Nanoscale-Agglomerate-Mediated Heterogeneous Nucleation - Nano

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Nanoscale-Agglomerate-Mediated Heterogeneous Nucleation Hyeongyun Cha, Alex Wu, Moon-Kyung Kim, Kosuke Saigusa, Aihua Liu, and Nenad Miljkovic Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b03479 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 29, 2017

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Nanoscale-Agglomerate-Mediated Heterogeneous Nucleation Hyeongyun Cha1,3, Alex Wu1, Moon-Kyung Kim1, Kosuke Saigusa3, Aihua Liu1, and Nenad Miljkovic1,2,3,* 1

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Department of Mechanical Science and Engineering, University of Illinois, Urbana, Illinois 61801, USA

Frederick Seitz Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801, USA

International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan

*Corresponding Author E-mail: [email protected]

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ABSTRACT: Water vapor condensation on hydrophobic surfaces has received much attention due to its ability to rapidly shed water droplets and enhance heat transfer, anti-icing, water harvesting, energy harvesting, and self-cleaning performance. However, the mechanism of heterogeneous nucleation on hydrophobic surfaces remains poorly understood and is attributed to defects in the hydrophobic coating exposing the high surface energy substrate. Here, we observe the formation of high surface energy nanoscale agglomerates on hydrophobic coatings after condensation/evaporation cycles in ambient conditions. To investigate the deposition dynamics, we studied the nanoscale agglomerates as a function of condensation/evaporation cycles via optical and field emission scanning electron microscopy (FESEM), microgoniometric contact angle measurements, nucleation statistics, and energy dispersive X-ray spectroscopy (EDS). The FESEM and EDS results indicated that the nanoscale agglomerates stem from absorption of sulfuric acid based aerosol particles inside the droplet and adsorption of volatile organic compounds such as methanethiol (CH3SH), dimethyl disulfide (CH3SSCH), and dimethyl trisulfide (CH3SSSCH3) on the liquid-vapor interface during water vapor condensation, which act as preferential sites for heterogeneous nucleation after evaporation. The insights gained from this study elucidate fundamental aspects governing the behavior of both short and long term heterogeneous nucleation on hydrophobic surfaces, suggest previously unexplored microfabrication and air purification techniques, and present insights into the challenges facing the development of durable dropwise condensing surfaces. KEYWORDS: heterogeneous nucleation, condensation, hydrophobic, durability, nanoscale agglomerate, volatile organic compounds


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Water vapor condensation is ubiquitous in nature and has a large influence on the performance of industrial systems including building environmental control,1 power generation,2 high-heat-flux thermal management,3, 4 and thermally driven water desalination.5 When water vapor condenses on non-wetting hydrophobic surfaces, it undergoes dropwise condensation typified by the formation of small liquid droplets that grow,6 coalesce, and shed via gravitational body forces, and clear the surface for renucleation,7 allowing for an order of magnitude enhancement in heat transfer performance when compared to filmwise condensation on wetting hydrophilic surfaces.8,

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addition to condensation heat transfer, hydrophobic surfaces have been demonstrated to reduce condensate retention and frost on heat exchanger components,10-18 and enable atmospheric water and energy harvesting coatings.19-23 However, the long term durability (> 5 years) of hydrophobic surfaces has not been demonstrated, resulting in a lack of industrial implementation.24-28 Indeed, the unexpected nucleation of water droplets on hydrophobic surfaces at supersaturations (𝑆 ≈ 1) well below the critical supersaturation for low surface energy hydrophobic coatings (𝑆cr ≈ 3) is currently explained by the presence of nanoscale ‘defects’ in the hydrophobic coating which act to expose the high surface energy substrate.29-33 Here, we elucidate previously unobserved mechanisms governing heterogeneous water nucleation on hydrophobic surfaces during atmospheric water vapor condensation in the presence of noncondensable gases (NCGs). After a number of condensation and evaporation cycles, we observe nanoscale agglomerate particles accumulating on the hydrophobic surface. By measuring the contact angle on the surface after condensation/evaporation cycles and characterizing the chemical composition of the particles, we show that atmospheric sulfur-based water-soluble volatile organic compounds accumulate on the liquid-vapor interface during condensation, deposit on the surface after evaporation, and act as preferential sites for heterogeneous nucleation during subsequent 4 ACS Paragon Plus Environment

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condensation cycles. The insights gained from this study have implications for developing novel biphilic surface microfabrication techniques, purifying air, and increasing the longevity of hydrophobic surfaces, an important step towards enhancing the efficiency of a wide range of energy and water applications. Hydrophobic functionalization was obtained by depositing heptadecafluorodecyltrimethoxysilane (HTMS) on a patterned silicon (Si) wafer. Contact angle measurement of ≈100 nL droplets on the hydrophobic Si surface showed intrinsic advancing and receding contact angles of 𝜃a = 111.5 ± 0.8°/𝜃r = 103.5 ± 1.6°, respectively. To study the condensation nucleation behavior, we interfaced a top-view optical microscopy setup with a monochrome microscope camera. To induce water droplet condensation and evaporation in an atmospheric laboratory environment having an air temperature 𝑇air = 22 ± 0.5°C and a relative humidity 𝛷 ≈ 50 ± 1%, samples were horizontally placed on a cold stage and transiently reduced to a temperature of 𝑇w = 1 ± 0.5°C for 100 seconds, followed by increasing the temperature to 𝑇w = 30 ± 0.5°C for 150 seconds. Condensation and evaporation durations were specifically chosen in order to not let the condensate droplets coalesce during the droplet growth for the purpose of determining the nucleation sites accurately and to let the condensate droplets to evaporate completely, respectively. Droplet condensation initiated at critical supersaturations 𝑆 ≈ 1.02 ± 0.05. A description of the experimental setup with procedures is detailed in the Supporting Information.


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Figure 1. Top-view optical microscopy of agglomerate deposition after (a) 0, (b) 25, (c) 50, and (d) 100 condensation/evaporation cycles on the HTMS coated Si wafer. The deposition and growth of the agglomerates had cycle-dependent size and acted as preferential sites for heterogeneous nucleation as shown in (e). The insets show a microscopic droplet in advancing state on the HTMS coated Si wafer. Inset scale bars are 50 µm each. (f) Advancing and receding contact angles as a function of the number of condensation/evaporation cycles, showing no significant difference in contact angle after condensation/evaporation cycles. A cold stage was used to reduce the sample temperature of 𝑇w = 1 ± 0.5°C to induce water droplet condensation and 𝑇w = 30 ± 0.5°C to induce evaporation in the laboratory environment condition having an air temperature 𝑇air = 22 ± 0.5°C and a relative humidity 𝛷 ≈ 50 ± 1%.

Figure 1a through d show top-view optical images of the HTMS coated Si wafer as a function of condensation/evaporation cycles. After complete evaporation of the condensed water droplets, renucleation was observed to occur at identical locations where the previous droplets evaporated (see Supporting Information, Video S1). Increase of the evaporation time or stage temperature to ensure removal of potentially pinned liquid films did not affect the results. The identical nucleation site distribution during subsequent condensation experiments along with the low critical


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supersaturation suggested the existence of defects in the HTMS coating which helped to initiate phase transition.29-33 Interestingly, repetition of condensation/evaporation cycles revealed deposition and growth of nanoscale agglomerates having cycle-dependent size and acting as preferential sites for heterogeneous nucleation (Figure 1e). To investigate whether the observed nanoscale agglomerates stem from degradation of the HTMS hydrophobic coating,34 contact angles were measured after 0, 25, 50, and 100 condensation/evaporation cycles (Figure 1f). The intrinsic advancing and receding contact angles, along with the contact angle hysteresis (∆𝜃 = 𝜃a − 𝜃r ) did not show a statistically significant change after 100 condensation/evaporation cycles, indicating no significant degradation of the hydrophobicity even in the presence of nanoscale agglomerates. In contrast to previous work,34 the surfaces here were functionalized using vapor deposition of HTMS, resulting in the development of a highly conformal, ultra-thin, ultra-smooth, and covalently bonded coating (see Supporting Information, Figure S1a). The results indicate that the nanoscale agglomerates were not formed via the detachment of HTMS molecules, rather water soluble vapor-phase or solid-phase particles in the atmosphere self-assembled on the liquid-vapor interface or absorbed into the liquid droplet during condensation, densified during evaporation, and remained on the surface after evaporation. Furthermore, the condensation or pure water vapor from the atmosphere removed any interaction of liquid water with solid surfaces prior to its presence on the hydrophobic coating, indicating that agglomerate formation was not analogous to the deposition and densification of water borne contaminants during evaporation of deposited droplets from a surface (see Supporting Information, Figure S1b).35


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Figure 2. Top-view (a) optical microscopy, (b) field emission scanning electron microscopy, (c) atomic force microscopy (AFM) of agglomerate deposition after 100 condensation/evaporation cycles. (d) High resolution AFM scan of the dotted region in (c). (e) Height profile of individual nanoscale agglomerates along the black dotted-line trace in (d) as a function of condensation/evaporation cycles. 8 ACS Paragon Plus Environment

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To determine the nature of the agglomerate deposition observed with optical microscopy (Figure 2a), we preformed field emission scanning electron microscopy (FESEM) of the nanoscale agglomerates (Figure 2b). The FESEM images revealed that the agglomerates look analogous to deflated crumpled membranes. Average heights and profiles of the nanoscale agglomerates as a function of number of condensation/evaporation cycles were measured by atomic force microscopy (AFM, Figure 2c, d). Figure 2e shows a height profile of an individual agglomerate as a function of condensation/evaporation cycles. The results show that agglomerates grow in size as the number of condensation/evaporation cycle increases, agreeing with the optical microscopy results in Figure 1a through d. In contrast to previously established theory,33 analysis of the FESEM and AFM images corroborate that condensation and evaporation cycles did not form defects sites or expose high surface energy areas for the renucleation on the hydrophobic HTMS coatings used here.


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Width [m] Figure 3. (a) Top-view field emission scanning electron microscopy of agglomerate deposition after 100 condensation/evaporation cycles. (b) Energy dispersive X-ray spectroscopy (EDS) line scans of carbon, oxygen, sulfur, and fluorine along the red dotted-line trace in (a). The results indicate that the agglomerate deposition phenomena is governed by atmospheric sulfur-based hydrocarbon contamination on the liquid-vapor interface composed of carbon, oxygen, and sulfur, not by detachment of fluorine based hydrophobic coating.

To determine the chemical composition of agglomerate molecules, we first performed a wide band EDS point analysis on a single agglomerate and its peripheral hydrophobic surface area. The EDS


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results revealed the existence of significantly higher concentrations of carbon (C), oxygen (O), and sulfur (S) on the agglomerates when compared to the adjacent hydrophobic area (see Supporting Information, Figure S2). By performing EDS line scan analysis for each element along the line trace across the agglomerates (Figure 3a), we ensured that deposited agglomerates are indeed composed of C, O, and S (Figure 3b). The results show that the concentration of C and S are relatively higher than that of O across all agglomerates. Furthermore, the fluorine (F) signal was identical for the agglomerate and adjacent areas, indicating that nanoscale agglomerates were not formed due to the detachment of the fluorine-based hydrophobic coating. In order to understand the source of the nanoscale agglomerates, we analyzed previously characterized aerosols and volatile organic compounds (VOCs) that contain chemical elements including C, O, and S.36, 37 Sulfur based hydrocarbons in the atmosphere stem from sulfuric acid (H2SO4), which is formed during the oxidation of sulfur dioxide (SO2) emissions during fossil fuel combustion, a major precursor of new airborne compounds.38 Furthermore, agricultural practices and biological processes generate organosulfur compounds (OSCs), which can lead to formation of SO2 and methanesulfonic acid (CH3S(O)2OH) in the air through the atmospheric oxidation of OSCs.39-43 In addition, the human breath contains OSCs,44, 45 and livestock and farming practices are a significant source of methanethiol (CH3SH), dimethyl disulfide (CH3SSCH3), and dimethyl trisulfide (CH3SSSCH3) airborne hydrocarbons.46-50 Thus, as vapor phase VOCs are oxidized in the atmosphere, low-volatility solid-phase aerosols are generated, consisting of only of a few molecules and having diameters between 1 and 2 nm,51 resulting in agglomerate formation during evaporation of water droplets.52,

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emissions and our EDS results suggest that the observed agglomerate deposition phenomenon is


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governed by absorption of aerosol particles inside the droplet or adsorption of volatile VOCs on the liquid-vapor interface. To test the validity of the aerosol absorption and VOC adsorption hypothesis, we used scaling to estimate the maximum possible size of agglomerates. Assuming that oxidized VOC molecules are fully adsorbed on the entire water droplet surface area, 𝐴 = 2𝜋𝑅 2 (1 − cos 𝜃), where 𝑅 (≈ 10 µm) and 𝜃 are the droplet radius and advancing contact angle, respectively, and the length of molecules to be 𝑙 ≤ 1 nm,54 we can estimate the total adsorbed volume of the self-assembled molecules as 𝑉 = 𝐴𝑙. After the evaporation of the water droplet, the radius of hemispherical agglomerate is 𝑅 = (3𝑉/(2𝜋))1⁄3 . The theoretical maximum agglomerate size after 100 condensation/evaporation cycles is 𝑅 ≈ 3 µm, much larger than the experimentally measured 𝑅exp ≈ 500 nm. The lower experimental size is due either to incomplete coverage of the VOC molecules on the liquid-vapor interface during condensation, or more likely due to volumetric absorption of aerosol particles governing the agglomerate formation instead of interfacial adsorption or self-assembly. To determine if particle absorption was the main mechanism of agglomerate formation, we utilized Henry’s law to determine the maximum size of agglomerates after 100 evaporation/condensation cycles. For absorbed aerosol particles (H2SO4) in liquid water, Henry’s law solubility constant is 𝐾 ≈ 2.9 x 107 mol/(m3·Pa). Typical atmospheric concentration of H2SO4 can range from 105 to 107 molecules/cm3 (or 4 x 10-10 to 4 x 10-8 Pa at 298 K).55 Assuming clean laboratory air with a lower bound of vapor concentration (4 x 10-10 Pa), the equilibrium concentration in liquid is 𝐶H2 SO4 = 0.0116 mol/m3 or 1.14 g/m3. Multiplying the equilibrium concentration with the total condensed water during 100 cycles, we obtain a mass of agglomerated H2SO4 particles of 𝑚H2 SO4 = 4.77 x 10-13 g. Relating the mass to volume through the density (𝜌H2 SO4 = 1.84 g/cm3), we obtain an 12 ACS Paragon Plus Environment

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agglomerate volume of 𝑉 = 2.6 x 10-19 m3, and corresponding radius of 𝑅 = (3𝑉/(2𝜋))1⁄3 = 498.2 nm, strikingly similar to the experimentally measured value of 𝑅exp ≈ 500 nm. The excellent agreement between the absorption agglomerate radius estimate and the experimental radius lends credence to the theory of aerosol absorption in the droplet bulk as the main driving mechanism of agglomerate formation. To further test the absorption theory, we conducted additional experiments by increasing the number of condensation/evaporation cycles to 1200. Due to the finite solubility of H2SO4 in water, the size of agglomerates formed on the surface should theoretically cease to grow once the solubility limit is reached. Indeed, experimental results with 1200 cycles revealed that agglomerate growth decreases substantially after 300 cycles with agglomerates ceasing to grow above 1200 cycles (see Supporting Information, Figure S3). Furthermore, the individual droplet evaporation rates were found to be independent of evaporation/condensation cycle (see Supporting Information, Figure S4), indicating that the liquid-vapor interface was devoid of VOC molecules which may act to alter surface tension and evaporation dynamics. The deposited nanoscale agglomerates enable preferential heterogeneous nucleation at fixed spatial locations as shown by the deviation of the cumulative probability distribution during the overlay of 50 and 100 condensation/evaporation cycles from the predicted Poisson distribution, 𝑃 = 1 − exp(−𝑁𝜋𝐿2 ) , where 𝑁 and 𝐿 are the nucleation density and mean separation distance, respectively (see Supporting Information, Figure S5). Note, the initial nucleation behavior prior to particle accumulation remained spatially random,56, 57 and location of agglomerates depends on initial nucleation sites (cycle 1), following Poisson statistics (inset of Figure S5).


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As a further demonstration of agglomerate absorption in the droplet bulk during condensation, water solubility of the agglomerates was demonstrated during droplet coalescence (see Supporting Information, Figure S6). Maintaining the sample temperature of 𝑇w = 1 ± 0.5°C for more than 100 seconds during condensation resulted in coalescence of neighboring droplets (Figure S6c). Coalescence of droplets having distinct agglomerates after evaporation resulted in agglomerate coalescence (Figure S6d), indicating agglomerate particle solubility inside the condensate droplet. Furthermore, cleaning of the surface with deionized (DI) water after agglomerate formation resulted in removal or dissolution of all hydrophilic agglomerates. However, cleaning of the surface with non-hydrogen-bonding solvents such as isopropanol and acetone did not remove any nanoscale agglomerates, supporting the hypothesis of water solubility of hydrophilic agglomerates (see Supporting Information, Figure S7). Indeed, the relatively high Henry’s law constants for H2SO4 and sulfur based compounds indicate hydrophilicity and significant solubility in water.58

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Figure 4. Top-view optical microscopy of (a) water condensation on an agglomerate deposited surface, (b) a fresh surface after cleaning of the nanoscale agglomerates with DI water, (c) condensation on the fresh surface after cleaning, and (d) condensation on the fresh surface after cleaning twice. (e) Droplet size distribution for agglomerate deposited surface and cleaned surface. Blue dotted-circle indicate possible defects sites where the nucleation repeats on identical location. Nucleation initiates uniformly on the agglomerates due to high surface energy of agglomerates, while nucleation initiates consecutively on a cleaned surface. (f) Nucleation behavior, showing that nucleation sites are not repeatable after cleaning the agglomerates.

Given the previously unknown agglomerate deposition mechanism and the role it plays on heterogeneous nucleation of atmospheric water vapor condensation, we were interested in determining what governs the initial droplet distribution immediately after nucleation. Without the presence of condensate on the surface (0 cycle), water soluble agglomerate particles would not be present until after the first condensation/evaporation cycle was complete. Figure 4a through d show top-view optical microscopy images of water vapor condensation on an agglomerate deposited surface, a fresh surface after cleaning of the nanoscale agglomerates with DI water, condensation on the fresh surface after cleaning, and condensation on the fresh surface after cleaning twice, respectively. In presence of agglomerates, nucleation initiated on all agglomerates simultaneously (𝑆 ≈ 1.02) due to high surface energy of accumulated particles, resulting in a discrete droplet size distribution (i.e. all droplets were approximately the same size at any given time). Even after one cycle of condensation/evaporation when particles are not visible yet, we observed the same condensation phenomena as observed at later stages on the agglomerate deposited surface where the most of nucleation sites are activated simultaneously. However, in the absence of nanoscale agglomerates, heterogeneous nucleation sites are activated in sequence and at higher subcooling corresponding classical nucleation theory (CNT),59 resulting in a more random droplet size distribution (Figure 4e, see Supporting Information, Video S2). Although the surface was devoid 15 ACS Paragon Plus Environment

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of nanoscale agglomerates after cleaning, a few repeatable nucleation sites at identical location were observed after multiple cleaning cycles, indicating that defect sites in the hydrophobic HTMS coating do play a key role in heterogeneous nucleation. On the defect sites of a clean hydrophobic surface, water droplets condensed initially at low supersaturations (𝑆 ≈ 1.02), followed by growth of secondary microdroplets at randomly distributed sites at higher supersaturations (𝑆 ≈ 1.37 and 1.72) still below those predicted by CNT on homogeneous smooth hydrophobic surfaces (≈ 3). Thus, defects sites did exist on the surface along with a number of randomly distributed nucleation sites which still showed nucleation at low critical supersaturations (𝑆 ≈ 1.02 ± 0.05), as shown by the statistically significant deviation of the cumulative probability distribution overlay of agglomerate and cleaned surfaces from the predicted Poisson distribution (Figure 4f). The source of the secondary and non-spatially correlated nucleation sites remains to be pinpointed, as larger dust particles or weakly adsorbed aerosol particles could have initiated the process. As a check to ensure that the observed behavior was not unique to the fabricated HTMS surface or SAM surfaces, all aforementioned experiments were conducted on a non-SAM hydrophobic surface. Hydrophobic Si wafers created through deposition of a ≈150 nm thick layer of C4F8 resulted in identical agglomerate and nucleation dynamics, indicating that the observed phenomenon was not unique to our HTMS surface (see Supporting Information, Figure S8). The findings reported here have important implications toward the potential development or long lasting functional coatings. The agglomerate deposition mechanism shows that even for defectfree, infinite lifetime coatings, the effects of the surrounding environment play an important role on the condensation process. Indeed, even for systems which may not operate in ambient conditions such as vapor chambers,60 heat pipes,61 and industrial power plant condensers,62 VOCs


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have been shown to exist in ultra-clean, and ultra-high vacuum conditions.63 Agglomerate formation may not be observable for systems employing steady conditions without condensation/evaporation cycles, however the VOC-driven aerosol absorption process in the pure liquid droplets remains, resulting in soluble VOC accumulation in the condensate stream. The VOC accumulation poses a threat to working fluid purity and water side fouling. Furthermore, the results presented here have important implications for past and future nucleation and heat transfer studies

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condensation/evaporation cycles to study the heterogeneous nucleation process and droplet growth has been widely utilized for decades.33, 57, 64-73 The previously unidentified agglomerate deposition mechanism shown here was most likely present in previous studies, casting doubt on the validity of previous experimental nucleation data. Although we clearly show here that hydrophobic defect sites do exist, in accordance to currently established theory, the degree to which nucleation sites are defects remains to be determined. Unlike previous studies, our work shows that heterogeneous nucleation during condensation of water is dictated by an intricate balance of agglomerate formation, coating defect dynamics, presence of weakly adsorbed aerosol or VOC particles on the surface prior to initial condensation, and classical nucleation theory. Furthermore, the observation of VOC accumulation has broad implications for the experimental determination of the accommodation or condensation coefficient of water. Previous works have noted that ultra-clean water and surrounding conditions must be used due to high sensitivity of molecular contamination on the liquid-vapor interface during measurement.74 Although demonstrated here to be H2SO4based water soluble aerosol particles which should not affect the surface tension of water,75, 76 the presence of other non-H2SO4 organosulfur VOCs could have a surfactant-like effect stemming from the atmosphere.77, 78 17 ACS Paragon Plus Environment

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In the future, it would be interesting to utilize the agglomerate deposition dynamics as a tool to foster low cost and scalable micro/nanofabrication. By controlling the condensation/evaporation cycles, humidity, and air quality, a wide variety of surfaces can be made with the agglomerates as the micro/nanotextures, from nanoscale bumps with relatively wide spacing (low supersaturations, few cycles), to microscale hills with dense packing (high supersaturations, many cycles). Furthermore, the innate high surface energy of the agglomerate coatings provide a novel method to create simple and scalable bi-philic surfaces.27,

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Instead of utilizing lithography based

micro/nanofabrication techniques which are particularly difficult to use for the creating of nanoscale hydrophilic areas, a cold stage in the atmospheric environment can be used to create an array of surfaces that can spatially control the heterogeneous nucleation of water. In addition, the removal of VOCs and aerosol particles from the laboratory atmosphere through subsequent condensation/evaporation cycles presents a unique and unexplored method for air purification.84 Lastly, in the future, it would be interesting to repeat the experiments reported here in drastically differing locations, where the atmospheric conditions may differ substantially that that of typical Midwestern towns such as Champaign, Illinois, USA. The ubiquitous presence of sulfur based VOCs and aerosols in the atmosphere across the globe casts doubt that differing results will be observed, however agglomerate growth dynamics may differ due to local concentration differences. In summary, we demonstrated agglomerate-deposition-mediated heterogeneous nucleation during atmospheric water vapor condensation on hydrophobic surfaces. We showed that atmospheric VOCs, composed of C, O, and S, absorb in the liquid droplets during condensation and accumulate on the surface during evaporation, acting as preferential heterogeneous nucleation sites. The outcomes of this work not only show the important effects of atmospheric deposition of hydrocarbon-based agglomerates on the condensation process, they paint a more complex picture 18 ACS Paragon Plus Environment

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of heterogeneous nucleation during water vapor condensation in the presence of NCGs, which are inexorably linked to the degradation and longevity of hydrophobic surfaces.


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ASSOCIATED CONTENT Supporting Information The Supporting information is available free of charge on the ACS Publication website at DOI: ##/###. Further information about experimental methods, AFM images of HTMS coated smooth Si wafer, EDS point analysis, agglomerate growth rate, evaporation rate, nucleation behavior, water solubility and solvent insolubility of nanoscale agglomerates, agglomerate deposition on polymercoated surfaces, and the wafer patterning (PDF) Two videos showing the water droplet condensation on the agglomerate deposited and cleaned hydrophobic surface, Video S1 (MPG) Video S2 (MPG)

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

ORCID Hyeongyun Cha: 0000-0001-7157-7315


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Alex Wu: 0000-0003-4369-1908 Moon-Kyung Kim: 0000-0003-0913-7655 Kosuke Saigusa: 0000-0002-3244-5245 Aihua Liu: 0000-0003-3096-7469 Nenad Miljkovic: 0000-0002-0866-3680

Note The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors acknowledge Dr. H. Jeremy Cho of MIT for discussion on absorption theory and the liquid-vapor interface. The authors gratefully acknowledge funding support from the Office of Naval Research (ONR) with Dr. Mark Spector as the program manager (Grant No. N00014-16-12625) and the National Science Foundation under Award No. 1554249. N.M. gratefully acknowledges funding support from the International Institute for Carbon Neutral Energy Research (WPI-I2CNER), sponsored by the Japanese Ministry of Education, Culture, Sports, Science and Technology. Atomic force microscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy were carried out in part in the Frederick Seitz Materials Research Laboratory Central Facilities, University of Illinois.


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