Condensation Heat Transfer Performance of Thermally Stable

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Condensation Heat Transfer Performance of Thermally Stable Superhydrophobic Cerium Oxide Surfaces Jaehwan Shim, Donghyun Seo, Seungtae Oh, Jinki Lee, and Youngsuk Nam ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09597 • Publication Date (Web): 23 Aug 2018 Downloaded from http://pubs.acs.org on August 26, 2018

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ACS Applied Materials & Interfaces

Condensation Heat Transfer Performance of Thermally Stable Superhydrophobic Cerium Oxide Surfaces Jaehwan Shim1, Donghyun Seo1, Seungtae Oh1, Jinki Lee2, and Youngsuk Nam1* 1

Department of Mechanical Engineering, Kyung Hee University, Yongin 17104, Korea

2

Theomochemical Energy System R&D Group, Korea Institute of Industrial Technology,

Cheonan, 31056, Korea

Corresponding Author: Youngsuk Nam Department of Mechanical Engineering Kyung Hee University, Yongin, 446-701, Korea Tel: 82 (31) 201-3652 Email: [email protected]

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ABSTRACT

We introduce a thin (< 200 nm) superhydrophobic cerium oxide surface formed by a 1-step wet chemical process to enhance the condensation heat transfer performance with improved thermal stability compared to silane-treated surfaces. The developed cerium oxide surface showed a superhydrophobic characteristic with a low (< 5º) contact angle hysteresis due to the unique surface morphology and hydrophobicity of cerium oxide. The surface was successfully incorporated to popular engineering materials including copper, aluminum and steel using. Thermal stability of the surfaces was investigated by exposing them to hot (~100ºC) steam condition for 12 hours. The introduced Ceria surfaces could maintain active dropwise condensation after the thermal stability test, while silane-treated surfaces completely lost their hydrophobicity. The heat transfer coefficient was calculated using the thermal network model incorporating the droplet size distribution and morphology obtained from the microscopic measurement. The analysis shows that the suggested cerium oxide surfaces can provide approximately 2 times and 5 times higher heat transfer coefficient before and after the thermal stability test, respectively, mainly due to the decrease in the thermal conduction resistance across droplets. The results indicate that the introduced nanostructured cerium oxide surface is a promising condenser coating to enhance the droplet mobility and resulting condensation heat transfer performance for various thermal and environmental applications especially those being exposed to hot steam condition.

KEYWORDS: rare earth oxide, cerium oxide, superhydrophobic surface, thermal stability, dropwise condensation, heat transfer enhancement


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1. Introduction Enhancing condensation heat transfer has a potential to improve the overall efficiency of a wide range of energy and environmental applications including air-conditioning1, water harvesting2, power generation3, thermal management4 and desalination5. Condensation may occur in two distinct modes including the filmwise condensation on hydrophilic surfaces and dropwise on hydrophobic or superhydrophobic ones. Previous studies have demonstrated that the dropwise condensation can offer significantly higher heat transfer coefficient than filmwise due to the facilitated removal of condensates via gravity6-7 or spontaneous droplet jumping8 and resulting decrease in the thermal conduction resistance9-12.

To promote such dropwise condensation, previous studies have investigated various surface modification techniques based on fluoropolymers13, self-assembled monolayers (SAMs)14-23, noble metals24-26 and ion implantation27-28. Fluoropolymers such as PTFE, PFA and PVDF can provide active dropwise condensation and a fair durability when the coating thickness becomes larger than a few microns. However, such thick polymer coatings induce a large parasitic thermal resistance and limit the benefit of dropwise condensation. By introducing thin ( 300 nm) regions.

Table 1 shows the dynamic contact angles measured on the samples shown in Figures 1a-1c. All the contact angles were measured 4 days after the Ceria deposition to induce the superhydrophobicity (See section 4-1 for further discussion). Although the advancing contact angles are not strongly affected by such non-uniformity, the receding angles decrease below 10° due to the defect regions when the chemical composition of the solution or dipping time is not optimized. Considering the quality, thermal resistance and manufacturing cost of the coating, the optimal composition of the solution is determined to be 41.68 g of cerium nitrate and 25.64 mL of hydrogen peroxide per 1000 mL of DI water.


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After the deposition, the particles of 100 to 300 nm diameters covered the entire surfaces (Figure 1). To verify the thickness of the coating, we applied the focused ion beam (FIB) (Quanta 3D FEG, FEI) milling technique and the results are provided in Figure 2a. The average Ceria coating thickness was measured to be approximately 100 nm - 200 nm.

To investigate the surface oxidation state for Ceria-Cu samples, the high-resolution X-ray photoelectron spectroscopy (XPS) (K-alpha+, Thermofisher Scientific) spectra of the Ce 3d region for the samples was provided (Figure 2b). We observed four component peaks (u, u´, u0 and u0´) representing the Ce3+ profile multiplet and six component peaks (v, v´, v˝, v0, v0´ and v0˝) associated with the Ce4+ profile multiplet due to the presence of mixed oxidation states of cerium oxides. The result containing all the component peaks is provided in the supplementary material (Figure S1). Figure 2b shows that the mixed phase oxides including Ce2O3 and CeO2 are deposited on Cu substrate, which corresponds to the previous results obtained with the sintered pellets of cerium oxide39. The energy dispersive spectroscopy (EDS) (S-4800, Hitachi) was also conducted and the result is provided in the supplementary material (See Table S2 in the supplementary information).

3. Experiments 3-1. Wettability test To measure the dynamic contact angle, the volume of water droplet placed on the sample was gradually increased or decreased using a computer controlled micro-syringe controller (SYS-Micro 4, WPI). Then the droplet images were recorded with a high speed CCD (Miro M110, Phantom) and analyzed by Image J software. The contact angles were measured on



three different spots of five different samples for each case and the average value was reported with the standard deviation.

3-2. Thermal stability test Figure 3a shows the schematic of hot-steam based thermal stability test setup applied to evaluate the thermal stability of each sample. Each sample is attached to a cold plate placed in a vertical orientation within an acrylic chamber. The temperature of the coolant was set to be 5ºC using a temperature-controlled flow circulator and hot steam (~100ºC) was supplied to the samples. During the test, the temperature of the test surface was measured at ~29ºC due to the hot steam. The supersaturation level (S) is determined by S=Pv /Psat(Ts) where Pv is the vapor pressure and Ts is the surface temperature. To conduct the accelerated thermal stability test, very high supersaturation level (~25.3) was maintained throughout the entire test.

3-3. Microscopic condensation experiment Figure 3b shows the microscopic condensation experimental setup. The samples are placed within the temperature-humidity controlled stage connected to water-saturated nitrogen flow. The temperature of the test sample was maintained at ~5ºC with the flow circulator. The ambient temperature was ~20ºC and the relative humidity was ~60%. The supersaturation level is set to be ~1.6 throughout the entire experiment. The microscopic condensation behaviors are observed from the top using a high speed microscopy (Miro M110, Phantom) connected to Olympus BX5354.


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4. Results and discussion 4-1. Wetting characteristics of Ceria samples The temporal changes in the dynamic contact angle of air-exposed Ceria sample were provided in Figure 2c. Initially, the advancing contact angle rapidly increased while the receding angle remained near constant. After ~2 days, the receding angle also rapidly increased, and the sample showed excellent superhydrophobicity with less than 5° hysteresis after ~3 days. Since the sample was placed in the ambient environment where various VOCs exist42-44, the spontaneous adsorption mechanism should play a major role for the contact angle increase. In order to further investigate the role of adsorption on the hydrophobicity of Ceria, we exposed the Ceria samples to the hydrocarbon-rich environment and observed the contact angle changes. We put 1 mL of 1-octadecene (C18H36) solution (90%, Alfa Aesar) to a petri-dish, and then the Ceria samples and the petri-dish were put into an airtight container with preventing the samples from contacting the 1-octadecene solution. Then the container was maintained at 60°C. Figure 2d shows that the hydrocarbon-treated Ceria-Cu turned into superhydrophobic in ~1 day, 3 times faster than the air-exposed one, which implies that the hydrocarbon (or VOC) adsorption is a crucial mechanism for the increase in the hydrophobicity of investigated Ceria surfaces. We note that the 1-octadecene-treated CeriaCu showed the equivalent level of thermal stability (Table S3), droplet mobility (Figure S2) and heat transfer coefficient (Figure S3) in the present work. In addition, the identical hydrocarbon treatment did not cause any meaningful changes in the wettability of untreated Cu substrate (Figure S4a).

Combined with the hydrophobicity of Ceria, the unique particle-like morphology of Ceria surface (See Figure 1) induced the superhydrophobicity by capturing air pocket underneath a



water droplet. The rapid increase in the receding angle after ~2 days implies that the transition to Cassie wetting morphology occurred during the time period. The effective solid fraction (f) was estimated to be as low as ~6% from the Cassie-Baxter equation (f=(cosθa+1)/(cosθs+1)) incorporating the apparent contact angles of the Ceria samples (θa~162°) and smooth Ceria surface (θs~99°)31, 46. We note that no meaningful change was observed for the contact angles of untreated Cu samples exposed to the equivalent condition.

Table 2 summarizes the contact angles of investigated superhydrophobic Ceria-Cu and FAS samples obtained before and after the thermal stability test. Before the thermal stability test, Ceria-Cu samples show the excellent superhydrophobicity with a small (~5°) contact angle hysteresis. FAS treated Cu shows 40~50° of contact angle hysteresis due to the absence of structures that capture the air-pocket. Table 2 shows that Ceria-Cu samples could maintain near-identical dynamic contact angles even after 12 hours of the accelerated thermal stability test, while FAS-Cu lost their hydrophobicity and showed very low (< 10°) receding contact angle.

We also conducted a longer period of thermal stability test for Ceria samples. In the present work, we presented the data obtained after 12 hours of thermal stability test since FAS samples completely lost their hydrophobicity within 12 hours (See the supplementary Figure S5). However, the Ceria samples could provide active dropwise condensation even after 2 weeks of thermal stability test, which clarifies the superior thermal stability of the suggested Ceria samples.

4-2. Microscopic condensation behaviors


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Figure 4 shows the microscopic condensation behaviors of investigated samples obtained before and after the thermal stability test, respectively. The number density and size distribution of condensed droplets are provided in Figure 5. Before the thermal stability test (Figures 4a and 4b), active dropwise condensation occurs on both FAS-Cu and Ceria-Cu. Small condensates initially form on the entire substrate, merge each other and grow into larger droplets. The large droplets hinder the condensation heat transfer due to the thermal conduction resistance across the droplets, therefore the areas covered by the large droplets are typically considered as inactive55. As the coalescing event occurs between a large and small droplets on FAS-Cu, the liquid-solid interfacial area continuously increases due to the relatively large contact angle hysteresis of the surface, which reduces the active nucleation sites that provide small droplets. Therefore, the number of small (< 30 µm) droplets rapidly decreases in the early stage of microscopic condensation test (Figure 5a).

When the coalescing event occurs between droplets on Ceria-Cu, the increase in the solidliquid interfacial area is limited due to the high contact angle and small hysteresis55. Consequently, a large number of active nucleation sites can be maintained, and a large number density of small (< 30 µm) droplets were observed throughout the entire microscopic condensation experiments (Figure 5b). We note that previously-reported coalescence-induced droplet jumping54, 56-57 was rarely occurred on Ceria-Cu despite its superhydrophobicity. Such droplet jumping is affected by the morphology and length scale of surface structure58. More specifically, recent study showed that the condensation-induced adhesion force between droplets and substrate increases according to the increase in the structure size, surface energy and nucleation density59. In our case, droplet jumping was not actively occurred on Ceria surfaces since relatively large microscopic surface non-uniformity such as tiny cracks (See



Figure 1) increases the condensation-induced adhesion between the droplets and surface, which limits the coalescence-induced jumping.

In the present experiment, we did not focus on facilitating drop jumping since the drop jumping was mostly reported only at low supersaturation level (S < ~1.2)8, 18, 60-63, while industrial condensers typically operate at a relatively high supersaturation level (S > ~1.5). However, recent study demonstrated drop jumping at relatively high supersaturation level using 3-D superhydrophobic Cu nanowire networks64, which implies that further optimization of surface morphology may enable us to use the drop removal mechanism for real world condensers.

After the thermal stability test, the condensation behaviors on FAS-Cu significantly changed as in Figure 4c. Due to the significant increase in contact angle hysteresis (See Table 2) and resulting contact line pinning, the coalescing event rapidly expands the liquid-solid interfacial area. As a result, large (> 100 µm) droplets cover most of the FAS-Cu surface after ~60 min (Figure 5c). In case of Ceria-Cu, however, the condensation behaviors and resulting droplet number density are nominally same before and after the thermal stability test (Figures 4d and 5d), which clarifies the enhanced thermal robustness of Ceria-Cu compared with FAS-Cu.

4-3. Heat transfer performance To investigate the heat transfer performance, we applied the thermal-network-based model as in previous studies including ours18, 54, 65-67. Figures 6a and 6b show the schematics of thermal resistance networks for FAS and Ceria samples, respectively. For FAS, heat is transferred from the saturated vapor to the substrate through the thermal resistances


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associated with droplet curvature (Rc), vapor-liquid interface (Ri), conduction through the droplet (Rd) and hydrophobic coating (Rhc) (Figure 6a). The temperature drops across each thermal resistance are calculated as follows:

∆Tc =

rmin (Tsat Tds ) = 2Tsatσ r rhfg ρw

∆Ti = Tsat

Ti =

qind 2πr hi (1 cosθ )

∆Td = Ti Tb =

2

qind θ 4πrkw sin θ

∆Thc = Tb Ts =

qind δhc πr khc sin2 θ 2

∆TTotal·FAS = Tsat − Ts = ∆Tc＋∆Ti＋∆Td＋∆Thc

(6)

(7)

(8)

(9)

(10)

where ∆T represents the temperature drop associated with each thermal resistance. ∆TTotal·FAS is the total temperature difference through an individual droplet on FAS samples.

Ceria has additional thermal resistances associated with cerium oxide nanostructures (Rn), water column formed within the nanostructures (Rw) and bottom cerium oxide layer (Rl) instead of the resistance of the hydrophobic coating (Rhc) (Figure 6b). The additional temperature drops across the cerium oxide nanostructures (∆Tn) and bottom cerium oxide layer (∆Tl) are obtained as follows:

∆Tn = Tb1 Tb2 =

qind δn 1 2 2 πr sin θ fkn + (1 f )kw


(11)


∆Tl = Tb2 Ts =

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qindδl πr sin2 θkn

(12)

2

where Tb1 is the temperature of a droplet on cerium oxide nanostructures and Tb2 is the temperature of cerium oxide layer. Required parameters applied to the model are summarized in Table 3. The thickness of cerium oxide nanostructure and cerium oxide layer was estimated from the FE-SEM and FIB measurement. Then the total temperature drop across the individual droplet on Ceria surface (∆TTotal·Ceria) is obtained as below: ∆TTotal·Ceria = Tsat − Ts = ∆Tc＋∆Ti＋∆Td＋∆Tn＋∆Tl

(13)

From the temperature drop, the heat transfer rate through an individual droplet (qind) on each surface is obtained as follows:

qind·FAS =

qind·Ceria =

∆ ·

& ! " " !) #$ %! $ %!

& ! & " " % " !) #$ %! %! $% *)$ % !

%$ ∆ ·'(

(14)

(15)

Figures 7a and 7b show the total thermal resistance (Rtotal) and conduction thermal resistance (Rd=rθ/4kwsinθ) through an individual droplet on each sample before and after the thermal stability test, respectively. Total thermal resistance was extracted by dividing the total temperature drop by the heat flux across a droplet and the droplet radius was obtained from the images using the high speed microscopy (Rtotal=∆TTotal πr2/qind). Before the thermal


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stability test, the total thermal resistance gradually increases on both FAS and Ceria samples mainly due to the increase in the conduction thermal resistance. As in Figure 7, over 98% of the total thermal resistance is due to the conduction thermal resistance across the droplet in all cases. After the thermal stability test (Figure 7b), the total thermal resistance of FAS rapidly increases mainly due to the significant increase in the thermal conduction resistance. As in Figure 5c, the increase in contact angle hysteresis and resulting contact line pinning rapidly decrease the number of small droplets in ~60 min. As a result, rapid increase in the thermal conduction resistance was observed from ~60 min. On the other hand, Ceria samples can maintain their low contact angle hysteresis and high droplet mobility even after the thermal stability test, which maintains the low thermal resistance after the test.

The average heat transfer rate through an individual droplet (qind·ave) before and after the thermal stability test was provided in Figure 8. To obtain the average heat transfer rate, the heat transfer rate (qind) through each droplet was calculated using Equations (14) and (15) based on the microscopic experimental data, and then the sum of each heat transfer rate was divided by the number of droplets (N) (qind·ave=∑qind/N). Since the conduction thermal resistance is dominant in all cases, the heat transfer rate is approximately proportional to the radius of a droplet (r) (qind~r). Before the thermal stability test, the average heat transfer rate on Ceria is an order of magnitude smaller than that on FAS due to the large number of small droplets on Ceria. After the thermal stability test, the average heat transfer rate through an individual drop on Ceria does not change significantly while that of FAS becomes an order of magnitude larger than before due to the significant decrease in droplet mobility and resulting increase in the droplet radius.



Figure 9a shows the calculated overall heat transfer coefficient (hoverall) obtained on each surface. The ratio between the heat transfer coefficient of Ceria-Cu and FAS-Cu is plotted in Figure 9b. The overall heat transfer coefficient on each sample is determined by dividing the sum of calculated heat transfer rate of individual droplets by the total temperature drop for unit surface area (1 m2) (hoverall=∑qind/(∆TTotal ·A)). Before the thermal stability test, the overall heat transfer coefficient gradually decreases along the time in all cases because small droplets grow and merge into larger droplets (Figure 4). Due to the large number of small droplets, Ceria shows approximately ~2 times higher overall heat transfer coefficient compared with FAS (Figure 9b) after ~300 min, which shows the importance of providing large number of small droplets for effective heat transfer. After the thermal stability test, the overall heat transfer coefficient of FAS rapidly decreases in the beginning stage (up to ~60 min) due to the rapid decrease in droplet mobility and resulting increase in droplet size. The heat transfer coefficient of FAS after the thermal stability test was similar with the one obtained with untreated Cu substrates, which implies the effects of SAM coating was almost disappeared after the thermal stability test. On the other hand, the overall heat transfer coefficient value of Ceria was maintained almost same after the thermal stability test. After ~300 min of condensation test, the overall heat transfer coefficient of Ceria samples was calculated to be ~5 times higher compared with FAS (Figure 9b).

We note that the reported heat transfer coefficients were calculated using the thermalnetwork-based model incorporating experimentally obtained droplet size distribution and wetting morphology. Consequently, the results include several simplifications. First, the effects of non-uniform liquid vapor temperature and Marangoni forces were ignored. Recently, more accurate numerical approaches were conducted to incorporate the non-


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uniform liquid vapor interface temperatures68 and the Marangoni forces69 in the heat transfer rate calculation. In the present work, however, we applied the simplified analytical model ignoring such effects since over 50% of heat transfer occurs through small (d < ~30 µm) droplets for Ceria surfaces. For such small droplets, the analytical model and numerical approach show good agreement due to the negligible conduction resistance (i.e., small Bi number, Bi < ~5) and low Marangoni number (Ma < 100). Second, the suggested model did not incorporate the diffusion resistance associated with the presence of non-condensable gases. In the present analysis, the improvement of heat transfer coefficient on Ceria surfaces was predicted mainly due to the decrease in the thermal conduction resistance across the droplet associated with the enhanced droplet mobility. When the diffusion resistance due to the presence of non-condensable gases plays a significant role, the increase in the heat transfer coefficient should significantly decrease. Despite these limitations, the present work shows that the suggested Ceria coating can provide stable dropwise condensation even after long exposure to hot (~100°C) steam environment, which shows that the suggested Ceria coating may increase the heat transfer performance of various industrial condensers.

4-4. Applying to other substrates Nanostructured Ceria coating was demonstrated on other popular engineering materials including aluminum and steel using the developed 1-step wet-chemical process. Commercially available aluminum substrates (A1050, 99.5% purity, 3 cm × 3 cm × 0.8 mm) are used as starting substrates (The detailed chemical composition is provided in Table S1). The pretreated Al surfaces were immersed in the identical solutions described in 2-1 (41.68 g of cerium nitrate powders (99.5%, Alfa Aesar), 25.64 mL of hydrogen peroxide (35%, Alfa Aesar) per 1000 mL DI water) but the optimized dipping time was longer (120 min) than Cu



surfaces since the formation of Ce conversion layers on Al surface may delay the coating reaction.

Figures 10a and 10b show the top and cross-sectional FE-SEM images of the manufactured Ceria-Al surfaces, respectively. The FIB milling technique was applied to obtain the crosssectional view. Figure 10a shows that the particles of diameter of 100 nm - 300 nm cover the entire substrate and the average coating thickness is approximately 100 nm - 200 nm for Al substrate, which is similar with the morphology obtained with Cu substrate. Both XPS and contact angle measurement show the nearly identical trend with Ceria-Cu (Figures 10c and 10d), which implies the effects of substrate are negligible after incorporating the Ceria coating. As in Ceria-Cu samples, the hydrocarbon (1-octadecene) treated Ceria-Al shows superhydrophobicity 3-4 times faster compared with the air-exposed one.

Ceria-Al also shows the improved thermal stability and heat transfer coefficient compared with FAS-Al. Before the thermal stability test, the overall heat transfer coefficient of CeriaAl is approximately ~2 times higher than that of FAS-Al, which is similar with the case of Cu (Figures 11a and 11b). After the thermal stability test, the overall heat transfer coefficient value of FAS-Al decreases more sharply than that of FAS-Cu according to the significant increase in contact angle hysteresis (Table S4). As in Figure S7, the Ceria-Al maintained dropwise morphology with larger number density of small (< 30 µm) droplets after the thermal stability test, while FAS-Al lost its dropwise morphology. As a result, the overall heat transfer coefficient value of Ceria-Al was calculated to be up to ~5 times higher than that of FAS-Al after ~300 min of test (Figure 11b). More detailed information obtained with Ceria-Al including the microscopic condensation behaviors (Figure S7), the droplet number density with size distribution (Figure S8), the total and conduction thermal resistance (Figure


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S9) and the average heat transfer rate of an individual droplet (Figure S10) are provided in the supplementary information.

We note that the suggested manufacturing process was successfully applied to another popular engineering material, steel. Figures S11a and S11b provide the SEM images and contact angles of Ceria-Steel obtained with the same recipe applied to Ceria-Al. The results are similar with the results obtained with Cu and Al substrates. The results show that the suggested method can incorporate thermally robust superhydrophobic coatings to popular engineering materials including Cu, Al and steel, which shows the strong potential for the suggested coating for industrial applications.

5. Conclusion We introduced thermally stable nanostructured cerium oxide coating formed by a 1-step wet chemical process on popular engineering materials including copper, aluminum and steel. Due to the unique particle-like morphology, the suggested thin (< 200 nm) Ceria coating provided excellent superhydrophobicity with less than 5° of contact angle hysteresis. Microscopic condensation experiments showed that the suggested ceria surface could provide a significantly larger number of small (< 30 µm) condensates compared with FAS-treated hydrophobic surface due to the higher contact angle and enhanced droplet mobility. After 12 hours of accelerated thermal stability test conducted with hot (~100°C) steam, the Ceria surface maintained its original microscopic condensation behaviors and resulting heat transfer performance while FAS-treated surface showed significant reduction in droplet mobility and heat transfer performance. The thermal-network model incorporating the microscopic



experimental data predicts that the suggested cerium oxide surfaces can provide approximately 2 times and 5 times higher heat transfer coefficient compared with the silanetreated surfaces before and after the thermal stability test, respectively. The results show that the suggested Ceria coating may provide a promising solution to enhance the heat transfer performance and thermal stability of various industrial condensers.

ASSOCIATED CONTENT Supporting Information. The Supporting information is available free of charge on the ACS Publication website at DOI: ##/###. Further information about chemical compositions of substrates (Table S1), EDS data of Ceria-Cu (Table S2), contact angles of Air and Hydrocarbon-exposed Ceria-Cu and Ceria-Al before and after the thermal stability test (Table S3), contact angles of FAS and Ceria-coated aluminum surfaces before and after the thermal stability test (Table S4), parameters applied to the thermal-network-based model (Table S5), additional high resolution XPS data of Ceria-Cu (Figure S1), microscopic condensation behaviors (Figure S2) and calculated overall heat transfer coefficient (Figure S3) of Air and Hydrocarbon-exposed Ceria-Cu before and after the thermal stability test, changes in contact angle of hydrocarbon-exposed untreated Cu and Al (Figure S4), condensation behaviors of FAS-Cu and Ceria-Cu during the thermal stability test (Figure S5), XPS data of Ceria-Al (Figure S6), microscopic condensation behaviors (Figure S7), droplet number density (Figure S8), thermal resistance (Figure S9) and average heat transfer rate (Figure S10) of Ceria-Al before and after the thermal stability test, surface morphology and contact angles of Ceria-Steel (Figure S11) (PDF)


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AUTHOR INFORMATION Corresponding Author Youngsuk Nam Email: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by Space Core Technology Program (2014M1A3A3A02034818), Fundamental Technology Research Program (2014M3A7B4052202), Basic Research Laboratory Program (2016R1A4A1012950) funded by the Ministry of Science and ICT, and also by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Ministry of Trade, Industry & Energy (MOTIE) (No. 20153030091420).

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(32) Enright, R.; Miljkovic, N.; Alvarado, J. L.; Kim, K.; Rose, J. W. Dropwise Condensation on Micro- and Nanostructured Surfaces. Nanoscale and Microscale Thermophysical Engineering 2014, 18 (3), 223-250. (33) An, T.; Deng, X.; Gao, Y.; Liu, S.; Dou, C.; Ju, J. Preparation of Highly Hydrophobic CeO2 Films Using Glancing Angle Deposition. Materials Letters 2018, 216, 147-149. (34) Azimi, G.; Kwon, H.-M.; Varanasi, K. K. Superhydrophobic Surfaces by Laser Ablation of Rare-Earth Oxide Ceramics. MRS Communications 2014, 4 (3), 95-99. (35) Oh, I.-K.; Kim, K.; Lee, Z.; Ko, K. Y.; Lee, C.-W.; Lee, S. J.; Myung, J. M.; LansalotMatras, C.; Noh, W.; Dussarrat, C.; Kim, H.; Lee, H.-B.-R. Hydrophobicity of Rare Earth Oxides Grown by Atomic Layer Deposition. Chemistry of Materials 2015, 27 (1), 148-156. (36) Khan, S.; Azimi, G.; Yildiz, B.; Varanasi, K. K. Role of Surface Oxygen-to-Metal Ratio on the Wettability of Rare-Earth Oxides. Applied Physics Letters 2015, 106 (6), 061601. (37) Pedraza, F.; Mahadik, S. A.; Bouchaud, B. Synthesis of Ceria Based Superhydrophobic Coating on Ni20Cr Substrate via Cathodic Electrodeposition. Physical Chemistry Chemical Physics 2015, 17 (47), 31750-31757. (38) Carchini, G.; García-Melchor, M.; Łodziana, Z.; López, N. Understanding and Tuning the Intrinsic Hydrophobicity of Rare-Earth Oxides: A DFT+U Study. ACS Applied Materials & Interfaces 2016, 8 (1), 152-160. (39) Lundy, R.; Byrne, C.; Bogan, J.; Nolan, K.; Collins, M. N.; Dalton, E.; Enright, R. Exploring the Role of Adsorption and Surface State on the Hydrophobicity of Rare Earth Oxides. ACS Applied Materials & Interfaces 2017, 9 (15), 13751-13760. (40) Külah, E.; Marot, L.; Steiner, R.; Romanyuk, A.; Jung, T. A.; Wäckerlin, A.; Meyer, E. Surface Chemistry of Rare-Earth Oxide Surfaces at Ambient Conditions: Reactions with Water and Hydrocarbons. Scientific Reports 2017, 7, 43369. (41) Fu, S.-P.; Rossero, J.; Chen, C.; Li, D.; Takoudis, C. G.; Abiade, J. T. On the Wetting Behavior of Ceria Thin Films Grown by Pulsed Laser Deposition. Applied Physics Letters 2017, 110 (8), 081601.



(42) Gligorovski, S.; Abbatt, J. P. D. An Indoor Chemical Cocktail. Science 2018, 359 (6376), 632-633. (43) McDonald, B. C.; de Gouw, J. A.; Gilman, J. B.; Jathar, S. H.; Akherati, A.; Cappa, C. D.; Jimenez, J. L.; Lee-Taylor, J.; Hayes, P. L.; McKeen, S. A.; Cui, Y. Y.; Kim, S.-W.; Gentner, D. R.; Isaacman-VanWertz, G.; Goldstein, A. H.; Harley, R. A.; Frost, G. J.; Roberts, J. M.; Ryerson, T. B.; Trainer, M. Volatile Chemical Products Emerging as Largest Petrochemical Source of Urban Organic Emissions. Science 2018, 359 (6377), 760-764. (44) Cha, H.; Wu, A.; Kim, M.-K.; Saigusa, K.; Liu, A.; Miljkovic, N. NanoscaleAgglomerate-Mediated Heterogeneous Nucleation. Nano Letters 2017, 17 (12), 7544-7551. (45) Wei, X.-L.; Li, N.; Yi, W. J.; Li, L.-J.; Chao, Z.-S. High Performance SuperHydrophobic ZrO2-SiO2 Porous Ceramics Coating with Flower-Like CeO2 Micro/NanoStructure. Surface and Coatings Technology 2017, 325, 565-571. (46) Preston, D. J.; Miljkovic, N.; Sack, J.; Enright, R.; Queeney, J.; Wang, E. N. Effect of Hydrocarbon Adsorption on the Wettability of Rare Earth Oxide Ceramics. Applied Physics Letters 2014, 105 (1), 011601. (47) Fu, S.-P.; Sahu, R. P.; Diaz, E.; Robles, J. R.; Chen, C.; Rui, X.; Klie, R. F.; Yarin, A. L.; Abiade, J. T. Dynamic Study of Liquid Drop Impact on Supercooled Cerium Dioxide: Anti-Icing Behavior. Langmuir 2016, 32 (24), 6148-6162. (48) Hamlaoui, Y.; Tifouti, L.; Remazeilles, C.; Pedraza, F. Cathodic Electrodeposition of Cerium Based Oxides on Carbon Steel from Concentrated Cerium Nitrate. Part II: Influence of Electrodeposition Parameters and of the Addition of PEG. Materials Chemistry and Physics 2010, 120 (1), 172-180. (49) Hamlaoui, Y.; Pedraza, F.; Remazeilles, C.; Cohendoz, S.; Rébéré, C.; Tifouti, L.; Creus, J. Cathodic Electrodeposition of Cerium-Based Oxides on Carbon Steel from Concentrated Cerium Nitrate Solutions: Part I. Electrochemical and Analytical Characterisation. Materials Chemistry and Physics 2009, 113 (2–3), 650-657. (50) Zhitomirsky, I.; Petric, A. Electrochemical Deposition of Ceria and Doped Ceria Films. Ceramics International 2001, 27 (2), 149-155.


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(51) Golden, T. D.; Shang, Y.; Wang, Q.; Zhou, T. Electrochemical Synthesis of Rare Earth Ceramic Oxide Coatings. In Advanced Ceramic Processing; Mohamed, A., Ed.; InTech: Rijeka, 2015; p Ch. 04. (52) Wang, H.; Yu, J.; Wu, Y.; Shao, W.; Xu, X. A Facile Two-Step Approach to Prepare Superhydrophobic Surfaces on Copper Substrates. Journal of Materials Chemistry A 2014, 2 (14), 5010-5017. (53) Yang, Z.; Wu, Y.-Z.; Ye, Y.-F.; Gong, M.-G.; Xu, X.-L. A Simple Way to Fabricate an Aluminum Sheet with Superhydrophobic and Self-Cleaning Properties. Chinese Physics B 2012, 21 (12), 126801. (54) Kim, H.; Nam, Y. Condensation Behaviors and Resulting Heat Transfer Performance of Nano-Engineered Copper Surfaces. International Journal of Heat and Mass Transfer 2016, 93, 286-292. (55) Rykaczewski, K.; Scott, J. H. J.; Rajauria, S.; Chinn, J.; Chinn, A. M.; Jones, W. Three Dimensional Aspects of Droplet Coalescence during Dropwise Condensation on Superhydrophobic Surfaces. Soft Matter 2011, 7 (19), 8749-8752. (56) Nam, Y.; Seo, D.; Lee, C.; Shin, S. Droplet Coalescence on Water Repellant Surfaces. Soft Matter 2015, 11 (1), 154-160. (57) Nam, Y.; Kim, H.; Shin, S. Energy and Hydrodynamic Analyses of CoalescenceInduced Jumping Droplets. Applied Physics Letters 2013, 103 (16), 161601. (58) Cha, H.; Xu, C.; Sotelo, J.; Chun, J. M.; Yokoyama, Y.; Enright, R.; Miljkovic, N. Coalescence-Induced Nanodroplet Jumping. Physical Review Fluids 2016, 1 (6), 064102. (59) Mouterde, T.; Lehoucq, G.; Xavier, S.; Checco, A.; Black, C. T.; Rahman, A.; Midavaine, T.; Clanet, C.; Quéré, D. Antifogging Abilities of Model Nanotextures. Nature Materials 2017, 16, 658. (60) Enright, R.; Miljkovic, N.; Sprittles, J.; Nolan, K.; Mitchell, R.; Wang, E. N. How Coalescing Droplets Jump. ACS Nano 2014, 8 (10), 10352-10362.



(61) Miljkovic, N.; Preston, D. J.; Enright, R.; Wang, E. N. Electric-Field-Enhanced Condensation on Superhydrophobic Nanostructured Surfaces. ACS Nano 2013, 7 (12), 11043-11054. (62) Miljkovic, N.; Preston, D. J.; Enright, R.; Wang, E. N. Electrostatic Charging of Jumping Droplets. Nature Communications 2013, 4, 2517. (63) Miljkovic, N.; Wang, E. N. Condensation Heat Transfer on Superhydrophobic Surfaces. MRS Bulletin 2013, 38 (5), 397-406. (64) Wen, R.; Xu, S.; Ma, X.; Lee, Y.-C.; Yang, R. Three-Dimensional Superhydrophobic Nanowire Networks for Enhancing Condensation Heat Transfer. Joule 2018, 2 (2), 269-279. (65) Carey, V. P. Liquid-Vapor Phase-Change Phenomena, Hemisphere, New York, NY (United States): United States, 1992. (66) Hiroaki, T.; Takaharu, T. A Microscopic Study of Dropwise Condensation. International Journal of Heat and Mass Transfer 1984, 27 (3), 327-335. (67) Miljkovic, N.; Enright, R.; Wang, E. N. Modeling and Optimization of Superhydrophobic Condensation. Journal of Heat Transfer 2013, 135 (11), 111004-11100414. (68) Chavan, S.; Cha, H.; Orejon, D.; Nawaz, K.; Singla, N.; Yeung, Y. F.; Park, D.; Kang, D. H.; Chang, Y.; Takata, Y.; Miljkovic, N. Heat Transfer through a Condensate Droplet on Hydrophobic and Nanostructured Superhydrophobic Surfaces. Langmuir 2016, 32 (31), 7774-7787. (69) Phadnis, A.; Rykaczewski, K. The Effect of Marangoni Convection on Heat Transfer during Dropwise Condensation on Hydrophobic and Omniphobic Surfaces. International Journal of Heat and Mass Transfer 2017, 115, 148-158. (70) Kim, S.; Kim, K. J. Dropwise Condensation Modeling Suitable for Superhydrophobic Surfaces. Journal of Heat Transfer 2011, 133 (8), 081502-081502-8.


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Table 1. Contact angles of each sample obtained by varying the concentration of solution and dipping time. The concentration of cerium nitrate, the concentration of hydrogen peroxide and the dipping time were modified by fixing the other parameters at the optimal level. The contact angles values are obtained after 4 days of exposure to an ambient condition.

Cerium nitrate*

Hydrogen peroxide*

Dipping time

Advancing C.A. (º)

Receding C.A. (º)

< 41.68 g

158.4±0.7

< 10

Optimal

164.0±0.5

159.4±0.8

> 98.9 g

159.9±0.7

< 10

< 25.64 mL

147.1±2.9

< 10

Optimal

164.0±0.5

159.4±0.8

> 41.67 mL

159.9±0.7

< 10

< 20 min

114.9±1.6

< 10

Optimal

164.0±0.5

159.4±0.8

> 40 min

149.4±1.7

< 10 *


per 1000 mL of DI water


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Table 2. Contact angles of the investigated surfaces before and after the thermal stability test.

Advancing CA (º)

Receding CA (º)

Before

After

Before

After

FAS-Cu

122.5±2.0

78.1±1.1

73.1±5.6

< 10

Ceria-Cu

164.0±0.5

159.0±1.1

159.4±0.8

153.3±0.5


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Table 3. The values of each parameter applied to the thermal-network-based model. We applied the average thickness of cerium oxide layer and the cerium oxide nanostructures based on the cross-sectional FE-SEM images.

Thermal conductivity of hydrophobic coating (khc)

0.2 W/mK22, 70

Thermal conductivity of cerium oxide (kn)

11.7 W/mK46

Thickness of hydrophobic coating (δhc)

1 nm

Thickness of cerium oxide nanostructure (δn)

110 nm

Thickness of cerium oxide layer (δl)

130 nm



Figure 1. Field emission scanning electron microscopy images (FE-SEM) of the top view of cerium oxide layer deposited on Cu substrate by varying (a) the concentration of cerium nitrate, (b) the concentration of hydrogen peroxide and (c) the dipping time. The values were based on 1000 mL of DI water. (The concentration of cerium nitrate, the concentration of hydrogen peroxide and the dipping time were modified by fixing the other parameters at the optimal level.)


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Figure 2. (a) Field emission scanning electron microscopy (FE-SEM) images of crosssectional view of focused ion beam (FIB) milled Ceria-Cu, (b) X-ray photoelectron spectroscopy (XPS) results of ceria 3d reference spectra for Ce2O3 and CeO2 on Ceria-Cu. The mixed phase oxides including Ce2O3 (u, u´, u0 and u0´) and CeO2 (v, v´, v˝, v0, v0´ and v0˝) are observed. Dynamic contact angles as a function of time for Ceria-Cu exposed to (c) an ambient condition (temperature ≈ 24º, relative humidity ≈ 45%) and (d) hydrocarbon (1octadecene)-rich environment.



Figure 3. (a) Schematics of hot (100ºC) steam based thermal stability test setup. High supersaturation level (S~25.3) was maintained to conduct the accelerated thermal stability test during condensation. (b) Schematics of microscopic condensation experiment setup with a moderate supersaturation level (S~1.6).


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Figure 4. Time lapse images of condensed droplets on FAS-Cu and Ceria-Cu samples before (a-b) and after (c-d) the thermal stability test. Note that the scale bar is different for (c) FAS-Cu due to the significant increase in the size of condensed droplets.



(a)

(b) 1.5x1010 d ≤ 30µm 30µm ≤ d < 100µm d ≥ 100µm

FAS-Cu 4.0x109 2.0x109 4.0x108 2.0x108

Number of droplet (1/m2)


6.0x109

1.0x1010

d ≤ 30µm 30µm ≤ d < 100µm d ≥ 100µm

Ceria-Cu

5.0x109

4.0x108 2.0x108 0.0

0.0 0

60

120

180

240

300

0

60

Time (min)

120

180

240

300

Time (min)

(c)

(d) 1.4x10

9

1.2x10

9

1.5x1010 d ≤ 30µm 30µm ≤ d < 100µm d ≥ 100µm

FAS-Cu 1.0x109 4.0x108 2.0x108



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 38 of 46

1.0x1010

d ≤ 30µm 30µm ≤ d < 100µm d ≥ 100µm

Ceria-Cu

5.0x109

4.0x108 2.0x108 0.0

0.0 0

60

120

180

240

300

0

Time (min)

60

120

180

Time (min)


240

300

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Figure 5. Droplet number density with size distribution for FAS-Cu and Ceria-Cu surfaces before (a-b) and after (c-d) the thermal stability test.



0123

(a) 0. r Coating

04

6

(b)

δ:

6

+7

048

Substrate 049

Vapor 0123 +, - +. 0. +/ 04 +5, 01 Substrate Vapor 0123 +, - +. 0. +/ 048

0123

0.

r Ceria

δ5,

01

Substrate

Page 40 of 46

01

δ;

+;

+:

049

01 Substrate

Figure 6. Schematics of the thermal network model of an individual droplet on (a) FAS and (b) Ceria surfaces.


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Figure 7. Total and conduction thermal resistance through an individual droplet on each surface (a) before and (b) after the thermal stability test.



Figure 8. Average heat transfer rate through an individual droplet on each case before and after the thermal stability test.


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(a) 2.0x104

Coefficient (W/m2K)

Calculated Overall Heat Transfer

FAS-Cu (Before) FAS-Cu (After) Ceria-Cu (Before) Ceria-Cu (After)

1.5x104

1.0x104

5.0x103

0.0

0

60

120

180

300

240

Time (min)

(b) 6

hCeria-Cu / hFAS-Cu

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Before After

5 4 3 2 1

0

60

120

180

240

300

Time (min) Figure 9. (a) The calculated overall heat transfer coefficient of each sample according to the microscopic condensation time, (b) the ratio between the heat transfer coefficient of Ceria-Cu and FAS-Cu obtained before and after the thermal stability test. The overall heat transfer coefficient of Ceria-Cu is approximately 2 times and 5 times higher than that of FAS-Cu before and after the thermal stability test, respectively. 43 ACS Paragon Plus Environment


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Figure 10. Field emission scanning electron microscopy images (FE-SEM) of (a) top view of Ceria-Al and (b) cross-sectional view of focused ion beam (FIB) milled Ceria-Al. (c) X-ray photoelectron spectroscopy (XPS) results of ceria 3d reference spectra for Ce2O3 and CeO2 on Ceria-Al and (d) dynamic contact angles as a function of time for Ceria-Al exposed to an ambient condition (temperature ≈ 24º, relative humidity ≈ 45%) and hydrocarbon (1-octadecene).

44 ACS Paragon Plus Environment

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(a) 2.0x104

Coefficient (W/m2K)

Calculated Overall Heat Transfer

FAS-Al (Before) FAS-Al (After) Ceria-Al (Before) Ceria-Al (After)

1.5x104

1.0x104

5.0x103

0.0

0

60

120

180

240

300

Time (min)

(b) 6

Before After

5

hCeria-Al / hFAS-Al

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4 3 2 1

0

60

120

180

240

300

Time (min) Figure 11. (a) The calculated overall heat transfer coefficient of each sample according to the microscopic condensation time, (b) the ratio between the heat transfer coefficient of Ceria-Al and FAS-Al obtained before and after the thermal stability test. The overall heat transfer coefficient of Ceria-Al is calculated to be approximately 2 times and 5 times higher than that of FAS-Al before and after the thermal stability test, respectively. 45 ACS Paragon Plus Environment


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For Table of Contents Only

46 ACS Paragon Plus Environment

Condensation Heat Transfer Performance of Thermally Stable

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