Hierarchical Superhydrophobic Surfaces with Micropatterned

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Hierarchical Superhydrophobic Surfaces with Micropatterned Nanowire Arrays for High-Efficiency Jumping Droplet Condensation Rongfu Wen, Shanshan Xu, Dongliang Zhao, Yung-Cheng Lee, Xuehu Ma, and Ronggui Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14960 • Publication Date (Web): 07 Dec 2017 Downloaded from http://pubs.acs.org on December 8, 2017

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ACS Applied Materials & Interfaces

Hierarchical Superhydrophobic Surfaces with Micropatterned Nanowire Arrays for HighEfficiency Jumping Droplet Condensation Rongfu Wen,1 Shanshan Xu,1 Dongliang Zhao,1 Yung-Cheng Lee,1 Xuehu Ma,2 and Ronggui Yang1,3,4,* 1

Department of Mechanical Engineering, University of Colorado, Boulder, CO 80309, USA

2

Liaoning Key Laboratory of Clean Utilization of Chemical Resources, Institute of Chemical

Engineering, Dalian University of Technology, Dalian 116024, P. R. China 3

Materials Science and Engineering Program, University of Colorado, Boulder, CO 80309, USA

4

Buildings and Thermal Systems Center, National Renewable Energy Laboratory, Golden, CO

80401, USA

KEYWORDS: superhydrophobic, hierarchical nanostructured surface, condensation, nucleation control, spontaneous droplet movement, jumping droplets

*

Corresponding author: [email protected]

1 Environment ACS Paragon Plus

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

ABSTRACT: Self-propelled droplet jumping on nanostructured superhydrophobic surfaces is of interest for a variety of industrial applications including self-cleaning, water harvesting, power generation, and thermal management systems. However, the uncontrolled nucleation-induced Wenzel state of condensed droplets at large surface subcooling (high heat flux) leads to the formation of large pinned droplets, which results in the flooding phenomena and greatly degrades the heat transfer performance. In this work, we present a novel strategy to manipulate droplet behaviors during the process from the droplet nucleation to growth and departure through a combination of spatially controlling initial nucleation for mobile droplets by closely spaced nanowires and promoting the spontaneous outward movement for rapid removal using micropatterned nanowire arrays. Through the optical visualization experiments and heat transfer tests, we demonstrate greatly improved condensation heat transfer characteristics on the hierarchical superhydrophobic surface including the higher density of micro-droplets, smaller droplet departure radius, 133% wider range of surface subcooling for droplet jumping, and 37% enhancement of critical heat flux for jumping droplet condensation, compared to the-state-of-art jumping droplet condensation on nanostructured superhydrophobic surfaces. The excellent water-repellency of such hierarchical superhydrophobic surfaces can be promising for many potential applications such as anti-icing, anti-fogging, water desalination, and phase-change heat transfer.


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INTRODUCTION Vapor to liquid condensation is a ubiquitous phase-change process in nature1-3 and has been exploited in many energy-intensive industrial systems from power generation, water treatment and harvesting, to thermal management of electronics.4-7 The wetting behavior, usually characterized as the contact angle for a liquid co-existing with its vapor or air on a solid surface, is the key that determines the phase-change heat transfer efficiency.8, 9 It is well known that dropwise condensation on a hydrophobic surface can have a heat transfer coefficient an order-ofmagnitude higher than that in filmwise condensation on a hydrophilic surface.10, 11 Such a big difference in heat transfer efficiency can be understood in the viewpoint of thermal resistance of liquid condensate between the vapor and solid surface: filmwise condensation exhibits a continuously thick liquid film while the solid surface is refreshed and in direct contact with vapor in the dropwise condensation when the discrete droplets fall off the surface.7 Recently, there have been significant efforts in developing superhydrophobic surfaces to promote the self-removal of condensed droplets in a smaller size.12, 13 Micro-droplets on such surfaces can undergo self-propelled jumping when two or more smaller droplets coalesce where the decrease in the total surface area transfers the excess of surface energy into kinetic energy for droplet jumping. Such self-propelled droplet jumping offers a potential new route to further enhance condensation heat transfer.14, 15 However, to achieve jumping droplet condensation, the wetting state of droplets should be manipulated to ensure the mobile Cassie state or partially wetting state, as opposed to the pinned Wenzel state, which results in flooding condensation.16-19 On this basis, many hierarchical superhydrophobic surfaces have been designed to promote the dewetting transition of condensed droplets by utilizing gradients of Laplace pressure or surface free energy.20-22 However, the visualization experiments have showed that the flooding



phenomena can occur as the surface subcooling (heat flux) is increased past a critical value, even on the superhydrophobic surfaces that initially exhibit jumping droplet condensation.17, 18, 23 This is attributed to the uncontrolled droplet nucleation within the micro/nanostructures of condensing surfaces with the decreased nucleation size that is much smaller than the structure scale at large surface subcooling (large heat flux), according to the classical nucleation theory.24, 25 Despite that many visualization experiments of droplet condensation using environmental scanning electron microscopy,16, 26, 27 focal plane shift imaging,28 and high-speed imaging,5, 12 have shown self-propelled droplet movement on the superhydrophobic surfaces, quantitative measurements of enhanced condensation heat transfer performance on such functionalized surfaces remain limited. Moreover, most current designs of superhydrophobic surfaces can lose their efficacy due to the uncontrolled droplet nucleation when a practically high heat flux level or large surface subcooling is reached.12, 16, 29, 30 An alternate approach, known as slippery liquidinfused porous surface (SLIPS),26, 31, 32 shows improvement in droplet departure and heat transfer performance. However, the heat transfer efficiency is still lower than that of jumping condensation on superhydrophobic surfaces.33 In addition, to preserve the lubricating fluid required for SLIPS, the non-condensable gases (NCGs) cannot be removed completely, which results in a significant degradation in heat transfer efficiency that could be even worse than filmwise condensation without NCGs.23 Recently, a 30% enhancement in heat transfer coefficient by droplet jumping was first demonstrated on a superhydrophobic surface consisting of CuO nanostructures, compared to the dropwise condensation on the plain hydrophobic surface.18 However, this work was performed under a very small surface subcooling, < 0.8 K, to avoid the flooding phenomenon.34 Despite that a considerable amount of studies have focused on promoting dropwise condensation by


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micro/nanostructured and hierarchical superhydrophobic surfaces,11,

16, 18, 23, 35

almost all the

existing structured surfaces cannot prevent nucleation-induced flooding condensation at large surface subcooling. To delay the uncontrolled flooding on superhydrophobic surfaces, a hybrid surface was reported to promote self-organization of microdroplets.36 Although the flooding is delayed on the hybrid surface, condensation still transitions from the jumping droplet mode to gravity-driven shedding mode with the increase of surface subcooling. To further expand the range of surface subcooling for higher heat fluxes utilizing droplet jumping, a hydrophobic surface with closely spaced copper nanowires was exploited to control droplet nucleation to be just on the top surface of the nanowires.37 A 100% enhanced heat transfer coefficient in comparison with dropwise condensation on the plain hydrophobic surface was demonstrated, with an enhanced heat flux (140 kW·m-2) and extended range of surface subcooling (< 8 K), which were indeed the highest heat flux and widest range of surface subcooling for jumping droplet condensation achieved by then. Even though that work nicely demonstrated the formation of mobile droplets by spatially controlling nucleation at the top of nanowires, further structural design for accelerating droplet removal is critical to enhance condensation on such nanowired superhydrophobic surfaces. Here we present a novel strategy to manipulate droplet behaviors from the nucleation in the order of a few nanometers to the departure size typically in the order of tens to hundreds of micrometers through a combination of spatially controlling nucleation by closely spaced nanowires and promoting spontaneous outward movement of droplets for rapid removal using micropatterned nanowire arrays. Significantly different from the previous strategies for accelerating droplet removal alone by regulating the liquid-solid interface,3, 8, 20, 38, 39 we take advantage of the reduced vapor permeability into the narrow separations between closely spaced



nanowires to control initial nucleation to be only on the top of nanowires for mobile droplets. By further arranging these hydrophobic nanowires with different lengths into microscale patterns, the micro-valleys between the long nanowire arrays are created to promote the spontaneous outward movement of the mobile droplets formed on the nanowires within micro-valleys. When the droplets in the micro-valleys grow larger than the critical size (the width of micro-valleys), the micro-valleys can push the droplets to the top of micropatterned nanowire arrays by outward Laplace pressure. Considering that each micropatterned nanowire array is composed of thousands of closely spaced nanowires, the sides of micropatterned nanowire arrays have a very large surface roughness for reducing contact angle hysteresis during droplet outward movement. In this study, we aim to achieve condensation heat transfer enhancement in a wide range of surface subcooling on the micropatterned nanowire arrays. Compared to the shallow microvalleys, the deep micro-valleys are conducive to the accumulations of surface energy by droplet deformation for dewetting transitions at small surface subcooling.21 However, the deep microvalleys could increase surface adhesion of pinned droplets due the droplet nucleation within nanostructures at large surface subcooling.18, 37 Thus, the shallow micro-valleys are adopted in this study to promote spontaneous outward movement of droplets at small surface subcooling and reduce surface adhesion of droplets at large surface subcooling. Such a hierarchical nanowired superhydrophobic surface substantially outperforms other hydrophobic and superhydrophobic surfaces developed for enhancing condensation, such as higher density of micro-droplets, smaller departure radius of droplets, and most importantly wider range of surface subcooling and enhanced critical heat flux for jumping droplet condensation. RESULTS AND DISCUSSION


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By leveraging the spatial control of droplet nucleation using closely spaced nanowires and the spontaneous outward movement of the growing droplets by outward Laplace pressure using micro-valleys, a hierarchical nanowired superhydrophobic surface is designed with micropatterned nanowire arrays. Figure 1a-b shows the schematic of vapor condensation on the hierarchical nanowired superhydrophobic surface from the initial nucleation to departure of condensed droplets. Figure 1a shows a schematic to control droplet formation by exploring the spatial confinement effect, which takes advantage of the reduced molecular permeability of vapor into the narrow separations between the closely spaced hydrophobic nanowires. By regulating the density difference of water vapor between inside and outside of the separations, droplet nucleation tends to occur at the top of nanowires due to the larger density of vapor (ρout > ρin), which mitigates droplet formation in separations between nanowires. In addition, the closely spaced hydrophobic nanowires increase the energy barrier of nucleation for the droplets in the narrow spacings (130-160 nm) compared to that on the top of nanowires.40 As a result, suspended droplets form on the top of nanowire arrays, which results in highly mobile droplets for subsequent droplet movement. To accelerate droplet departure for refreshing the condensing surface, Figure 1b shows that the micro-valleys consisting of short nanowire arrays between long nanowire arrays are designed to promote the spontaneous droplet movement by the outward Laplace pressure. Taking into account that the size of micro-valleys (~100 µm) is far larger than the nucleation size (typically a few nanometers) of condensed droplets, the initial droplets could be randomly formed on the short and long nanowire arrays. For the droplets formed in the micro-valleys, they can rapidly grow to a critical dimension where the droplet size approaches to the micro-valley. With the further growth of these droplets, the lower part of the confined droplet in the micro-valleys



remains under a high pressure Pup with a small radius R1. Meanwhile, the upper part of the droplets expands on the micropatterned nanowire arrays and the pressure inside decreases to Pdown, which is due to the increased radius from R1 to R2. As a result, the outward Laplace pressure, ∆P = Pup – Pdown, can push the growing droplets out of the micro-valleys, which accelerates the droplet removal and surface renewal.

Figure 1. Design of the hierarchical superhydrophobic surface with micropatterned nanowire arrays for the nucleation control and rapid departure of condensed droplets. (a) Schematic showing the closely spaced nanowires for controlling initial nucleation to be only on the top surface of nanowire arrays, i.e., reducing the density of water vapor in the separations between nanowires and thus inhibiting nucleation due to the lower vapor density, ρin > ρout. (b) Microvalleys between long nanowire arrays for the spontaneous outward movement of growing droplets by the outward Laplace pressure, ∆P = Pup - Pdown, which is attributed to the deformation of the growing droplet by the confinement of micro-valleys, R1 < R2. (c) SEM images of the hierarchical nanowired superhydrophobic surface consisting of micropatterns of long nanowire arrays surrounded by short nanowires, which forms micro-valleys. Insets are the magnified view of closely spaced short nanowire arrays (yellow dash rectangle) and long


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nanowire arrays (red dash rectangle). (d) The apparent contact angle 167 ± 3° of a 5 uL water droplet on the hierarchical nanowired superhydrophobic surface.

To fabricate the hierarchical superhydrophobic surface with micropatterned nanowire arrays, we first fabricated the copper micropillars on the condensing block by a standard photolithography and electroplating processes. Closely spaced copper nanowires were then grown on the micropillared surface using a two-step porous anodized aluminum oxide (AAO) template-assisted electroplating method. A self-assembled hydrophobic coating was further implemented on the copper nanowired surface using the solution of n-octadecanethiol in ethanol for achieving the superhydrophobicity (see details in Supporting Information S1). Figure 1c shows the scanning electron microscopy (SEM) images of the hierarchical nanowired superhydrophobic surface. The width and the center-to-center spacing of micropatterned square long nanowire arrays are 200 µm and 300 µm, respectively. The length (height) of long nanowire arrays is 40 µm and the diameter and center-to-center spacing of long nanowires are ~220 nm and ~350-380 nm, respectively. Between the long nanowire arrays lie the short nanowires with ~220 nm in diameter, ~300-350 nm in pitch, and 20 µm in length (height). As such, the depth of micro-valleys between long nanowire arrays is 20 µm resulting from the length difference between the long nanowire arrays and short nanowire arrays. For comparison, the uniform nanowired superhydrophobic surface is also fabricated to demonstrate that the spontaneous outward movement of droplets only happens on the hierarchical nanowired superhydrophobic surface. In addition, other two different kinds of hierarchical nanowired superhydrophobic surfaces, consisting of nanowire arrays and plain regions, are also designed to illustrate the importance of initial nucleation control for enhancing condensation heat transfer (see Supporting Information S1). Figure 1d shows that the apparent contact angle and contact angle hysteresis of 9 Environment ACS Paragon Plus


a 5 µL water droplet on the hierarchical nanowired superhydrophobic surface in the ambient condition are 167 ± 3° and 3 ± 3°, respectively. This indicates by the micropatterned nanowire arrays improved hydrophobicity compared to uniform nanowired superhydrophobic surface, which has an apparent contact angle and contact angle hysteresis of 151 ± 3° and 5 ± 3°, respectively. Condensation Modes and Droplet Behaviors. To show the advantages of dropwise condensation with control through a combination of spatial nucleation control and spontaneous outward movement of droplets, we investigated the condensation modes and droplet behaviors on three test surfaces: plain hydrophobic surface, uniform nanowired superhydrophobic surface and hierarchical nanowired superhydrophobic surfaces. The condensation modes and droplet behaviors were visualized in-situ on the condensing surfaces, which are vertically mounted in a custom-made condensation heat transfer test chamber (see Supporting Information S2). Condensation experiments were conducted at the water vapor pressure Pv = 60 ± 0.5 kPa, which is the common conditions for multi-effect desalination, low temperature heat pumps, and heat pipes.41-44 As the driving force for vapor condensation, the surface subcooling ∆T is defined as the temperature difference between vapor temperature Tv and surface temperature Tw (see Supporting Information S3). The concentration of non-condensable gas in the experimental system is ~ 0.5%. For the plain hydrophobic surface, Figure 2a shows the continuous droplet formation, growth, and sliding, driven by gravity when the condensing surface is oriented vertically, where the droplet departure diameter (~2 mm) is comparable to the capillary length to overcome the contact line pinning force (circle in red).10,

11

Droplets with a size ranging from a few

micrometers to millimeters are discrete on the plain hydrophobic surface and the departure


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frequency of droplets increases with the increased droplet growth rate at a larger surface subcooling (Supporting Information Video S1 and Video S2, also seen Supporting Information S4 for details). This is known as the conventional dropwise condensation that has attracted much attention since its discovery due to the advantage of the higher heat transfer coefficient compared to that of filmwise condensation.9,

10

However, for the uniform nanowired superhydrophobic

surface, Figure 2b shows that a very large number of small droplets leave the surface by selfpropelled jumping at small surface subcooling of ~ 2 K, with a size < 250 µm (Supporting Information Video S3). As the surface subcooling increases to 9 K, condensed droplets grow to a larger size and the merged droplets do not directly leave the surface by self-propelled jumping but rapidly slip on the surface, which results in refreshing a large surface area by the sweeping (Supporting Information Video S4). When the surface subcooling increases to 18 K, large condensed droplets (> 2 mm) pin on the surface and the condensation mode completely degrades to flooding condensation (Supporting Information Video S5). This is due to the wetting transition of condensed droplets from the Cassie state at small surface subcooling to the Wenzel state at large surface subcooling, which greatly increase the surface adhesion for droplet departure. The significant variation of condensation modes and droplet behaviors on the uniform nanowired superhydrophobic surface is attributed to the wetting transition of droplets from the mobile Cassie state at 2 K to pinned Wenzel state at 18 K. At small surface subcooling of 2 K, droplet nucleation occurs on top of the nanowire arrays for the formation of mobile droplets and they exhibit strong coalescence-induced self-propelled jumping. However, as the surface subcooling increases to 18 K, the critical size for initial nucleation greatly reduces and the nucleation density greatly increases, which together result in high-density of small droplets nucleated in the separation between nanowires.37 The subsequent rapid droplet growth and



droplet coalescence result in the large pinned droplets and flooding condensation. Between the Cassie state and Wenzel state of condensed droplets, the droplets in the partially wetting state (∆T = 9 K) can rapid slip on the surface independent of gravity. The kinetic energy of the sweeping droplet comes from the free energy released during droplet coalescence, which is insufficient to directly push the droplet off the surface by self-propelled jumping.45-47 Note that the direction of droplet sweeping is independent on gravity and the sweeping droplet leave the surface at a smaller size (600-800 µm), which is significantly different from the droplet sliding driven by gravity on the plain hydrophobic surfaces. More importantly, the sweeping of merged droplets can rapidly refresh the surface for re-nucleation of next-generation droplets. Compared to the uniform nanowired superhydrophobic surface, Figure 2c shows that the jumping droplet condensation is indeed maintained on the hierarchical nanowired superhydrophobic surface even when the surface subcooling is increased from 2 K (Supporting Information Video S6) to 9 K (Supporting Information Video S7). Despite of the transition from the jumping droplet condensation into the sweeping droplet condensation as the surface subcooling increases to 18 K (Supporting Information Video S8), the flooding condensation does not appear for the range of subcooling experimented. The improved droplet removal on the hierarchical nanowired superhydrophobic surface is attributed to the spontaneous outward movement of micro-droplets in micro-valleys, as discussed above. It is worth noting that even for the hierarchical nanostructured superhydrophobic surfaces with micro-valleys, the condensation changes into flooding condensation with the increase of surface subcooling, which is due to the uncontrolled nucleation on the plain region without nanowires (see Supporting Information S5 for details). We hereby clearly demonstrate the importance in coupling nucleation control by closely spaced


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nanowires with the spontaneous outward movement of droplet growth in micro-valleys to achieve rapid droplet removal in a wide range of surface subcooling.

Figure 2. Condensation modes and droplet behaviors on three test surfaces under different surface subcooling. (a) Conventional dropwise condensation on the plain hydrophobic copper surface. (b) The transition from jumping droplet condensation to flooding condensation on the uniform nanowired superhydrophobic surface. (c) Jumping droplet condensation and sweeping droplet condensation on the hierarchical nanowired superhydrophobic surface. The red dash circles highlight the contour of condensed droplets on the surface. The yellow dash circles highlight areas of the surface right after droplet coalescence. The yellow arrows indicate the trajectory of droplet removal. See also Figure S9.

To gain a quantitative understanding, we characterized the droplet size distribution and droplet departure size on the three test surfaces. The droplet density in different sizes is defined



as droplet number per unit area in a field of view of 3.5 mm × 3.5 mm. The number counts of droplets were averaged over 30 images in a 30 s period of condensation after the onset of nucleation at t = 0 s. The departure radius of droplets was estimated from the removed droplets by comparing the condensation images recorded at 1/60 s intervals.

Figure 3. Droplet size distribution and departure radius on the three test surfaces. Droplet size distribution at ∆T = 2 K (a) and ∆T = 18 K (b). (c) Effect of surface subcooling on droplet departure radius. Error bars indicate the range of multiple measurements on the same sample.

As expected, compared to the plain hydrophobic surface, the high frequency droplet removal by self-propelled jumping at a small surface subcooling (∆T = 2 K) decreases the emergence of


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large droplets (> 300 µm) on both uniform and hierarchical nanowired superhydrophobic surfaces. As a result, the density of small droplets (20-50 µm) on both superhydrophobic surfaces is more than three times that on the plain hydrophobic surface, as shown in Figure 3a. As the surface subcooling is increased to 18 K, the density of small droplets (20-50 µm) on the uniform nanowired superhydrophobic surface is significantly reduced to be similar to that on the plain hydrophobic surface, as shown in Figure 3b, which is due to the formation of large pinned droplets caused by uncontrollable nucleation. However, the hierarchical nanowired superhydrophobic surface is still covered by high-density small droplets (60 mm-2), which greatly reduces the thermal resistance of condensation between the vapor and surface. Figure 3c shows the droplet departure radius as a function of surface subcooling on the three test surfaces. The departure radius of condensed droplets on the plain hydrophobic surface ranges from 0.9 mm to 1.1 mm, which is almost independent of the surface subcooling. This is because the removal of droplets on the plain hydrophobic surface are driven by gravity after growing upto the critical size, rc ~ (γ/ρlg)1/2, where γ is the surface tension, ρl is liquid density, and g is gravitational acceleration. However, there is a strong correlation between droplet departure radius and surface subcooling on both the uniform and hierarchical nanowired superhydrophobic surfaces. As the surface subcooling increases from 0.4 K to 29.5 K, the departure radius of droplets on the uniform nanowired superhydrophobic surface monotonously increases from ~20 µm to ~1.2 mm, which is even larger than that on the plain hydrophobic surface. The smallest droplet departure radius, 0.02-0.75 mm, is achieved on the hierarchical nanowired superhydrophobic surface at various surface subcooling, 0.4 K < ∆T < 30 K, which is also obviously smaller than that the reported droplet departure radius of 0.87-1.5 mm on plain hydrophobic surface vertically mounted at 101 kPa,48 1.2-1.4 mm on graphene-coated cylindrical



tubes at 2-5 kPa,49 1.1 mm on smooth oleophobic surface vertically mounted at 50 kPa,50 and 0.98 mm on the oil-infused heterogeneous immersion cylindrical tubes at 2-3 kPa.33 The reduced departure radius of droplets on the hierarchical nanowired superhydrophobic surface can greatly accelerate the surface refreshing (see Supporting Information S4 for more data comparison). Heat Transfer Performance. Figure 4 shows the condensation heat flux q as a function of surface subcooling on the three test surfaces. For the plain hydrophobic surface, the heat flux increases monotonically with the increase of surface subcooling due to the gravity removal. As the driving force of condensation, the increased surface subcooling facilitates the growth of condensed droplets, which results in accelerating surface renewal at higher heat flux. Compared to the dropwise condensation on the plain hydrophobic surface, at each surface subcooling, the heat flux is enhanced on both uniform and hierarchical nanowired superhydrophobic surfaces. The heat transfer curves of both the nanowired superhydrophobic surfaces are highly dependent on droplet behaviors under various surface subcooling. For the uniform nanowired superhydrophobic surface, the heat flux rapidly increases with the increase of surface subcooling, and reaches a maximum value of 136 kW·m-2 (the critical heat flux for jumping droplet condensation) at ∆T = 6 K, and then slightly reduces with the increase of surface subcooling up to 15 K. As the surface subcooling further increases, the heat flux increases again, following the similar trend as conventional dropwise condensation on the plain hydrophobic surface. Based on the trend of heat flux with surface subcooling, the heat transfer curve can be divided into three regimes, corresponding to the droplet behaviors from the self-propelled jumping, rapid sweeping, to the flooding, which is due to the nucleation-induced droplet wetting transition from Cassie state at small surface subcooling to Wenzel state at large surface subcooling. Compared to the plain hydrophobic surface, the significantly enhanced heat flux at small surface subcooling (∆T
15 K), the increase of heat flux benefits simply from the improved droplet growth rate under larger surface subcooling, similar to conventional dropwise condensation. Different from the flooding-induced heat transfer degradation on other nanostructured superhydrophobic surface in previous studies,18, 30 the heat flux of uniform nanowired surface is always better than that on the plain hydrophobic surface under a wide range of surface subcooling. This is due to the increase of effective heat transfer area for condensation using long nanowires even though the Wenzel state increase the droplet departure size at large surface subcooling. Despite that the similar trend in heat flux with surface subcooling is observed on the hierarchical nanowired superhydrophobic surface, the heat flux is increased at each surface subcooling, compared to that on the uniform nanowired superhydrophobic surface. Specifically, the critical heat flux for jumping droplet condensation is increased by 37% from 136 kW·m-2 to 186 kW·m-2, which is the highest heat flux for jumping droplet condensation achieved so far. Meanwhile, the region of surface subcooling for droplet jumping is expanded by 133% from 6 K to 14 K. By combining the nucleation control and spontaneous outward movement for rapid



droplet removal, such a hierarchical nanowired superhydrophobic surface outperforms both the state-of-the-art plain hydrophobic and uniform nanowired superhydrophobic surfaces we recently developed for enhancing condensation heat transfer (see Supporting Information S4 for more data comparison).37

Figure 4. Condensation heat transfer performance. Heat flux q (left) and heat transfer coefficient h (right) as a function of surface subcooling on the three test surfaces, undergoing the state-ofthe-art dropwise condensation on the plain hydrophobic surface, and the jumping droplet and sweeping droplet condensation on the uniform and hierarchical nanowired superhydrophobic surfaces.

Condensation Mechanisms on Hierarchical Nanowired Superhydrophobic Surface. Due to the limitations of the existing visualization techniques, such as environmental SEM with limited surface subcooling range under low vapor pressure (allowed upper limit of vapor pressure < 1kPa),53 insufficient scanning frequency to cope with the fast-growing droplets,17 and


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the heating effect on condensed droplets at high magnification,54 it is challenging to directly capture the cross-section images of condensation in the narrow spacings (130-160 nm) between nanowires in a wide range of surface subcooling (< 28 K). In addition, it is difficult to prevent vapor condensation on the sidewall of copper nanowires, which hinders the observation of condensation in nanowire arrays. To gain an understanding of condensation mechanisms on the hierarchical nanowired surface, here we alternatively quantify the droplet departure behaviors to evaluate the wetting transition-induced falling resistance of droplets, which indirectly reflects the wetting state of condensed droplets on the hierarchical nanowired surface under different surface subcooling.

Figure 5. Dynamic behaviors of droplets on the hierarchical nanowired superhydrophobic surface under different surface subcooling. (a) Schematic showing droplets formed on the top of nanowires at small ∆T. (b) Sufficient surface energy is released for coalescence-induced droplet jumping. (c) Formation of partially wetting droplets due to the initial nucleation in the separations. (d) Released surface energy during coalescence drives the rapid droplet sweeping. 19 Environment ACS Paragon Plus


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(e) Pinned droplets formed at large ∆T. (f) Increased departure size of sweeping droplets. The shadows in (a, c, e) indicate the depth of droplet immersion (pinning) due to the nucleation. The yellow dash circles in (b, d, f) highlight the falling droplets and the blue dash regions denote the refreshing area on the surface.

Figure 5a shows that the nucleated droplets tend to form on the top of nanowires at low surface subcooling due to the spatial confinement effect of the closely spaced nanowires, which results in highly mobile droplets on the surface. Once the droplets grow large enough to merge with the neighboring droplets, the coalescence-induced droplet jumping can be observed. This can happen both in the micro-valleys and on the top of the long nanowire arrays. More interestingly, the droplets in the micro-valleys between long nanowire arrays could grow and be pushed upward to the top of the long nanowire arrays due to the outward Laplace pressure even without coalescence. Figure 5b shows time-lapse images of jumping droplet condensation on the hierarchical nanowired superhydrophobic surface at the surface subcooling of 2 K (also see Supporting Information Video S6). Benefiting from the large contact angle and small solid-liquid adhesion of droplets on the nanowire arrays, the excess surface free energy released during droplet coalescence process drives the merged droplet to jump off the condensing surface at a small size (< 250 µm). With the increase of surface subcooling, Figure 5c shows nucleation can happen in the spacing between nanowires due to the decreased critical nucleation size and increased nucleation density, according the classical nucleation theory.17,

37

Although the

partially wetting state is favorable for improving individual droplet growth by reducing the droplet-base thermal resistance,27 the increased surface adhesion can reduce droplet dynamic characteristics. This is manifested as the transition of droplet departure mode from the selfpropelled jumping to the droplet sweeping when the surface subcooling increases to 18 K (Figure


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5d, also see Supporting Information Video S8). Limited by the insufficient kinetic energy released during the coalescence of droplet in partially wetting state, the merged droplets cannot jump off but rapidly slip on the condensing surface. Different from the droplet departure mode in conventional dropwise condensation, the sweeping droplets have a larger initial kinetic energy although the droplets keep contacting with the surface in both cases. Further increase of surface subcooling (Figure 5e) leads to increased surface adhesion and droplet departure size (Figure 5f, also see Supporting Information Video S9).

Figure 6.

Droplet growth and departure on the hierarchical nanowired superhydrophobic

surface. (a) Quantification of droplet growth rate and departure size. (b) Droplet departure mode



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and falling velocity. (c) Effect of surface subcooling on the falling acceleration and hysteresis of droplets.

To quantify the effect of surface subcooling on wetting transition and departure velocity of droplets on the hierarchical nanowired superhydrophobic surface, we characterized droplet growth rate and falling velocity under different surface subcooling. Figure 6a shows the droplet volume over time and the maximum volume when it begins to leave the condensing surface. Compared to the droplets growth rate at ∆T = 2 K, the growth rate increases nearly 63% at ∆T = 28 K. This is due to the increased surface subcooling (driving force) for vapor condensation as well as the reduced the droplet-base thermal resistance of the immersed droplets.27 However, the droplets formed at larger surface subcooling pin on the surface without removal until it reached a large departure volume (Vdmax = 0.264 mm3), a value about twenty times larger than the droplet departure volume (Vdmax = 0.013 mm3) at the surface subcooling of 2 K. Figure 6b shows the droplet falling trajectory at different surface subcooling including the coalescence-induced droplet jumping and droplet sweeping. By calculating the second derivative of the falling distance of droplets sf, the falling acceleration of droplets af can be obtained as ܽ୤ =

ୢమ ௦౜ ୢ௧ మ

.

Benefiting from the self-propelled jumping, the falling droplet at the surface subcooling of 2 K exhibits almost constant accelerated motion at af = 9.6 m·s-2 without a contact with the condensing surface, which is similar to the free fall motion. As the surface subcooling increases to 28 K, the falling acceleration of droplet is reduced to af = 1.6 m·s-2 due to the increased surface adhesion caused by the wetting transition. Here, we should emphasize that the falling acceleration af can be expressed as maf = mg - Fhys, where m and Fhys are the droplet mass and hysteresis force due to the surface adhesion, respectively. The ratio of hysteresis force and


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gravity of the falling droplet, ηhys = Fhys/mg, is used to describe the falling resistance caused by surface adhesion. For the jumping droplets at small surface subcooling, the falling acceleration af is close to the gravity acceleration. However, for the droplet departure by sweeping on the surface, the hysteresis force and falling acceleration are dependent of surface subcooling, owing to the variation of droplet wetting state as surface subcooling increases. As such, the wetting transition induced falling resistance ηhys can be easily deduced from droplet departure behaviors. Figure 6c shows the droplet falling acceleration af and resistance ηhys as a function of surface subcooling ∆T. The falling acceleration decreases with surface subcooling while the falling resistance shows an opposite trend. In the range of small surface subcooling (∆T < 12 K), the large falling acceleration (> 9 m·s-2) is due to the jumping behavior, which is almost independent of surface subcooling. Once the sweeping mode of droplet departure occurs with the increase of surface subcooling (∆T > 12 K), the hysteresis force remarkably increases with surface subcooling and reaches a common plateau, a value more than ten times larger than that at small ∆T (∆T < 12 K). These experimental results underscore the importance of the surface subcooling for droplet wetting state and droplet departure on the hierarchical nanowired superhydrophobic surfaces. By coupling the spatial nucleation control on closely spaced nanowires and the spontaneous outward movement of droplets in the micro-valleys, we demonstrated a significantly enhanced condensation heat transfer on a hierarchical nanowired surface compared to that on the plain hydrophobic surface. Nevertheless, as the surface subcooling increases up to 14 K, the droplet departure mode changes from self-propelled jumping to sweeping, which is attributed to the reduced nucleation size and increased nucleation density under large surface subcooling. Therefore, it appears that more closely spaced nanowires are required to promote stable jumping



droplet condensation. Moreover, the micro-valleys composed of the nanowire arrays with different lengths can promote spontaneous outward movement of droplets for rapid removal. Compared to the shallow micro-valleys, the deep micro-valleys are beneficial to the accumulation of surface energy for droplet dewetting transition.21 It is noted that this is based on the suspended state of droplets on the nanowires within micro-valleys. Once the droplets are immersed into the nanowires under large surface subcooling, the shallow micro-valleys could reduce the surface adhesion of pinned droplets due to the smaller solid-liquid contact area compared to the deep micro-valleys. In addition, the droplet outward movement in micro-valleys can further enhance by optimizing the shape of micropillars such as micro-cones. Future surface optimization for maximizing the condensation heat transfer enhancement such as the spacing and length of nanowires and the depth and shape of micro-valleys, requires the development of more advanced models and numerical simulations to match the complicated dynamic process, especially for the nucleation-induced wetting transition of droplets and the rapidly growing droplets at large surface subcooling. While limitations exist, the hierarchical surfaces with micropatterned nanowire arrays for enhanced condensation heat transfer have been deepening our fundamental knowledge and stetting new benchmarks for dropwise condensation heat transfer performance. CONCLUSIONS To summarize, we reported a novel hierarchical superhydrophobic surface with micropatterned nanowire arrays to manipulate droplet behaviors from the initial formation to growth and departure for enhancing condensation heat transfer. By combining the spatial control of nucleation for mobile droplets using closely spaced nanowires and the spontaneous outward movement of droplets for rapid removal using micropatterned nanowire arrays, such surface


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substantially outperformed the state-of-the-art hydrophobic and superhydrophobic surfaces developed for enhancing condensation heat transfer. By the optical visualization and heat transfer test, we demonstrated that this hierarchical nanowired superhydrophobic surface led to the higher density of micro-droplets, smaller departure radius of droplets, 133% wider range of surface subcooling and 37% enhanced critical heat flux for jumping droplet condensation, compared to dropwise condensation on the uniform nanowired superhydrophobic surface. The excellent water-repellency of such hierarchical superhydrophobic surfaces can be promising for many potential applications such as anti-icing, anti-fogging, water desalination, and phasechange heat transfer. ASSOCIATED CONTENT Supporting Information. Fabrication of Nanowired Surfaces, Experimental System for Condensation Heat Transfer Measurements, Data Reduction and Measurement Accuracy, Droplet Departure on the Plain, Uniform Nanowired, and Hierarchical Nanowired Surfaces, and Condensation on Hierarchical Superhydrophobic Surfaces with Different Micropatterned Nanowire Arrays. (PDF) Nine videos showing the conventional dropwise condensation on the plain hydrophobic surface, the jumping droplet condensation, sweeping droplet condensation, and flooding condensation on the uniform and hierarchical nanowired superhydrophobic surfaces under different surface subcooling. (AVI) AUTHOR INFORMATION Corresponding Author *

Email: [email protected]



Author Contributions R.W. and R.Y. conceived the research. R.Y. supervised the research. R.W. and S.X. designed and carried out the surface fabrication. R.W. and D.Z. carried out the condensation experiments. R.W., S.X., D.Z., and R.Y. analyzed the data. R.W., X.M., Y.C.L., and R.Y. wrote the manuscript. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

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Hierarchical Superhydrophobic Surfaces with Micropatterned

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