Nanograssed Zigzag Structures To Promote Coalescence-Induced

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Article Cite This: Langmuir 2019, 35, 9093−9099

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Nanograssed Zigzag Structures To Promote Coalescence-Induced Droplet Jumping Taeyang Han,† Ho Jae Kwak,§ Jong Hyun Kim,‡ Jeong-Tae Kwon,∥ and Moo Hwan Kim*,†,§ †

Division of Advanced Nuclear Engineering, POSTECH, Pohang 37673, Gyeongbuk, Republic of Korea Department of Mechanical Engineering, POSTECH, Pohang 37673, Gyeongbuk, Republic of Korea ‡ Pohang Accelerator Laboratory (PAL), POSTECH, Pohang 37673, Gyeongbuk, Republic of Korea ∥ School of Mechanical Engineering, Hoseo University, Asan 31499, Chungnam, Republic of Korea Downloaded via KEAN UNIV on July 24, 2019 at 03:12:47 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

§

S Supporting Information *

ABSTRACT: To increase the efficiency of jumping-droplet condensation, this study proposes a hierarchical superhydrophobic surface that promotes coalescence-induced jumping. Inspired by the phenomenon in which a growing droplet moves spontaneously within a superhydrophobic V structure, we fabricated nanograssed zigzag structures on the surface to induce the spontaneous motion of condensed droplets. The direction of the motion was parallel to the surface, so the condensed droplets easily coalesced on it. Compared with a conventional nanograssed superhydrophobic surface, the proposed surface increased the frequency of coalescence-induced jumping by ≥17 times and increased the cumulative volume of jumping droplets by ∼1.8 times. The proposed surface has great potential to increase the efficiency of applications such as water- and energy-harvesting and cooling systems that exploit jumping-droplet condensation.

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INTRODUCTION Vapor condensation has been widely utilized in industries including power generation,1 water desalination,2 water harvesting,3 and thermal management.4,5 Methods to increase the condensation heat and mass transfer would reduce the consumption of energy and natural resources. The efficiency of condensation depends on the wettability of the condenser surface. When the surface is wettable (hydrophilic), a film-like liquid condensate covers the surface and disturbs the formation of new nuclei. In contrast, on nonwettable (hydrophobic) surfaces, hemispherical droplets form and are removed by gravity when the droplets reach a capillary length of ∼2.7 mm.6 The heat transfer coefficient is an order of magnitude higher on hydrophobic surfaces than on hydrophilic surfaces because the condensate acts as thermal resistance during condensation.7 On superhydrophobic surfaces, the adhesion energy Wa between condensates and surfaces is much lower than that on hydrophobic surfaces. This characteristic enables condensate to be easily removed from superhydrophobic surfaces. When two or more microsized droplets (1−100 μm) coalesce, © 2019 American Chemical Society

the resulting coalesced droplet can jump up from the surfaces because excessive surface energy is converted to kinetic energy.8−11 The reduced size of condensed droplets results in an increase in the heat transfer coefficient.12−16 Jumping droplets can transfer heat and electric charge,17 so jumpingmode condensation is applied to cooling18−21 and energyharvesting22 systems. Extensive research has been conducted to increase the efficiency of jumping-mode condensation. Biphilic surfaces combining superhydrophobic and hydrophilic surfaces have been suggested to increase the growth rate and number density of condensed droplets.23−25 The energy barrier for the heterogeneous nucleation of condensation is lower in the hydrophilic area than in the superhydrophobic area, so nucleation is promoted by hydrophilic patterns on a superhydrophobic surface.26,27 In addition, the control of the nucleation site using the biphilic surfaces has great potential to Received: April 12, 2019 Revised: June 17, 2019 Published: June 18, 2019 9093

DOI: 10.1021/acs.langmuir.9b01065 Langmuir 2019, 35, 9093−9099

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Langmuir delay flooding in high supersaturation conditions.28 Using an electric field is also an effective method to increase the efficiency of jumping-mode condensation. The jumping droplets are positively charged by the formation of the electric double layer at the droplet−surface interface,17 so applying an electric field can prevent the jumping-droplet return.29 Next, micro/nanohierarchical-structured surfaces can promote droplet jumping. The nucleation density increased in the hierarchically structured surfaces due to the increased heat transfer area and decreased nucleation energy barrier at the corner of the microstructures,30 so the number of coalescence events of the condensed droplets increased.31,32 Furthermore, the condensed droplets can move up to the top of the microstructures spontaneously due to the Laplace pressure gradient.32−36 This expulsion reduces Wa due to the minimized liquid−solid contact area. In this study, we suggested a hierarchically structured surface, which consisted of microzigzag structures and bladelike CuO nanostructures (nanograss), to increase the efficiency of jumping-mode condensation through the fast removal of condensed droplets. The suggested surface induced the spontaneous motion of condensed droplets in parallel with the substrate, whereas the spontaneous motion on the previous hierarchical surfaces was out-of-plane. Therefore, the number of coalescence events of the condensed droplets could be increased further because the condensed droplets were gathered in a certain area. The effect of the in-plane motion of growing condensates was reported in a previous study, which used asymmetric bumps that mimicked cactus spines to induce spontaneous motion.37,38 The growth rate of the moving droplets increased because the spontaneous motion promoted coalescence with other droplets. Therefore, the moving droplets were removed quickly by gravity. However, this method cannot be applied to jumping-mode condensation because spontaneous motion is not induced when the surface of the asymmetric bumps is superhydrophobic. To overcome this limitation, we exploited the phenomenon in which a growing droplet moves spontaneously within a V shape when its inner surface is superhydrophobic.39,40 We designed nanograssed zigzag structures, which consisted of superhydrophobic V shapes, to induce the in-plane motion of the condensates. In the nanograssed zigzag structures, the growing condensates inside the V shapes moved out spontaneously and coalesced with other droplets (Figure 1a). Then, the coalesced droplets jumped up from the surface due to the excessive surface energy released during the coalescence (Figure 1b).

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Figure 1. Concept of nanograssed zigzag structures. Zigzag structures consist of superhydrophobic V shapes. The zigzag structures are arranged to face each other. (a) During condensation, growing droplets inside the V shapes move to the opening of the V. The movement of the droplets promotes the coalescence of droplets. (b) The coalesced droplets can jump up from the surface because the excess surface energy that the droplets release is converted to kinetic energy. To make the entire surface superhydrophobic, we used chemical oxidation to fabricate bladelike CuO nanograss on the surface.13,31 The cleaned surface was immersed in a hot alkaline solution for the oxidation. The solution consisted of NaClO2, NaOH, Na3PO4· 12H2O, and deionized water (DIW) (3.75:5:10:100 wt %), and its temperature was maintained at approximately 100 °C. After 20 min, bladelike nanostructures with a height of ∼1 μm covered the entire surface including the sidewalls of the microstructures. Finally, we conducted hydrophobic functionalization of the surface by using fluorinated silane (trichloro(1H,1H,2H,2H-perfluorooctyl)silane, Sigma-Aldrich). The surface was placed with ∼3 μL of the silane in a vacuum chamber, which was connected to a vacuum pump. The vacuum pump depressurized the chamber for 15 min, and then the pressure inside the chamber was maintained below −0.1 MPa (gauge pressure) for 35 min. Then, the pressure was returned to ambient, and the surface was rinsed with DIW and dried using an air gun. Surface Characterization. To characterize the wettability of the surface, advancing and receding contact angles of DIW droplets on the bladelike nanograssed superhydrophobic surface were measured using a goniometer (SmartDrop, Femtobiomed). Initially, we placed a water droplet (2−5 μL) on the surface, and then the advancing and receding contact angles were measured by water injection and withdrawing methods. The advancing contact angle was measured when the triple line of the droplet advanced by the injection, and the receding contact angle was measured when the triple line receded by the withdrawing. The advancing contact angle was 172.4 ± 2.6°, and the receding contact angle was 167.9 ± 1.6°. The surface geometry was characterized using a field emission scanning electron micrograph (SU6600, Hitachi). The beam power used was 10 kV. Experimental Setup. We visualized the condensation process on the superhydrophobic surface that had nanograssed zigzag structures. To capture the behavior of microsized condensates, the condensation experiment was performed using an optical microscope (Axioskop 2, Carl Zeiss) with a CMOS camera (Ace acA4096-30uc, Basler). The resolution of the camera is 4096 × 2168 pixels. A customized cooling plate was installed on a stage of the microscope to keep the surface temperature below the dew point. The cooling plate consisted of an aluminum jacket and a chiller (RW-3040G, Jeiotech). To measure the temperature of the cooling plate, a k-type thermal couple (diameter, 1 mm) was inserted into the aluminum jacket. A resistance temperature detector (RTD) and a hygrometer (HF532, Rotronic) were installed above the cooling plate to measure the air temperature and humidity, respectively. The surface was placed on the cooling plate, and topview images were captured. The microscope was tilted 90° to install the surface perpendicularly to the ground because returned jumping droplets can disturb the visualization. (Experimental Setup; see Figure S1 of the Supporting Information). The condensation experiment was conducted under atmospheric conditions. A thermo-hygrostat (ACTH-050 AU) maintained the air

EXPERIMENTAL SECTION

Surface Fabrication. We used electrochemical deposition to fabricate zigzag copper microstructures that were composed of microwalls with a width of ∼5 μm and a height of ∼15 μm.41,42 A copper plate with a thickness of ∼500 μm was used as a substrate for the electrochemical deposition. To make a mold that had ∼20 μm thickness, a negative photoresist (PR) (KMPR 1050, Microchem) was coated on the substrate by spin-coating for 30 s at 4800 rpm. The PR layer was soft-baked at 65 °C for 5 min and at 100 °C for 20 min then illuminated with UV for 21 s at 14 mW/cm2. The PR layer was then baked at 65 °C for 5 min and 100 °C for 4 min then developed for 3 min. After development, the surface was baked at 110 °C for 1 min. The electrochemical deposition was a two-step current density process (∼1 mA/cm2 for 1 h then ∼7 mA/cm2 for 2 h) to make uniform microstructures with a height of ∼15 μm. The remaining mold was removed using a PR remover (mr-Rem 700, micro resist technology). 9094

DOI: 10.1021/acs.langmuir.9b01065 Langmuir 2019, 35, 9093−9099

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Langmuir temperature at 23.8 ± 0.2 °C and humidity at 76.1 ± 1.7%. The temperature of the cooling plate was 9.2 ± 0.05 °C. We performed the condensation for 10 min. After precondensation for 5 min to reach the steady state, the condensation process was recorded for 10 min at 1 frame per second.

escaped from the V shape completely or coalesced with other droplets (Figure 2b). Three Departure Modes in Nanograssed Zigzag Structures. We recognized that the average departure diameter was ∼30 μm on the bladelike nanograssed superhydrophobic surface in low supersaturation condition (see Experimental Setup. To reduce the departure diameter to ≤20 μm, we designed the V shapes to have 20 μm maximum spacing and arranged them to face each other with a 20 μm gap (G) (Figure 2a); that is, we designed the nanograssed zigzag structures that can gather droplets that have a diameter of ∼20 μm into the gap to promote their coalescence. Our experiments revealed the intended phenomenon. The growing droplets inside the V shapes spontaneously moved toward the gap then were launched from the surface by the energy released by coalescence with other droplets (mode 1) (Figure 3a); these droplets had an average departure diameter

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RESULTS AND DISCUSSION Spontaneous Motion of Droplets in Nanograssed V Shapes. Wettability of the surface strongly influences the spontaneous motion of a growing droplet inside a V shape. The contact angle θin of the inner meniscus of the droplet decreases as the droplet grows because the growth decreases the curvature of the droplet. During this process, if θin becomes smaller than the receding contact angle θr of the surface, the droplet exits the V shape. This spontaneous motion of the growing droplet can occur when the surface of a V shape is superhydrophobic; in a superhydrophobic V shape, θ in becomes less than θr easily due to the low contact angle hysteresis.40 In this research, we fabricated nanograssed zigzag structures on a copper substrate. The zigzag structure consists of V shapes that have a 30° cross-sectional angle. As shown in Figure 2a, the entire surface is covered by bladelike nanograss

Figure 2. (a) SEM images of the nanograssed zigzag structures. The zigzag structures face each other with a ∼20 μm gap (G), and the maximum spacing inside the V shape (L) is ∼20 μm. In each image, the area in the yellow dashed box is magnified in the image to its right. The entire surface is covered by bladelike nanograss. (b) Time-lapse optical images of a droplet inside a V shape during condensation. The time in the upper left corner of the images represents the time duration from the nucleation. The droplet grew continuously, and the growing droplet spontaneously moved to the mouth of the V. Yellow dashed line: center position of droplet.

Figure 3. Time-lapse optical images of condensed droplets inside the nanograssed zigzag structures. Times in lower left corners: time elapsed since nucleation. Yellow dashed circles: moving droplets inside V shapes. The moving droplets coalesced with other droplets and jumped up from the surface and disappeared from the view. (a) Jumping mode 1: a moving droplet coalesced with other droplets in the gap between the zigzag structures. (b) Jumping mode 2: a moving droplet inside a V shape coalesced with other droplet in the shape. (c) Jumping mode 3: a droplet moved inside a V shape, and then another droplet (yellow dashed circle) also moved inside the V shape and caught the first droplet.

with a height of ∼1 μm. The surface was coated by a fluorinated silane to induce superhydrophobicity. Consequently, a nanograssed superhydrophobic surface completely covered the zigzag structures, including the sidewalls. The expected size of droplets inside the V shapes was 17 times the density of 1800 mm−2 on the nanograss. Then, on the zigzag, DAVG = 13.3 μm, whereas the nanograss had DAVG = 28.7 μm. These results prove that the nanograssed zigzag structures promoted the coalescenceinduced jumping. Due to the vigorous jumping, the average

Figure 4. Average departure diameter and cycle time of jumping modes. X-axis represents the droplet jumping modes. Nanograss represents the droplet jumping on a nanograssed surface without the zigzag structures. Left-side y-axis, gray bars represent the droplet departure diameter. Right-side y-axis, red squares represent the cycle time; i.e., measured time between the nucleation and the jumping of a droplet. Error bars represent standard deviation, n = 10.

Increased Efficiency of Jumping-Mode Condensation. To confirm the effect of the nanograssed zigzag structures on the efficiency of condensation, we compared

Figure 5. (a) Time-lapse images of condensed droplets. Times in lower left corners: time elapsed since the start of condensation. The images compare condensation trends with and without the zigzag structures. The area covered by yellow dashed line represents the inside area between the zigzag structures. (b) Size distribution of departing droplets. Red bars: result from the area inside the zigzag structures and blue bars: result from the area without the zigzag structures. (c) Average diameter of condensed droplets inside the area between the zigzag structures (red triangles) and the area without the zigzag structures (blue squares). Error bars: standard deviation. (d) Cumulative volume of the jumping droplets inside the area between the zigzag structures (red triangles) and the area without the zigzag structures (blue squares). Error bars: uncertainty (see Section S1 of the Supporting Information). 9096

DOI: 10.1021/acs.langmuir.9b01065 Langmuir 2019, 35, 9093−9099

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Langmuir diameter of condensed droplets on the zigzag was kept smaller than that on the nanograss (Figure 5c). As a result, the cumulative volume of the departure droplets was approximately 1.8 times higher on the zigzag than on the nanograss (Figure 5d). The increased efficiency was due to the multiple effects of the zigzag structures. The nucleation site density on the zigzag was approximately four times higher than that on the nanograss (see Figure S2a of the Supporting Information), so the frequency of the coalescence was higher on the zigzag. Furthermore, the spontaneous motion on the zigzag also promoted coalescence-induced jumping. On the nanograss, the condensed droplets remained fixed before they coalesced with each other. Therefore, the average departure diameter D was similar to the average distance S between the closest droplets on the surface (i.e., S/D, ∼1). However, S/D was larger than 2.5 on the zigzag (see Figure S2b of the Supporting Information). This result proved that the spontaneous motion on the zigzag promoted coalescence-induced droplet jumping. Effect of Depinning Motion to Adhesion Energy. Condensed droplets can be pinned on nucleation sites during condensation on nanograssed superhydrophobic surfaces.11,43,44 We also recognized that a condensed droplet was initially pinned inside the V shape. However, the pinned droplet was depinned in the V shape when it grew enough (Figure 6; see Video S2 of the Supporting Information).

Figure 7. (a) Schematic diagram of a partially pinned droplet on a nanograssed surface. The total base is the region of contact, and the pinned base is the pinned region. R is the radius of curvature and θapp a is the apparent advancing contact angle. State 1: a pinned droplet on a nanograssed superhydrophobic surface. State 2: detached state of the pinned droplet. (b) Effect of diameter of partially pinned droplet on the ratio (blue line) of pinned base to the total base and the ratio (red line) of adhesion energy of the pinned base to the total adhesion energy.

and φ is the solid fraction that represents the proportion of the liquid−solid contact area in which the droplet is in the Cassie− Baxter state. To simplify, we put the Young’s equation into eq 1 to obtain Wwenzel = A pγlv(φ cos θ + 2 − φ) Figure 6. Time-lapse ESEM images of a condensed droplet inside a V shape. The droplet was initially pinned inside the V shape (∼10 s); the stretched shape of the droplet implies the pinning. After 12 s, the droplet was changed to a sphere due to the depinning.

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where θ is intrinsic contact angle. In the nonpinned area, the liquid−solid interfacial area is changed to the liquid−vapor area and to the solid−vapor interfacial area when the droplet detaches. The adhesion energy WCassie of the second area is expressed as

To understand the effect of the depinning motion on the droplet-jumping phenomenon, we calculated the adhesion energy of a pinned droplet.45 The calculation assumed that the pinned base of the droplet remained after the droplet detached (Figure 7a). The base of the pinned droplet has two areas: pinned area Ap and nonpinned area (Ab − Ap), where Ab is the total projection area of the contact area between the droplet and the surface. In Ap, the solid−liquid interface is converted to a solid−vapor area and a liquid−vapor area when the droplet detaches from the surface; this process also generates a liquid−vapor interfacial area. The adhesion energy Wwenzel of Ap is

WCassie = (Ab − A p)φ(γsv + γlv − γsl)

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which can be simplified using Young’s equation to WCassie = (Ab − A p)φγlv(1 + cos θ )

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The total adhesion energy Wa is the sum of Wwenzel and WCassie Wa = A pγlv(φcosθ + 2 − φ) + (Ab − A p)φγlv(1 + cos θ ) (5)

We can calculate Ab as Ab = πR2(sin θaapp)2

Wwenzel = A p[φ(γsv + γlv) + (1 − φ)(γlv × 2)] − A pφγsl

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θapp a

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where R is the radius of the pinned droplet, and is the apparent advancing contact angle on the superhydrophobic surface. We can calculate Ap as

where γsv, γlv, and γsl represent the interfacial energy of solid− vapor, liquid−vapor, and solid−liquid interfaces, respectively, 9097

DOI: 10.1021/acs.langmuir.9b01065 Langmuir 2019, 35, 9093−9099

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Langmuir A p = πrp2

where rp is the radius of the pinned base. For the calculation, we used the characteristics of the CuO bladelike nanostructured superhydrophobic surface. We assumed that the θapp a value was 172.4°, which is the advancing contact angle of the superhydrophobic surface because the droplets grow until they are removed from the surface. The intrinsic contact angle θ is between the advancing contact angle θa=123.4° and receding contact angle θr= 81.2°, so we assumed that θ = 102.3°, which is the average of θaand θr on a hydrophobically coated smooth surface.13 The solid fraction was calculated as

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Moo Hwan Kim: 0000-0002-7193-7189 Author Contributions

φ = (cos θaapp + 1)/(cos θa + 1) ≈ 0.02

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T.H. conceived the study, conducted the experiments and analysis, and wrote the manuscript. H.J.K. assisted in the visualization. J.H.K. provided the process for making the surface. M.H.K. and J.T.K. made comments and reviewed the manuscript.

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Then, Ap depends on the characteristics of the surface. The spacing of the bladelike nanograss was 0.1−1 μm (Figure 2a), so we conservatively assumed that the radius of a pinned base is 0.1 μm. We used the characteristics of the bladelike nanograssed superhydrophobic surface to calculate the ratio of the adhesion of the pinned base to the total adhesion Wp/Wa. Figure 7b shows that as D increases, the area of total base increases, whereas the area of the pinned base is constant. Therefore, Wp/Wa decreases as D increases. At D = 13.3 μm, which is the DAVG on the zigzag, Wp takes ∼62% in the total Wa, whereas the area of the pinned base is only ∼1.3% in the area of the total base. Even though the conservatively assumed pinned base radius was used for the calculation, the adhesion of the pinned base exceeds half of the total Wa. This result implies that once the spontaneous depinning motion is achieved, the adhesion energy between the droplet and surface can be decreased greatly. Therefore, the depinned droplets can be propelled easily by coalescence with other droplets.

Funding

The National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF2017R1A2B2010115) supported this research. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank all the members of Moo Hwan Kim’s lab (two-phase flow laboratory in POSTECH), Jae Man Park and Kanghyun Kim. The experimental sample was fabricated at the 9D lithography beamline of Pohang Accelerator Laboratory (PAL).

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REFERENCES

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CONCLUSIONS In summary, we promoted coalescence-induced jumping using nanograssed zigzag structures to increase the efficiency of jumping-mode condensation. In the V shapes of the zigzag structures, growing droplets spontaneously moved in the direction parallel to the surface, and the moving droplets jumped up from the surface through the coalescence with other droplets. This spontaneous motion facilitated the more frequent coalescence of droplets and reduced the cycle time between the nucleation and the jumping of a droplet. As a result, the proposed surface increased the frequency of the coalescence-induced jumping by ≥17 times and increased the cumulative volume of jumping droplets by ∼1.8 times compared with the conventional nanograssed superhydrophobic surface. These results provide a way to enhance the efficiency of jumping-droplet condensation and will contribute to increasing the efficiency of various applications that exploit condensation, including the hot-spot cooling of high-power electronics20,21 and water-harvesting in arid regions.25 Furthermore, the results yielded insight that will help to develop efficient condenser surfaces for various applications.

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Data reduction method, uncertainty analysis, and experimental setup using ESEM (PDF) Comparison of condensation trends with and without zigzag structures (AVI) Depinning motion of a condensed droplet in a V shape (AVI)

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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.9b01065. 9098

DOI: 10.1021/acs.langmuir.9b01065 Langmuir 2019, 35, 9093−9099

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DOI: 10.1021/acs.langmuir.9b01065 Langmuir 2019, 35, 9093−9099