Wetting Criteria of Intrinsic Contact Angle To Distinguish between

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Wetting Criteria of Intrinsic Contact Angle To Distinguish between Hydrophilic and Hydrophobic Micro-/Nanotextured Surfaces: Experimental and Theoretical Analysis with Synchrotron X‑ray Imaging Dong In Yu,† Ho Jae Kwak,‡ Chulmin Park,§ Chiwoong CHOI,∥ Narayan Pandurang Sapkal,† Jiwoo Hong,‡ and Moo Hwan Kim*,‡,§ Downloaded via WEBSTER UNIV on February 28, 2019 at 17:57:17 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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Department of Mechanical Design Engineering, Pukyong National University, Busan 48547, Republic of Korea Department of Mechanical Engineering and §Division of Advanced Nuclear Engineering, POSTECH, Pohang 37673, Republic of Korea ∥ Division of Thermal Hydraulic and Severe Accident Research, KAERI, Daejeon 34057, Republic of Korea ‡

S Supporting Information *

ABSTRACT: In this study, the existing knowledge on the wetting criterion, that is, the intrinsic contact angle, for distinguishing between hydrophilic and hydrophobic textured surfaces is verified experimentally. A precise apparent contact angle is measured on micro-, nano-, and micro-/nanotextured surfaces to quantitatively define the surface-wetting conditions. In particular, X-ray tomography is introduced to measure precise geometric morphologies of nano- and micro-/nanotextured surfaces, and the wetting state of the textured surfaces is clearly visualized using synchrotron X-ray imaging. By comparing previous theoretical models and experimental results, it is verified that the intrinsic contact angle for distinguishing between hydrophilic and hydrophobic textured surfaces should be corrected from 90° to 43°. In addition, nonwetting phenomena in the region of the intrinsic contact angle between 43° and 90° are discussed.

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recently proposed by Kang−Jacobi.30 On the basis of this model, an intrinsic contact angle θC ≈ 48° was suggested to distinguish between hydrophilic and hydrophobic structured surfaces. In our previous study,31 the wetting criteria suggested by previous researchers were verified experimentally on microtextured surfaces, and the wetting criterion suggested by Kang−Jacobi was adjusted from θC ≈ 48° to θC ≈ 43°. In this study, following our previous research, the wetting criterion of the intrinsic contact angle is investigated in detail, to distinguish between hydrophobic and hydrophilic surfaces in the extended cases of micro-, nano-, and micro-/nanotextured surfaces. We conducted experiments and analyzed the wetting tendencies of textured surfaces with an intrinsic contact angle between two different wetting criteria (43° < θ0 = 70° < 90°); within this range, the surface is classified as hydrophilic within the Wenzel model but hydrophobic in the Kang−Jacobi model. The relationship between the apparent contact angle and the roughness ratio was investigated in the region of the intrinsic contact angle (θ0 = 70°), and a wetting

INTRODUCTION In 1936, Wenzel postulated that the effect of a roughened surface is to magnify the wetting characteristic of the solid;1,2 that is, that increasing the surface roughness will increase the wetting tendency of a hydrophilic surface and decrease the wetting tendency of a hydrophobic surface. On the basis of this wetting model, an intrinsic contact angle θC = 90° was suggested as a wetting criterion to distinguish between hydrophilic and hydrophobic structured surfaces. This criterion has been referenced in a number of reports, including those on self-cleaning,3−7 hydrodynamic friction reduction,8−12 anti-biofouling,13−15 and thermal and energy systems,16−24 and hence this wetting criterion is widely accepted in a range of research and engineering fields. The Wenzel model was derived thermodynamically under the assumption that the energy loss at a moving contact line can be neglected when a liquid droplet reaches an equilibrium state on a solid surface. On the basis of this model, the apparent contact angle θA can be predicted theoretically from the intrinsic contact angle θ0 and roughness ratio f. However, energy loss at a moving contact line has been reported by a number of previous researchers,25−29 and a wetting model that considers the energy loss at the moving contact line was © XXXX American Chemical Society

Received: October 9, 2018 Revised: November 21, 2018 Published: February 18, 2019 A

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Figure 1. Test sections for experiments: (a) equilibrium contact angle on the flat silicon surface (intrinsic contact angle) and (b−l) SEM images of micro-, nano- and micro-/nanotextured surfaces. and the pitch p and gap g between micropillars were measured using a 3D profiler, as shown in Figure 2a. Through the comparison of measures by a 3D profiler and scanning electron microscopy (SEM), about one of the test sections (M02), it is confirmed that the measures of the 3D profiler are equivalent with the measures of SEM within ±6.66%. Then, because the micropillars are arranged periodically, the concept of the unit cell was used to calculate the roughness ratio based on the Wenzel and Cassie−Baxter wetting states

criterion for distinguishing between hydrophilic and hydrophobic surfaces was established. By comparing the experimental results with previous wetting models, we investigated the assumption that the energy loss at the contact line can be neglected as a liquid droplet reaches an equilibrium state on a solid surface. In addition, nonwetting phenomena for intrinsic contact angles in the range of 43°−90° are discussed.

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EXPERIMENTS

Preparation of Test Sections. Surface wetting conditions are generally expressed using the chemical composition and geometric morphology of the surface. 1,2 The chemical composition is quantitatively defined using the intrinsic contact angle, which is an apparent contact angle on an ideal smooth surface, and the geometric morphology is quantitatively defined as the roughness ratio f, which is the ratio of the wetted area Awetted to the projected area Aprojected. To verify the intrinsic contact angle for distinguishing between hydrophilic and hydrophobic textured surfaces, test sections to measure the surface wetting conditions quantitatively were prepared using a microelectro-mechanical-systems technique. A well-polished silicon wafer surface (θ0 = 70°) was used as a reference test section, and uniformly arrayed micro-, nano-, and micro-/nanotextured surfaces were fabricated (Figure 1a−l) on smooth silicon surfaces using photolithography and a conventional dry-etching method. Because of the limitations of fabricating nanometer-scale patterns using photolithography, nanometer-scale pillars were fabricated on the nanoand micro-/nanotextured surfaces using the black silicon method.32 All test surfaces were cleaned with piranha solution and O2 plasma. The test sections were then baked in a vacuum chamber at high temperature (220 °C, −0.1 MPa, 1 day) to remove temporary hydroxyl groups on the surfaces. Full details of the fabrication and cleaning processes are provided in the Supporting Information.33 Measurement of the Geometric Morphologies of Test Sections. The roughness ratios of the micro-, nano-, and micro-/ nanotextured surfaces were measured for quantitative analysis. For the microtextured surface, the diameter d and height h of the micropillars,

fW |m = =

A wetted A projected

A wetted A projected

=1+C unit cell

= unit cell

πd m 2 4pm 2

πdmhm , fC−B |m pm 2

(1)

where f W and f C−B are the roughness ratios in the Wenzel and Cassie−Baxter states, respectively, and C(= π/2) is the correction coefficient for the scallop shape at the sides of the micrometer-scaled textures.34 For the nanotextured surfaces, because it is difficult to measure the geometries of randomly arrayed nanopillars using a 3D profiler, a 3D tomography technique developed using synchrotron X-ray imaging with high spatial resolution (40 nm/pixel) was used instead. The penetration depth of the X-ray beam source δsilicon was ∼30 μm, and on the nanotextured surface, nanometer-scaled textures outside the measured area (25 × 25 μm2) are shaved away by the focused ion beam, as shown in Figure 2b. Synchrotron X-ray images were obtained from the prepared surface at 0.5° azimuthally using a nanometer-scale spatial resolution X-ray beam [7C beam line in Pohang Acceleration Laboratory (PAL)]; image post-processing was then used to obtain the 3D geometric morphologies of the nanopillars over a specific volume (6.5 [width] × 6.5 [column] × 6.8 [height] μm3), as shown in Figure 2c. The roughness ratios of the nanotextured surfaces were obtained from the 3D geometric morphologies of the nanopillars, and the roughness ratios of the B

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Figure 2. Measurement of geometric morphology: (a) 3D profiler image (microtextured surface, dm = 20 μm, pm = 20 μm, hm = 16 μm), (b) SEM image (surfaces fabricated by focused ion beam), and (c) synchrotron X-ray tomography image (nanotextured surface). micro-/nanotextured surfaces were then calculated based on the roughness ratios of both the micro- and nanotextured surfaces, as shown in eq 2. The specific geometric morphologies of the test sections are given in Table 1.

fW |mn = fW |n + (1 − fW |m ) = fW |n + C = fC−B |n · fC−B |m = fC−B |n ·

where subscript m, n, and m/n are micro-, nano-, and micro-/ nanotextured surfaces, respectively. Full details of the post-processing of a 3D tomography are provided in the Supporting Information.33 Visualization of the Wetting State of a Water Droplet on Test Sections. When a water droplet rests on a textured surface, various wetting states can be observed depending on the surface wetting conditions. The wetting state can be defined by the geometric contact shape angle of the liquid droplet and the solid surface. For estimation of the apparent contact angle under each wetting model, a wetting state should be estimated to define the wetted area of the roughness ratio. Among the various wetting states shown in Figure 3a, with the exception of special cases, both the Wenzel and Cassie− Baxter states are generally observed on hydrophobic and hydrophilic textured surfaces. In the Wenzel state, the pillars of the surfaces are fully wetted by a water droplet, and in the Cassie−Baxter state, the pillars are partially wetted. To verify the wetting state on the surfaces experimentally, it is necessary to establish the liquid−vapor interfaces between textures visually. Comparing with the visualization technique by visible ray, the synchrotron X-ray imaging has the merits to identify the wetting state on the micro-/nanotextured surfaces such as relatively high spatial resolution and reduced disturbance at the liquid−vapor interface. Therefore, the wetting states of water droplets on the test sections were established using synchrotron X-ray imaging, as shown in Figure 3b. On the basis of the field of view and the spatial resolution, two different synchrotron X-ray beam lines were used. The 6C beam line in PAL has a 2 × 2 mm2 field of view and a 1.6 μm/ pixel spatial resolution and was used to measure the wetting state of the test sections with relatively large spacings between pillars (M02− M05), and the 7C beam line in the laboratory has a 40 × 40 μm2 field

πdmhm , fC−B |mn pm 2

πd m 2 4pm 2

(2)

Table 1. Specific Geometrical Morphology of Test Sections no. flat M01 M02 M03 M04 M05 nano MN01 MN02 MN03 MN04 MN05

dm [μm]

im [μm]

pm [μm]

hm [μm]

4 20 20 40 40

4 20 60 40 120

8 40 80 80 160

16 16 16 16 16

4 20 20 40 40

4 20 60 40 120

8 40 80 80 160

16 16 16 16 16

f W|m/f C−B|m []

f W/f C−B []

1.00/1.00 5.93/0.20 1.98/0.20 1.25/0.049 1.49/0.20 1.12/0.049 1.00/1.00 5.93/0.20 1.98/0.20 1.25/0.049 1.49/0.20 1.12/0.049

1.00/1.00 5.93/0.20 1.98/0.20 1.25/0.049 1.49/0.20 1.12/0.049 16.67/0.114 21.60/0.0022 17.65/0.0022 17.16/0.0056 17.91/0.0022 17.79/0.0056 C

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Figure 3. Wetting state (a) schematics, (b) synchrotron X-ray imaging (microtextured surfaces), and (c) synchrotron X-ray imaging (nanotextured surface). source (δsilicon ≈ 30 μm), a 15 μm width groove was fabricated on the nanotextured surfaces, and the wetting state on the groove was measured using synchrotron X-ray imaging, as shown in Figure 3c. Because it is difficult to statically place a liquid droplet on the micro-/ nanotextured surfaces, the wetting state on the surfaces could not be measured using synchrotron X-ray imaging. However, on the surfaces (M01, Nano cases) with relatively large f C−B and small f W than micro-/nanotextured surfaces, a liquid droplet is in Cassie−Baxter state. According to the previous research,35,36 as f C−B increases and f W decreases, the Cassie−Baxter state is more favorable thermodynamically. Therefore, it is likely that droplets on the micro-/nanotextured surfaces are in the Cassie−Baxter state. The mixed state and hemiwicking state are not shown in all test sections. Measurement of Apparent Contact Angle on Test Sections. The apparent contact angle of a 3 μL distilled ionized water droplet on the prepared surfaces was measured by automatic goniometry under the controlled temperature and humidity conditions (22 ± 1 °C, 42 ± 3%). Each experiment was repeated three times, and the deviations of the measured apparent contact angles were within ±3°.

of view and a 20 nm/pixel spatial resolution and was used to measure the wetting state of the test sections with relatively small spacings between pillars (M01 and nano). Detailed information on the beam lines in PAL is provided in Table 2. On the uniformly arrayed

Table 2. Specific Information on Beam Lines in PAL value parameter peak (main) energy photon flux pixel resolution field of view

6C BMI

7C XNI

10−55 [keV]

6.75 [keV]

1010−1012 [photons/mm2/s] >1.74 μm/pixel 8 × 5 mm2

109 [photons/μm2/s] >20 nm/pixel 30−150 μm2

microtextured surface, if the micrometer-scaled textures are wellaligned on the surfaces, imaging of the wetting state is straightforward because the intensity of the X-ray beam source barely attenuated little by the silicon textures. However, on the randomly arrayed nanotextured surfaces, imaging of the wetting state is difficult because the X-ray beam source is significantly attenuated by the silicon pillars. Hence, based on the penetration depth of the synchrotron X-ray beam

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RESULTS AND DISCUSSION Wetting Criterion of Intrinsic Contact Angle on Hydrophilic and Hydrophobic Textured Surfaces. If the D

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Langmuir temperature and volume of the three-phase system are constant, the equilibrium process of a liquid droplet in air to a liquid droplet on a test section can be described based on the Helmholtz free energy. During the equilibrium process, if the energy loss because of the moving contact line is neglected, the Wenzel model1,2 and the Cassie−Baxter model37 can be derived theoretically based on the Wenzel and Cassie−Baxter states, as shown in eqs 3 and 4. cos θA = fW cos θ0

(3)

cos θA = − 1 + fC−B (cos θ0 + 1)

(4)

Considering a liquid droplet in the Wenzel state, the relationship between the intrinsic and apparent contact angles is a cosine function with a roughness ratio given by eq 4. The relationship between the roughness ratio and the apparent contact angle changes at an intrinsic contact angle θ0 = 90°. However, in the previous research, it has been reported that an additional energy loss is generated because of the friction resistance at moving contact lines during the time when a droplet in air contacts the solid surface, and it places on the surfaces in thermodynamic equilibrium condition.25−29 If energy loss because of friction loss at the moving contact line is generated during the equilibrium process, the Kang− Jacobi model30 can be derived based on the wetting state, as shown in eqs 5 and 6. ÄÅ É Å (2 + cos θA)(1 − cos θA)2 ÑÑÑ2/3 ÑÑ 1 − cos θA − 2ÅÅÅ 4 ÅÅÇ ÑÑÖ 2 sin θA ÅÄÅ (2 + cos θ0)(1 − cos θ0)2 ÑÉÑ2/3 ÑÑ 1 − cos θ0 − 2ÅÅÅÅ ÑÑ 4 Å ÑÖ Ç = fW 2 sin θ0 (5) ÄÅ É Å (2 + cos θA)(1 − cos θA)2 ÑÑÑ2/3 ÑÑ 1 − cos θA − 2ÅÅÅ 4 ÅÅÇ ÑÑÖ 1 + (1 − fC−B ) 2 2 sin θA ÅÄÅ (2 + cos θ0)(1 − cos θ0)2 ÑÉÑ2/3 ÑÑ 1 − cos θ0 − 2ÅÅÅÅ ÑÑ 4 Å ÑÖ Ç = fC−B 2 sin θ0 (6)

Figure 4. Comparison between theoretical models and experimental results: (a) apparent contact angle and (b) wetting state. At Figure 4a, Cassie−Baxter state cases (M01, nano, MN01−05) are noted with hollowed circles.

This equation demonstrates that if the energy loss at a moving contact line is considered, the relationship between the intrinsic and apparent contact angles is more complex. On the basis of the Kang−Jacobi model for the Wenzel state (eq 5), the relationship between the roughness ratio and the apparent contact angle changes at an intrinsic contact angle of θ0 ≈ 43°.31 For the Wenzel state cases (M02−M05), it can be demonstrated that the measured apparent contact angle on the test sections is larger than the intrinsic contact angle on the silicon surface (θ0 = 70°), and that the apparent contact angle increases as the roughness ratio increases, as shown in Figure 4a. In addition, the Kang−Jacobi model (eq 5, within 7.53% error) provides a better estimate of the experimental results than the Wenzel model (eq 3, within 58.22% error). For the Cassie−Baxter state cases (M01, nano, MN01−05), both the Cassie−Baxter model (eq 4) and the Kang−Jacobi model (eq 6) provide good estimates of the experimental results to within 4.14%. Although these models are based on different assumptions regarding the energy loss at the moving contact

line, because a liquid droplet in the Cassie−Baxter state is in partial contact with the solid surface and the energy loss at the moving contact line is relatively small, the experimental results are similar to both of these models. Previous studies have provided thermodynamic models that estimate the wetting state between the Wenzel and Cassie− Baxter states depending on the surface wetting conditions.35,36 These models differ in their assumptions of the energy loss at the moving contact line: the energy loss is neglected in Bico et al.’s model (eq 7)35 but included in that of Yu et al. (eq 8).36 ∴ FC−B ≡

fC−B − 1 fW − fC−B

− cos θ0 ⇒ [FC−B > 0

(Cassie−Baxter state), FC−B < 0 (Wenzel state)] E

(7)

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Figure 5. Nonwetting phenomena on the micro-/nanotextured surface with the intrinsic contact angle (θ0 = 70°) (MN01 case): (a) dosed liquid droplet and (b) hung droplet at a needle of syringe.

∴ FC*−B ≡

fC−B − 1

180°) was estimated for a droplet in the Wenzel state on the surfaces with a high Wenzel state roughness ratio (f W). For θ0 = 70°, a water droplet in the Wenzel state is in the nonwetting condition on a surface with f W ≈ 3.1. However, according to eq 8, a water droplet on the surface should be in a Cassie− Baxter state because this state is more stable under this surface wetting condition. If a droplet is in the Cassie−Baxter state, the apparent contact angle increases as the Cassie−Baxter state roughness ratio (f C−B) decreases, as shown in Figure 4a. On the textured surfaces, the roughness ratios of both states (f W, f C−B) generally decrease with the increasing pitch between textures, as shown in eqs 1 and 2. Therefore, under conditions wherein the roughness ratio of the Wenzel state (f W) is sufficiently large for a Wenzel droplet, the roughness ratio of the Cassie−Baxter state (f C−B) is sufficiently small for the nonwetting condition. However, on hierarchically textured surfaces, such as micro-/nanotextured surfaces, a droplet could be in the nonwetting state because the surfaces can overcome the limitations of the geometric relationship between the roughness ratios of the Wenzel and Cassie−Baxter states, as shown in eq 2. In this study, nonwetting phenomena were observed on the micro-/nanotextured surfaces. In particular, because of superhydrophobicity of the surface, a liquid droplet could not be placed statically on the micro-/nanotextured surfaces, as shown in Figure 5a and Video 1a. In addition, although a hung liquid droplet at a needle syringe is forcefully

fW − fC−B ÄÅ É Å (2 + cos θ0)(1 − cos θ0)2 ÑÑÑ2/3 ÑÑ − 2(1 − cos θ0) 4ÅÅÅÅ 4 ÑÑÖ + ÅÇ 2 sin θ0 ⇒ [FC*−B > 0 (Cassie−Baxter state), FC*−B < 0

(Wenzel state)]

(8)

As shown in Figure 4b, only Bico et al.’s model predicts a Wenzel state. Yu et al.’s model is in good agreement with the X-ray imaging data for all cases in this study. In particular, a Cassie−Baxter state on the micro-/nanotextured surfaces (these cases are noted with hollowed circles in Figure 4b) is estimated by Yu et al.’s model; our conjecture of a wetting state on the micro-/nanotextured surfaces, in the absence of imaging data, is therefore reasonable. Through comparison of the apparent contact angle and wetting state between the experimental data and theoretical models, it is verified that energy loss at the moving contact line is significant in the wetting phenomena. Nonwetting Phenomena on the Textured Surfaces in the Region of the Intrinsic Contact Angle 43°< θ0 < 90°. On the basis of the Kang−Jacobi model in the intrinsic contact angle range 43°< θ0 < 90°, the nonwetting condition (θA ≈ F

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would like to thank J. Lim, S. Lee, Y. Kim, and J.-H. Lim in PAL for conducting the experiments with the synchrotron Xray beam lines (6C, 7C).

pressed at the surfaces, it is detached easily on the micro-/ nanotextured surfaces when a needle moves out, as shown in Figure 5b and Video 1b. Through Figure 5a,b and Video 1a,b, it was verified that nonwetting phenomena can be realized on hydrophilic textured surfaces based on the Wenzel model.

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(1) Wenzel, R. N. Resistance of solid surfaces to wetting by water. Ind. Eng. Chem. 1936, 28, 988−994. (2) Wenzel, R. N. Surface roughness and contact angle. J. Phys. Colloid Chem. 1949, 53, 1466−1467. (3) Neinhuis, C.; Barthlott, W. Characterization and distribution of water repellent, self-cleaning plant surfaces. Ann. Bot. 1997, 79, 667− 677. (4) Nakajima, A.; Hashimoto, K.; Watanabe, T.; Takai, K.; Yamauchi, G.; Fujishima, A. Transparent superhydrophobic thin films with self-cleaning properties. Langmuir 2000, 16, 7044−7047. (5) Blossey, R. Self-cleaning surfacesvirtual realities. Nat. Mater. 2003, 2, 301−306. (6) Fürstner, R.; Barthlott, W.; Neinhuis, C.; Walzel, P. Wetting and self-cleaning properties of artificial superhydrophobic surfaces. Langmuir 2005, 21, 956−961. (7) Cao, W.-T.; Liu, Y.-J.; Ma, M.-G.; Zhu, J.-F. Facile preparation of robust and superhydrophobic materials for self-cleaning and oil/water separation. Colloids Surf., A 2017, 529, 18−25. (8) Choi, C.-H.; Ulmanella, U.; Kim, J.; Ho, C.-M.; Kim, C.-J. Effective slip and friction reduction in nanograted superhydrophobic microchannels. Phys. Fluids 2006, 18, 087105. (9) Choi, C.-H.; Kim, C.-J. Large slip of aqueous liquid flow over a nanoengineered superhydrophobic surface. Phys. Rev. Lett. 2006, 96, 066001. (10) Leong, J. Y.; Reddyhoff, T.; Sinha, S. K.; Holmes, A. S.; Spikes, H. A. Hydrodynamic Friction Reduction in a MAC-Hexadecane Lubricated MEMS Contact. Tribol. Lett. 2012, 49, 217−225. (11) McHale, G.; Newton, M. I.; Shirtcliffe, N. J. Immersed superhydrophobic surfaces: Gas exchange, slip and drag reduction properties. Soft Matter 2010, 6, 714−719. (12) Gose, J. W.; Golovin, K.; Boban, M.; Mabry, J. M.; Tuteja, A.; Perlin, M.; Ceccio, S. L. Characterization of superhydrophobic surfaces for drag reduction in turbulent flow. J. Fluid Mech. 2018, 845, 560−580. (13) Marmur, A. Underwater superhydrophobicity: theoretical feasibility. Langmuir 2006, 22, 1400−1402. (14) Liu, Y.; Zhao, Y. J.; Teng, J. L.; Wang, J. H.; Wu, L. S.; Zheng, Y. L. Research progress of nano self-cleaning anti-fouling coatings. IOP Conf. Ser.: Mater. Sci. Eng. 2018, 284, 012016. (15) Du, T.; Ma, S.; Pei, X.; Wang, S.; Zhou, F. Bio-inspired design and fabrication of micro/nano-brush dual structural surfaces for switchable oil adhesion and antifouling. Small 2016, 13, 1602020. (16) Boreyko, J. B.; Chen, C.-H. Self-propelled dropwise condensate on superhydorphobic surfaces. Phys. Rev. Lett. 2009, 103, 184501. (17) Miljkovic, N.; Enright, R.; Nam, Y.; Lopez, K.; Dou, N.; Sack, J.; Wang, E. N. Jumping-droplet-enhanced condensation on scalable superhydrophobic nanostructured surfaces. Nano Lett. 2012, 13, 179− 187. (18) Parin, R.; Martucci, A.; Sturaro, M.; Bortolin, S.; Bersani, M.; Carraro, F.; Del Col, D. Nano-structured aluminum surfaces for dropwise condensation. Surf. Coat. Technol. 2018, 348, 1−12. (19) Preston, D. J.; Wilke, K. L.; Lu, Z.; Cruz, S. S.; Zhao, Y.; Becerra, L. L.; Wang, E. N. Gravitationally driven wicking for enhanced condensation heat transfer. Langmuir 2018, 34, 4658− 4664. (20) Ahn, H. S.; Jo, H. J.; Kang, S. H.; Kim, M. H. Effect of liquid spreading due to nano/microstructures on the critical heat flux during pool boiling. Appl. Phys. Lett. 2011, 98, 071908. (21) Chu, K.-H.; Joung, Y. S.; Enright, R.; Buie, C. R.; Wang, E. N. Hierarchically structured surfaces for boiling critical heat flux enhancement. Appl. Phys. Lett. 2013, 102, 151602.

CONCLUSIONS We experimentally verified the existing knowledge on wetting phenomena, including the wetting criterion of the intrinsic contact angle for distinguishing between hydrophilic and hydrophobic textured surfaces. On the basis of synchrotron Xray imaging and tomography, we obtained the following insights: • In contrast to the existing knowledge on wetting phenomena (based on the Wenzel model), on hydrophilic textured surfaces with an intrinsic contact angle in the range 43° < θ0 < 90°, hydrophobic characteristics and nonwetting phenomena are observed. • These differences between the existing knowledge and the experimental results arise from the inclusion of the energy loss at a moving contact line, a phenomenon that has been neglected in previous research. Therefore, for good estimation of interfacial phenomena with dynamic contact lines, the energy loss at a moving contact line should be considered, and this amends the intrinsic contact angle for distinguishing between hydrophilic and hydrophobic structured surfaces from θC = 90° to θC ≈ 43°.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b03407. Specific processes for preparation of test sections and synchrotron X-ray tomography for nanometer-scaled pillars (PDF) Liquid droplet not placed statically on the micro-/ nanotextured surfaces (AVI) Hung liquid droplet forcefully pressed at the surfaces detaches easily on the micro-/nanotextured surfaces (AVI)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +82-54-279-8157. ORCID

Moo Hwan Kim: 0000-0002-7193-7189 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean Government (MOE) (NRF-2018R1D1A1B07048332). This work was supported by “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (20184010201700). Experiments at the Pohang Light Sources were supported in part by MOE and POSTECH. We G

DOI: 10.1021/acs.langmuir.8b03407 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.8b03407 Langmuir XXXX, XXX, XXX−XXX