Surface Nanodroplets: Formation, Dissolution, and Applications

Invited Feature Article Cite This: Langmuir XXXX, XXX, XXX−XXX

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Surface Nanodroplets: Formation, Dissolution, and Applications Jiasheng Qian,† Gilmar F. Arends,† and Xuehua Zhang*,†,‡ †

Department of Chemical and Materials Engineering, University of Alberta, Alberta T6G 1H9, Canada Physics of Fluids Group, Max-Planck-Center Twente for Complex Fluid Dynamics, Mesa+ Institute and J. M. Burgers Centre for Fluid Dynamics, Department of Science and Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands

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‡

ABSTRACT: Droplets at solid−liquid interfaces play essential roles in a broad range of fields, such as compartmentalized chemical reactions and conversions, high-throughput analysis and sensing, and super-resolution near-field imaging. Our recent work has focused on understanding and controlling the nanodroplet formation on solid surfaces in ternary liquid mixtures. These surface nanodroplets resemble tiny liquid lenses with a typical height of 1, eq 4 can be converted to eq 5

dθ 4D = Pe(1 + cos θ )2 f (θ ) cs(t ) ζ(t ) dt ρL2

(5)

where it can be integrated from tc(L,Pe) to ∞ and a contact angle of θc at the beginning of the CR mode to the final contact angle θf. Assuming the oversaturation pulse contains a Gaussian shape, we can describe the oversaturation pulse cs(t) ζ(t) by a maximum concentration cs,0ζmax and the pulse width τ. The integration of eq 5 becomes eq 6 ΔG : =

∫θ

c

E

θf

Dcs,0ζmaxτ dθ Pe(1 − α(L , Pe)) ≈ 2 f (θ )(1 + cos θ) ρL2 (6) DOI: 10.1021/acs.langmuir.9b01051 Langmuir XXXX, XXX, XXX−XXX

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Invited Feature Article

Figure 8. Droplet formation around a microcap. (A) AFM images of multiple nanodroplets around one microcap with symmetric N-gons (N = 2− 7). Scale bar: 5 μm. (B) Representation of the coalescence model when N = 9, 6, 3. The respective angles ϕ between the droplets before and after the coalescence process are also shown. (C) Angle ϕ (light blue) and average angle ϕ̅ (red) between droplets vs r/R as revealed from the model. Reproduced with permission from ref 41, copyright 2015 American Chemical Society, and ref 42, copyright 2018 Royal Society of Chemistry.

Figure 9. Growth dynamics of droplet branches. (A) Schematic illustration of the fluid cell setup for the formation of the nanodroplet branches. (B) Optical image of typical nanodroplet branch formation. (C) Optical images of the branches formed under different flow rates of 100 and 200 μL/min. (D) Droplet branches for enhanced colloidal particle transport in the quasi-2D channel. (E) PDFs of the angles between two merged branches over their full range at different flow rates and component ratios. Reproduced with permission from ref 20. Copyright 2017 National Academy of Sciences.

where ΔG is the integration with the contact angle function

ΔG =

and (1−α(L, Pe)) is the fraction of time that the droplet grows in CR mode. The equation can be simplified by casting the

Lτ 2 L2

Pe(1 − α(L , Pe))

(7)

where Lτ ≈ Dcs,0ζmaxτ /ρoil is the diffusive length scale based on the time τ of the oversaturation pulse for the oil of solubility cs,0 and density ρ. This work paves the way for manipulating

constants in a dimensionless form and naming the quantity Lτ (eq 7) F

DOI: 10.1021/acs.langmuir.9b01051 Langmuir XXXX, XXX, XXX−XXX

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nanodroplet on the microcap and θfs is the contact angle of the nanodroplet on the flat surface).

the size and morphology of surface nanodroplets in highly ordered arrays. The volume of an as-formed droplet can be controlled on the femtoliter or even attoliter level. On the basis of this work, the latest study on microbubbles grown on a prepatterned substrate demonstrated that it is feasible to control the growth of microbubbles in similar surroundings by a simple solvent-exchange process.39 Collective Interactions of Growing Surface Nanodroplets. In the period of droplet formation before reaching the final state, the droplets interact with their neighbors through the surrounding flow field. The collective interactions between droplets could further influence the surface coverage and the bare zone around the droplets. Previous works revealed that the surface coverage increases to a value in the range of 35−50% along with increases in the flow rate, channel height, and oil concentration.40 The number density reaches a maximum value when the surface coverage is around 25−30%, indicating the occurrence of mutual interaction and merging between the droplets. The Voronoi tessellation indicates that the depleted area A around the droplet and the droplet footprint area a show a linear relationship: A ∝ a in the case of high surface coverage (30%). In the event of low surface coverage, the value of A is dispersed over a large span even for a given value of a. The collective interaction is universal, which may provide a simple way to tune the droplet size and coverage. Nonplanar microstructures were used to investigate the local concentration effect for the collective interaction of growing droplets through coalescence. As reported by Peng et al., the growth of surface nanodroplets around a microcap leads to further self-organization into symmetrical arrangements with respect to position, size, and mutual distance.41 The AFM images of multiple nanodroplet formation around a microcap with symmetric N-gons (N = 2−7) are shown in Figure 8A. Initially, the droplets nucleated simultaneously at the rim of the microcap where their size was almost the same as those of isolated ones. The difference in droplet size occurred only after a while and kept increasing with time. The coalescence among droplets around a microcap was studied by utilizing a TIRF microscopy technique.42 The coalescence process occurred between two neighboring droplets around a microcap. They touched each other and coalesced into an ellipse droplet. A dynamical model of the coalescence process was proposed. As shown in Figure 8B, the merging process obeys the rule that two coalescing drops are removed and the final location of as-coalesced droplet is determined by the scaling law: Δθ1/Δθ2 ≈ (r1/r2)−2 (where Δθ1 and Δθ2 are the angular differences between two parent droplets and the final position of an as-coalesced droplet and r1 and r2 are the diameter of the two parent droplets), which is mostly influenced by the larger parent droplet, eventually leading to nearly symmetrical arrangements of droplets around the microcap with time. Figure 8C shows the numerical results from this model. The overall features of ϕ ∝ r/R (ϕ = 2π/N, r and R are the radius of the new droplet and the microcap, respectively) are well represented by the dynamical model. Peng et al. also investigated the position of the droplets sitting on the rim of the microcaps.22 The nucleation location of droplets is determined by the different wetting properties, including interfacial tension, contact angles, and droplet volume. A droplet may form on the rim of a microcap only when the contact angle of microcap (α) is larger than the absolute value of |θfs − θmc| (θmc is the contact angle of the

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UNIVERSAL DROPLET BRANCH FORMATION IN QUASI-2D CONFINEMENT Lu et al. investigated the droplet growth confined in quasi-2D geometry.20 The illustration of the fluid cell setup is shown in Figure 9A. The central area is denoted as the main channel with a depth of only about 20 μm. The channels on both sides of 1.7 mm in depth are denoted as the side channels. At the beginning, the main channel was entirely filled with an aqueous ethanol solution of the oil (i.e., ternary liquids solution). After that, solution B, the poor solvent (i.e., water), was injected into the side channel from the inlet and flowed to the outlet. Perpendicular to the external flow, solution B diffused from the side channel into the main channel. The optical image of a typical nanodroplet branch is displayed in Figure 9B. If the flow rate, oil concentration, or wettability of the channel walls are changed, then the branch formation remains similar to each other, as shown in Figure 9C. The mean branching angle analyzed from 660 angles was 74 ± 2°, which was independent of flow rate, composition of the droplet, and wettability of the substrate. This universal branching angle was found to be similar to the growth and merging characteristics of the ramification of stream networks, where the experimental bifurcating angle was around 72°, as shown in Figure 9E. In the theoretical model, this angle was rationalized by the growth of the 1D streams in the network controlled by 2D diffusion. Such processes can be analyzed by treating the harmonic field with the 2D Laplace equation in conjunction with the Löwner transformation.43 A significantly enhanced mobility of the colloidal particles can be observed from Figure 9D after loading the colloidal particles into the ternary liquids system. The colloidal mobility is possibly due to diffusiophoresis driven by the local chemical concentration gradient generated from droplet formation.

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DISSOLUTION OF SURFACE NANODROPLETS Four dissolution modes have been reported for droplets on the solid/liquid interface:44 CR mode, CA mode, stick-slide (SS) mode, and stick-jump (SJ) mode. Following the calculation for evaporating droplets by Popov,45 the droplet dissolution can be described as below. Assuming that droplet dissolution in the liquid phase is a diffusion-controlled process, then the mass loss (dM/dt) can be calculated (eqs 8 and 9): dM π = − LD(cs − c∞)f (θ ) dt 2 f (θ) =

sin θ +4 1 + cos θ dξ

∫0

∞

(8)

1 + cosh 2θξ tanh[(π − θ)ξ ] sinh 2πξ (9)

In CR mode, the contact angle varying with time (dθ/dt) can also be described by eq 4. Meanwhile, the lateral diameter (L) varying with time (t) in the CA mode can be described by eqs 10 and 11: L2(t ) = L0 2 − κ(θ )t

(10)

with slope κ (θ ) = G

8Dcsζ f (θ ) ρ 3g (θ )

(11) DOI: 10.1021/acs.langmuir.9b01051 Langmuir XXXX, XXX, XXX−XXX

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Figure 10. Droplet dissolution on flat and prepatterned surfaces. (A, B) Confocal microscopy images of MMA nanodroplets in saturated water. The lateral diameter of MMA nanodroplets in (A) are observed to change over time, as seen after 70 min in (B). (C) Contact angle profiles of HDODA nanodroplets as a function of lateral diameter. (D) Time evolution of droplet dissolution from experimental results (left) and numerical simulations (right). The scale bar is 45 μm. Reproduced with permission from refs 44 and 47. Copyright 2015, 2018 Royal Society of Chemistry.

where L0 is the initial lateral droplet extension and f(θ)/3g(θ) is the wall correction factor. The SS mode could be considered to be a combination of CR-CA modes. Typically, the contact angle continues to decrease in the CR mode first, which could be attributed to the pinning effect.17 After the droplet shrinks to a certain size, the CR mode converts to the CA mode where the lateral diameter starts to decrease with a constant contact angle. The SJ mode is similar to the SS mode: The droplet disolution switches between sticking and jumping. The contact angle of the drop increases after jumping because of the volume conservation and the sudden shrinkage in the drop lateral diameter. The dissolution mode is closely related to the surface micro/ nanostructures. Recently, a novel droplet dissolution mode referred to as zipping-depinning (ZD) was unveiled by Escobar et al.46 During the dissolution in ZD mode, two zippingdepinning fronts (ZDFs) recede along the rings on the substrate at a controlled velocity until detaching from the outer ring. Finally, the fronts meet each other and the contact line shrinks to the inner ring. The nanodroplet dissolution in the mixed mode was further confirmed by experimental measurements.44 The AFM images of as-polymerized MMA nanodroplets at different times are shown in Figure 10A,B. It is clear that after a period of time in still water some smaller nanodroplets decreased in size or almost entirely disappeared, leading to a decrease in the number density. The center of the droplet shifted with the dissolution, suggesting a pinning effect. Figure 10C shows the variation of the contact angle of 1,6-hexanediol diacrylate (HDODA) nanodroplets with the lateral diameter. When in the water flow, the droplet dissolution is much faster. The authors concluded that there is droplet individuality in the

dissolution rate due to the surface heterogeneity and cooperative effect. Bao et al. reported the dissolution dynamics of surface nanodroplets in a highly ordered array under a uniform flow.47 The real-time images and numerical simulations of the droplet array grown in a confined area dissolved in a flow of pure water are shown in Figure 10D. The dissolution initially starts from the corner of the array and gradually spreads to the inner part, which agrees well with the simulation results. The transition droplet diameter from the CR to the CA mode depends on both the pinning effect and the contact angle.48 Furthermore, the dissolution rate increases if the interdroplet spacing or the flow rate increases. It was found that sequential dissolution along the flow direction can occur in closely packed arrays as a result of the increase in the local concentration gradient for the droplets downstream by the dissolving upstream neighbors. Additionally, these effects are more pronounced for outer droplets at first because outer droplets have a shielding effect on the inner droplets. When an external flow is applied, the total dissolution time decreases at higher Reynolds (Re) numbers. As the Re number was further increased using the same closely packed array, each drop at any given location on the array began to dissolve simultaneously, indicating that the reduction of the local concentration gradient becomes more uniform by the faster external flow. From experimental data, a scaling law reflecting a laminar flow of the Prandtl−Blasius−Pohlhausen type was obtained for the dissolution time as Ti ∝ Re−3/4.

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EVAPORATION OF A SESSILE MULTICOMPONENT DROP AT SOLID INTERFACES The evaporation dynamics of an Ouzo drop on a substrate was studied by Tan et al.25 The parent drop contains water, H


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ethanol, and anise oil with an initial contact angle of 90°.50 An Ouzo drop was formed with an initial contact angle of ∼150° on a superamphiphobic substrate. The difference between this work and the prior one is that the Ouzo effect was observed to be triggered at the top of the drop, which could be attributed to two reasons: (i) the maximum local evaporation rate and (ii) the highest water concentration caused solutal Marangoni convection. Hence, the oil droplets may coalesce on the surface and finally form an oil shell wrapping around the Ouzo drop. Diddens et al. worked on the thermal effect and flow conditions during droplet evaporation.51 Taking advantage of the axisymmetric finite element method model with the influence of thermal convection transport, axial symmetry breaking was pointed out to be due to the Marangoni instability.

Figure 11. Surface wettability characterization by surface nanodroplets. (A) Microwetting of pH-sensitive surface and anisotropic MoS2 surfaces. (B) Contact angle on SLG, HOPG, and OTS surfaces versus ethanol concentration in the liquid phase. Reproduced with permission from refs 63 and 62. Copyright 2014, 2016 American Chemical Society.

indicating a high hydrophobicity. However, the ME surface at the same pH value as for the MF surface shows a contact angle of as high as >60°, implying that the layered microstructure may influence the surface properties, further demonstrating the anisotropic wettability of the MoS2 crystal. When graphene layers were used as the substrate, the polymerizable oil nanodroplets were formed on them through the solvent exchange process and were polymerized for morphology measurements.63 Figure 11B shows the contact angle profiles of the droplets produced and polymerized in various ethanol aqueous solutions on OTS-Si, highly oriented pyrolytic graphite (HOPG), and SLG surfaces, respectively. If no ethanol is added, then the contact angle values on the three surfaces are similar, suggesting the similar wettability of the oil on three substrates. If 5 or 10% ethanol is added, then the HOPG surface’s wettability is similar to that of the OTS-Si surface, while the contact angle of droplets produced on the SLG surface can be higher than those on the OTS-Si and HOPG surfaces, indicating less hydrophobicity of the oil on the SLG surface in the ethanol aqueous solution. Functional Surface Microstructures. Surface nanodroplets can be converted into permanent and stable structures. To this end, a polymerizable monomer was used as the oil phase. After the formation of surface nanodroplets by the solvent exchange process, by directly immersing the substrate64 or by adding surfactant,65 the monomer is polymerized by in situ photopolymerization under ultraviolet (UV) light. Finally, microlenses (MLs) are randomly formed on the surface with high transparency, large-scale production, and controlled geometries. Later on, the scalable production of a highly ordered microlense array on a prepatterned surface was reported for the enhanced performance of optical applications. Taking advantage of the photolithography technique, Yu et al. enlarged the production of the microlens array to a 4-in.-scale Si wafer.34 The production rate of 106 nanodroplets per second was achieved on either planar or bent substrates. Furthermore, a polymer-capped microstructure array, such as a micropillars array, can be produced on the basis of droplet formation along with the etching process afterward, as shown in Figure 12C,D. The surface nanodroplets could not only be implemented for wide applications in light harvesting, as shown in Figure 12A,B, but also can serve as a versatile structural template in surface engineering, for example, in tailoring the wettability and morphology.2,66 On the basis of the microlens array,

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POTENTIAL APPLICATIONS OF SURFACE NANODROPLETS As mentioned in the Introduction, droplets on surfaces have potentials in a wide range of applications.52−57 The above understanding of surface nanodroplets by solvent exchange has provided the basis for the scalability of surface droplets with controllable geometries and morphological properties for different implementations. These will be introduced in this section. Visualization of Microwettability on the Microscopic Scale. The morphology of surface nanodroplets is affected by surface wettability. Morphological characteristics of surface nanodroplets may be examined to compare the features of the local microwettability of the surface. Xu et al. coated the substrate with a methyl-terminated alkanethiol monolayer with different alkyl chain length to investigate the effect of static wettability on the surface nanodroplets and the origin of threephase contact line pinning.58 With the increase in the alkyl chain length from 8 to 14 methyl groups, the lateral diameter of the surface nanodroplets also increases while the contact angle and number density of the as-formed surface nanodroplets decrease, demonstrating that the morphology of surface nanodroplets is sensitive to the detailed molecular structures of the monolayer on the substrate. The study of surface wettability was extended to the layered materials.59−61 As shown in Figure 11A, The droplet-based protocol was employed in the MoS2 wettability study.62 The oil droplets show significantly different contact angles under acidic condition on the MoS2 face and edge (denoted as MF and ME, respectively) surfaces, respectively. Compared to the pH-sensitive surfaces (carboxylic acid monolayer on a Au substrate) under various pH conditions, the contact angle of the oil droplets on the MF surface at pH 3.0 is close to 0°, I


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Figure 12. Microlenses, micropillars, and other 3D microstructures produced by polymerized surface nanodroplets. (A, B) Photographs of two lens arrays on a glass substrate against a blue background and diffraction patterns generated by the corresponding 5 × 5 μm2 highly ordered nanolens array. SEM images of the arrays of hybrid micropillars of (C) 5 and (D) 10 μm in thickness. (E, F) Top-view SEM images of complex micropillars by the templating of polymerized dissolving droplets on circular patterns. Scale bars are 30 μm. Side-view SEM images of mushroom structure by etching the silicon substrates decorated with polymer lenses (G) and droplet on lenses (H). Reproduced with permission from ref 34, copyright 2016 American Chemical Society; ref 48, copyright 2017 American Chemical Society; and ref 66, copyright 2017 John Wiley & Sons, Inc.

Taking advantage of the synergistic effect between the focusing effect of the microlens and the strong response of the Au nanoparticles (NPs) to light, bubbles are able to nucleate and grow on the surface of the microlens. The snapshots of the bubble growth in Figure 13B show that the bubbles can grow to 1.5 μm over 128 ms as a result of the localized effect of plasmonic Au NPs, demonstrating an extensive potential for the microlens-based applications. Colloid Assembly Driven by the Dissolution or Evaporation of Ternary Drops. Some applications are based not only on droplet formation but also on the shrinkage of droplets, such as in the assembly of nanomaterials.68 The dissolution of droplets containing graphene oxide (GO) was investigated because GO can be easily dispersed in water and assembled into GO sheets.69 The GO aqueous droplet was first formed on the OTS-Si substrate, followed by dissolution in an ethanol/toluene mixture. As a result, GO assembled into a snowball-like structure, which could be attributed to the dissolution dynamics of the droplet and the influence of shear flow. Additionally, the pinning effect on the morphology of the GO droplet after dissolution was discussed in another article.70 By adjusting the ethanol concentration in the surrounding

biomimetic microstructures can be built up to form mushroomlike, droplet-on-lens, droplet-on-mushroom, core− shell, or even more complicated structures with controllable geometrical parameters by the prepatterning process, as shown in Figure 12G,H. On the other hand, the shape of the microlens can be manipulated by multiple domains on the surface.48 Figure 12E,F shows 3D micropillar arrays after three and four droplets coalesced together. This work developed an alternative approach to the construction of surfaces with complex morphology in a liquid system. The latest work on microlenses is on the focusing effect reported by Deytt et al.67 As depicted in Figure 13A, the surface microlenses were placed in the evanescent wave field which was produced by the total internal reflection (TIR) of light. In this area, a sharp light intensity enhancement was demonstrated within a certain focal distance from the microlens. Overall, a local minima of light intensity can be observed at the apex of the microlens, while the light intensity sharply increases when the position moves across and away from the microlens. The effect of the microlens on plasmonic bubble formation was evaluated by using Au-decorated PLMA microlenses on a glass substrate, as shown in Figure 13C. J


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Song et al. achieved the encapsulation of inorganic salt by GO in a water/toluene/ethanol ternary system.71 Interestingly, the curve of the dissolution rate with time indicates a two-stage process. The dissolution rate in the first stage is much faster than in the second stage. Besides, the droplet size, salt concentration, and temperature influence the transit time, which may facilitate the control of the dissolution process for the nanomaterials package. The assembly of different nanocolloids, such as nanospheres, nanocellulose, and silk fibers, was all possibly achieved by the droplet dissolution process.72 An example was the dissolution of an aqueous droplet containing SiO2 NPs on the OTS-Si substrate in the mixture of ethanol/toluene. By controlling the ethanol concentration, the SiO2 NPs assembled to spherical and toroidal structures, along with the arrangement of SiO2 NPs changing from ordered (hexagonal and square array) to disordered. The approach can also be extended to other types of particle suspensions with different initial concentration or different materials and shapes. Not only through the dissolution process, the porous supraparticle assembly was also achieved through selflubricating evaporating colloidal Ouzo drops.73 On the basis of the evaporation mechanism of an Ouzo drop,25 a ternary system containing anise oil, ethanol and water may serve as the carrier and template for different nanoparticles while also forming a lubricating layer to overcome the pinning effect during evaporation. The synergistic effect leads to the scalable assembly of colloidal particles with tunable shapes and high porosity on hydrophobic surfaces, which could be regarded as a versatile tool for the production of porous functional materials for various applications. Deytt et al. reported the crystallization of femtoliter lipid nanodroplets immobilized on the prepatterned surface.74 Through the synchrotron small-angle X-ray scattering (SAXS) measurments, the authors found that the surface lipid nanodroplets have a lower onset temperature compared to that of the bulk lipids. Meanwhile, compared to the larger lipid droplets, the smaller ones have a higher surface-to-volume ratio, resulting in a faster crystallization rate. Reactive Droplets for Online Microanalysis. The ongoing work for droplet-based protocols turns to the online

Figure 13. Focusing effect of a surface nanolens. (A) Threedimensional representation of a TIRF image for an individual microlens. The approximate focal length and enhancement are schematically depicted by the arrows. (B) Series of optical images captured in bright field, with tilted incident illumination. The size of the lenses is D ≈ 2.9 μm. The scale bar is 1 μm. (C) Tilted SEM of a representative gold-decorated microlens and schematic representation of the bubble growth via plasmonic heating. The scale bar is 1 μm. Reproduced with permission from ref 67, copyright 2018 American Chemical Society.

solvents, the contact angle of the GO aqueous droplet changes, leading to the assembly of different nonspherical morphologies including a capsule, dim sum, olive, peasecod, and mushroomlike architecture.

Figure 14. Reactive droplets for supersensitive online microanalysis. (A) Schematic of the programmed flow sequence for the automation of formation of functionalized surface droplets. (B) Schematic of the extraction based on surface droplets. Reproduced with permission from ref 26. Copyright 2018 John Wiley & Sons, Inc. K


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pollutant analysis in water through SERS detection.26 As depicted in Figure 14, binary surface nanodroplets containing vitamin E and octanol (VE/OCT) were first produced through solvent exchange. Then the Ag salt solution was added to react with vitamin E at the water/oil interface to form Ag NPs. These Ag NP-decorated droplets may concentrate the analyte for an enhanced response by in situ SERS analysis. As shown in Figure 15A, using rhodamine 6G (R6G) solution to serve as


AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xuehua Zhang: 0000-0001-6093-5324 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are extremely grateful for our close collaborators. These findings would not have been possible without extremely valuable input from Detlef Lohse, John M. Shaw, Harold J. W. Zandvliet, Christian Diddens, Leslie Yeo, Bat-El Pinchasik, Yoshiyuki Tagawa, Chao Sun, Yuliang Wang, and others. This work is attributed to years of effort from former group members Ziyang Lu, Lei Bao, Shuhua Peng, Miaosi Li, Huanshu Tan, Haitao Yu, Chenglong Xu, Brendan Dyett, and others. We are thankful for funding support from the Natural Sciences and Engineering Research Council of Canada (NSERC), Future Energy Systems (Canada First Research Excellence Fund), and The Netherlands Center for Multiscale Catalytic Energy Conversion (MCEC).

Figure 15. SERS of hydrophobic micropollutants in water. (A) Schematic of the in situ detection based on surface droplets. Bottom: SEM image of Ag NPs (left) and fluorescent image of the AgNPfunctionalized droplets in contact with an R6G aqueous solution (right). The scale bar is 100 nm. (B) log−log plot of peak intensity at 1508 cm−1 as a function of the R6G concentration. Reproduced with permission from ref 26. Copyright 2018 John Wiley Sons, Inc.

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REFERENCES

(1) Zhang, X.; Ren, J.; Yang, H.; He, Y.; Tan, J.; Qiao, G. G. From Transient Nanodroplets to Permanent Nanolenses. Soft Matter 2012, 8, 4314−4317. (2) Lei, L.; Li, J.; Yu, H.; Bao, L.; Peng, S.; Zhang, X. Formation, Growth and Applications of Femtoliter Droplets on a Microlens. Phys. Chem. Chem. Phys. 2018, 20, 4226−4237. (3) Hou, J.; Zhang, H.; Yang, Q.; Li, M.; Song, Y.; Jiang, L. Bioinspired Photonic-Crystal Microchip for Fluorescent Ultratrace Detection. Angew. Chem., Int. Ed. 2014, 53, 5791−5795. (4) Kang, H.; Heo, Y. J.; Kim, D. J.; Kim, J. H.; Jeon, T. Y.; Cho, S.; So, H.-M.; Chang, W. S.; Kim, S.-H. Droplet-Guiding Superhydrophobic Arrays of Plasmonic Microposts for Molecular Concentration and Detection. ACS Appl. Mater. Interfaces 2017, 9, 37201−37209. (5) Zhu, Y.; Fang, Q. Analytical Detection Techniques for Droplet Microfluidics  A Review. Anal. Chim. Acta 2013, 787, 24−35. (6) Lampronti, G. I.; Artioli, G.; Oliva, L.; Ongaro, A.; Maretto, S.; Bonino, F.; Barbera, K.; Bordiga, S. Role of Phosphate Species and Speciation Kinetics in Detergency Solutions. Ind. Eng. Chem. Res. 2012, 51, 4173−4180. (7) Dresselhuis, D. M.; Klok, H. J.; Stuart, M. A. C.; de Vries, R. J.; van Aken, G. A.; de Hoog, E. H. A. Tribology of O/W Emulsions under Mouth-Like Conditions: Determinants of Friction. Food Biophys 2007, 2, 158−171. (8) Qiao, W.; Zhang, T.; Yen, T.; Ku, T.-H.; Song, J.; Lian, I.; Lo, Y.H. Oil-Encapsulated Nanodroplet Array for Bio-molecular Detection. Ann. Biomed. Eng. 2014, 42, 1932−1941. (9) Yang, S.; Dai, X.; Stogin, B. B.; Wong, T.-S. Ultrasensitive Surface-Enhanced Raman Scattering Detection in Common Fluids. Proc. Natl. Acad. Sci. U. S. A. 2016, 113, 268−273. (10) Wang, L.; McCarthy, T. J. Capillary-bridge-derived Particles with Negative Gaussian Curvature. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 2664−2669. (11) Kuang, M.; Wang, L.; Song, Y. Controllable Printing Droplets for High-Resolution Patterns. Adv. Mater. 2014, 26, 6950−6958. (12) Rigas, G.-P.; Payne, M. M.; Anthony, J. E.; Horton, P. N.; Castro, F. A.; Shkunov, M. Spray Printing of Organic Semiconducting Single Crystals. Nat. Commun. 2016, 7, 13531. (13) Chan, E. P.; Crosby, A. J. Fabricating Microlens Arrays by Surface Wrinkling. Adv. Mater. 2006, 18, 3238−3242. (14) Kang, D.; Pang, C.; Kim, S. M.; Cho, H. S.; Um, H. S.; Choi, Y. W.; Suh, K. Y. Shape-Controllable Microlens Arrays via Direct

the pollutant, the Ag NPs-functionalized droplets may extract R6G from the solution and show a strong fluorescence response. A linear relationship between the peak intensity of SERS and R6G concentration on log−log plots could be obtained at 1508 cm−1, as shown in Figure 15B. The linear range of the R6G concentration may vary from 1 nM to 1 μM in the solution, demonstrating the applicability of this approach to quantitative analysis with high reproducibility.

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CONCLUSIONS AND OUTLOOK Significant progress has been made in the quantitative understanding of the formation of surface nanodroplets from liquid−liquid phase separation by solvent exchange. This apparently simple process may be developed into a general solution-based approach for producing single or multiple component droplets over a large surface area or in highly confined spaces. Except for the ternary liquid system, the phase separation is expected to be triggered in other multiphase systems, such as a polymer/solution mixture.75 The potential applications of these droplets have been demonstrated in microwettability characterization, chemical microanalysis, the self-assembly of porous nanomaterials, and engineering a wide range of surface microstructures with desirable optical or wetting properties. Many other potential applications remain to be explored. The growth dynamics of nanodroplets reflect several characteristics of early stage liquid−liquid phase separation, which are influenced by both the thermodynamic properties of the solutions and substrates and kinetic features of the mixing flow. Several intriguing dynamic processes are associated with the droplet branch development in confinement, the evaporation of the ternary liquid mixture, and chemical reactions with surface nanodroplets. Fully understanding and utilization of these phenomena will require joint research efforts from the areas of physics of fluids and colloids and interfaces. L


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Surface Nanodroplets: Formation, Dissolution, and Applications

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