturized by passing the gas through water yielding humidified nitrogen, whereas the second part of flow remains dry. The desired relative humidity, measured by a SHT75 sensor (Sensirion, Eval. Kit EK-H2),29 is adjusted by the mixing the proportions of the wet and dry gas flow. Mie scattering serves for measuring the particle size during levitation and variations of the trapping conditions, i.e. humidity and temperature.22 Excitation is accomplished by a helium neon laser (= 632.8 nm). As long as the injected droplet stays liquid, the scattered light consists of a characteristic stripe pattern. This pattern vanishes upon phase transition, yielding a speckle pattern of the polycrystalline solid.22, 28 Particle charging is performed for electrodynamic and optical trapping as follows: Liquid microdroplets are generated by a piezo nozzle (Microdrop), where charging is done during the ejection process from the glass capillary via electrical influence and electrokinetic charging, reaching up to excess charges of typically q < 300 fC. The magnitude of this charge is controlled by shifting the phase between the ac-voltage relative to the arrival of the particles in the trap (cf. ref.

30, 31

). For optical levitation electrical charging of the droplets is


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accomplished by an electrode, that is connected to a high voltage charger (FuG), so that high voltages up to 3.5 kV can be applied. This voltage is applied to the liquid samples that are stored in a connected vial. In this work, we focus on particles carrying negative excess charges. This approach yields particle charges up to -2.10.2 pC. Note that systematic studies on the particle charge as a function of applied high voltage yield a linear change below 1.65 kV (particle charge < -700 fC), whereas above this value a jump in particle charge is observed yielding particle charges ≤ -2.1 pC. The particle charge is determined by a collection electrode, at which the microdroplets are discharged, similar to ref.

32

. The collected charges are measured by an electrometer

amplifier (Keithley 6514), which can determine charges  10 fC. The nucleation experiments are performed as follows: At constant temperature, the crystallization time is determined for single levitated droplets that are sequentially trapped either by electrodynamic or optical levitation. These studies are performed at a constant relative humidity (0.5%) for typically 40-60 droplets, similar to earlier work.22 This required to wait after adjustment and stabilization of the relative humidity for at least for 2-3 min before starting the measurements. The results yield a significant change in crystallization time that depends on the excess charges of the trapped particles and the relative humidity at ambient temperature (T = 2984 K). Note that all measurements are carried out at ambient pressure. All initial aqueous solutions are prepared from ultrapure water (Millipore, Milli-Q) and NaCl (Roth, purity 99.5%). The solutions are prepared with an initial concentration of 2.9 mol/L.

Molecular Dynamics Calculations The formation of critical clusters is analyzed by molecular dynamics simulations (MD). The software package CP2K33 is used in conjunction with many-body-potentials in order to calculate classical molecular dynamic trajectories. A Coulomb pair potential is used to describe the electrostatic interactions with neutral water by taking the charges q = +0.417 e on hydrogen and q = -0.834 e on oxygen, where e is the elementary charge. For OH- the charges are taken from Mulliken charge distributions calculated by Gaussian09,34 using qO = -1.317 e and qH = 0.317 e. The charges on Na+ and Cl- are chosen as qNa+ = 1.0 e and qCl- = 1.0 e, respectively. Conventional Lennard-Jones 6-12-type potentials are used for simulating the water-water and water-ion short-range interactions. Born-Mayer potential with Van-derWaals corrections taken from ref.

35

are used for determining the non-bonding ion-ion

interactions. The resulting potential is expressed by eq. (1):


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q q    i j ij  4 ij  V     rij ij  rij   where  ij 

i   j 2

12 6    ij          r     ij   

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(1)

and  ij   i  j .

The corresponding Van-der-Waals parameters are compiled in Table 1.

Table 1: Van-der-Waals parameter of the species under study.35 atom σi (Å)

εi (kcal//mol)

Na

3.734

0.047

Cl

4.940

0.150

O

3.536

0.152

H

0.400

0.046

The harmonic force constant of the H-O bond in water is taken as 553.0 kcal/(mol·Å2) with an equilibrium bond length of 0.9572 Å. The elementary cell used for periodic boundary conditions is on average 18 × 18 × 18 Å3 and includes 165 water molecules. For seeding experiments the cell is increased to a size of 27 × 27 × 27 Å3, containing up to 505 water molecules and 64 ions. The cutoff constant in the Ewald summations α is equal to 0.44 and the cutoff radius for the short-range interactions is 11.0 Å. A Nosé thermostat is used to equilibrate the system at room temperature (T = 298 K). The total simulation time is set to 12.5 ps.

Results and Discussion Figure 1 shows a series of microdroplet observation times of differently charged optically trapped microdroplets, from which the efflorescence humidity is determined by the significant drop in observation time if the humidity in the trap chamber is slowly decreased. This is possible, since solid NaCl-microparticles cannot be stored in the optical trap and get readily lost after nucleation and is due to small trapping forces of the anisotropic microparticles.28 In the case of electrodynamic levitation angle-resolved Mie scattering patterns sensitively indicate the phase transition.22 The efflorescence of NaCl microdroplets containing no excess


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charges has been reported before to be ~45% relative humidity.36 Note that a larger value range has been reported for differently sized particles, ranging between 43% and 54% relative humidity,37 which has been subsequently interpreted by model calculations.38 The reported humidogram of NaCl is less steep for efflorescence than for deliquescence,39 which means that the size change upon efflorescence might occur at a less well defined regime of relative humidity than deliquescence. It has been suggested that this is due to the statistical nature of the efflorescence process, so that the relative humidity, that corresponds to the phase transition from the metastable supersaturated solution into the solid, is not well defined.40

Figure 1: Observation times of neutral and negatively charged NaCl solution microdroplets levitated in the optical trap. The microdroplets get lost after efflorescence, limiting the observation times. The relative humidity values, corresponding to the steep drop in observation time correspond to the efflorescence humidity. The dashed curves are used to guide the eye, the dashed-dotted line indicates the maximum observation time that was arbitrarily set to 600 s. The inset shows the charge dependence of the relative nucleation humidity at the efflorescence point. The results from optical trapping shown in the inset are presented in red color, additional data points using the electrodynamic trap are indicated in cyan color (see inset). The charge values are calibrated according the procedure detailed in the text.


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Microparticles without any excess charges are investigated by optical trapping, since electrodynamic trapping requires excess charges. The humidity surrounding the droplets is carefully varied. This yields an abrupt change in crystallization time at 46.11% relative humidity. We called this value in our earlier work 'critical partial water vapor pressure'22 or 'critical humidity', which corresponds to the efflorescence humidity, below which the trapped droplet undergoes spontaneous nucleation, yielding solid sodium chloride particles. If the humidity is further lowered, immediate phase transitions are observed, which cannot be temporally resolved by the present setup (t ≤ 1 s). However, if the humidity is increased from this critical partial water vapor pressure, then the droplet remains liquid within the selected time period of 600 s that is chosen for observing each liquid droplet. Note that we used in previous work 2000 s as an upper limit of the observation time, which yielded comparable changes in nucleation behavior,22 since the droplets may remain above the effloresence humidity even for hours in the liquid state. The humidity window between these limits, where the efflorescence time varies strongly with humidity, corresponds to the efflorescence relative humidity. This is consistent with previous efflorescence studies on NaCl, indicating the validity of the present approach.37 As the particle charge, i.e. the number of negative excess charges, is increased one observes for a given relative humidity that the crystallization time increases. These distinct changes are compiled in Figure 1. This charge effect is small for excess charges < -300 fC. Note that additional studies were performed, in which the sign of the charges was changed from negative to positive (not shown).30 These results were received from electrodynamic trapping along with a phase-dependent injection of the charged particles, so that either particles with a positive or negative excess charge were stored in the trap.30 These studies give within the experimental error limit of 1% for the efflorescence relative humidity no distinct changes, indicating that both, positive and negative excess charges do not influence selectively the experimental results, if the microparticles are charged < 300 fC. The data points gained from electrodynamic trapping are indicated in the inset of in Figure 1 by cyan color. The slightly displaced values gained by electrodynamic levitation as compared to the optical levitation are most likely due to different setups using different humidity sensors (see Experimental Section). However, if the particle charge is further increased to -40040 fC for negative excess charges, which can only be done by optical levitation (see inset of Figure 1 indicated by red color) due to limitations of the ac-voltage generator used for electrodynamic levitation, then one observes a significant increase in critical efflorescence humidity to 53.21.0%. If the particle charge is further increased to -2.1 pC, then the critical efflorescence humidity is even higher, occurring at 61.31.0%, as shown in Figure 1.


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We discuss in the following plausible reasons for these changes in relative efflorescence humidity, as shown in Figure 1. Firstly, the location of excess charges is of importance, since homogeneous nucleation, which is typically investigated by levitation techniques, is sensitive to the size of the stored particles. Specifically, it has been shown that there are such size effects for nanoscopic droplets.37 This leads to an increase of the relative efflorescence humidity with decreasing particle size, reaching up to ca. 54% relative humidity and has been ascribed to the influence of the Kelvin effect, as follows from related theoretical work.38 Certainly, the present results must have a different origin compared to the earlier results on nanoparticles, which is due to the fact that the present particle size exceeds the Kelvin regime. Furthermore, it is known that electrical excess charges are located on the surface of microparticles.41 This creates a potential, which can lower the surface tension.42 If these would be the essential reasons to rationalize the present results, then one would have to find the same excess charge dependent changes in pure water as well as electrolyte solutions, which is evidently not the case. Recently, Rzesanke et al. have discussed the role of surface charges for homogeneous freezing of supercooled water droplets.24 Their work goes beyond earlier studies,19 where the range of atmospheric charge densities up to 200 elementary charges per m2 of cloud droplets was covered in order to determine the possible role of related nucleation processes in the ambient environment. There, neither an influence of the surface charge on nucleation nor for filamentation-induced freezing was observed. This already hints that there are distinct differences compared to the highly concentrated, supersaturated electrolyte solutions, which are under study in this work.24 In addition, homogeneous nucleation is a bulk phase effect, since the nucleation rate is proportional to the droplet volume, whereas charge localization on the droplet surface is related to the liquidgas interface. In order to derive structural and size-dependent information on the molecular moieties, which contribute to the observed charge-induced nucleation process, we have performed molecular dynamics simulations on clusters containing negative excess charges starting from metastable supersaturated solutions. It is known from previous work that these solutions contain pre-nucleation clusters, which tend to grow in size as supersaturation increases with decreasing humidity in the vicinity of the levitated droplets.29 Once the critical size is reached, they spontaneously grow. In addition, at decreased distance between such clusters at high excess charges, they may merge, also overcoming the critical size for spontaneous growth. It is of central interest to identify possible carriers of negative excess charges and their properties in comparison to neutral droplets. This concerns on the one hand the radial properties of such species indicating how possible growth mechanisms in clusters occur, as


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shown in Figure 2. On the other hand, plausible cluster structures are discussed, which are shown in Figure 3. The free energy profiles of different ionic pairs are calculated by a standard procedure, described in the documentation of the CP2K code.33 The force f(x) needed to constrain the ions at a given distance ranion-cation, is obtained from averaging the values of Lagrangian multipliers over 100000 steps of a molecular dynamics simulation run. The free energy profile of a Bjerrum pair GBjerrum is derived by integrating the average force and using the closest distance dmin as a lower integration limit:

GBjerrum 

d

 f ( x)dx

(2)

d min

The energy profile of NaCl dissociation, shown in Figure 2(a), has a barrier separating the associated pair region (ap) from the solvent separated pair region (ssp).14 The substantial barrier of ∆GBjerrum ≈ 3.8 kT is preventing the solution from efflorescence even in a saturated solution. Both ions are localized in their solvation shells and a phase transition can only occur at high supersaturation levels. In contrast, in the case of droplets carrying excess charges near the droplet's surface, the carrier for negative charges is identified to be OH-. The solutes cannot carry the excess charges, as mentioned above, since their heats of formation are expected to be significantly higher than the corresponding value for OH-. Furthermore, OH- is known to have a high mobility.43,

44

In case of OH-,, the carrier for

negative excess charges, interacting with Na+ there is a vanishingly small free energy barrier of 58%), as indicated by the cluster structure shown in Figure 3(d). The relative contribution of G to the barrier-free Na+ - OH– coupling, reducing the total G in the critical cluster, is approaching values ranging between 67% and 78% (cf. Figure 3(c), (d)). Several Na+-Cl– distances in the critical embryo are then stretching rNa+-Cl- to values ranging between 3.2 Å and 3.6 Å, which is rather typical for the cluster shown in Figure 3(d). Furthermore, the total shift in efflorescence humidity, observed in Figures 1 and 5(a), reaches up to 16% in relative humidity with increasing droplet charge, as compared to the neutral droplets. Clearly, the steep change in this property is reproduced by this model. This effect turns into saturation, which is due to the limited number of sites at which a replacement of Cl– by OH- can occur. The structural model developed in this work indicates that the critical cluster cannot accommodate more OH-, so that a further increase in negative excess charge will not change the structure of the critical nucleus, which results in an almost constant value of RHeffl (see inset to Figures 1 and 5(b)).

The dashed black curve in Figure 5(b) is showing the pure charge effect on the surface tension





by

taking

 q   0  1  4q q coul 

1  G (q ) G   q )   0  in 3



2 1 2

as stated in ref

determined from the fissibility factor

55

X q

eq.

(4)

and

by

calculating

42

. The value of the Coulomb limit qcoul can be

2

64

2

3  0 rdroplet  and in the present case (rdroplet

= 12.5 μm) it is exceeding unity already at -1.5 pC. The excess charge value of -2.1 pC is showing saturation in Figures 1 and 5, which is related to the charges accommodated on the droplets during ejection, as measured on the collection electrode. There are deviations of the experimental data points from the dashed black curve shown in Figure 5(b). These deviations from the experimental findings are most likely due to charge effect on the chemical activity and the density of the solution.


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Conclusions Efflorescence measurements on single levitated microdroplets indicate that negative excess charges lead to a significant increase in relative efflorescence humidity (RH). The values of the RH-shift range up to 16% for NaCl, which is used as a model system. Low excess charges (≤-250 fC) leave the relative efflorescence humidity almost unchanged. Higher values lead to a characteristic threshold with a turning point near -400 fC and saturation above -700 fC. The experimental results are assigned by calculating plausible model structures of NaCl clusters in an aqueous environment by MD-calculations. These results indicate that OH- is evidently the carrier of negative excess charges in the surface layer of negatively charged microdroplets. Structural models based on MD-calculations indicate that the OH--moieties become part of the critical clusters as well as water that allow for spontaneous growth at higher relative humidity than is possible for electrically neutral microdroplets. This structural model that is coupled to a charge-induced nucleation model clearly goes beyond homogeneous nucleation. It is rather shown that the modeling approach of heterogeneous nucleation is most suitable to rationalize the experimental findings. The change of surface tension with increasing excess charge appears to play an important though not exclusive role on the reported phenomena. Finally, we note that efflorescence studies were performed on neat water clusters.19,

24

In

none of these studies a significant change in efflorescence humidity was observed. This implies that the neat solvent cannot form clusters that are of similar stability and size, as observed in the present study. Evidently, the solute NaCl plays a crucial role for the formation of stable clusters overcoming the nucleation barrier, as is expected to be relevant for the tropospheric aerosol in the marine environment. This process can also be promoted by efficient aggregation of such clusters, if their density is sufficiently high, corresponding to a decreased distance between the clusters. In addition, we expect that for other highly concentrated salt solutions similar effects should occur with increasing the number of excess charges, as observed in this study that focuses on NaCl as a model system. However, this goes beyond the scope of this work. One might also extend such studies to positive excess charges, where H3O+ moieties are expected to be active as charge carriers in aqueous solutions.


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Acknowledgments Financial support by DFG within Collaborative Research Center 1109 (project B03) is gratefully acknowledged. We are grateful to the team of High-Performance Computing at ZEDAT (Freie Universität Berlin) for their assistance in support of the model calculations. We thank Prof. Baron Peters University of California, Santa Barbara, for valuable discussions.

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Figure 1



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Figure 2


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Figure 4


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Figure 5


Charge Effects on the Efflorescence in Single Levitated Droplets - The

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