Dissolution of Sessile Microdroplets of Electrolyte and Graphene

Sep 14, 2016 - Manipulating the way a droplet shrinks by evaporation or dissolution is an effective approach for assembling dissolved nanomaterials. I...
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Dissolution of Sessile Microdroplets of Electrolyte and Graphene Oxide Solutions in an Ouzo System Yuting Song,†,‡,§ Ziyang Lu,∥ Haijun Yang,⊥ Suojiang Zhang,*,‡ and Xuehua Zhang*,∥ †

Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, China Institute of Processing Engineering, Chinese Academy of Sciences, Beijing 100190, China § University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100190, P. R. China ∥ Soft Matter & Interfaces Group, School of Engineering, RMIT University, Melbourne, Victoria 3001, Australia ⊥ Shanghai Institute of Applied Science, Chinese Academy of Sciences, Shanghai100190, China ‡

S Supporting Information *

ABSTRACT: Manipulating the way a droplet shrinks by evaporation or dissolution is an effective approach for assembling dissolved nanomaterials. In this work, we investigate the dissolution dynamics of a submicroliter sessile droplet of electrolyte aqueous solution and of graphene oxide suspension immersed in a binary mixture of solvents, among which one is miscible and the other is immiscible with water (i.e., an Ouzo system). Our measurements reveal an interesting two-stage dissolution of the droplet: a fast initial stage and a slow second stage. The duration of the first stage is longer at a lower temperature, leading to a counterintuitive result that the dissolution completes faster at reduced temperature. The presence of graphene oxide in the droplet dramatically alters the dissolution dynamics, possibly due to its enrichment at the droplet surface. The finding from this work provides useful guideline for designing conditions to pack nanomaterials by dissolving droplets, especially for those temperature sensitive components.

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INTRODUCTION

It is known that manipulating the way a suspension droplet shrinks is effective for assembling nanomaterials dispersed in the droplet.10,11 In an ouzo system, varying the ratio of the miscible and immiscible solvents can give rise to dramatically different solubility for the droplet of the third solvent. Hence the droplet can potentially dissolve at a rate continuously adjusted by the composition in the surrounding phase. The dissolution power of an ouzo system may be harnessed to assemble nanomaterials for a myriad of ordered nanostructures that are determined by the shrinking dynamics of a suspension droplet. Our recent work has demonstrated this possibility of assembling nanosheets in a dissolving droplet as a complementary approach to other existing processes for making corrugated microstructures,12 for example, by freeze-drying and spray drying.13,14 The wide regime of dissolution rate of a droplet in an ouzo system is highly attractive for tuning the nanostructures of assembled materials. On the other hand, production of designed microstructures requires comprehensive understanding and good control of the dissolution dynamics of the sessile microdroplet in an ouzo system. In contrast to a large body of

When water is added into a clear binary solution of ethanol and oil, the solution becomes cloudy due to the spontaneous formation of oil microdroplets. This phenomenon is referred to as “ouzo” effect, named after an ancient Greek drink.1 Recently, this daily phenomenon has drawn increasing research interest because of its rich and complex fundamental aspects and practical relevance to flavor of drinks, lifetime of perfumes, and fabrication of functional nanomaterials and pharmaceutical product.2 Research efforts have been devoted to understanding the mechanisms behind the formation, stability, and interactions of nanodroplets in the ternary system.3 In their latest work, Zemb et al. has proposed a general principle to explain the stability of microemulsions in the ternary mixture, based on the microstructures in the mixture revealed by X-ray and neutron scattering and molecular dynamics.4 Different from classical micellar systems, the stability of the detergent-free ouzo emulsion is attributed to the balance of the hydration versus entropy.4 Tan et al. has focused on the dynamics of an evaporating ouzo microdroplet and identified phases during droplet evaporation.5 Under flow conditions, the mixing front of an oil (or gas) of an ouzo system can be applied to produce nanodroplets (or nanobubbles) at solid−liquid interface in a controlled way.6−9 © 2016 American Chemical Society

Received: July 30, 2016 Revised: September 12, 2016 Published: September 14, 2016 10296

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Figure 1. (a) Snapshots of a typical dissolving droplet as a function of time in 8% ethanol/toluene solution, scale bar 100 μm. (b) Plot of square of the droplet diameter as a function time. The initial droplet volume is 0.37 μL. (c) Plot of the droplet volume change as a function of time normalized by the droplet radius.

behavior for solution droplets of salts either soluble or insoluble in the surrounding phase and attributed it to the convective flow induced by the fast outflux from initial droplet dissolution. This work has significantly advanced the understanding from our previous work,12 where two-stage dissolution for water droplets was observed but neglected because the lifetime of droplets containing GO in water was predominately determined by the extended second stage dissolution. The insights from this work will be helpful for design of the dissolution conditions for packing nanomaterials or concentrating components in ouzo systems.

literature on the dissolution of a sessile microdroplet in immiscible or partially miscible liquid phase,15−17 there are only several quantitative experimental studies performed in an ouzo system.12,18 The dissolution of a droplet in an ouzo system is far from equilibrium, dominated by rich and complex mass transfer processes due to the large chemical potential of the system. The dynamical events, such as kicking at the interface, self-splitting, or self-propelling of the droplet, may occur.18,19 The dissolution behavior of a droplet in an ouzo system can be further complicated by the presence of colloidal particles and electrolytes in the droplets. Our recent work has shown that the salt concentration is an important parameter for the final morphology of the assembled graphene oxide-salt hybrid microsphere.20 The nanosheets stacked the least at a medium concentration of salts, indicating more than screening effects of electrostatic repulsion from the salt on the assembled structures.20 As nanoparticles in suspension stabilized by surface charges, GO sheets bear residue polar groups. Simple electrolytes in the nanoparticle suspension may have important implications on the droplet dissolution, as the electrolytes can induce osmotic pressure or influence the interfacial enrichment of the nanoparticles and packing of the nanoparticles. The salt may also reduce the solubility of water in the surrounding binary solution, inducing ouzo effect and nanodroplet formation at the droplet surface.1 It is of great importance to understand the droplet dynamics in the presence of salt for reliable production and optimization of the GO assembled nanostructures. This work will examine the droplet dissolution under controlled conditions to experimentally reveal the effects of several main parameters on the droplet dissolution, including the salt type and concentration, droplet initial size, temperature, and initial GO concentration. To the best of our knowledge the influence of those basic parameters on the droplet dissolution in the ouzo system has not been systematically investigated in literature. We found that the dissolution dynamics is influenced by temperature, droplet initial size, and coupled effects of GO and salt. We observed an interesting two-stage dissolution

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RESULTS

Two-Stage Dissolution of a Sessile Droplet in an Ouzo System. In our experiments, a submicroliter droplet of potassium chloride (KCl) or lithium chloride (LiCl) aqueous solution was deposited on a hydrophobilized silicon substrate immersed in a binary mixture of ethanol and toluene. The volumetric concentration of ethanol in the mixture was 8% in all the experiments. As shown in the time course snapshots in Figure 1a, the shape of the droplet was close to a sphere during the entire process of the droplet dissolution, while the contact angle of the droplet was measured to be around ∼154°. At temperature of 298 K, the size of the droplet of KCl solution (D) is plotted as a function of dissolution time in Figure 1b, showing two distinct dissolution stages. There are a very fast first stage and a slow second stage. The square of the droplet diameter at both stages is fitted linearly with D2t = D20-at, where D0 is the initial droplet diameter, Dt is the droplet diameter at the time of t, and a is a constant. At the transit from the first to the second stage, the dissolution rate changed from a1 of 1.5 × 10−3 mm2 s−1 to a2 of 2.1 × 10−4 mm2 s−1. There is no noticeable difference in the droplets of KCl solution and of LiCl solution. As KCl is insoluble but LiCl is soluble in the surrounding phase, the two-stage dissolution cannot be attributed to the increased salt concentration in the droplet with the dissolution. The osmotic pressure in the droplet containing KCl is not related to the transition of the dissolution 10297

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Figure 2. (a) Plot of square of the droplet diameter with different initial volumes as a function time. (b) Transit time from the first to the second stage as a function of the initial volume of the droplet. A fitting line was drawn to show the trend.

Figure 3. Plot of square of the droplet diameter as a function of time at different LiCl concentration (a) 0.5, (b) 2.5, and (c) 5 mg/mL. Inset figures for each plot highlight the transition points. (d) Dissolution rate of LiCl droplets in first stage and second stage as a function of LiCl concentration.

in Figure 1c, which reflects the water undersaturation level around the droplet. The undersaturation level decreased very sharply from 0.64 to 0.07 at the first stage and then remained almost constant until the completion of droplet dissolution. Effects of Droplet Size, Salt Concentration, and Temperature. Below we will show how the duration of the first stage is affected by several parameters. First of all, as the initial volume of the sessile droplet was varied from 0.06 to 0.50 μL, all of the dissolution curves possess the feature of two-stage dissolution in Figure 2a. The transit time postponed from 61 to 363 s with increase in the initial volume in Figure 2. Second, there are clear two stages in the dissolution of solution droplets with different concentration of salt, as shown in Figure 3. For a given initial droplet size, the first stage dissolution was not influenced much by the concentration of LiCl in the droplet solution. The droplet with a lower concentration of salt dissolves slightly faster at the second stage. Consequently the lifetime of a 0.5 mg/mL LiCl solution is shorter than the droplet of 5 mg/mL solution. The most unexpected effect on the droplet dissolution is from temperature. The two-stage dissolution behavior was also observed at different temperature. For a given initial droplet volume of 0.37 μL at five different temperature, the square of the droplet diameter is plotted in Figure 4a,b. Interestingly, the

phases. Below the dissolution dynamics is focused on the droplet containing LiCl. The two-stage dissolution is also verified from the shrinkage rate in the droplet volume. Assuming a sessile droplet with a contact angle θ, the volume change of a droplet during the dissolution can be described by21−23 −4RD(cs − c∞) dV = f(θ) ρ dt

(1)

We define the time scale τ = and t ̃ = t/τ, volume ratio change is Ṽ = V/Vo, and radius change is R̃ = R/Ro. Here f(θ) is a geometrical factor, which is constant as the droplet is almost a sphere, and the shape is constant during the considered dissolution period.24 Combined with eq 1 (ρR2o)/(csD)

c − c∞ dV ̃ =4 s R ̃ dt ̃ cs

(2)

Here D is the diffusion coefficient, ρ is the density of the droplet liquid (the density ρ of the droplet is assumed to be constant during the dissolution) and cs and c∞ are the solubility and concentration of water in 8% ethanol in toluene solution, respectively. The derivative of the droplet volume over time normalized by the droplet radius is plotted as a function of time 10298

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Figure 4. Square of diameter of (a) LiCl droplets with same initial volume 0.37 μL as a function of time at different temperatures. Inset of (a) highlights the transition points. (b) Transit time for LiCl droplets as a function of temperatures.

Figure 5. Diameter square of GO/LiCl solution droplets with different GO and LiCl concentration as a function of time: (a) 0 mg/mL GO, 0.5 mg/ mL LiCl, (b) 0.5 mg/mL GO, 0.5 mg/mL LiCl (c) 2.5 mg/mL GO, 0.5 mg/m LiCl, (d) 2.5 mg/mL GO, 0 mg/mL LiCl, and (e) 2.5 mg/mL GO, 2.5 mg/mL LiCl. Insets for each plot highlight the transition points. Dissolution rate of GO/LiCl droplets in first stage and second stage as a function of (f) GO concentration and (g) LiCl concentration.

10−3 mm2 s−1 when the temperature change from 283 to 298 K. On the second stage, the dissolution rate varied in a range from 1.2 × 10−4 to 2.2 × 10−4 mm2 s−1. However, the duration of the first stage varied with temperature (Figure 4b). The transition took place at 545 and 100 s at 283 and 303 K. Clearly, the duration of the first stage of dissolution is shorter at higher temperature. As the

overall time for the droplet dissolution is longer with an increase in the temperature. For example, the LiCl droplet dissolution took twice long at 303 K compared to 283 K. Our analysis shows that the dissolution rate of the droplets in each stage did not change much as the temperature increased from 283 to 298 K. In the first stage for a LiCl solution droplet, the rate was varied in a very small range from 1.48 × 10−3 to 1.67 × 10299

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Figure 6. Snapshots of dissolving a GO/LiCl droplet with 2.5 mg/mL GO and 2.5 mg/mL LiCl as a function of time in the 8% ethanol/toluene solution; scale bar is 200 μm.

Figure 7. SEM images of dry GO snowballs assembled by the dissolution of a GO-LiCl droplet. The concentration of GO and LiCl is (a,b) 2.5 mg/ m, 0 mg/mL; (c,d) 2.5 mg/mL, 0.5 mg/mL; (e,f) 2.5 mg/mL, 2.5 mg/mL.

dissolution rate at the first stage is one order higher than that at the second stage, a shorter duration in the first stage results in a longer dissolution time for a droplet at higher temperature.

This is opposite to diffusion-dominated droplet dissolution or evaporation where usually the higher the temperature is, the faster the droplet dissolution is, due to a reduced viscosity and a 10300

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water in the surrounding liquid and even leads to separation of water-rich and toluene-rich phases. The change in the liquid composition was actually evident in the experiments: some waves were visible near the bottom of the droplet due to the refractive index difference from newly formed water-rich surrounding phase and the original binary mixture (Supporting Information). The increase in the water content near the droplet gives rise to local difference in liquid density, which may trigger the instability of the fluid and the onset of hydrodynamic convection, as sketched in Figure 8a.

faster diffusion rate at higher temperature. Such temperaturedependent lifetime of dissolving droplet in the binary mixture may be useful for assembling temperature-sensitive components. Dissolution of a GO Suspension Droplet. GO was added into the droplets at the initial concentration of LiCl at 0.5 mg/ mL. As the concentration of GO varied from 0, 0.5, to 2.5 mg/ mL in Figure 5a−c, respectively, there are still two stages in the dissolution. However, the presence of GO reduced the rate of the first stage dissolution shown in Figure 5f. The reduction is even more pronounced at a higher concentration of LiCl. The dissolution rate at the first stage decreased with an increase in LiCl concentration (Figure 5g), which is in contrast to the negligible effect from only salt shown in Figure 3. As shown in Figure 5e, the initial fast dissolution vanished at the concentration of LiCl at 2.5 mg/mL. Instead the droplet expanded slightly at the beginning and then shrank with time. Figure 6 shows the time course snapshots of a dissolving droplet of GO in electrolyte solutions. The expansion is evident at time of 40−100s. The dissolution at the second stage is not influenced noticeably by GO or LiCl concentration in Figure 5f,g. The droplet dissolved as D2 linearly with time. At the later stage from 2000 s onward, the droplet became irregular, different from the smooth droplet at a lower concentration of GO and LiCl.20 These results show interplay of GO and salt on the droplet dissolution dynamics. The coupled effects were also observed in the dissolution of the droplet containing GO and KCl (Supporting Information). In this case, the droplet expands in the first stage. Although no significant difference was observed in the dissolution dynamics of KCl and LiCl droplets, the final assembled GO/electrolyte composites by the dissolution exhibit a difference in their microstructures, particularly in the size of salt crystals. Microstructures of the assembled GO snowballs in the presence of LiCl are shown in Figure 7. No crystals formed on the snowball at the LiCl concentration at 0.5 and 2.5 mg/mL, which is consistent with previous observation.20 The difference between LiCl and KCl is that for the latter some small needle-like crystals were observed around the assembled structures (Supporting Information). Interestingly, the KCl crystals were accumulated more near the foot of the GO/KCl sphere, which may be due to nonuniform distribution of KCl in the droplet at a late stage dissolution. Discussion. What drives the two-stage dissolution of an electrolyte solution droplet in an ouzo system? In the previous work, the dissolution behavior of liquid droplet in a partially miscible liquid phase was attributed to convection effects generated by the droplet self-propelling.18 The self-generated motion of the dissolving droplet was driven by Korteweg forces at the droplet interface.19 The motion modified the mass transfer from the droplet due to the droplet being in contact with fresh liquid medium all the time, and hence the droplet radius decreased linearly with time.19 However, the sessile droplet in our experiments remained at the same location throughout the dissolution process, so the constant contact of the droplet with the fresh liquid cannot explain the fast dissolution at the early stage. We propose the following mechanism to account for the two-stage dissolution of an electrolyte solution droplet: The solubility of water is high in the surrounding phase due to the miscibility with ethanol, which leads to rapid flux of water from the droplet that is freshly in contact with the liquid phase. The dissolved water creates a sharp increase in the concentration of

Figure 8. (a) Three phase diagram of water, ethanol and toluene at different temperature. Point A and B are initial conditions for the surrouding phase and water droplets and point B is where the interface equilibrium established. Scheme of the dissolution of (b) electrolyte droplet and (c) GO/electrolyte droplet. The black dots show the edge of the boundary layer in which water diffusion from the surface to the surrounding phase. The rapid out flux of water from the droplet leads to the heavier water-rich liquid going down to the substrate. The fast loss of water from the droplet dissolution causes the enrichment of GO nano sheets due to their large size, which slows down the establishment of the water-rich zone around the droplet.

To estimate whether the convection takes place, we define a Rayleigh number as below Δρg Ra =

3

( 2h )

μDw,t

(3)

Here the density difference Δρ is 0.14 g/ml between water and 8% ethanol in toluene solution, g is the local gravitational acceleration, μ is the dynamic viscosity of toluene solution (0.59 mPa s), and Dw,t is the mass diffusion coefficient of water in toluene (0.62 × 10−9 m2/s at 298 K). The characteristic length-scale of the convection is assumed to be half of the 10301

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large GO sheets at the interface between the dissolving droplet and the liquid phase. The interfacial enrichment of GO could slow down the outflux so much that the droplet started the intake of the surrounding solution at a high concentration of LiCl. Hence there was an initial expansion of the droplet size. Such significant influence from the presence of GO suggests that the interfacial properties of the droplet are important for the two-stage dissolution in the ouzo system. The interfacial enrichment of GO is also supported from our previous observation that empty microcapsules formed around a GO snowballs assembled in the same ouzo system. Those microcapsules were suspected to come from GO-coated toluene nanodroplets at the interfacial zone.20 We also observed the formation of nanocellulose crystals at the droplet surface, driven by their interfacial enrichment during the droplet initial dissolution.28

droplet heights, which are 0.3 and 0.23 mm for droplets with volume of 0.12 and 0.6 μL.25 Physical properties and the Ra number at different temperature are listed in Table 1. On the basis of eq 3, the Table 1. Properties of Solvents and Ra Numbers of the Droplet at Different Temperature30−34 T (K)

ρwater g ml−1

ρmix 102 g ml−1

Dw,t 10−10 m2 s−1

Δρ g ml−1

Ra 105

283 293 298

0.9997 0.9982 0.997

0.87 0.861 0.854

5.1 5.8 6.2

0.1297 0.1372 0.143

1.26 1.46 1.5

initial Ra number calculated from the droplet top is all above the critical value for three different temperatures, so the convection effect take place in all three cases. The convection effects is also recently reported for a dissolving sessile droplet in a single partially soluble solvent.26 There the droplet liquid density was lighter than the bulk liquid, opposite to our systems. As the sessile droplet remained at the same location in our experiment, the local concentration of water eventually built up from the droplet dissolution. The Ra number became smaller and the convection settled down with the droplet dissolution, once a layer of water-rich interfacial zone was established around the droplet. This layer stabilized the dissolution rate, allowing for more diffusive mixing of water in the binary mixture. Such water-rich interfacial zone is possible for the ternary system in our system. Through the interfacial layer, the water fraction may vary from the droplet surface to the bulk phase, while the ratio of toluene and ethanol remains the same.27 As shown in the three-phase diagram in Figure 8a, the water content can lie between the phase boundary (toluene rich) and the boundary of the ouzo zone (water rich), while the solution does not undertake macroscopic phase separation. In this way, the duration of the first stage is determined by the time required to establish the interfacial zone. Once the dissolution enters the second stage, water transports from this interfacial zone to the outside toluene-rich phase. The square of the droplet radius decreases much slowly and linearly with time, suggesting a diffusion-limited dissolution process at the second stage. Why is the first stage of droplet dissolution longer at lower temperatures? Building up the interfacial zone may be slaved to the time scale for the increase in water concentration. Longer time is required due to a smaller diffusion coefficient at lower temperature. We note that the difference in the duration of the first stage cannot be attributed to the change of water solubility in the binary mixture with temperature. As shown in Figure 8, the solubility of water in the binary mixture decreases at lower temperature, so the water-rich interfacial zone would need less dissolved water and shorter time to establish. The first dissolution stage of a smaller droplet was shorter, probably because of a smaller volume of the interfacial zone. The presence of salt in the droplet may facilitate the phase separation, due to lower solubility of electrolyte solution in the bulk phase. Meanwhile, the osmotic pressure also counteracts the liquid outflux from the droplet, slowing down the droplet dissolution. The presence of GO in the droplet slows down the droplet dissolution further. Our proposed mechanism for the dramatic effects of GO is sketched in Figure 8b. The rapid outflux of water from the droplet likely led to accumulation of

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CONCLUSIONS

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EXPERIMENTAL SECTION

In this work, we have studied the dissolution behavior of a sessile microdroplet in an ouzo system. The droplet was an electrolyte solution with and without graphene oxide and the surrounding phase was a binary mixture of toluene and ethanol. We observed two distinct dissolution regimes for the electrolyte solution droplet. The first dissolution regime was much faster than the second regime, possibly due to convection effects. The transition from the first stage to the second was attributed to buildup of an interfacial zone around the droplet. The extension of the first regime at lower temperature led to the counterintuitive result: the droplet dissolution completed faster at reduced temperature. The presence of graphene oxide in the droplet drastically changed the droplet dissolution behavior, possibly due to the interfacial enrichment of GO. The finding from this work may provide useful guideline for designing conditions to pack nanomaterials by dissolving droplets, especially for those temperature sensitive components.

Chemicals and Solutions. Potassium chloride (KCl, AR) and lithium chloride (LiCl, AR) were purchased from Sinopharm Chemical Reagent Co. Ethanol (GR) and toluene (AR) were purchased from Beijing Chemical Reagents Company. Octadecyltrichlorosilane (OTS, >90%) was purchased from Sigma-Aldrich. Silicon wafers were purchased from Mitsubishi Silicon (U.S.A.). The hydrophobic substrates were prepared and cleaned by following the protocol reported in previous work.20 All of aqueous solutions were prepared with Milli-Q Water (18.2 M Ω). The concentrations of both electrolytes aqueous solutions were 0.5 mg/mL. GO aqueous solutions were prepared according to our previous report.29 To prepare GO in KCl solution (m1/m2 = 1), KCl (1 mg/mL) aqueous solution was added into GO aqueous solution (1 mg/mL) of the same volume, and the obtained mixture was sonicated with a Branson Digital Sonifier (S450D, 45W, 30% amplitude) for 20 min. The mass concentration of GO was 0.5 mg/mL. GO in LiCl solution (m1/m3 = 1) aqueous solution was prepared in the same way. Monitor the Droplet Dissolution. As sketched in Figure 9, to in situ investigate the process of droplet dissolution a 10 mm path-length cuvette (3.5 mL; Nova Biotech, El Cajon, CA) was used as the container. A drop of the electrolytes or GO/electrolytes solutions were dipped on a hydrophobic substrate (octadecyltrichlorosilane-coated silicon, OTS-Si) which was immersed in 2 mL mixture of ethanol and toluene. The container was then sealed with a lid to avoid any evaporation during the droplet dissolution. The concentration of the ethanol was fixed with 8 vol % in the surrounding liquid phase. The dissolving droplet was monitored from the side-view by the contact angle meter (DSA100-Kruss). The diameter of the droplets was 10302

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automatically analyzed by self-written MatLab codes. From the videos, the edge of the droplets was detected and fitted with a circle. The diameter of the circle was plotted as a function of time. We followed the droplet diameter until the side-view of the droplet shape became not spherical. At this point, the concentration of GO in the droplet already was already very high, close to the completion of droplet dissolution. The properties of water, ethanol, and toluene at different temperature are listed in Table 1. A video was recorded at an interval of 30 s. The completion of the droplet dissolution was determined by monitoring the size of the droplet and determining that it had no longer changed with time. Characterize the Structures of GO Assemblies. The condensed GO/electrolyte droplet was taken out from liquid phase with a pipet (Eppendorf research) and then deposited on a positively charged mica coated with poly- (diallyldimethylammonium chloride) (PDMA) for SEM imaging. All GO/electrolyte ball images were obtained by an environment scanning electron microscopy (Quanta; FEI Company, Hillsboro, Oregon, U.S.A.). All the samples were scanned under a low-vacuum mode at an acceleration voltage of 10 kV.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b02837. Additional figures (PDF)

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REFERENCES

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Figure 9. Sketch of the setup to visualize the dissolving droplet. The videos were taken from the side view of the droplet and the droplet diameter was extracted from imaging process.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are extremely grateful for the valuable suggestion from Chao Sun and Detlef Lohse on the calculation of the Rayleigh number. X.H.Z gratefully acknowledges the support from Australian Research Council (FFT120100473). The authors also acknowledge the access to facilities and the associated technical support at the RMIT Micro-Nano Research Facility and the Microscopy and Microanalysis Facility in RMIT University. 10303

DOI: 10.1021/acs.langmuir.6b02837 Langmuir 2016, 32, 10296−10304

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DOI: 10.1021/acs.langmuir.6b02837 Langmuir 2016, 32, 10296−10304