Rationally 3D-textured copper surfaces for Laplace pressure

Aug 1, 2018 - Enhancing the thermal efficiency of a broad range of condenser devices requires means of achieving sustainable dropwise condensation on ...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 29127−29135

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Rationally 3D-Textured Copper Surfaces for Laplace Pressure Imbalance-Induced Enhancement in Dropwise Condensation Chander Shekhar Sharma,†,§,∥ Christos Stamatopoulos,‡,∥ Reto Suter,†,‡ Philipp Rudolf von Rohr,*,‡ and Dimos Poulikakos*,† †

Laboratory of Thermodynamics in Emerging Technologies, Department of Mechanical and Process Engineering, and ‡Transport Processes and Reactions Laboratory, Department of Mechanical and Process Engineering, ETH Zurich, 8092 Zurich, Switzerland

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S Supporting Information *

ABSTRACT: Enhancing the thermal efficiency of a broad range of condenser devices requires means of achieving sustainable dropwise condensation on metallic surfaces, where heat transfer can be further enhanced, by harvesting the advantage of the sweeping action of vapor flow over the surface, facilitating a reduction in the droplet departure diameter. Here, we present a rationally driven, hierarchical texturing process of copper surfaces, guided by fundamental principles of wettability and coalescence, which achieves controlled droplet departure under vapor flow conditions and thus significantly enhances phase change thermal transport. The desired texture is attained by fabricating an array of 3D laser-structured truncated microcones on the surface, covered with papillae-like nanostructures and a hydrolytically stable, low surface energy self-assembled-monolayer coating. Passive droplet departure on this surface is achieved through progressive coalescence of droplets arising from microcavities formed by the microcone array, resulting in depinning and subsequent departure of the depinned condensate drops through vapor shear. The synergistic combination of vapor shear and the sustained dropwise condensation on the hierarchical copper surface results in a nearly 700% increase in heat transfer coefficients as compared to filmwise condensation from identical, standard unstructured surfaces. KEYWORDS: dropwise, flow condensation, hierarchical, copper, superhydrophobic, nanostructure

1. INTRODUCTION

Maintaining the superhydrophobicity of a surface during condensation requires periodic ejection of condensate droplets nucleating within the surface texture.16,17 This can be achieved through the creation of gradients of Laplace pressure within the droplets, progressive droplet coalescence, and coalescenceinduced droplet jumping7,18−21 and has been recently demonstrated on random hierarchical surfaces.7 It is also possible to control the droplet ejection behavior through the design of well-defined microfeatures, and a number of model silicon surfaces with microfeatures such as micropillars and microcones have been explored for controlled droplet departure.12−15,18,22 However, silicon is not a substrate suitable for realistic applications as metallic surfaces, primarily copper, aluminum, and brass, comprise the bulk of heat exchange interfaces for surface condensers.1 Hence, means of passive, controlled droplet departure during dropwise condensation are required on metallic surfaces in order to significantly improve the efficiency of surface condensers used in a wide range of applications.23

Filmwise heterogeneous condensation, wherein a continuous film of condensate is formed over the solid surface, is commonly observed on all metallic surface condensers. The insulating effect of the condensate film between vapor and cold surface presents a primary impediment to the overall heat transfer. It is well known that avoiding the formation of this condensate film by causing water to condense and remove itself as drops before film formation, a mode termed as dropwise condensation, can significantly increase the efficiency of the phase change heat transfer.1−3 This mode of condensation is observed in nature4,5 and has also been demonstrated for a number of artificial surfaces through modification of the surface wettability.2 A large fraction of these substrates are superhydrophobic surfaces as superhydrophobicity ensures small droplet departure diameters because of the inherently low contact angle hysteresis. Although conventional superhydrophobicity can be achieved by only microtexturing the surface coupled with low surface energy coatings,6 superhydrophobicity during condensation requires the presence of nanotexture7−10 and a broad palette of such surfaces, both nanostructured8,9,11 and hierarchical, already exists in the literature.7,12−15 © 2018 American Chemical Society

Received: May 31, 2018 Accepted: August 1, 2018 Published: August 1, 2018 29127

DOI: 10.1021/acsami.8b09067 ACS Appl. Mater. Interfaces 2018, 10, 29127−29135

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Working principle of a condensate droplet propulsion in a tapered microcavity. The droplet is able to move when contact angle at the top, θt, becomes equal to θA (left inset) and contact angle at the bottom, θb, becomes equal to the θR (right inset). Middle inset of (a) depicts the specific geometrical parameters considered for the microstructuring: base diameter D, height H, opening angle α of each truncated microcone, and the pitch L. (b) SEM images showing (i,ii) microcone array fabricated with 3D laser micromachining and (iii) nanotexturing, which resembles papillae, was imparted to the walls of the microcones with wet chemistry etching. Inset shows a magnified view of the nanopapillae. (c) EDX analysis of the superhydrophobic hierarchical copper-based surface.

droplet pinning during condensation on the side walls of the microfeatures, we introduce a nanoroughness, in the form of papillae-like nanofeatures, that overlays these microfeatures and functionalizes the hierarchical surface with a hydrolytically stable low surface energy self-assembled monolayer (SAM). We demonstrate the surface performance through in situ observations of the microscale condensate dynamics, macroscale fluid dynamics, and heat transfer investigations. The hierarchical surface achieves dropwise condensation through coupled effects of droplet deformation, droplet coalescenceinduced depinning, and droplet departure under vapor shear. Additionally, the surface is able to successfully maintain dropwise condensation under accelerated endurance test conditions of super-atmospheric saturated steam flow across the surface. The collaborative effect of droplet ejection and vapor shear results in a 700% increase in heat transfer coefficients compared to filmwise condensation.

In recent years, a number of metallic surfaces have been presented for enhancement in heterogeneous condensation. These include nanostructured surfaces,11,20,24−28 liquid-impregnated slippery surfaces,23,29 biphilic surfaces,30 and chemical vapor-deposited graphene coatings,31 among others. Most of these results pertain to quiescent vapor conditions with eventual droplet departure from surface dependent on either coalescence-induced droplet jumping, gravity, or through coalescence with areas of filmwise condensation on a biphilic surface. Vapor flow-induced shear across the surface can significantly influence the heterogeneous condensation dynamics and is an important factor in commercial surface condensers. Several classical studies have explored the effect of vapor flow on dropwise condensation using hydrophobic surfaces.32−34 Vapor shear effects can reduce the droplet departure diameter compared to quiescent vapor conditions,25 and a coupling between low wettability and vapor shear has the potential to achieve significant enhancement in condensation heat transfer. Here, we exploit this phenomenon and demonstrate an exceptional enhancement in condensation heat transfer on copper through a synergistic combination of controlled passive droplet ejection and subsequent removal through vapor flow. We utilize laser-assisted 3D-microstructuring to generate welldefined truncated microconical features in copper. The diverging cavities formed by the microcones not only achieve enhanced droplet ejection,35 by creation of favorable Laplace pressure gradients within the condensate microdroplets, but also increase the surface area available for overall thermal transport. In order to achieve high contact angles and lower the

2. RESULTS AND DISCUSSION The working principle of the adopted strategy is presented in Figure 1a. Our approach consists of utilizing capillarityinduced droplet motion in tapered geometries. The effect of capillarity in tapered geometries has been reported by previous researchers in tapered tubes and cavities36−38 as well as in biological applications.39 Adopting a similar approach, we use this property to induce motion of the droplets toward the divergent part of microcavities40 with the help of hydrophobic walls. During condensation, the droplets growing inside microcavities on a hydrophobic surface can bridge with the droplets lying on top of the microfeatures.41 Diverging 29128

DOI: 10.1021/acsami.8b09067 ACS Appl. Mater. Interfaces 2018, 10, 29127−29135

Research Article

ACS Applied Materials & Interfaces

Figure 2. (a) Microscale condensation dynamics involving coalescence of droplets in adjacent microcavities and resulting progressive depinning. Coalescence of drops of similar size results in pinned coalesced droplets (see droplets marked 1 + 2, 3 + 4, and 5 + 6 in panel (ii)). Droplets of dissimilar size coalesce and result in depinning of smaller drops from microcavities (see droplets coalescences D1 = (1 + 2) + (3 + 4) + (5 + 6) + 7 + 8 + 9 + 10 and D2 = 11 + 12 + 13 + 14 + 15 + 16 in panels (ii,iii)). The renewed microcavities lead to formation of fresh condensate droplets that occupy the microcavities and expand outward to repeat the cycle (example, droplets N1, N2, and N3). Scale bar represents 100 μm. (b) Net pressure difference along a growing condensate droplet ΔPin vs normalized position of top meniscus contact disc zt/H of truncated microcone array with nanostructured walls (left). The nanostructured walls allow the formation of a meniscus at the bottom of the cavity formed by adjacent microcones (inset i), contrary to the case of smooth walls, see (inset ii). (c) Schematic depicting the different states that a droplet undergoes: (i) droplet grows inside the microcone cavity until ΔPin becomes slightly positive and begins to move toward its diverging part of the cavity. (ii) Upper meniscus contact disc of the droplet reaches the top of the microcavity with simultaneous movement of the lower meniscus contact line. (iii) Further growth leads to a critical upper meniscus radius of curvature Rt,cr = L/2 above which coalescence with an adjacent droplet occurs. (iv) Coalescence of a droplet Ωcr with a larger droplet of at least ∼4Ωcr. Mean pressure Pin(Ωcr) inside the small droplet is much larger compared to the mean pressure Pin(∼4Ωcr) inside the larger droplet. (v) This coalescence induces a pressure difference that drags the small droplet out of the microconical cavity. (d) Pressure difference along a coalesced droplet ΔPin vs normalized position of bottom meniscus’ contact disc zb/H. Scenario explored: coalescence of two droplets with dissimilar sizes Ωcr and ∼4Ωcr (red line). Dashed line shows instantaneous increase in ΔPin in the coalesced drop immediately after coalescence. Solid line shows change in ΔPin as droplet lower meniscus rises.

induces a positive capillary force toward the divergent part of the cavity,7,19,37,39 which causes the droplet to move with θt and θb being equal to the advancing (θA) and receding (θR) contact angle, respectively, on the nanostructured hydrophobic walls of the microcavity (see left and right insets of Figure 1a, see also Supporting Information, Section S1 for details on the estimation of ΔPin and criterion for movement of the droplet). On the basis of this principle, the investigated artificial roughness consists of an array of truncated cones (Figure 1) termed as the microstructure. As the proposed surface is intended for heat transfer enhancement, we sought to create these features on copper, a widely used metal in heat transfer applications because of its high thermal conductivity λCu = 401 W/(m·K). The truncated microcone array is initially designed with the help of a CAD software in order to define all the geometrical parameters that include the base diameter D, the height H, the opening angle α of each truncated microcone, and the pitch L (Figure 1a, middle inset). We implement a rational approach in surface design to maximize the pressure difference ΔPin as a function of opening angle α. Interestingly, we find that ΔPin is a non-monotonic function of α and reaches a maximum for an opening angle α(ΔPin,max) ≈ 7° (see Supporting Information, section S2). However, because of the limitations of the laser micromachining method per se, the closest possible value of the opening angle to α(ΔPin,max), which can be reliably fabricated,

hydrophobic cavities can exploit capillarity to significantly enhance this bridging of the condensate, which should prevent the flooding of the surface42 and thus enhance the associated heat transfer.1,23 To simplify the description of the droplet ejection mechanism, it is assumed that the process occurs inside an axisymmetric diverging microcavity (see Supporting Information, section S1). After the onset of condensation, small droplets grow in size, coalesce, and finally occupy the microcavity, resulting in the formation of two menisci of different radii of curvature, Rt (for top meniscus) and Rb (for bottom meniscus), which result in unequal Laplace pressure differences across the top and bottom menisci. Assuming that viscous forces are negligible,40 the resulting capillary force acting on the drop is estimated by the order of magnitude of the integral of the Laplace pressure gradient over the volume of the drop, that is, ΔPin/ztΩ, where zt is the size of the droplet along the depth of the cavity, Ω is the total volume of the droplet, and ΔPin is the overall Laplace pressure difference.37 ΔPin can be estimated as ΔPin = Pin,b − Pin,t = 2σw,v(cos(π + α − θb)/rb − cos(α + θt − π)/rt), where Pin,t and Pin,b are pressures inside the top and bottom part of the drop, σw,v is the surface tension of water in its vapor environment, and θt and θb are the instantaneous contact angles of the two menisci with the cavity wall. The contact angles are governed by the wetting property and geometry of the cavity (See Figure 1a) with θt ≤ θA and θb ≥ θR. A favorable pressure gradient (Pin,b > Pin,t) 29129

DOI: 10.1021/acsami.8b09067 ACS Appl. Mater. Interfaces 2018, 10, 29127−29135

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ACS Applied Materials & Interfaces is α ≈ 26°. The following values of the truncated microcone geometrical parameters are obtained for this work: D = 78 μm, H = 50 μm, α = 26°, and L = 81 μm. In order to materialize the fabrication of these microfeatures on copper, laser micromachining is employed. Laser micromachining is a powerful method that enables the fabrication of intricate 3D structures on a wide range of materials including metals (see Supporting Information, section S3 for further details about the laser micromachining process).43,44 Additionally, for a given α, the presence of nanoroughness increases ΔPin, by achieving high values for θA and θR and thus increasing the radii of curvature of the two menisci compared to a surface that exhibits lower contact angles. Therefore, on top of the truncated microcone array, nanostructuring is imparted with the employment of wet chemical etching, where an aqueous solution of ammonium persulfate ((NH4)2S2O8) and sodium hydroxide (NaOH) is used.45,46 The nanostructures resemble nanopapillae (Figure 1b). The aforementioned process results in a hierarchically micro-/nanotextured solid substrate that consists of truncated microcone array as the microstructure overlayed with nano-papillae-like nanostructures. Subsequently, the interfacial energy of the solid substrate is reduced by applying a SAM of perfluorodecanethiol (PFDT). Energy dispersive X-ray spectroscopy (EDX) analysis (Figure 1c) shows the presence of oxygen (O) and copper (Cu), which is indicative of the oxidation process that occurs because of the use of ammonium persulfate and sodium hydroxide and results in the formation of nanopapillae. Additionally, the presence of fluorine (F), carbon (C), and sulfur (S) confirms that the copper samples are coated with PFDT, which results in high water repellency for the hierarchically structured surface (see Methods, for further details on surface fabrication). For the microcavity walls, nanoroughness and PFDT coating achieves θR = 150.6° and θA = 164.8°, estimated from the macroscopic receding and advancing contact angles on the plain nanostructured surface. We test this hierarchical surface for condensation performance through in situ investigations at the microscale using environmental scanning electron microscopy (ESEM). Microscale condensation dynamics on the hierarchical superhydrophobic surface with nanopapillae are illustrated in Figure 2a. The sample is oriented at 45° with respect to the electron beam. The sample temperature is controlled through the use of a cooling stage, and the chamber pressure is increased until the onset of condensation (refer to Methods section for details of ESEM experiments). Condensate droplets nucleate at the base and on the lateral surface of microcones. These droplets combine to form a single distorted droplet that occupies the diverging microcavity formed by four adjacent microcones as shown in panel (i). The geometry of the microcavity results in droplets that expand outward and develop a favorable Laplace pressure gradient directed outward from the cavity. This pressure gradient is created by the increase in the upper meniscus radius of curvature along with the growth of the droplet (see Supporting Information section S6 for an example of such an increase in Rt). Because of the symmetrical geometry of the microcavity array, droplets of nearly identical size can grow in adjacent cavities (see droplets 1 to 6 in panel (i)). These droplets eventually coalesce but the resulting coalesced droplet remains pinned inside the respective microcavities leading to the formation of water bridges (see droplets marked as 1 + 2, 3 + 4, and 5 + 6 in panel (ii)). This shows that the pressure gradient induced by this symmetrical

coalescence event is not sufficient to overcome the droplet pinning at the base or at the lateral walls of the microcavities. Additionally, we speculate that such coalescence also does not induce sufficient interfacial instability to overcome the localized droplet pinning.7 However, when droplets of dissimilar size in adjacent cavities coalesce, the Laplace pressure imbalance and the coalescence-induced interfacial instability are able to progressively remove the smaller droplets from the respective microcavities. This is clearly shown by coalescence of droplets 1 + 2, 3 + 4, and 5 + 6 with droplets 7 to 10 in panel (ii) to form the droplet marked as D1 in panel (iii). Similarly, droplets 11 to 16 in panel (ii) coalesce to form droplet D2 in panel (iii). It is evident from panel (ii) that the formation of droplets D1 and D2 results in clearing of all the surrounding microcavities, thus allowing for a fresh start of dropwise condensation cycle in these cavities. Subsequently, further coalescence of droplets D1 and D2 with surrounding droplets yields a coalesced drop, whose pinning is confined to only a few microcavities and tops of truncated microcones. In essence, unsymmetrical coalescence results in progressive decrease in the droplet pinning, thus enabling the renewal of microcavities for nucleation of fresh droplets. The new condensate droplets coalesce, occupy the microcavities, and expand outward to repeat the condensation cycle. Refer, for example, droplets N1, N2, and N3 marked in panel (iv) (also see Video S1). We utilize the axisymmetric microcavity-based model to highlight the effect of wetting in Figure 2b, where variation of ΔPin versus the normalized contact position zt/H of the upper meniscus due to droplet growth is shown. We compare the performance of hydrophobicity of the microcavity side walls achieved by nanoroughness in our fabrication with that of smooth hydrophobic microcavities. Similar to the case of nanorough microcavities, the wetting behavior of the smooth microcavities is estimated based on the macroscopic dynamic contact angles of the plain smooth surface, that is, θR = 50.9° and θA = 145.1°. It should be noted that the droplet can reach a suspended state by forming a lower meniscus only when the criterion θR − π/2 − α > 035 is satisfied. For the smooth microcone walls, a bottom meniscus cannot be formed because θR − π/2 − α = −65.1° < 0 (Figure 2a, inset (ii)), contrary to the case of the nanostructured walls, where θR − π/2 − α = 34.6° > 0 (Figure 2a, inset (i)). As a result, for the case of smooth walls, a pressure gradient ΔPin within the droplet cannot be established. On the other hand, for the case of the nanostructured wall, ΔPin increases as the upper meniscus rises with droplet growth and the droplet movement is initiated as soon as ΔPin becomes marginally positive (Figure 2b) (see Supporting Information, section S1 for details of calculation of ΔPin as a function of zt/H for the nanostructured microcavity). This infinitesimally positive, favorable, net pressure difference enables the movement of the lower and upper menisci toward the diverging part of the cavity (Figure 2c(i)). The continuous growth of the droplet maintains ΔPin slightly above zero and its upper meniscus eventually reaches the top edge of the microcavity (Figure 2c(i,ii)). Thus, this difference in state of the droplet (suspended or not) between the nanostructured hydrophobic walls and the smooth hydrophobic walls emphasizes the need for nanotexturing.7 Nevertheless, even for the case of nanostructured hydrophobic microcavity, as the top meniscus reaches the top of microcavity, the lower meniscus only reaches zb/H ≈ 0.3, which means that a considerable part of the droplet still 29130

DOI: 10.1021/acsami.8b09067 ACS Appl. Mater. Interfaces 2018, 10, 29127−29135

Research Article

ACS Applied Materials & Interfaces

Figure 3. Macroscale condensate droplet dynamics on hierarchical (microcones + nanopapillae) surface: (a) high-speed droplet dynamics during flow condensation under vapor shear from saturated steam flowing across the surface at ∼3 m/s at 111 °C. Scale bar represents 1 mm and arrow indicates direction of vapor flow. (b) Dropwise condensation sustainability under prolonged exposure to flow condensation and effect of vapor velocity on droplet departure. For panels (i−iii), steam velocity is ∼3 m/s; for panels (iv−vi), steam flows at ∼9 m/s. Reduction in the droplet departure diameter with increase in steam velocity is evident. Additionally, gradual degradation of surface is reflected by appearance of filmwise condensation regions on prolonged exposure to steam at high velocity. Scale bar represents 5 mm and arrow indicates direction of vapor flow.

inducing and enhancing droplet ejection. It should be noted that the droplet ejection from microcavities is also energetically favorable as the Cassie−Baxter state is a more stable wetting state for the hierarchical hydrophobic surface considered here (see Supporting Information, section S4). We next combine this passive coalescence-induced droplet ejection from microcavities with vapor shear to achieve enhanced shedding of the drops sitting on top of the microcones. The application of vapor flow instead of quiescent conditions reduces the droplet departure diameter as the velocity squared, that is, V2Ddrop = constant, where V is vapor velocity and Ddrop is droplet diameter.25 This, coupled with the fact that condensation heat flux Q″ improves with the reduction in Ddrop,47 promises enhanced condensation heat transfer with increase in vapor shear. Additionally, truncated microcones also contribute to heat transfer enhancement by increasing the net available area for heat transport.48 The roughness factor r increases with decreasing opening angle α, and achieving the minimum possible opening angle of α ≈ 26° results in r = 1.80, that is, the hierarchical surface has 80% additional active surface area available for heat transfer (see Supporting Information S5). We test the surface at the macroscale under vapor shear using a custom-built natural circulation flow-condensation loop. Here, the surface is exposed to a flow of steam, at a saturation temperature of 111 °C, across the sample at two velocities ∼3 and 9 m/s in a custom-built flow condensation chamber (refer to Methods and Supporting Information section S7 for further details on vapor flow condensation experiments). Coupled with vapor shear, the surface is able to achieve and sustain dropwise condensation under the flow of steam. Figure 3a shows macroscale condensate droplet departure from the hierarchical surface during flow condensation. The image sequence shows frames from a highspeed video of droplet coalescence and departure. Panel (i) shows a group of adjacent droplets in the projected diameter range of 1−1.5 mm on the surface. The regular microcone array on the surface is clearly visible through the droplets because of the droplet lensing effect. As the droplets grow and coalesce, the resulting depinning clears the surface in the vicinity of the drops (panels (ii−v)). Eventually, the coalesced droplet departs and the dropwise condensation cycle restarts

remains in the asperities of the surface. At this point, further growth of the droplet results in a stationary triple phase contact line at the top edge (Figure 2c(ii)). As a result, while the lower meniscus displacement is negligible, the growing droplet reaches a critical volume Ωcr, corresponding to an upper meniscus critical radius of curvature Rt,cr = L/2 that enables the potential coalescence with an adjacent droplet of the same (Figure 2c(iii)) or dissimilar size (Figure 2c(iv)). In order to estimate a sufficient postcoalescence droplet size Ωej, at which the coalesced droplet will be able to reach the top of the microconical cavity and finally be removed from it (Figure 2c(iv,v), i.e., the lower meniscus will be situated at zb/H ≈ 1 postcoalescence), we explore a scenario where droplets of dissimilar size Ωcr and Ωej − Ωcr coalesce. To simplify the analysis, we assume that a droplet with size (Ωej − Ωcr) drags the adjacent droplet of size Ωcr, resulting in the formation of a larger droplet that is Ωej. We estimate that the smallest possible size Ωej of a droplet that will enable its lower meniscus contact line to finally reach position zb/H ≈ 1 is ∼5Ωcr (see Supporting Information, section S1 for details of growth of droplet beyond the top edge, coalescence of adjacent droplets and the estimation of Ωej). In this line, Figure 2d shows the change of ΔPin in the droplet formed as the result of the coalescence of two droplets with size Ωcr and ∼4Ωcr versus the displacement of lower meniscus contact line. Assuming that, immediately after coalescence, the lower meniscus contact line for droplet of size Ωej ≈ 4Ωcr + Ωcr resides at the same position as for droplet of size Ωcr before coalescence namely zb/H ≈ 0.3 (inset of Figure 2d), the instantaneous increase of the droplet volume leads to an abrupt rise of ΔPin. This results in an upward movement of the lower meniscus and increase of the upper meniscus radius of curvature leading to decrease of ΔPin until reaching ΔPin ≈ 0. At this point, bottom meniscus reaches position zb/H ≈ 1, which indicates that coalescence of two droplets of dissimilar size, Ωcr and ∼4Ωcr, is sufficient to achieve droplet self-ejection from the microcavities (see Video S1). On the other hand, a droplet smaller than ∼4Ωcr cannot bring a drop of size Ωcr to the top leading to the formation of water bridges (see Video S1). Therefore, our simple model based on axisymmetric microcavity geometry explains qualitatively the ESEM observations shown in Figure 2a that the coalescence of droplets of dissimilar sizes plays a key role for 29131

DOI: 10.1021/acsami.8b09067 ACS Appl. Mater. Interfaces 2018, 10, 29127−29135

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ACS Applied Materials & Interfaces

Figure 4. Condensation heat transfer performance of the hierarchical surface (green symbols) as compared to nanostructured hydrophobic (orange symbols), plain hydrophobic (blue symbols), and hydrophilic surface (red symbols). (a) Heat flux as a function of surface subcooling. The empty and solid symbols correspond to a steam velocity of ∼3 m/s (bottom) and ∼9 m/s (top), respectively. (b) Average heat transfer coefficients. The colors correspond to Figure 4a. Inset figures show representative images of flow condensation on 5 mm × 5 mm area of the hydrophilic (filmwise), plain hydrophobic (dropwise), nanostructured hydrophobic (dropwise), and hierarchical hydrophobic (dropwise) surfaces (refer Supporting Information, Figure S7, for heat transfer coefficients as a function of surface subcooling). Error bars represent net uncertainty in measurements (refer Supporting Information section S8 for further details).

condensation is evident from the significantly reduced slope of the surface subcooling versus heat flux curves. The reduction in the slope for dropwise condensation curves is caused by a marked suppression of surface subcooling, while the overall temperature difference between vapor and coolant remains practically the same between filmwise and dropwise condensation in our experiments. This is attributed to the fact that the resistance of the cooler and the contact resistance between the sample and the cooler are the dominating components in the total resistance to the flow of heat flux from steam to coolant (refer Supporting Information, section S7 for further details). Hence, surface subcooling is chosen as the ordinate in Figure 4a because of the high sensitivity of dropwise condensation heat flux to this parameter.1,24 Figure 4b plots average heat transfer coefficients for the four surfaces and shows that the hierarchical surface achieves nearly 7-fold higher heat transfer coefficients compared to filmwise condensation under vapor shear (see Supporting Information sections S7 and S8 for details on estimation of average heat transfer coefficients and uncertainty estimation). As expected, the vapor velocity has a significant effect on the thermal performance of the hierarchical surface and the heat transfer coefficient at the high vapor velocity is about 37% higher compared to low vapor velocity. This increase is directly correlated with the decrease in the droplet departure diameter with the increase of vapor flow.25 The average droplet departure diameter on the hierarchical hydrophobic surface reduces from ∼2.5 to ∼1.3 mm as the vapor velocity increases from 3 to 9 m/s. These droplet departure diameters are similar to the ones measured for the nanostructured hydrophobic surface. However, the hierarchical surface achieves about 50 and 90% improvement in heat transfer compared to nanostructured surface at low and high vapor flow velocity, respectively. Additionally, both the hierarchical and nanostructured superhydrophobic surfaces show better performance as compared to the plain hydrophobic surface, thus highlighting the need for structuring the surface. The hierarchical surface is able to utilize the increase in heat transfer area obtained with the array of microcones, by achieving droplet ejection from the diverging cavities through a synergistic combination of droplet deformation-induced Laplace pressure gradient, droplet coalescence, and flow-mediated droplet

(panels (v,vi), also see Video S2). The surface is exposed to this high-temperature, vapor shear environment for over 9 h and no visible degradation in dropwise condensation on the surface is noticed. Figure 3b shows results from a prolonged exposure of the surface. An extended durability test has been performed, wherein the surface is exposed to steam flow at a velocity of ∼3 m/s for first 6 days (Figure 3c, panels (i−iii)), and subsequently, the steam flow is increased to ∼9 m/s for the next 5 days (Figure 3c, panels (iv−vi)). Refer Supporting Information section S7 for details on control of vapor flow and section S10 for details of the extended durability test. The surface maintains dropwise condensation and shows a significant reduction in droplet departure diameter as a result of increased vapor shear (see Video S3 for comparison of droplet departure at high and low vapor shear).25 However, the surface shows degradation with the appearance of distinct filmwise condensation regions in the upstream end and central region of the sample (Figure 3c, panels (v,vi)) after exposure to high velocity steam for 5 days. We emphasize here that the exposure of surface to condensation under shear flow of high temperature vapor is an accelerated endurance test for the robustness of the surface in terms of sustainability of dropwise condensation. The surface performance in these conditions underlines the promising potential of this surface for industrial surface condenser applications, which operate at much lower saturation temperatures and quiescent vapor conditions and hence milder conditions.49,50 As a result of this sustained dropwise condensation, the hierarchical (microcones + nanopapillae) surface achieves exceptional enhancement in condensation heat transfer performance. Figure 4 compares the thermal performance of the hierarchical hydrophobic surface with a nanostructured hydrophobic surface consisting of the same nanopapillae as in the case of hierarchical surface, a plain hydrophobic surface, and a hydrophilic surface. The hydrophilic surface expectedly results in filmwise condensation, while plain, nanostructured, and hierarchical superhydrophobic surfaces sustain dropwise condensation. The thermal performance of the surfaces is shown at two steam velocities of ∼3 m/s (shaded symbols and bar graphs) and ∼9 m/s (solid symbols and bar graphs). Figure 4a shows heat flux versus subcooling for the four surfaces. The improved thermal transport for the dropwise 29132

DOI: 10.1021/acsami.8b09067 ACS Appl. Mater. Interfaces 2018, 10, 29127−29135

Research Article

ACS Applied Materials & Interfaces

nanotextured hydrophobic surface was fabricated by adding step 4 as well to impart hydrophobicity (Figure 1, see also Supporting Information, section S9). The plain hydrophobic surface was fabricated by employing steps 2 and 4. The plain hydrophobic sample exhibited θA = 145.1 ± 1.8 °C and θR = 50.9 ± 6.8 °C. The plain nanotextured hydrophilic sample exhibited θA = 10.4 ± 4.3 °C, with no measurable receding contact angle of the contact disc (due to high pinning forces), and the plain nanotextured hydrophobic surface showed θA = 164.8 ± 1.3 °C and θR = 150.6 ± 7.6 °C. 4.2. Surface Characterization. Wettability characterization of the surfaces was conducted by measuring the advancing and receding contact angles employing the dynamic sessile drop technique using a commercial goniometer Drop Shape Analzyer-DSA25 by Krüss (for further details about the contact angle measurements, see Supporting Information section S9). Morphology of the surfaces was examined with a scanning electron microscopy (SEM) system (Quanta 200F FEI). Surface chemical characterization was done using EDX with the system EDAX Octane Super embedded on the SEM system. Information and data about characterization of surface topology by SEM, before and after exposure to vapor shear, for the plain nanostructured hydrophilic, plain nanostructured hydrophobic, and hierarchical superhydrophobic samples is given in Supporting Information section S9 and Figure S9. 4.3. Microscale Condensation Experiments. In situ observations of condensation at the microscale were performed using an FEI Quanta 600 ESEM system with a gaseous back scattered electron detector. Droplet growth from within the microcones and subsequent ejection toward the top of the microfeatures were investigated by mounting the samples on custom made copper stubs such that the sample was oriented to the electron beam at a beam incidence angle of 45°. A Peltier cooling stage (Emott AG) was used to control the sample temperature. The cooling stage was set to 2 °C and the chamber pressure was slowly increased until the onset of condensation, following which the chamber pressure was kept constant at ∼1 kPa. A beam voltage of 20 kV, a spot current of 0.16 nA, and a viewing area above 550 μm × 550 μm were used to minimize the beam heating effects for all the experiments.7,51 The obtained images were analyzed using ImageJ. 4.4. Macroscale Flow Condensation Experiments. Macroscale condensation experiments were performed on 2 cm × 2 cm samples, wherein the surface was exposed to steam at a temperature of 111 °C, pressure of 1.42 bar, and flowing across the sample at two velocities ∼ 3 and 9 m/s in a custom-built flow condensation chamber. Condensation dynamics on the sample were observed by using high-speed imaging, heat flux through the sample was measured using a copper block embedded with a series of temperature sensors, and the temperature of the sample was measured separately for estimation of the overall heat transfer coefficient (refer Supporting Information section S7 for further details on vapor flow condensation experiments).

departure, thus improving the thermal performance compared to even the nanostructured superhydrophobic surface.

3. CONCLUSIONS In conclusion, we have achieved exceptional enhancement of condensation heat transfer on a rationally textured metallic surface through a synergistic combination of droplet coalescence-induced droplet depinning and vapor shear. We have exploited 3D laser microstructuring to obtain a microstructure consisting of a regular array of microcones and achieved enhancement of droplet ejection through additional surface nanostructuring in the form of nanopapillae and a low-surface energy coating. The droplet ejection from the surface texture is achieved through coalescence of dissimilar size drops observed through high-resolution in situ imaging using ESEM. The overall mechanism of droplet ejection is explained through a simple analytical model, providing rational guidelines for the needed surface texturing. The surface is able to sustain dropwise condensation under harsh vapor shear conditions and achieve enhancement in thermal performance compared to a nanostructured hydrophilic surface that promotes filmwise condensation and a plain hydrophobic nanostructured surface that promotes dropwise condensation. The condensation conditions endured by the surface can be regarded as an accelerated endurance test for the surface in terms of sustenance of dropwise condensation mode. Laser micromachining can be used to create a range of complex 3D microstructures on metallic surfaces and is not limited by the type of metallic substrate. This functionality allows rational designing of microstructural features on metals and provides similar flexibility to photolithography-based microstructuring of silicon. 4. METHODS 4.1. Surface Fabrication. The fabrication steps of the hierarchical superhydrophobic surface are described here briefly. During the first step, as received rectangular plain copper samples (EN CW004A, Metall Service Menziken Inc., Switzerland), 20 mm × 50 mm and 1 mm thick, were used as a substrate to generate an array of truncated microcones with the employment of laser microstructuring (Fraunhofer, Institute for Laser Technology (ILT); see also Supporting Information, section S2 for details about the configuration of the laser microstructuring process). Subsequently, during the second step, the microcone-structured samples were cleaned sequentially in hydrochloric acid, acetone, isopropanol, and finally water. Next (third step), nanotexturing was imparted to the sample employing a facile wet chemical etching process (Figure 1, see also Supporting Information section S9).45,46 Specifically, the samples were immersed in an aqueous solution of 77.5 mM sodium hydroxide (Sigma-Aldrich) and 3.1 mM ammonium persulfate (Sigma-Aldrich) for 25 min resulting in the formation of nanoscale features that covered uniformly the entire surface area of the truncated microcones (Figure 1, see also Supporting Information section S9). Finally, in the fourth step, the samples were immersed in an ethanolic solution of 1 mM PFDT (Sigma-Aldrich) to minimize their surface energy, thus imparting high water repellent behavior and droplet mobility. The microcone-structured samples, after the final step, exhibited advancing and receding contact angle, θA = 161.6 ± 1.5 °C and θR = 160.4 ± 1.3 °C, respectively. For comparison and to highlight the exceptional performance of hierarchical superhydrophobic surface, three additional types of copper-based surfaces were tested: a plain nanotextured hydrophilic, a plain hydrophobic, and a plain nanotextured hydrophobic surface. The plain nanotextured hydrophilic surface was fabricated by employing fabrication steps 2 and 3 described previously (Figure 1, see also Supporting Information section S3), whereas the plain

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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/acsami.8b09067. Details on pressure gradient calculation inside a microconical cavity and coalescence-induced ejection; details on laser micromachining; variation of pressure difference inside the cavity as a function of opening angle; calculation of roughness factor; energetically favorable wetting state of droplet on the surface; details on macroscale condensation experiments and data processing; optimization of surface fabrication parameters; surface characterization; and extended durability test (PDF) 29133

DOI: 10.1021/acsami.8b09067 ACS Appl. Mater. Interfaces 2018, 10, 29127−29135

Research Article

ACS Applied Materials & Interfaces

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Microscale condensate droplet dynamics on the hierarchical (microcones + nanopapillae) copper hydrophobic surface (AVI) Microscale droplet departure from the hierarchical (microcones + nanopapillae) copper hydrophobic surface (AVI) Effect of vapor shear on microscale droplet departure from the hierarchical (microcones + nanopapillae) copper hydrophobic surface (AVI)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +41 44 632 24 88. Fax: +41 44 632 13 25 (P.R.v.R). *E-mail: [email protected]. Phone: +41 44 632 27 38. Fax: +41 44 632 11 76 (D.P.). ORCID

Chander Shekhar Sharma: 0000-0002-6193-6457 Christos Stamatopoulos: 0000-0002-8482-1150 Dimos Poulikakos: 0000-0001-5733-6478 Present Address §

Department of Mechanical Engineering, Indian Institute of Technology Ropar, Rupnagar 140001, India. Author Contributions ∥

C.S.S. and C.S. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the funding from Commission for Technology and Innovation (CTI) under the Swiss Competence Centers for Energy Research-Efficiency of Industrial Processes (SCCER-EIP) program (grant no. KTI.2014.0148) and Swiss National Science Foundation (SNSF) Grant (grant no. 200021_162847/1). We thank Asel Maria Aguilar Sanchez from the Institute for Building Materials, ETH Zurich, for her support of ESEM measurements and Dr. Kunze Karsten from Scientific Center for Optical and Electron Microscopy, ETH Zurich, for his support in the acquisition of SEM and EDX data. We also thank Jovo Vidic and Peter Feusi, ETH Zurich, for their assistance in construction of macroscale condensation experimental setup and Andreas Dohrn from Fraunhofer Institute for Laser Technology (ILT), Aachen for laser micromachining. The authors would like to thank Prof. Dr. Nicholas Spencer from the Laboratory of Surface Science and Technology, ETH Zurich, for his insightful comments within the context of this work.

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