Rationally Designed Nanostructure Features on Superhydrophobic

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Rationally design nanostructure features on superhydrophobic surfaces for enhancing self-propelling dynamics of condensed droplets Yizhou Shen, Yuehan Xie, Jie Tao, Haifeng Chen, Chunling Zhu, Mingming Jin, and Yang Lu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05780 • Publication Date (Web): 31 Dec 2018 Downloaded from http://pubs.acs.org on January 4, 2019

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ACS Sustainable Chemistry & Engineering

Rationally design nanostructure features on superhydrophobic surfaces for enhancing self-propelling dynamics of condensed droplets

Yizhou Shen†,*, Yuehan Xie†, Jie Tao†,‖,* , Haifeng Chen‡, Chunling Zhu§, Mingming Jin†, Yang Lu†

†

College of Materials Science and Technology, Nanjing University of Aeronautics and

Astronautics, Nanjing 210016, P. R. China ‡

Department of Materials Chemistry, Qiuzhen School, Huzhou University, 759, East

2nd Road, Huzhou 313000, P. R. China §

College of Aerospace Engineering, Nanjing University of Aeronautics and

Astronautics, Nanjing 210016, P. R. China ‖

Jiangsu Collaborative Innovation Center for Advanced Inorganic Function

Composites, Nanjing 210009, P. R. China

Yizhou Shen and Yuehan Xie contribute equally to this work. * Professor Jie Tao, E-mail: [email protected]. A/Professor Yizhou Shen, E-mail: [email protected]. KEYWORDS: superhydrophobic; designed nanostructures; water adhesion force; self-propelling ability

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ABSTRACT: Self-propelling ability towards achieving more efficient dropwise condensation intensively appeals to researchers due to its academic significance to explain some basic wetting phenomena. Herein we designed and fabricated the two types of microstructure superhydrophobic surfaces, i.e., sealed layered nanoporous structures (SLP-surface) and open nanocone structures (OC-surface). As a consequence, the resultant surfaces exhibit the robust water repellency, and the water droplet nearly suspends on the superhydrophobic surfaces (CA=158.8°±0.5°, SA=4°±0.5° for SLPsurface and CA=160.2°±0.4°, SA=1°±0.5° for OC-surface, respectively). Meanwhile, the impacting droplets can be rapidly rebounded off with shorter contact time of 11.2 ms and 10.4 ms (impact velocity V0 = 1 m/s). The excellent static-dynamic superhydrophobicity is mainly attributed to the air pockets captured by the both microscopic rough structures. Regarding the self-propelling ability of condensed droplets, it is found that the droplet microscopic pining effect of SLP-surface severely weakens dynamic self-propelling ability of condensed droplets. The capillary adhesive force induced by the sealed layered nanoporous structures is up to 16.0 μN. However, the open nanocone structures cause lower water adhesive force (~4.1 μN) under the action of flowing air pockets, producing higher dynamic self-propelling ability of condensed droplets. As a consequence, the open nanocone structure superhydrophobic surface displays a huge potential of inhibiting attachment of condensed droplets.

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INTRODUCTION

The formation and accumulation of dews are inevitable in the cold and humid environments, and the condensation is preferentially on the solid surface rather than in the bulk due to the reduced energy barrier of nucleation.1 Dropwise condensation on non-wetting substrates has attracted more attentions since it was firstly recognized by Schmidt et al. in the early 1930s, which is considered to play a huge role in phasechange heat transfer field.2 Many literatures have reported that the dropwise condensation has a 5 to 10-fold enhanced heat transfer efficiency comparing with that on wetting (hydrophilic) solid surface.3-5 Self-propelling dynamics always takes place during dropwise condensation on some superhydrophobic surfaces,6 which is caused by releasing the excess surface energy in the coalescing process of microdrops.7,8 The coalescing process on the nanostructure surface will reduce the solid-liquid contact fraction and breaks the symmetry of the coalesced droplets, leading to removing the large coalesced droplets.9,10 The efficient dropwise condensation is of great significance to many practical applications including anti-frosting/icing, water desalination and treatment, condensation water collection, and condensation heat transfer.11-15 Comparing with the traditional gravity-induced shedding of dropwise condensate droplets, the coalescence-induced propelling on the superhydrophobic surfaces can reduce the size of departure condensate droplets from millimeter scale down to micrometer scale and greatly increases the renewal frequency of condensate droplets, leading to the entire efficiency increase of heat transfer.16-17 The dropwise condensation on such function superhydrophobic surfaces can be 3



clearly observed and divided into three stages: condensate droplet initial nucleation and growth, immobile coalescence of droplets (no propelling), and mobile coalescence to bounce off surfaces (condensed microdroplets self-propelling).5,18 Previous researches have shown that the self-propelling ability is related to the Cassie-Baxter wetting state of condensate droplets on superhydrophobic surfaces, which is determined by a synergistic action of microscopic rough morphology and surface free energy.19-20 However, the Cassie-Baxter wetting state at room temperature cannot always be maintained during the condensation period. The microdroplets that are comparable in size to the surface features may nucleate and grow internally rather than growing on the textures, and the Wenzel wetting state replaces the Cassie-Baxter wetting state, destroying the superhydrophobicity.21-24 The droplets pinned in this way remains stuck as dew accumulates, even affecting roll-off of large drops because of the liquid bridges provided by the wet patches between rough stustures.25,26 Therefore, study of the dynamic evolution process of condensate micro-droplets on microscopic structure superhydrophobic surfaces is crucial for developing a new-type multifunctional material surface applied to dropwise condensation. Herein, we reported two kinds of sealed layered nanoporous (SLP) and open nanocone (OC) microscopic rough structures, which were designed for experimental comparison. The excellent non-wettability was systematically discussed and confirmed by static apparent contact angles, dynamic sliding angles, and bouncing behavior. Prior to evaluating the dynamic self-propelling ability, we measured the water adhesion force and made theoretically calculation and deduction of the energy condition. Subsequently, 4


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we carried out the condensation experiments, and focused on the dynamic evolution process of condensate micro-droplets on both as-constructed microstructure superhydrophobic surfaces.

EXPERIMENTAL SECTION

Materials Aluminium substrates (99.9%) were cut into square pieces with the size of 20 mm × 20 mm × 1 mm (obtained from the Beijing Nonferrous Metal Research Institute, Beijing, China). Ultrapure deionized water was obtained via Milli-Q system in our laboratory. All the chemicals used in this experiment, such as zinc acetate (Zn(CH3COO)26H2O),

ammonium

hydroxide

(NH3H2O),

zinc

nitrate

(Zn(NO3)26H2O), hexamethylenetetraamine (C6H12N4), acetone and ethanol, were analytical grade and provided by Sinopharm Chemical Reagent Co., Ltd, China. In addition, as the modifying agent, fluoroalkylsilane (FAS-17, purchased from Tokyo Chemical Industry Co., Ltd, Japan) was used to modify the hierarchically structured surfaces. Preparation of structure-designed superhydrophobic surfaces The superhydrophobic surface with designed nanostructure features were prepared via a very simple and economic method. The entire process only consists of the following three main steps:

5



Sample pretreatment: the square sample sheets were firstly smoothed with metallographic sandpapers (No. 0-6) and polishing-cloth till scratches was disappeared. Subsequently, the samples were cleaned ultrasonically in acetone, ethanol, and deionized water for 15 min sequentially and dried under the cold-wind condition. Hydrothermal treatment: Preparation of sealed layered nanoporous structure: the pretreated samples were placed in an autoclave with 30 mL mixture solution of 0.04 mol/L Zn(NO3)2·6H2O and C6H12N4 in a volume ratio of 5:1 at 90 °C for 1h. For the preparation of open nanocone structures, the mixture solution was composed of 6 mM Zn(CH3COO)26H2O and a little amount NH3H2O (about 4~5 drops to every 50 mL solution) and the pretreated samples was reacted at 90 °C for 5h to grow a layer of nanocone structures. After the as-treated samples being cooled, we used deionized water to rinse the sample surfaces and dried them in the cold-wind condition. Fluorinated modification: Subsequently, the samples were immersed in 1 wt% FAS-17 ethanol solution for 24 h and then dried in a 120 °C oven for 2 h to obtain the final superhydrophobic samples. Characterizations and wettability test The surface morphologies of the samples were characterized by field emission scanning electron microscope (FE-SEM; Hitachi S4800, Japan). The surface roughness were obtained from a portable surface roughness meter (SJ-410, Mitutoyo, Japan). Furthermore, both apparent contact angle (APCA) and sliding angle (SA) were measured using a contact angle analyzer (Kruss DSA100, Germany). The APCA and SA reported here were the average values measured with a 4 μL reference water droplet 6


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at five different positions on each sample. In order to ensure the 4 μL water droplet successfully dripping on the surfaces, we chose the ultrafine syringe needle with an inner diameter of only 0.03 mm, which was also modified with FAS-17. Regarding the dynamic contact process of impact droplets, a high-speed camera (Phantom Mini 100, USA), filming at a 5,000 frames per second, was used to record the process of spreading, contraction and rebounding when the droplet impacted on the sample surfaces. In this measurement, the droplets were released from a fixed height of 50 mm over the samples, and the initial impact velocity was approximately 1 m/s, following the formula v= 2gh . Water adhesion force test To further characterize the moving ability of water droplets on the as-constructed microscopic rough superhydrophobic surfaces, the droplet adhesion force test was performed via a self-made experimental apparatus, which mainly includes a movable stage and a high-sensitivity microbalance detector connected with a metal circle. In this test process, a reference droplet with volume of 6 μL was suspended on the metal ring, and the sample moved up or down with the movable stage. The force change during the contact and detachment of the sample with the droplet is recorded as a curve. Self-propelling ability evaluation Regarding the dynamic self-propelling ability, we carried out the condensation experiments, and placed these samples on a horizontally cooling stage, whose temperature was controlled at 2 ± 0.5 °C through a low-temperature circulating chiller. 7



It should be noted that the laboratory temperature was 25 °C (dew point = 19 °C) and the relative humidity was ~70%. A high-speed camera (Phantom Mini 100, USA) and a HDMI HD digital microscope (UM016, China) were used to record the condensation process.

RESULTS AND DISCUSSION

Morphologies of the superhydrophobic surfaces Two kinds of sealed and open microscopic rough structures were designed to reveal the action of microscopic structure on wettability and self-propelling ability of condensed droplets. Figure 1(a-d) illustrates the morphologies of the two types of nanostructures, i.e., the layered nanoporous structure surface (SLP-surface) and the nanocone structures surface (OC-surface). It can be seen that SLP-surface is evenly covered by the well-arranged nanoporous structures with the diameter of ~1.5 μm, yet the nanocone structures (diameter = ~600 nm) form the same spacing dimension on OC-surface. In addition, it is found that a layer of secondary nanostructures (look like flocculent) cover the surface of the open nanocone structures, as shown in Figure 1(d). Afterwards, a fluorination modification was performed on the both sample surfaces, causing the superhydrophobicity. It mainly attributes to the low-free-energy groups self-assembled onto the surfaces of microscopic rough structures, and the related analyses of chemical compositions are revealed in Figure S1. It can be seen that the sample possesses high intensity peaks of the F1s and FKLL after modifying with FAS8


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17. Besides, in the high resolution spectrum of C1s (see Figure S1(b) and (d)), the peaks corresponding to –CF2 and –CF3 are observed, indicating that the fluoroalkyl groups of FAS-17 have been self-assembled onto the surfaces of the structures. Under the inspiration of lotus leaf effect, the type of hierarchical nanostructure is endowed with higher specific surface area that is in a favor of grafting the low-energy groups to

form

the

stable

synergistic

system,27,28

and

achieve

the

robust

superhydrophobicity.

Figure 1. SEM images of the as-prepared microscopic rough structures (a,b) SLPsurface and (c,d) OC-surface. Insets illustrate the wettability of condensed droplets on the corresponding sample surfaces. Non-wettability Contrary to the intrinsic hydrophilicity of bare aluminum substrate, the resultant samples with certain roughness exhibit the robust superhydrophobicity, and the water droplet can nearly suspend on the surfaces (see the insets in Figure 1). The APCAs 9



reach 158.8° ± 0.5° for SLP-surface and 160.2° ± 0.4° for OC-surface, respectively. The SAs of water droplets on the both microstructure superhydrophobic surfaces are only 4° ± 0.5° and 1° ± 0.5°, respectively. The excellent superhydrophobicity is mainly attributed to the air pockets captured by the both microscopic rough structures, and the interfacial contact mechanism also conforms to the typical wetting model, i.e., CassieBaxter wetting model.29 In order to further evaluate the non-wettability of superhydrophobic surface with the different nanostructures, we recorded the dynamic process of droplet impacting on the structure surface via a high-speed camera. For the untreated aluminum substrate, the impact droplets cannot rebound off., where a stable Wenzel wetting model is formed to result in higher kinematic viscosity.30 However , the impacting droplets can be rapidly rebounded off with shorter contact time of 11.2 ms and 10.4 ms (impact velocity V0 = 1 m/s), as shown in Figure 2 and Video S1. It can be obviously observed that the entire contact process of the water droplet impacting on the solid surface mainly consists of spreading and retracting processes. During the spreading, the kinetic energy of the droplet is converted into surface tension energy. Then, the increased surface tension, in turn, is converted into kinetic energy that forces the droplets to retract and bounce off the surface. The energy consumption of the whole process is mainly to overcome the liquid adhesion in the contact process. Furthermore, the SLP-surface and OC-surface belong to the sealed and open-type topography respectively which exhibit different abilities to handle the trapped air pockets. Regarding the wettability, the detailed characterization results 10


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are listed in Table 1. The distinction of superhydrophobic property, including static water contact angle, dynamic sliding angle, and bouncing behavior, between the both microscopic rough structures is considered to be mainly related to moving state of the air pocket which will be confirmed below.

Figure 2. Image of contact processes of water droplet on the (a) Flat Al substrate, (b) SLP-surface, and (c) OC-surface. Table 1. The detailed information of static / dynamic wettability and surface roughness. Properties

Roughness

APCA

SA

Contact time

Water adhesion

/ μm

/°

/°

/ ms

/ μN

Al Substrate

0.114

73.57 ± 1.2

> 90

--

317.3

SLP-surface

0.284

158.8 ± 0.5

4 ± 0.5

11.2

16.0

OC-surface

0.929

160.2 ± 0.4

1 ± 0.5

10.4

4.1

Water adhesion Apart from that, the water adhesion force was directly measured by a highsensitivity micro-electromechanical balance system (see Figure 3(a).) to further characterize the moving ability of water droplets on the as-constructed microscopic 11



rough superhydrophobic surfaces. In this test process, the force-distance curve can be obtained, and the test results of water adhesion forces on the SLP-surface and OCsurface are shown in Figure 3(c,d). It can be deduced that the water adhesion forces are 16.0 μN and 4.1 μN, respectively for SLP-surface and OC-surface. Furthermore, the water adhesion force on the flat Al substrate is far greater than the above-mentioned values, reaching 317.3 μN, as shown in Figure 3(b). This indicates that the superhydrophobic function modification greatly reduces the water adhesion (~two orders of magnitude). Meanwhile, the water droplets can rapidly detach from the both microscopic rough superhydrophobic surfaces, and the detachment can be reflected by peak width of the force-distance curve at the force value of zero.31 As shown in Figure 3(c,d), the peak width for OC-surface is only 0.074 mm, yet that for SLP-surface is 0.127 mm, indicating that the open nanocone structures induce a lower viscous resistance of water droplet comparing with the sealed layered nanoporous structures. This is also in line with the results of water adhesion force, as analyzed above.

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Figure 3. (a) The schematic diagram of measurement device for water adhesive force. (b~d) Water adhesion force of a reference droplet as a function of moving distance of the as-prepared surfaces. (b) Flat Al substrate; (c) SLP-surface; (d) OC-surface. The distinction of viscous resistance and water adhesion are mainly caused by the liquid-solid contact ways at the microscopic scale. Different liquid-solid contact ways will induce the unequal capillary adhesive force,32-34 shown as Figure 4(a,b), which is also confirmed to be in a line with the principle of area-contact (flat Al substrate) > line-contact (SLP-surface) > point-contact (OC-surface). Furthermore, as shown in Figure 4(c,d), comparing with the open nanocone structures, the sealed layered nanoporous structures can easily form an action of negative pressure at liquid-level to further prevent the water droplet from detaching.32 Therefore, the OC-surface possesses lower water adhesion force (4.1 μN) and shorter contact time (10.4 ms) of impact droplets, displaying higher ability to repel the dynamic water droplets.

Figure 4. Schematic diagrams of three phases contact lines on different structures (a) SLP-surface, and (b) OC-surface. (c,d) Schematic diagram of negative pressure generation on SLP-surface. 13



Dynamic self-propelling ability evaluation Prior to evaluating the dynamic self-propelling ability of condensed droplets on the both as-constructed microstructure superhydrophobic surfaces, we theoretically calculated and deduced the energy condition of occurring self-propelling. From the water adhesive force-distance curve, the work done against detaching from the superhydrophobic surfaces can be obtained through integrating the distance of the peak width in a range of force greater than zero. The work done is the interface adhesive energy Eint, which is the minimum energy value for a condensed droplets to take place self-propelling.24 The calculated results show that the Eints of SLP-surface and OCsurface are 9.60 × 10-10 J and 1.39 × 10-10 J (for a big reference droplet with volume of 6 μL), respectively. It means that the condensed droplets on the OC-surface have higher probability to take place self-propelling phenomena comparing with those on the SLPsurface. Subsequently, we carried out the condensation experiments, and placed these sample surfaces on a horizontally cooling stage, whose temperature was controlled at 2 ± 0.5 °C through a low-temperature circulating chiller. The condensation process recorded by the HD digital microscope, as shown in Figure 5. Without the assistance of an external force, the condensed droplets grow continuously on the hydrophilic flat Al substrate surface and finally form a layer of water film under the action of high water adhesion at 15 min (see Figure 5(a)). However, the completely opposite situations occur on the as-constructed microscopic rough superhydrophobic surface (see Figure 5(b,c)), and the condensed droplets can spontaneously depart from the sample surfaces (i.e., 14


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self-propelling). Comparing with SLP-surface, OC-surface exhibits higher ability to remove the condensed droplets with a large “dry” area at 30 min. An obvious phenomenon is observed that a limited number of condensed droplets are gradually changing large due to the small condensed droplets coalescing with each other, as shown in Figure 5(c).

Figure 5. Time lapse images of condensation experiments on different texture solid surfaces. (a) Flat Al substrate; (b) SLP-surface; (c) OC-surface. The images are captured over the same area. Previous researches have revealed that condensation on such function surfaces mainly undergoes two distinct stages including the condensed droplets nucleating and growing, and the following coalescence with each other to change lager.18,36 The first stage without significant interactions on the entire condensation process is mainly developed during the first 30 s. In the following stage, the condensed droplets will take place the coalescence with each other, and the coalesced big droplets partly bounce off or sweep the surface to form bigger droplets.37 For the hydrophilic flat Al substrate, the 15



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condensed droplets only occur the coalescence to form a water film rather than a bigger droplet, and the surface coverage continues to increase over time. However, after the coalescence of condensed droplets on the superhydrophobic surfaces, a bigger droplet can be formed and partly bounce off (or horizontally move) the surface under the increased kinetic energy converted from the reduction in surface energy during the coalescence process. Although the as-prepared superhydrophobic surfaces can induce the occurrence of self-propelling phenomena and show excellent anti-condensing performance, different microscopic rough structures will produce a distinction selfpropelling ability. As shown in Figure 6(a,b), it is easier for OC-surface to induce the occurrence of coalescence and self-propelling of condensed droplets comparing with SLP-surface. The reduction of temperature leads to a huge extent of deterioration of superhydrophobicity on the SLP-surface (APCA decreases from 158.8° to 113.79°), yet OC-surface still maintains superhydrophobic and the decrease in APCA is only ~3.5°. According to the Cassie-Baxter wetting model,38 the APCA (θ*) is formulated as follows: cos𝜃 ∗ = 𝑓1𝑐𝑜𝑠𝜃1 ― 𝑓2

(1)

where θ1 represent the wetting angle of water on the smooth solid surfaces. f1 and f2 are the area fraction of the water-solid and water-air interface, and f1 + f2 =1. The estimated contact area fraction between the solid surface and the beaded water droplet on SLPsurface was 0.465, whereas the value was only 0.067 for OC-surface. The distinction between the both values of contact area fraction further verify the ability to entrap air pockets

underneath

droplets.

Furthermore,

the

entrapped

16


air

pockets

on

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superhydrophobic microstructure surfaces are contracted to cause the change of wetting model from stable Cassie-Baxter model to Wenzel model. The layered nanoporous structures belonging the sealed system will induce to produce a negative pressure ∆P and compel the condensed droplet to infiltrate and pin onto the surface (see Video S2). As a consequence, the superhydrophobicity exhibits a certain extent of reduction. However, the open nanocone structures hardly create the negative pressure ∆P and continue to keep highly water-repellent, as shown in Figure 6(c).

Figure 6. Optical micrographs of condensed droplets on (a) SLP-surface and (b) OCsurface. Insets illustrate the wettability of condensed droplets on the corresponding sample surfaces. (c) Schematic diagram of the wetting mechanism of condensed droplets on SLP-surface and OC-surface. The distribution of condensed droplets on (d) SLP-surface and (e) OC-surface. (f) The entire coverage of condensed droplets on the sample surfaces. Following this, the distribution and entire coverage of condensed droplets on the sample surfaces was performed to further provide a support to the coalescence-induced 17



self-propelling on the microscopic structure superhydrophobic surfaces, as shown in Figure 6(d-f). All statistical data are obtained through a high-magnification digital camera to record the condensation process at an area of 2 mm × 2 mm. Evidently, the condensed droplets on the OC-surface mainly focus on the range of diameter less than 10 μm, and the ratio reaches ~90%. Only a small part of condensed droplets (less than 5%) belong to the range of diameter greater than 50 μm (see Figure 6(e)). However, the condensed droplets with diameter greater than 20 μm (even 50 μm) still has a higher ratio on the SLP-surface comparing with OC-surface (see Figure 6(d)). It is well known that the bigger condensed droplets are mainly derived from the coalescence of small droplets. This coalesced process produces a certain extent of kinetic energy (transferred from the decreased surface energy) to compel bigger condensed droplets to produce self-propelling movement and depart from the solid surface. Therefore, the open nanocone structures can produce higher self-propelling ability of condensed droplets comparing with the sealed layer nanoporous structures. As a result, the coalesced big condensed droplets can more rapidly depart from the OC-surface comparing with the SLP-surface,39 and the entire coverage of condensed droplets also shows the corresponding trend, as shown in Figure 6(f). The research results are considered to be applied to a broad range of water-harvesting and phase-change heat transfer applications.14 Theoretical analysis As shown in Figure 7(a,b), it can be obviously observed that the condensed droplets on the OC-surface take place the coalescence-induced self-propelling phenomenon. 18


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Three droplets are firstly coalesced and disappeared from the OC-surface, yet the coalesced droplet is viewed to bounce off the surface (see Figure 7(c) and Video S3). In terms of the underlying mechanism, the open nanocone structures can entrap firmly the air pockets underneath condensed droplets. This interfacial system can cause ultralow viscous force, and the condensed droplets can maintain well the natural movement status.40 After the coalescence of condensed droplets with each other, the coalesced droplet will rapidly change from unbalanced state to balanced state, and simultaneously produce the isotropic movement trends including bouncing off and sweeping on the surface, as shown in Figure 7(d). The bouncing movement means that the solid-liquid contact area reduces to zero, i.e., the radius 𝑟 = 0. This process can be mathematically described by d𝐸 d𝑟 = 𝐹P, where E is the solid-liquid interface energy, and 𝐹P is the propelling force of the triple-phase contact line.41-45 According to the law of energy conservation, the reduction of solid-liquid interface energy ―d𝐸 should be equal to the sum of the increased potential energy 𝑚𝑔dℎ, work done 𝐹v( ― d𝑟) against the viscous resistance of triple-phase contact line, and work done m(𝑑2h/d𝑡2)d ℎ against the inertia. This energy relation can be given by:46 𝐹P( ―d𝑟) = 𝐹v( ―d𝑟) +𝑚𝑔dℎ + m(𝑑2h/d𝑡2)dℎ

(2)

where m, g, h, and t are the mass of the droplet, acceleration of gravity, the risen height of droplet, and time, respectively. 𝐹v is the viscous force. Equation (2) can be rewritten by:

(

)

dℎ

𝐹p ― 𝐹v ―𝑚𝑔 ― d𝑟 =

𝑚d2ℎ 2

d𝑡

(― ) dℎ d𝑟

(3)

Furthermore, 𝐹p = 2𝜋𝑟𝜎(𝑐𝑜𝑠𝜃 ∗ ―𝑐𝑜𝑠𝜃′) and 𝐹v = 2𝜋𝑟𝜎𝑓(1 + 𝑐𝑜𝑠𝜃), where 𝜎 is 19



the surface tension of condensed droplets, 𝜃 ∗ and 𝜃′ are the apparent contact angle and balanced contact angle. 𝜃 is the intrinsic contact angle, and 𝑓 is the roughness factor of the open nanocone structures. Therefore, the bouncing of coalesced condensed dℎ

droplets is determined by the risen velocity Ve = d𝑡 under the condition of 𝑟 = 0. The bouncing height 𝐻 can be given by

1 dℎ 2 . 2𝑔( d𝑡 )

If the release of surface energy cannot

provide the enough energy to compel the droplet to rise and depart form the surface, i.e., 𝑟 > 0, the coalesced condensed droplets occur the sweeping phenomenon on the surface.

Figure 7. (a,b) The coalescence of condensed droplets on the OC-surface; (c) The bouncing track of condensed droplet under the action of coalescence-induced selfpropelling; (d) Schematic illustrations of coalescence-induced self-propelling including bouncing and sweeping on the OC-surface.

CONCLUSION

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In summary, we designed and fabricated the two types of microstructure superhydrophobic surfaces (sealed system of layered nanoporous structures and open system of nanocone structures), and the as-prepared superhydrophobic surfaces both display the excellent superhydrophobicity due to the trapped air pockets by microstructures. However, the water repellency cannot maintain stable under the condensate condition, especially for the sealed system of layered nanoporous structures. Regarding the self-propelling ability of condensed droplets, it is found that the droplet microscopic pining effect of SLP-surface severely weakens dynamic self-propelling ability of condensed droplets. The capillary adhesive force induced by the sealed layered nanoporous structures is up to 16.0 μN. However, the open nanocone structures cause lower water adhesive force (~4.1 μN) under the action of flowing air pockets, producing higher dynamic self-propelling ability of condensed droplets. Finally, the mathematical and physical relations are revealed in more details to deduce the energy conditions of taking place bouncing and sweeping of the coalesced condensed droplets on the open nanostructure superhydrophobic surface under the action of coalescenceinduced self-propelling.

○s Supporting Information Related analyses of chemical compositions of the both kinds of nanostructures with and without the low-free-energy modification. (PDF) Video S1 of the dynamic process of water droplets impacting on the nanostructure superhydrophobic surfaces; Video S2 of the condensing process of the microdroplets 21



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on the nanostructure superhydrophobic surfaces; Video S3 of the detaching process of the microdroplet under coalescence-induced self-propelling action. (AVI) AUTHOR INFORMATION Corresponding Author * Professor Jie Tao, Tel/Fax: +86-25-5211 2911. E-mail: [email protected]. A/Professor

Yizhou

Shen,

Tel:

+86-25-5211

2911.

E-mail:

[email protected]. Present Addresses †

College of Materials Science and Technology, Nanjing University of Aeronautics and

Astronautics, Nanjing 210016, P. R. China ‡

Department of Materials Chemistry, Qiuzhen School, Huzhou University, 759, East

2nd Road, Huzhou 313000, P. R. China §

College of Aerospace Engineering, Nanjing University of Aeronautics and

Astronautics, Nanjing 210016, P. R. China ‖

Jiangsu Collaborative Innovation Center for Advanced Inorganic Function

Composites, Nanjing 210009, P. R. China Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 51671105 and 51705244), the Natural Science Foundation of Jiangsu Province (No. 22


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BK20170790), General Project of Zhejiang Provincial Department of Education (Y201737320), the NUAA Innovation Program for Graduate Education (kfjj20170608, kfjj20180609) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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TOC Graphic: Two types of nanostructure superhydrophobic surfaces, i.e., sealed system of layered nanoporous structures and open system of nanocone structures, were designed to investigate the self-propelling dynamics of the condensed droplets for a potential application in the field of water-harvesting and phase-change heat transfer.

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Rationally Designed Nanostructure Features on Superhydrophobic

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