Liquid-Infused Smooth Surface for Improved Condensation Heat

Aug 21, 2017 - Center for Material Design Science, School of Integrated Design ... of low-roughness and high-roughness base layers; advancing contact ...
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Liquid-Infused Smooth Surface for Improved Condensation Heat Transfer Hirotaka Tsuchiya,†,‡ Mizuki Tenjimbayashi,†,‡ Takeo Moriya,† Ryohei Yoshikawa,† Kaichi Sasaki,† Ryo Togasawa,† Taku Yamazaki,† Kengo Manabe,† and Seimei Shiratori*,† †

Center for Material Design Science, School of Integrated Design Engineering, Graduate School of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522 Japan S Supporting Information *

ABSTRACT: Control of vapor condensation properties is a promising approach to manage a crucial part of energy infrastructure conditions. Heat transfer by vapor condensation on superhydrophobic coatings has garnered attention, because dropwise condensation on superhydrophobic surfaces with rough structures leads to favorable heat-transfer performance. However, pinned condensed water droplets within the rough structure and a high thermodynamic energy barrier for nucleation of superhydrophobic surfaces limit their heattransfer increase. Recently, slippery liquid-infused surfaces (SLIPS) have been investigated, because of their high water sliding ability and surface smoothness originating from the liquid layer. However, even on SLIPS, condensed water droplets are eventually pinned to degrade their heat-transfer properties after extended use, because the rough base layer is exposed as infused liquid is lost. Herein, we report a liquid-infused smooth surface named “SPLASH” (surface with π electron interaction liquid adsorption, smoothness, and hydrophobicity) to overcome the problems derived from the rough structures in previous approaches to obtain stable, high heat-transfer performance. The SPLASH displayed a maximum condensation heat-transfer coefficient that was 175% higher than that of an uncoated substrate. The SPLASH also showed higher heat-transfer performance and more stable dropwise condensation than superhydrophobic surfaces and SLIPS from the viewpoints of condensed water droplet mobility and the thermodynamic energy barrier for nucleation. The effects of liquid-infused surface roughness and liquid viscosity on condensation heat transfer were investigated to compare heat-transfer performance. This research will aid industrial applications using vapor condensation.

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INTRODUCTION Vapor condensation is broadly used in various fields ranging from energy infrastructures, such as nuclear power plants, power generation, and water harvesting, to daily life, including air conditioning, and refrigeration.1−5 Enhancing condensation heat transfer by surface treatment to realize efficient energy use has received much attention.6−8 The condensation state on a surface is strongly dependent on its wettability. On hydrophilic surfaces, thin water films are formed because of their high contact angle hysteresis; these films deteriorate heat-transfer performance, because of the high thermal resistance of water films. This state is called filmwise condensation. Unlike the situation on hydrophilic surfaces, discrete condensed water droplets repeatedly form and depart on hydrophobic surfaces. This state is called dropwise condensation. Dropwise condensation shows much higher heat-transfer performance than filmwise condensation, because of the decreased thermal resistance of condensed water droplets on a surface, compared with that of a water film.9−11 Therefore, many studies have focused on dropwise condensation on hydrophobic surfaces, especially superhydrophobic ones, because of their high water repellency.12−15 Some superhydrophobic surfaces promote jumping droplet mode and improve the condensation heat transfer over flat hydrophobic surfaces.4,6,16 © 2017 American Chemical Society

However, dropwise condensation on superhydrophobic surfaces has some limitations. Although a rough surface structure is necessary to attain superhydrophobicity,17−19 a rough surface structure results in problems during condensation heat transfer. The main problem is that, even if a surface is superhydrophobic to water at room temperature, in environments containing much hot vapor, vapor penetrates into the rough structures and large condensed water droplets form within them that become Wenzel states over time (Scheme 1A). Such a situation causes heat-transfer performance to deteriorate, because of the high heat resistance and low departure frequency of condensed water droplets.20 In addition, the thermodynamic energy barrier for nucleation and the conduction resistance between the condensed droplets and surface are higher on rough surfaces with higher static contact angle, which also lower heat-transfer performance.21,22 Moreover, the durability of superhydrophobic surfaces with rough structure in the vapor-filled environment is questionable, and it is obscure whether superhydrophobic surfaces with nanostructure keep their improved heat-transfer performance or not. Received: June 14, 2017 Revised: July 14, 2017 Published: August 21, 2017 8950

DOI: 10.1021/acs.langmuir.7b01991 Langmuir 2017, 33, 8950−8960

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The SPLASH is fabricated by infusing a liquid on a smooth base layer with abundant π-electrons through π-interactions, which work even at the liquid/solid interface, instead of using capillary force, as observed in SLIPS. The phenyl-groupmodified smooth base layer, which includes abundant πelectrons, is prepared via sol−gel reaction,31 and the smooth base layer interacts with organic materials by π-stacking, which even works in a liquid state, allowing various organic liquids to be superoleophilic on the smooth base layer (for detailed information on the coating and related studies, see our previous work in refs 30−32). We previously revealed that the SPLASH shows multifunctional properties, such as extraordinary water sliding ability, strong mechanical robustness, and defect-free self-healing. Furthermore, these properties were stable against the change of the liquid thickness. In addition, the method to prepare the SPLASH is easy and suitable for various substrates.32 The base layer of the SPLASH allowed the liquid to slide via the movement of liquidlike molecules on the surface, in the same manner as the case of liquidlike surfaces in previous studies.33−35 On the base layer of the SPLASH, phenyl molecular chain movement is restricted by π−π interaction,31 whereas alkyl molecular chains exhibit flexible movement, similar to that of a liquid, as described in a previous study.34 After infusing the liquid, the SPLASH shows high water sliding ability and stable sliding performance against changes in liquid thickness at room temperature, because of the lack of an exposable rough surface. We expect that the SPLASH has the possibility to increase heat-transfer performance, because of its high, stable droplet mobility, as shown in Scheme 1C, to overcome the limitations of superhydrophobic surfaces and SLIPS in vapor condensation. In this paper, a SPLASH using silicone oil as the infusing liquid is prepared on copper (Cu) for applications using condensation heat transfer. To clarify the advantages of this smooth liquid-infused surface, we study its water sliding speed, which is strongly related to the heat-transfer performance of liquid-infused surfaces with different roughness including the SPLASH. To compare the condensation heat transfer of the SPLASH, the SLIPS, and superhydrophobic surfaces, three types of the base layers, for the SLIPS were fabricateda smooth base layer for the SPLASH, a low-roughness base layer, and a high-roughness base layerand then silicone oil was infused into the base layers. The condensation heat-transfer properties of those surfaces were investigated from the viewpoints of surface structure, wettability, and water sliding ability. We also examine the effect of liquid viscosity on heat transfer in a vapor-filled environment by infusing the smooth surface with silicone oil of different viscosity to reveal the infused liquid best-suited for condensation heat transfer. This research aids functional surface design for not only potential applications using vapor condensation but also fundamental studies of liquid−solid interfaces, especially on liquid-infused surfaces.36−38

Scheme 1. Conceptual Images of Condensation on (A) a Superhydrophobic Surface, (B) SLIPS, and (C) SPLASHa

a

Large condensed water droplets were pinned within the rough structures on superhydrophobic surfaces and the SLIPS, while the SPLASH showed high and stable water sliding ability in the vaporfilled environment.

According to these points, an alternative approach to using superhydrophobic surfaces to improve condensation heat transfer is required. Recent studies have reported that liquid-infused surfaces represent such an alternative, because of their high water droplet mobility and surface smoothness originating from the liquid mobility.23−26 One type of liquid-infused surface is the slippery liquid-infused porous surfaces (SLIPS) inspired by the Nepenthes pitcher plant.27 SLIPS are prepared by infusing liquid immiscible with water onto rough base structures to allow condensed water droplets to slide upon the infused liquid. SLIPS possess favorable heat-transfer performance, because of their high condensed water droplet mobility and low heat resistance caused by condensed water droplets. However, the water sliding ability on SLIPS is unstable for their long-term use, because the rough base layer is exposed by the decrease of infused liquid thickness over time; thus, condensed water droplets are finally pinned in the rough structure after a long period.28,29 Therefore, in a vapor-filled environment, the mobility of condensed water droplets on the SLIPS becomes lower, and heat-transfer performance degrades after a long time (Scheme 1B). Superhydrophobic surfaces and SLIPS have difficulties with regard to increasong heat transfer, because of their surface roughness. Therefore, new approaches that do not use rough structures are required. Here, we introduce a liquid-infused smooth surface called “SPLASH” (surface with π-electron interaction liquid adsorption, smoothness, and hydrophobicity) that our group recently developed as a smooth liquid surface.30

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EXPERIMENTAL SECTION

Materials. Phenyltriethoxysilane (Shin-Etsu Chemical Co., Ltd., Tokyo, Japan), tetramethoxysilane (Junsei Chemical Co., Ltd., Tokyo, Japan), ethanol (Wako Pure Chemical Industries, Ltd., Osaka, Japan), deionized water, and hydrochloric acid (HCl, Kanto Chemical Co., Ltd., Tokyo, Japan) were used to produce the smooth base layer of the SPLASH. KF-96-30 cs (viscosity = 30 cSt), KF-96-100 cs (viscosity = 100 cSt), and KF-96-300 cs (viscosity = 300 cSt) silicone oils (ShinEtsu Chemical Co., Ltd.) were used as the infused liquids. Acetone (Wako Pure Chemical Industries, Ltd.) was used to clean the dried 8951

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Condensation Heat-Transfer System. A schematic diagram of the system used to measure the condensation heat-transfer coefficient of the samples is shown in Scheme S1 in the Supporting Information. The size of the acryl chamber used in this work was 25 cm × 25 cm × 25 cm. A steamer (Tiger Vacuum Bottle Co., Ltd., Osaka, Japan) was positioned in the closed acryl chamber, and it was turned on to fill the chamber with steam. Cu tubes with a heating length of 20 cm were placed in the chamber. An electric water pump (Fluojet, ITT Industries, New York) and silicone tube (inner diameter = 10 mm, outer diameter = 12 mm, As One, Co., Ltd.) were used to circulate water through the Cu tubes. A cooling thermal pump (Model CTP101, Tokyo Rikatai Co., Ltd., Tokyo, Japan) was used to maintain the temperature of the cooling water. The inlet water temperature (Tin) and outlet water temperature (Tout) were measured by thermometers. The vapor temperature (Tv) and the surface temperature of the Cu tubes (Tw) were measured by thermocouples. Photographs and movies of condensation states were captured by a digital camera (RICOH, Co., Ltd., Tokyo, Japan). Temperature in the chamber at the steady state was 66 °C and endurance tests in the vapor-filled environment on the SPLASH and SLIPS infused with silicone oil with a viscosity of 30 cSt were conducted under this condition. Procedure of Condensation Heat-Transfer Experiments. As the procedure of measuring the condensation heat-transfer coefficient, first, a cooling thermal pump was run and maintain the temperature of 20 L cooling water. Then, the Cu tubes were set and cooling water circulated around the system. After that, the steamer was turned on, and water vapor flowed into the closed chamber. After reaching the steady state, the inlet water temperature (Tin), the outlet water temperature (Tout), the vapor temperature (Tv), and the surface temperature of the Cu tubes (Tw) were measured. Tin and Tout were measured at the end point of Cu tubes. The typical value and error of the fixed physical quantity obtained in the experiments are shown in (Table S1 in the Supporting Information. Calculation of the Condensation Heat Transfer Coefficient on Cu Surfaces. Some mathematical formulas are necessary to calculate the condensation heat-transfer coefficient (hc) on the Cu surfaces. The energy applied to the Cu tubes was balanced by the enthalpy change of the cooling water,7

base layers. Silica nanoparticles (diameter = 50 nm, AEROSIL Co., Ltd., Tokyo, Japan) were used to fabricate the rough base layer of the SLIPS. A Cu plate (99.9% purity, 0.3 mm thick, Sanyo Rikagaku Co., Ltd., Tokyo, Japan) and Cu tube (99.9% purity, inner diameter = 10 mm, outer diameter = 12 mm, Sanyo Rikagaku Co., Ltd.) were used as substrates. Surface Preparation. Before fabricating the base layers on Cu substrates, ethanol and acetone were used to clean the Cu substrates and then the substrates were made hydrophilic via UV/O3 treatment (Model NL-UV253, Shoko Scientific Co., Ltd., (formerly Japan Laser Electron), Kanagawa, Japan). Fabrication of the SPLASH. The solution used to fabricate the smooth base layer of the SPLASH was prepared by mixing phenyltriethoxysilane (2.630 g), tetramethoxysilane (8.300 g), ethanol (26.460 g), deionized water (4.600 g), and HCl (12.20 μL) in order, and then the resulting mixture was stirred for 24 h to react. The smooth base layer of the SPLASH was fabricated on a Cu substrate by dip coating at a constant speed of 1 mm/s using the prepared solution. After dipping, the samples were dried for 24 h at room temperature and then acetone was used to clean the smooth base layer of the SPLASH. Silicone oil with viscosities of 30, 100, and 300 cSt was added dropwise onto the smooth base layer of the SPLASH to investigate the influence of oil viscosity on heat transfer. Each substrate was positioned vertically at room temperature, to remove excess liquid by gravity, and various thicknesses of oil (760, 840, and 910 nm) were formed for silicone oil with viscosities of 30, 100, and 300 cSt, respectively. The thickness of oil was calculated from the weight change of the samples between before and after lubrication. Fabrication of the SLIPS. The solution used to fabricate the rough base layer of the SLIPS was prepared by mixing silica nanoparticles (3.600 g), ethanol (45.000 g), phenyltriethoxysilane (0.275 g), tetramethoxysilane (1.096 g), deionized water (0.375 g), and HCl (3.00 μL) in order, and then the resulting mixture was stirred for 24 h to react. The low-roughness base layer of the SLIPS was fabricated on a Cu substrate by dip coating at a constant speed of 1 mm/s using the prepared solution. The high-roughness base layer was fabricated by spraying the prepared solution onto a Cu substrate, using a spray gun (nozzle diameter = 0.6 mm; Model XP7, Airtex Co., Ltd., Tokyo, Japan) and dried for 24 h at room temperature. The spraying pressure of the spray gun was 0.7 MPa, the amount of solution sprayed was 0.1 mL/cm2, and the spraying distance between the spray gun and Cu substrate was 50 cm. After dipping and drying, the samples were dried for an additional 24 h at room temperature and then silicone oil with a viscosity of 30 cSt was added dropwise onto the low- and highroughness base layer of the SLIPS. Each substrate was positioned vertically at room temperature to remove excess liquid by gravity and 950 and 1320 nm thickness of oil was formed on the SLIPS with lowroughness base layer and high-roughness base layer, respectively. The thickness of the oil was calculated from the weight change of the samples between before and after lubrication. Characterization. Field-emission scanning electron microscopy (FE-SEM) images were taken using a FE-SEM instrument (Model JSM-7600F, JEOL, Ltd., Akishima, Japan) with an accelerating voltage of 5 kV to characterize the surface morphologies of the base layer of the SPLASH and SLIPS. The surface smoothness was measured via atomic force microscopy (Model Nanoscope IIIa, Digital Instruments, USA). Laser scanning microscopy images were taken by a color 3D laser scanning microscope (Model VK-9710, Keyence, Japan). A commercial contact angle system (FACE, Kyowa Interface Science Co., Ltd., Niiza, Japan) was used to measure the static water contact angle, advancing contact angle, receiving contact angle, contact angle hysteresis, and water sliding angle of surfaces at room temperature. Distilled water (10-μL, surface tension: 72.8 mN/m) was used as the probe liquid. Static water contact angle was measured using the sessile drop technique. Advancing contact angle, receiving contact angle, and contact angle hysteresis were measured by needle-in-the-sessile-drop method. A high-speed camera (Model HAS-D3, Ditect Co., Ltd., Tokyo, Japan) was used to measure the water sliding speed on the liquid-infused surfaces.

Q = mC ̇ p(Tout − Tin)

(1)

where Q is the total condensation heat-transfer rate, ṁ is the cooling water mass flow rate inside the Cu tubes, and Cp is the specific heat of water. The overall heat-transfer coefficient U̅ is determined from

mC ̇ p(Tout − Tin) = UA ̅ ΔLMTD

(2)

where A is the outer surface area of the tube (A = πdODL, where dOD is the outer diameter of the tube, and L is its heating length). ΔLMTD is the log-mean temperature difference, obtained by

ΔLMTD =

Tout −Tin

(

ln

Ts − Tin Ts − Tout

)

(3)

Using eq 9, U̅ can be obtained by measuring Q experimentally. U̅ is written in the following form: U̅ =

mC ̇ p(Tout − Tin) AΔLMTD

(4)

Although U̅ is a measure of the overall heat-transfer performance from the vapor to cooling water, further calculations are required to determine hc, which is measured from the vapor to the tube outer surface. To obtain hc, the overall surface heat flux must be considered. Here, the Sieder−Tate correlation39 was used to calculate the internal flow condensation heat-transfer coefficient for laminar flow:

⎛ k ⎞ hi = ⎜ i ⎟Nu ⎝ dID ⎠

(5)

where 8952

DOI: 10.1021/acs.langmuir.7b01991 Langmuir 2017, 33, 8950−8960

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Figure 1. FE-SEM images (upper) and 3D laser microscope images (lower) of smooth, low-roughness, and high-roughness base layers. Black scale bars of FE-SEM image and 3D laser microscope image are 200 nm and 10 μm, respectively.

Table 1. Static Contact Angles and Surface Tensions of Water and 30 cSt Silicone Oil and Calculated Interfacial Tension and Spreading Coefficient between Water and 30 cSt Silicone Oil Water

30 cSt Silicone Oil

γwa [mN/m]

θ′w [°]

γwα oa [mN/m]

γoa [mN/m]

θ′o [°]

γαoa [mN/m]

γwo [mN/m]

ΔE1 [mJ/m2 ]

ΔE2 [mJ/m2 ]

Sow(a) [mN/m]

72.8 72.8

118.6 154.0

21.8 21.8

20.7 20.7

6.0 4.0

20.7 20.7

51.0 51.0

4.4 43.1

107.6 137.7

1.1 1.1

Rough L Rough H

1/3 ⎛ L ⎞ ⎛μ⎞ Nu = 1.86⎜Re· Pr· ⎟ ⎜⎜ ⎟⎟ dID ⎠ ⎝ μw ⎠ ⎝

0.14

13, 80, and 1077 nm, respectively. The very small Rrms value of the smooth base layer originated from the precisely controlled silane polymerization. The sprayed rough base layer showed higher roughness than the dipped rough base layer, because the spray coating promotes nanoparticle self-assembly to a greater extent than dip coating. The three surfaces were modified with phenyl groups, using silane technology. The static contact angles of water and silicone oil, interfacial tension between water, silicone oil, air, and the low- and highroughness base layers were measured to determine whether the base layers fulfilled the following SLIPS criteria:40

(6)

with Re =

ρvdID μ

(7)

Here, hi is the heat-transfer coefficient of the internal cooling water flow, ki the thermal conductivity of the cooling water, and dID the inner diameter of the Cu tube. Re is the Reynolds number of the cooling water flow, Pr the Prandtl number, ρ the cooling water density, μ the viscosity of the cooling water, and μw is the viscosity of the cooling water at the wall temperature of the Cu tube. After hi was calculated, all of the relevant temperature decreases (internal cooling water flow, radial conduction through the Cu tube, and radial conduction through thin films) were combined to obtain hc, using eq 8:

⎛ d A ln d ID ⎜1 A OD − hc = ⎜ − π U A h 2 Lk ̅ ⎜ Cu i i ⎝

ΔE1 = γoa cos θo′ − γwa cos θw′ − γwo > 0 ΔE2 = γoa cos θo′ − γwa cos θw′ + γwa − γoa > 0

(10)

where γij is the interfacial tension between two phases, with i and j denoting silicone oil (o), water (w), or air (a); and θ′o the static contact angle of silicone oil on the base layer, and θw′ the static contact angle of water on the base layer. γwo was calculated using the Fowkes equation:

( ) ⎞⎟⎟

−1

⎟ ⎠

(9)

(8)

where Ai is the internal tube surface area (Ai = πdIDL), and kCu is the Cu thermal conductivity. Observation of Microscale Dynamics of Condensation. Samples were set on Peltier element in a thermo-hygrostat in air with a temperature of 23 °C and the Peltier temperature was set at 2 °C. Photographic images were captured by a charge-coupled device camera. The Peltier stage was set vertically.

γwo = γoa + γwa − 2 γoa αγoa wα

γαoa

(11)

γwα oa

where and are the dispersion force components of the liquid surface tension. The dispersion force component of water is 21.8 mN/m. For a nonpolar liquid, such as silicone oil, γαoa ≈ γoa = 20.7 mN/m. The static contact angles of water and silicone oil were 118.6° and 6.0° on the low-roughness base layer, respectively, and those of water and silicone oil were 153.2° and 4.0° on the high-roughness base layer, respectively. Therefore, ΔE1 = 4.4 mJ/m2 and ΔE2 = 107.6 mJ/m2 on the low-roughness base layer, and ΔE1 = 43.1 mJ/m2 and ΔE2 = 137.7 mJ/m2 on the high-roughness base layer. Therefore, SLIPS that were designed by infusing silicone oil into low- and high-roughness base layers are thermodynamically stable. The criterion for cloaking is determined by the spreading coefficient,41,42

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RESULTS AND DISCUSSION Structure Analysis and Thermodynamic Stability of Liquid-Infused Surfaces. The morphologies of the designed smooth base layer, low-roughness base layer (Rough L), and high-roughness base layer (Rough H) were analyzed from FESEM images and laser microscopy images, as shown in Figure 1. AFM images of the smooth base layer and low-roughness base layer are shown in Figure S1 in the Supporting Information. Cross-sectional SEM images of low-roughness and highroughness base layers are shown in Figure S2 in the Supporting Information. The root-mean-square roughness (Rrms) of the smooth, low-roughness, and high-roughness base layers were

Sow(a) = γwa − γwo − γoa 8953

(12) DOI: 10.1021/acs.langmuir.7b01991 Langmuir 2017, 33, 8950−8960

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Figure 2. (A) Static water contact angles, (B) sliding angles, and (C) contact angle hysteresis on an uncoated Cu plate and smooth, low-roughness, and high-roughness base layers, along with those infused with 30 cSt silicone oil at room temperature.

A value of Sow(a) > 0 implies that the water droplet will be cloaked by the lubricant and that the lubricant can be easily lost through evaporation, whereas a value of Sow(a) < 0 implies that the water droplet will not be cloaked by the lubricant and that the lubricant will not evaporate. For SLIPS with rough base layers, Sow(a) = 1.1 mN/m (Table 1). Thus, the water droplet would be cloaked by the lubricant. However, the loss of silicone oil by evaporation would be very low, because the evaporation rate of silicone oil with a viscosity of 30 cSt was 52°. Therefore, the liquid-infused surfaces were completely covered with silicone oil, and the roughness of this oil layer was strongly affected by base layer structure. This difference of lubricant structure is crucial for droplet mobility. However, on the base layer of the SPLASH, 10 μL water droplets did not slide, because the movement of the phenyl molecular chain is smaller than that of the alkyl molecular chain in previous work.33−35 Contact angle hysteresis of the smooth base layer was lower than hydrophobic lowroughness base layer. These results show that the adhesion force of the smooth base layer is lower than that of the hydrophobic low-rough base layer and large droplets (>10 μL) would slide on the smooth base layer at room temperature. The sliding angle and contact angle hysteresis of the SPLASH (smooth base layer + silicone oil) was lower than that of the SLIPS (rough base layer + silicone oil), because the smoother oil layer supported the sliding and decreased the adhesion force of water droplets. Furthermore, the difference of static water contact angle can also affect the nucleation energy of condensed water. On the SPLASH, the water nucleation energy barrier would be lower than that on the SLIPS, because of its lower static water contact angle, leading to an increased condensation heat-transfer coefficient. Overall, a lower sliding angle with a smaller static contact angle is the most suitable to maximize heat-transfer condensation, which makes the SPLASH better than the superhydrophobic surface (highroughness base layer) in condensation heat transfer. Effect of Silicone Oil Viscosity on Surface Wettability. We studied how the silicone oil viscosity influenced the wettability of the smooth surface. Figure 3 depicts the static water contact angles, sliding angles, and contact angle hysteresis of the smooth base layer infused with silicone oil with different viscosities of 30, 100, and 300 cSt. The advancing contact angle and receding contact angle of them are shown in Figure S4 in the Supporting Information. In theory, the static wettability is affected by surface energy and morphology, not viscosity. All of the smooth base layers infused with silicone oil showed little variation in both static water contact angles, sliding angles, and contact angle hysteresis as the viscosity of the silicone oil was changed. Static water contact angles and sliding angles of the low-roughness and high-roughness base layers infused with silicone oil with different viscosities of 30, 100, and 300 cSt are shown in Figures S5 and S6, respectively, in the Supporting Information. Static water contact angles and sliding angles of those surface also showed little variation as the viscosity of the silicone oil was changed. Study of Droplet Mobility. Although the surface wettability is influenced by the roughness of the base layer but not the viscosity of silicone oil, the crucial point for condensation is how these parameters affect droplet mobi-

Figure 3. (A) Static water contact angles, (B) water sliding angles, and (C) contact angle hysteresis of smooth base layers infused with silicone oil with viscosities of 30, 100, and 300 cSt at room temperature.

lity.23−25,45 Figure 4 shows the droplet mobility on surfaces with different base layer roughness and different silicone oil viscosity. The mobility of the water droplet (1−10 μL) decreased with base layer roughness (Figure 4A) and the viscosity of silicone oil when the same base layer was used (Figure 4B). These results indicate that the base layer roughness can lower the mobility of silicone oil and this decrease of silicone oil mobility strongly affects droplet mobility on the SLIPS. When the droplet and oil are immiscible, the relationship between water sliding speed and the viscosity of infused oil is given by39,44,46 8955

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Figure 4. Water sliding speed of (A) smooth base layer with 30 cSt silicone oil (red diamonds), low-roughness base layer, and high-roughness base layer with 30 cSt silicone oil (blue triangles), and high-roughness base layer with 30 cSt silicone oil (green circles) and (B) smooth base layer with silicone oil with viscosities of 30 cSt (green diamonds), 100 cSt (blue triangles), and 300 cSt (red circles) and a tilt angle of 30°. (C) Water sliding speed versus silicone oil viscosity approximated with v = Cμ0−1, where v is the volume of the water droplet, C the fitting value, and μ0 the viscosity of the infused liquid. Green squares, red triangles, blue circles, and gray diamonds represent water volumes of 10, 5, 3, and 1 μL, respectively.

Figure 5. Condensation states on (A) uncoated Cu surface and smooth, low-roughness, and high-roughness base layers, along with those infused with 30 cSt silicone oil, and (B) smooth base layer without and with silicone oil with viscosities of 30, 100, and 300 cSt. Red flame border and blue frame border represent filmwise condensation and dropwise condensation, respectively. Black scale bar = 10 mm.

v∝

the most suitable for attaining the highest droplet mobility. Furthermore, decreasing silicone oil viscosity further increased droplet mobility. We found that eq 15 held for the case of not only the SLIPS but also the SPLASH, as illustrated in Figure 4C. Study of Condensation Heat-Transfer Properties. We then observed the condensation state on the surfaces of the smooth, low-roughness, and high-roughness base layer, as well as silicone oil-infused ones, to prove the above analysis; i.e., that wettability under ambient conditions can be adaptable to vaporfilled conditions. Figure 5 shows photographs of each steady state of samples 2 h after starting the experiment. Without

V(1 − ϕs) μ0

(15)

where v is the water sliding speed, V the volume of the water droplet, ϕs the area fraction of the base layer, and μ0 the viscosity of the infused liquid. The results for droplet mobility against surface fraction and the viscosity of silicone oil almost follow eq 15. This indicates that the smoothness of the base layer can increase the droplet mobility when the surface is completely covered with silicone oil. In wettability analysis, we found that the silicone-oil-infused surfaces were completely filled with silicone oil. Thus, in theory, the smooth base layer is 8956

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Figure 6. Total departed droplet volumes on (A) uncoated Cu surface and smooth, low-roughness, and high-roughness base layers, along with those infused with 30 cSt silicone oil, and (B) smooth base layer without and with silicone oil with viscosities of 30, 100, and 300 cSt.

Figure 7. Condensation heat-transfer coefficients on (A) uncoated Cu surface and smooth, low-roughness, and high-roughness base layers, along with those infused with silicone oil 30 cSt, and (B) smooth base layer without and with silicone oil with viscosities of 30, 100, and 300 cSt.

infusing silicone oil, smooth surfaces displayed dropwise condensation, while the rough surfaces exhibited filmwise condensation, because of the pinning of nucleated water. In the Wenzel state, the nanoroughness/microroughness works negatively to move the water−vapor−solid contact line, resulting in high hysteresis of droplets under filmwise condensation on the rough base layer. In contrast, condensed water droplets were formed on the smooth base layer, because the dynamic movement of the phenyl molecular chains allowed large condensed water droplets to shed from the smooth surface before becoming water films.33−35 Although 10 μL water droplets did not slide on the smooth base layer of the SPLASH at room temperature, because the dynamic energy of the phenyl groups was low (Figure 2B), larger condensed water droplets (>10-μL) slid on the base layer of the surface with the help of coalescence energy. In the vapor-filled environment, first, small condensed water droplets are formed and grow to water films (filmwise condensation) or larger water droplets (dropwise condensation). Filmwise condensation is observed when the mobility of the condensed water droplets is low. The mobility of the condensed water droplets\ on the smooth base layer was high enough to show dropwise condensation and condensed water droplets were departed from the surface before growing to water film. In contrast, all of the silicone-oilinfused surfaces showed dropwise condensation. Smaller condensed water droplets slid on the liquid-infused surfaces with the smoother base layer. Movies showing condensation on an uncoated surface, 30 cSt silicone oil with a smooth base layer, rough base layer L, and rough base layer H are available in Movie S1.

The condensation states on the SPLASH with silicone oil with different viscosities of 30, 100, and 300 cSt are presented in Figure 5B. All these samples showed dropwise condensation, and smaller condensed water droplets slid on the surfaces infused with silicone oil of lower viscosity (see Movie S2). We guessed that this is because the water mobility of the condensed water droplets was higher on the SPLASH with less viscous silicone oil and smaller condensed water droplets moving with higher speed, allowing them to depart before growing into larger condensed water droplets. Microscale dynamics of condensation was also observed by setting the samples on the Peltier element in a thermohygrostat in air at a temperature of 23 °C and the Peltier temperature was set at 2 °C. Photographic images were captured by a charge-coupled device camera. The Peltier stage was set vertically. The microscale dynamics of condensation on the surfaces of uncoated Cu surface and smooth, lowroughness, and high-roughness base layers, along with those infused with 30 cSt silicone oil, are shown in Figure S7 in the Supporting Information. Filmwise condensation was observed on the uncoated surface, whereas all other surfaces showed dropwise condensation. The microscale dynamics of condensation on the surfaces smooth base layer without and with silicon oil, with viscosities of 30, 100, and 300 cSt, are shown in Figure S8 in the Supporting Information. Smaller condensed water droplets were observed on the base layer with silicon oil of lower viscosity, because condensed water droplets went out of the stage before growing to larger droplets, because of higher condensed water droplets mobility and coalescence (detailed analysis is discussed in the Supporting Information). 8957

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Langmuir To quantify the total droplet mobility, the total volume of departed condensed water droplets from the surfaces was measured, as depicted in Figure 6. Lubrication of the smooth base coating with silicone oil drastically improved the total departure volume of droplets, because of the high droplet mobility of the lubricant. The roughness of the base layer (i) decreases droplet mobility and (ii) increases the nucleation potential with static contact angle (Figure 2A), resulting in a decrease of the total volume of departed droplets. The total volume of condensed water droplets departed from the smooth surface with silicone oil with viscosities of 30, 100, and 300 cSt is shown in Figure 6B. A less-viscous oil endows the surface with higher droplet mobility. A larger volume of departed condensed water droplets decreases the thermal resistance caused by condensed water droplets and leads to an increased condensation heat-transfer coefficient. The calculated condensation heat-transfer coefficients are provided in Figure 7. The influence of base layer roughness on heat-transfer performance (Figure 7A) indicated the SPLASH with silicone oil with a viscosity of 30 cSt showed the highest condensation heat-transfer coefficient of the samples. Its condensation heat-transfer coefficient was 175% higher than that of an uncoated Cu tube, which has a positive correlation with the total amount of condensed water droplets departed from the surface shown in Figure 7A. We assume that the condensation heat-transfer coefficient can be positively influenced by water droplet mobility and a low nucleation energy barrier, even under vapor-filled conditions. Although the SPLASH possessed a much higher condensation heat-transfer coefficient than the uncoated surface, the smooth base layer of the SPLASH showed only a slightly higher condensation heattransfer coefficient than that of the uncoated surface, because of its low volume of departed condensed water droplets caused by its poor water sliding ability. On the base layer of the SLIPS, the condensation heat-transfer coefficient was almost the same as that of the uncoated surface, because filmwise condensation occurred. Condensation heat-transfer coefficients on the SPLASH with silicone oil with viscosities of 30, 100, and 300 cSt are shown in Figure 7B. The SPLASH with silicone oil of lower viscosity showed a higher condensation heat-transfer coefficient than that of the SPLASH with oil of higher viscosity, because of the higher water sliding ability and larger total volume of departed water droplets. Thus, lower roughness and viscosity of the oil can provide better and more stable performance of the SPLASH. These positive relationships between droplet mobility at room temperature and condensation heat transfer under vapor-filled conditions are apparent on liquid-infused surfaces, but not on rough surfaces without lubrication. This is because the liquid-infused surfaces do not transfer to the Wenzel state by vapor under condensation conditions, whereas the rough base layers do. Stability During Continuous Use Under Vapor Conditions. So far, we have concentrated on enhancing the condensation heat-transfer coefficient on the SPLASH. Here, endurance tests in vapor-filled environments were conducted on the SPLASH and SLIPS infused with silicone oil with a viscosity of 30 cSt. The samples were set in a vapor-filled chamber at 66 °C, and four cycles of the experiment were conducted (each cycle took 2 h, and then the samples were dried after each cycle). Time-lapse images of each surface are shown in Figure 8. Initially, dropwise condensation was observed on both the SPLASH and SLIPS; however, condensed

Figure 8. Time-lapse images of continuous condensation in 66 °C steam on surfaces infused with 30 cSt silicone oil with (A) smooth, (B) low-roughness, and (C) high-roughness base layers until 8 h passed. Each cycle took 2 h. Samples were dried after each cycle.

water droplet mobility becomes lower as time passed and changed to filmwise condensation on the SLIPS. The SLIPS with low- and high-roughness base layers changed to filmwise condensation states after 8 and 4 h, respectively. This is because, after a long time in the vapor-filled environment, the rough base layer was exposed through the decrease of oil layer thickness, which led to large condensed water droplets being pinned on the SLIPS. In contrast, the SPLASH maintained its dropwise condensation state, even after 8 h, because of its stable water sliding properties against changes in liquid thickness. Such durability indicates that the surface roughness of a smoother surface can remain stable for a longer time under vapor conditions than a rougher surface. The results of extended the endurance tests until 16 h on the SPLASH with 30 cSt silicone oil is shown in Figure S9 in the Supporting Information. The SPLASH kept dropwise condensation and the condensation heat-transfer coefficient of the SPLASH after 16 h was 1009.1 W/(m2 K). The condensation heat-transfer coefficient after 2 h was 1015.7 W/(m2 K); therefore, the condensation heat-transfer coefficient of the SPLASH showed almost no degradation, because of the mobility of the stable condensed water droplets. If the silicone oil was lost, the condensed heat-transfer coefficient would be deteriorated, because the smooth base layer exhibited a much lower condensed heat-transfer coefficient derived from much lower mobility of the condensed water droplets than the SPLASH. This result showed that silicone oil on the SPLASH remained after the endurance test. However, the silicone oil would not remain on the surface permanently, and the condensation heattransfer coefficient would decrease after a very long time, because of the depletion of silicone oil. Although the SPLASH displayed improved the condensation heat transfer compared with that of the SLIPS, there still remains the challenge of realizing extremely long-term stability, because the eventual complete loss of silicone oil will degrade heat-transfer performance. As an additional test, water shower endurance test was conducted on the SPLASH and SLIPS infused with silicone oil with a viscosity of 30 cSt. In fact, in terms of water shower test, the roughness of base layer worked to immobilize lubricant. The detail of the water shower endurance test is discussed in the Supporting Information (Figure S10).

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CONCLUSIONS The effect of base layer roughness and liquid viscosity of liquidinfused surfaces on droplet mobility under ambient conditions 8958

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Langmuir Author Contributions

and condensation heat transfer were investigated. The water sliding ability of liquid-infused surfaces at room temperature had a positive correlation with condensation heat-transfer performance in vapor-filled environments. A smooth base layer and low-viscosity infusing liquid are crucial to enhancing droplet mobility and improving heat-transfer performance. In addition, the liquid-infused surface with a smooth base layer displayed longer-term stable dropwise condensation and heattransfer performance than that with a rougher surface, because of its stable water sliding ability against the decrease of liquid thickness. However, there still remains the challenge of preventing the depletion of the infused liquid. This study will aid basic research on thermodynamics, interfaces, and designing liquid-infused surfaces that may be used in various industrial applications.

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M.T. conceived and designed the experiments. H.T., M.T., T.M., K.S., and T.Y. carried out the experiments. H.T. and M.T. analyzed the data and wrote the paper. T.M., R.Y., K.S., R.T., T.Y., and K.M. provided experimental support and gave scientific advice. S.S. supervised the project and commented on the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI (Grant No. JP16J06070). We are deeply grateful to Dr. Yoshio Hotta and Dr. Kyu-Hong Kyung, whose comments were valuable to our study. We very much appreciate the support from Dr. Kouji Fujimoto, whose meticulous comments were an enormous help. M.T. is thankful for a predoctoral fellowship (DC1) from Japan Society of Promotion of Science (JSPS).

ASSOCIATED CONTENT

S Supporting Information *

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b01991. AFM images of smooth and low-roughness base layer; cross-sectional SEM images of low-roughness and highroughness base layers; advancing contact angle and receding contact angle on an uncoated Cu plate and smooth, low-roughness, and high-roughness base layers, along with those infused with 30 cSt silicone oil, and on smooth base layers infused with silicone oil with viscosities of 100, and 300 cSt at room temperature; static water contact angles and sliding angles of the lowroughness and high-roughness base layers infused with silicone oil with different viscosities of 30, 100, and 300 cSt; analysis of microscale dynamics of condensation on the surfaces; time-lapse images of continuous condensation in 66 °C steam on the smooth base layer with 30 cSt silicone oil until 16 h passed; and time-lapse water sliding angles of the smooth base layer, low-roughness base layer, and high-roughness base layer with 30 cSt silicone oil after water shower endurance test; schematic diagram of the system used to measure the condensation heattransfer coefficient of the samples, and typical value and error of the fixed physical quantity obtained in the experiments; (PDF) Movie showing a comparison of condensation on uncoated surface, along with 30 cSt silicone oil with smooth base layer, low-roughness base layer, and highroughness base layer (AVI) Movie showing a comparison of condensation on silicone oil with viscosities of 30, 100, and 300 cSt with a smooth base layer (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Mizuki Tenjimbayashi: 0000-0002-8107-8285 Kengo Manabe: 0000-0002-8601-8003 Seimei Shiratori: 0000-0001-9807-3555 Author Contributions ‡

H.T. and M.T. contributed equally to the research. 8959

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