Durable Lubricant Impregnated Surfaces for Water Collection under

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Functional Nanostructured Materials (including low-D carbon)

Durable Lubricant Impregnated Surfaces for Water Collection under Extremely Severe Working Conditions Xueshan Jing, and Zhiguang Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08885 • Publication Date (Web): 14 Aug 2019 Downloaded from pubs.acs.org on August 15, 2019

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ACS Applied Materials & Interfaces

Durable Lubricant Impregnated Surfaces for Water Collection under Extremely Severe Working Conditions Xueshan Jing, †,‡

Zhiguang Guo,*,†,‡

†Hubei Collaborative Innovation Centre for Advanced Organic Chemical Materials and Ministry of Education Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei University, Wuhan, People’s Republic of China. ‡State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, People’s Republic of China. *Corresponding author. Tel: 0086-931-4968105; Fax: 0086-931-8277088. Email address: [email protected]

Abstract It is worth noting that the multifunctional surfaces are highly desirable for water collection applications on droplet nucleation and removal. Although the superhydrophobic surfaces (SHSs) is beneficial to water collection due to easily shed liquid drops and favorable heat-transfer performance, the pinned condensed water droplets within the rough structure and a high thermodynamic energy barrier for nucleation severely limit the water collection efficiency. Recently, the liquid infused surfaces are significant for condensation heat transfer and droplet nucleation but have poor durability. In this work, under the UV light, polydimethylsiloxane was grafting onto ZnO nanorods (through Zn-O-Si bond) and the residual unbonded silicone oil was used as lubricant, so that form a hierarchical lubricant impregnated surfaces (LISs). Due to high viscosity of silicone oil and strong intermolecular force between silicone oil and PDMS brush, the lubricant can be firmly fixed in micro-nanosrtucture to form durable lubricant layer. For example, the LISs have outstanding properties for boiling water repellency, omniphobicity of various liquid and hot water resistant. Under a self-made hot vapor collection device, the surface can maintain good water collection capacity and there is no obvious change in the lubrication layer. After exposing in sunlight for 7 days and suffering 25 times heating/cooling cycles (heating at 150℃), the LISs exhibit excellent water collection and repairability. After measurement, the oil content in the water is 43mg/L, which is harmless to the human body. Via the high-water collection efficiency and durable 1

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lubricant layer, the LISs can be applied on a large scale in the water collection industry. Keywords: Lubricant Impregnated surface, Omniphobicity, Micro-nanosrtucture, Water Collection, Durability

1. Introduction Nowadays, the availability of fresh water is becoming an increasingly important worldwide problem, especially in arid regions.1-2 It is reported that more than one billion people living in arid regions will face the problem of grim water scarcity in 2025.3 Although water is the most abundant resource in the natural environment, the freshwater makes up only 36% of the total water on the earth. Therefore, the freshwater problem is worth studying for every scholar. In addition to rainfall, there are three main forms of “non-rainfall” water that can be used in daily life, including: fog deposition, dew formation, and water vapor adsorption.4 During above three main forms, it should be noting that fog, diameter is usually up to a few 10 μL in the atmosphere, is the main form of water in arid regions.5-7 Hence, collecting fog is an important way to solve water shortage problem by employing different strategies in arid regions. From the inspiration of the Namib Desert beetle Stenocara,8 spider silk9 and cactus10 for water collection behaviors of creatures in the deserts, biomimetic fog harvesting materials and devices are researched and fabricated to collect water.11-13 Usually, water capture, water supply and water removal are three significant parts for fog harvesting, 14-17 namely that the capture of tiny droplets on the surface of the fog collector, the coalescence between the droplets in the fog episode and the water droplet (film) on the surface of collector, and the elimination of harvesting water. As the previous researches, the surface topography and wettability of biomimetic material surfaces had great influence on fog harvesting.16 For the hydrophilic-hydrophobic chemistry pattern, Zhong et al.18 fabricated hydrophobic/hydrophilic Janus cooperative copper mesh by a simple liquidus modification for efficient fog harvesting. Zhang et al.19 developed a direct method to produce superhydrophilic micropatterns on SHSs based on inkjet printing technology to collect fog water. In addition, the reasonable surface topography is beneficial to fog harvesting. Guo et al. explained how surface chemistry and topography enhance fog harvesting efficiency based on the superwetting surface with patterned hemispherical bulges.16 There are many scholars study the preparations of 2


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water collection materials and mechanism. However, under extremely harsh working conditions such as high pressure, abrasion and droplets impacting, the surface wettability and topography of the water collection material will lose. And these super wetting materials have poor condensation heat-transfer efficiency. In the industrial applications, including the built environment, power generation and water collection systems, vapor condensation is an important process.20-23 According to enhancing condensation heat transfer by surface treatment, efficient energy use can be realized.24-25 The droplets detach from the non-wetting surface after many years of research , thereby cleaning the surface and allowing the droplets to re-nucleate/re-grow so that increasing the heat transfer rate.2629

To maximize the heat transfer coefficient, meeting these three characteristics such as low contact

angle hysteresis minimizes droplet deviation from radius, low contact angle reduces droplet conduction resistance, and high nucleation density are important for the high-performance droplet condensation surface.30-31 Nowadays, although the SHSs improve the droplet mobility and reduce the departure radii, the high apparent contact angles (CA) of condensing droplets on SHSs hinder the overall heat transfer performance.32-33 Therefore, inspired by Nepenthes pitcher plant,34 the liquids infused surfaces with easy droplet removal and low contact angles are proposed to enhance heat transfer.35-36 However, the rough base layer is exposed for the thickness of the infusion liquid decreases with time, thus water sliding characteristics cannot exist for a long time. In this work, according to grafting polydimethylsiloxane onto ZnO nanorods (via bonds of ZnO-Si) under UV light and the residual non-bound silicone oil served as a lubricant, a biocompatible durable LISs was fabricated. Although there have been articles reporting PDMS-impregnated surfaces, our articles had many unique innovations and highlights in preparation methods and applications. (See the Table S1) Via high viscosity and strong intermolecular force between silicone oil and PDMS brush, the lubricant can be reserved in slippery surface firmly, further resisting boiling water and hot liquids. Because water collection is related to surface structure and chemistry, thus surfaces with hierarchical structures collect more water than a single structure (nanostructure or microstructure). Herein, the ZnO nanowires were grew on micro-pyramids by simple hydrothermal method, which had better property to collect water. Compared to original Si wafer, micro-pyramids Si wafer and SHS, the LISs had superior ability in water capture, water supply and 3



water removal, thus they could collect more water than above three surfaces. In addition, the LISs had excellent ability for hot water vapor (105 ℃ ) collection based on stable lubricant layer. Furthermore, in order to explore the adaptability of harsh environments, the LISs were exposed in sunlight for seven days and suffered many heating/cooling cycles. The LISs still exhibited excellent water collection efficiency and reusability.

2. Experimental Section 2.1 Materials Zinc acetate dehydrate (Zn(Ac)2 ⋅2H2O≥ 99.0%) (AR) was bought from Tianjin Kemiou Chemical Reagent Co., Ltd., China. Purchasing Zinc nitrate hexahydrate (ZnNO3 ⋅6H2O ≥99.0%) (AR), ethanol (AR) from Tianjin Chemical Reagent Co., Ltd, China. Hexaethylamine (HMT≥ 99.0%) (AR) was purchased from Sinopharm Chemical Reagent Co. Inc., China. Purchasing Trimethylsiloxyterminated PDMS (silicone oil) from Shanghai Macklin biochemical technology co., Ltd. Purchasing Tetrahydrofuran (THF, ≥99.0%) from Chengdu Chron Chemicals Co. Ltd China. Purchasing N-type silicon (100) wafers with resistivity of 1–10 Ω.cm from Beijing hualide technology co. LTD.

2.2 Film fabrication: The whole fabricated process was shown in Figure 1a. and divided into three parts to describe: Si Etching:37 The Si wafers with 2×2 cm2 were washed by acetone and ethanol, then dried in a vacuum oven. To create micropyramids, the cleaned Si wafers were etched in mixed solution of KOH and isopropyl alcohol at 95 °C for 30 min. KOH (3 wt%), water, and isopropyl alcohol (10 vol%) constitute the etching solution.

ZnO-Nanowire Growth:38 The ZnO-Nanowire grew on micro-pyramids by hydrothermal reaction. Firstly, the ZnO seed layer was deposited onto the micro-pyramid Si wafer by soaking method. The etched Si wafer was immersed into zinc acetate dehydrate solution (0.15 M) and then it was annealed at 300 °C for 40 min to form a smooth ZnO film. Subsequently, the as-prepared substrate was vertically suspended in an aqueous solution containing zinc nitrate (0.025 M) and 4


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hexamethylenetetramine (0.025 M) at 95°C for 12 h. Finally, the prepared sample was cleaned by ethanol and dried in a vacuum oven.

Grafting PDMS onto the ZnO-nanowire film: Then the samples covered with a uniform oil layer were placed horizontally 10 cm below the UV lamp (20w, 254nm) for 40 min. The residual silicone oil acted as lubricant to form slippery surface.

2.3 Water collection measurements In our work, the prepared samples with 2×2 cm2, containing original Si wafer, the etched Si wafer with micropyramids, SHSs and slippery surface, were fixed in a self-made testing system to evaluate the fog harvesting performance. The SHSs were fabricated by cleaning the slippery surface to remove silicone oil. This system included a commercial humidifier and a large glass container with the humidifier.16 Herein, it is important to place a glass container to avoided the influence of air flow on fog and external wind. Then, the above different samples are fixed to the bracket in front of the mist outlet and perpendicular to the horizontal plane. By reason of gravity, water droplets collected remove from the surface of different samples. The distance between samples and fog outlet was 5 cm, and the distance between the fog outlet and the collection container was 15 cm. Placing a same fog collection container as a control group was necessary, which was beneficial to eliminate the influence of external fog. The flow rate and speed of this commercial humidifier were 0.07 g s1

and about 50 cm s-1 respectively. The temperature and relative humidity around the samples were

18°C and 90%. The water collection rates (WCR) can be described as the amount of water collected per unit area and per unit time.

2.4 Hot steam harvesting In our work, a self-made testing system was fabricated to evaluate the hot steam harvesting performance of the slippery surface and original surface. By heating the water intensively, a large amount of steam is brought along the nozzle to the surface of the samples. This system included a commercial humidifier and a large glass container with the humidifier. Herein, it is important to place a glass container to avoided the influence of air flow on fog and external wind, and the LISs 5



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and original sample are fixed to the bracket in front of the mist outlet and perpendicular to the horizontal plane. Then, by reason of gravity, water droplets collected remove from the surface of different samples. The distance between samples and fog outlet was 1.5 cm. Furthermore, the distance between the fog outlet and the collection container was 10 cm. The flow rate and speed of this commercial humidifier were 0.013 g s-1 and about 10 cm s-1 respectively. The temperature and relative humidity around the samples were 18°C and 90%.

2.5 Stable test The stability tests of the prepared slippery surface were divided into the following three sections: (i) The original Si wafer, the etched Si wafer with micropyramids, SHSs and slippery surface were rotated at different spin speeds from 1000 to 7000 for 60s to explore the water collection performance,

the

quality

change

(∆M),

the

contact

angle

and

sliding

angle.

∆M = the quality after centrifugation ― original quility (ii) The slippery surface was placed outside and horizontal with ground for 7 days at April 2019 in Lanzhou, China. (iii) To investigate the repellency of heating/cooling treatment, the samples were placed on an oven at about 150°C for 10 min, and then were immersed in deionized water at room temperature for 1min to cool. The contact angles, sliding angles, and sliding speeds of a 10 μL droplet on the samples were measured. The sliding speed was tested on 5° tilt samples. Herein, as the quality of the lubricant layer changes, the thickness H ( μ m) can be calculated according to the following formula: H=

(𝑚1 ― 𝑚0) 𝜌𝑠

× 104

-----------------------------------(1)

Herein, the m1 is the weight of silicone coated substrate at different spin rate; m0 is the weight of original substrate; 𝜌 is the density of silicone oil (ρ = 0.971 g/mL, 25 °C); s is the surface area of substrates that the equal is 4.14 cm2.

2.6 Characterization The microstructure and morphology of the samples were observed via field-emission scanning electron microscopy (FESEM, JSM-6701F). Using a JC2000D system to measure static water contact angle and sliding process of the specimens with a 5 μL droplet. Using X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi) to characterized their quantitative 6


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elemental composition. The crystal structure of the samples was characterized via X-ray diffraction (XRD) using an X’PERT PRO diffractometer with Cu Kα radiation of 1.5418 Å wavelength in the 2θ range of 5° to 90°. The droplet growth was recorded by a profession digital camera (SONY DSCHX200) and an industrial camera (DH-HV1351UM).

Figure 1. (a) Schematic of the LISs preparation. The ZnO nanowires grew on micro-pyramids by hydrothermal method and the PDMS brush grafted on ZnO nanowires by light-catalyzed reaction. (b) Etching process in KOH solutions. (c)The reaction of grafting in ZnO nanowires with PDMS. 7



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3 Results and Discussion 3.1 Preparation of lubricant impregnated surfaces with micro-nanostructure To prepare the LISs with micro-nanostructure, we adopted a simple hydrothermal method and photocatalytic reaction. The whole prepared process shown in Figure 1a. The cleaned Si wafers were immersed into mixed solution of KOH and isopropyl alcohol at 95 °C for 30 min to form micro-pyramids. That were formed by dissolving the Si (100) and (110) surfaces while reserving the (111) surface, shown in Figure 2b c.

37

The height of micro-pyramids was around 3.414 μm

(Figure 2c) and involving reaction shown in Figure 1b. In order to grow ZnO-nanowire on micropyramids uniformly, the hydrothermal reaction was induced. At the beginning, a thin ZnO seeding layer was deposited on micro-pyramids by immersing into zinc acetate dehydrate solution and then annealing at high temperature for 40min, which was important for the preparation of ZnO nanowires. Then the as-prepared substrate was immersed into mixed solution for 12h at 95℃.

38

Finally, the

hybrid ZnO–Si hierarchical nano/microsurface were prepared and displayed in Figure 2 (d-f). In addition, the ZnO nanowire was hierarchical, highly ordered and its height reached to 1.3μm. The presence of wurtzite ZnO was identified by analysis of the X-ray diffraction (XRD) pattern of the nanorod array, shown in Figure 2 g. All ZnO diffraction peaks were clearly seen and matched very well with those of wurtzite ZnO, which showed lots of purity ZnO nanorods grew from micropyramids. 38 In order to graft PDMS onto ZnO nanowires, the ZnO–Si hierarchical nano/micro surface covered silicone oil were placed horizontally 15 cm below a UV lamp for 40 min.39 Since the ZnO photocatalysts created reactive free radicals by generating electron–hole pairs under light irradiation, which caused oxidation or decomposition of most organic molecules and leaded to several secondary reactions.40-41 Therefore, the activated molecules partially cleaved the siloxane bonds of the surrounding silicone oil. These segmented siloxane-based chains formed a covalent bond with the ZnO photocatalysts based on a Zn–O–Si bond. And the residual silicone oil acted as lubricant to form the biocompatible slippery surface. Furthermore, to verify that polydimethylsiloxane was grafted on the ZnO nanorods successfully, X-ray photoelectron spectroscopy (XPS) was performed to explore the LISs that washed by THF. As shown in Figure 2 h, the Zn−O−Si bond (532.5),39 8


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causing by cross-linking of the silicone oil on the ZnO–Si hierarchical nano/micro surface, was confirmed. In addition, the contact angle on various surfaces (original Si wafer, the micro-pyramids surface, SHS and LISs) shown different states. Displayed in Figure S1, the original Si wafer exhibited hydrophobicity with 75° contact angle. Etched by KOH solution, the micro-pyramids surface had better hydrophobicity. Yet, the hybrid ZnO–Si hierarchical nano/microsurface changed into hydrophilicity that contact angle is 29°. After infusing silicone oil and irradiating under UV light, the LISs became hydrophobicity again. Herein, the SHSs were formed by cleaning the LISs with THF to expose the PDMS molecular brush to the surface. Since the PDMS grafted on ZnO surface had smaller surface tension (less than 25 mN m−1). The cleaned LISs exhibited superhydrophobicity that confirmed the silicone oil grafted on ZnO surface successfully after UV irradiation. If not, the LISs was still hydrophilic after washed by THF. Therefore, the slippery surface was fabricated on hydrophilic micro-nanosurface successfully.

9



Figure 2. (a) The SEM of original Si wafer. (b-c) The SEM of micro-pyramids on Si wafer after immersing in KOH solution. (d-f) The SEM images of ZnO nanowires grew on micro-pyramids. (g) XRD pattern of ZnO nanowires grew on micro-pyramids. (h) XPS analysis of O1s of the silicone oil-grafted ZnO nanowires.

3.2 Sliding ability for various liquids, superior omniphobicity at room-temperature, hot water repellency After growing ZnO nanowire on the micro-pyramids surface, the micro-nanostructure surface was prepared to LISs. To explore the sliding ability, the liquids with various surface tension were dropped on LISs. Herein, the uniform lubricating layer on LISs were formed by rotating under 3000 rpm and the thickness of lubricant layer was about 15 μm. The sliding angle of water and oleic acid were about 1.7° and 2.2° respectively, and the contact angle were 98° and 30°. In addition, the LISs exhibited superior omniphobicity for various liquid. The 10 µL drops of water and oleic acid 10


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can slide at speed of 0.23mm/s and 0.17m/s on 5° tilt surface. The sliding process of 10 µL water and oleic acid were shown as Movie S1 and S2 respectively. Furthermore, the speed of water droplets increased with the increase of water volume, demonstrated in Figure S2. When the volume of water is greater than 25 μL, the increase in sliding speed is not obvious. Because as the volume increases, the resistance to the droplets also increases. To explore the superior omniphobicity for various liquid at room-temperature, high contact angles (static, advancing, and receding contact angles) and a low sliding angle were used to characterize the properties of liquid repellency, shown in the following Table 1. The advancing and receding angles of water were 100° and 98.5° respectively, and the advancing and receding angles of hexadecane were 7.9° and 6.3° respectively. That proved the LISs had superior omniphobicity for low surface tension less than 30 mN m-1, such as hexadecane. In addition, due to the flexibility of the siloxane bond (Zn−O−Si), the LISs possessed low sliding angle (less than 5°) for the liquid with high surface tension and low surface tension. Therefore, the LISs possessed superior omniphobicity that various liquids with low surface tension can slide from them at smaller sliding angle. Table 1. Liquids repellency of the PDMS-grafted LISs. Liquid

γ [mN m−1 ]

Advancing

Receding

CA [°]

CA [°]

CA[°]

CA [°]

Sliding angle

Hysteresis [°]

[10 µL] Water

72.8

100

98.5

1.5

99.5

1.7

Glycol

48.4

74.6

73.2

1.4

73.7

1.57

Oleic acid

32

31.4

29.5

1.9

30.4

2.3

O-xylene

30

8.3

7.4

0.9

7.9

1.2

Hexadecane

27.5

7.9

6.3

1.4

7

1.74

11



Figure 3. (a) The dagram of sample immersed in boiling water. (b) The contact angle and sliding angle of water on original LISs, the LISs immersed in boiling water and re-oiled LISs. (c) The sliding speed of water and oleic acid on 5° tilt original LISs, the LISs immersed in boiling water and re-oiled LISs. (d) Contact angle and sliding speed of water (10 μL) under different temperature gradients. The LISs exhibited outstanding abilities for resisting boiling water and hot water. Since the PDMS brush was attached to the zinc oxide in the form of Zn-O-Si. Therefore, the silicone oil can reserve in slippery surface by strong intermolecular forces to form stable lubricating layer. As shown in Figure 3 a b, as the slippery sample was immersed into boiling water for 20 min, the contact angle and slide speed of water had no obvious change. In addition, the sliding speed of 10 µL water and oleic acid on 5° tilt surface decreased from 0.23mm/s to 0.15mm/s and decreased from 0.16mm/s to 0.11mm/s respectively. (Figure 3c) Because, the bubbles reserved in the lubricating layer blocked the droplets from moving. Whereas, as the LISs was re-oiled and centrifuged at 3000 rpm, a new uniform lubricating layer was formed and sliding speed of water and oleic acid restored original value, which shown in Movie S3. Herein, the sliding speed of water was faster than oleic acid. For 12


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the oleic acid droplet had a larger contact area with the slippery surface so that the resistance of oleic acid was larger than water. As for the sliding property of droplets with different temperature gradients on the LISS, the slippery surface kept ability for hot water repellency. As shown in Figure 3d, the contact angle had no obvious change that kept around 96°. However, the sliding speed of 10 µL water decreased as the water temperature increased and changed less than 30%. The sliding process of 10 µL 80℃ water was displayed in Movie S4. Because the condensed vapor of hot water on the lubricant layer hindered the movement of the water. Hence, the LISs had outstanding ability for resisting hot water, boiling water and kept superior omniphobicity at room-temperature.

3.3 The lubricant impregnated surfaces for water collection

Figure 4. (a) The schematic illustration of the fog harvesting system. (b) Water weight (WW) of the water container for 4 h on the water-collecting tests of all samples. (c) Water harvested of the water container for 30min on the water-collecting tests of all samples. (d) The fog capture of LISs in 10 s and 15 s (scale bar: 200 mm). (e) Images of the harvesting progress of the LISs in 5 min, 10 min and 15 min. (f) The fog capture of SHS in 10 s and 15 s (scale bar: 200 mm). (g) Images of the harvesting progress of the LISs in 5 min, 10 min and 15 min. (Herein, the water collected slippery samples were centrifuged at 3000 rpm.) As a form of water source, fog is a normal meteorological phenomenon and is of great 13



significance to arid areas. Hence, fog harvesting is a potential way to obtain freshwater in those arid areas where the fog is richer in water than the rain. In this work, the super durable slippery surface with superior omniphobicity at room-temperature was fabricated to investigate the ability of water collection at a home-made water harvesting device, where the samples were in the orthogonal direction of the generated fog. (shown in Figure 4a) Meanwhile, original Si wafer, micro-pyramids Si wafer, SHSs and LISs were placed in the same water harvesting device to explore the efficiency of water collection. Herein, the SHS was formed by removing the silicon oil on the LISs with THF. Since the grafted PDMS brush possessed low surface energy. Therefore, it was easy to prepare SHSs. Usually, fog harvesting can be divided into three parts: the water capture, the water supply and water removal. The whole fog harvesting in our work can be discussed and analyzed from these points. The fog capture processes of them (original Si wafer, micro-pyramids Si wafer, SHS and LISs) with initial 10 and 20 s were shown in Figure 4 d, f and Figure S3. At a certain time, the LISs harvested few dozen times more fog droplets than other three samples. While the SHSs showed a delayed capture process, and a film of water formed on micro-pyramids Si wafer surface and the smooth Si wafer. Because the nucleation energy barrier is lower for impregnated surfaces that consistent with the trends reported on bulk liquids under the identical conditions.42 Hence, condensation occurred first on the silicone oil impregnated surface and was delayed on the unimpregnated surface. As for water supply, there were many water droplets covered on SHSs and LISs. While the micropyramids surface and original Si wafer were only part covered by few water droplets and formed a water film that severely limited heat transfer,43 shown in Figure S4. The Microdroplets in the fog were captured by hydrophobic points or flat areas and grew at different rates. However, the growing rate of water droplets on rough surfaces (micro-pyramids Si wafer, SHSs and LISs) was faster than flat surface. For the bulges can capture more droplets than the flat at same time and accelerate the growth rate of droplets on them.

16

Furthermore, the growing rate of water droplets on LISs were

faster than SHSs, displayed in Figure 4 d f.16,

35

Since hydrophobic surface chemistry of SHSs

increased the nucleation thermodynamic energy barrier, which reduced the nuclear density and limited the heat transfer coefficient.44 And the condensed droplets on the SHSs exhibited the high apparent contact angle that leaded to an increase in the droplet conduction resistance, hindering the 14


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overall heat transfer performance. 44 Hence, the LISs possessed faster droplets grew rate than SHSs, shown in Figure 4 d f. Increasing the water supply of the droplets helped to capture more droplets by the coalescences between it and the growth droplets. It is worth noting that the water removal is important for fog harvesting. Due to special chemical and structure gradient that generated driving force, the water droplets can easily remove from surface. And under the action of Laplace pressure and surface energy, efficient water supply continuously produces slender water droplets.45 Then the droplets moved downward with different speed at driving by gravity. As shown in Figure S4, the water removed from surfaces with liquid film, which severely limited the water removed and fog harvesting. On the SHS and LISs, the water droplets existed in spherical and hemispherical forms. The growing droplet coalesced with its neighbor and then grew until its departure. Therefore, as shown in Figure 4 c, the LISs and SHSs (1.1261 g/30min and 0.9168 g/30min) collected more water than original Si wafer, micro-pyramids Si wafer and (0.3741 g/30min, 0.7986 g/30min respectively). In addition, the surfaces with micronanostructure (SHS and LISs) could collect more water than micro-pyramids surface. Because the two-layer surface roughness (see Figure 2 d-f.) can enhance the removal effect of the condensation droplets. 44 Herein, the water droplets were easier to remove on LISs than on SHSs. Although the SHSs exhibited excellent water repellency due to reserved air layer, the liquids will be in full contact with the surface textures and become highly pinned under high-humidity conditions.46 During the process of fog harvesting, the small water vapor adhered onto the surface and entered the Wenzel state to became pinned, resulting from the high Laplace pressure arising from the nanoscale spacing of the nanotextures. 46 That severely limited the movement of water on SHSs. The rate of water droplets on the SHSs were negligible until the droplets were large enough to move by gravity. Shown in Figure 4 e g., the water droplets were still fixed on the surface and the water droplets removed from LISs at 5 min. In addition, the gas layer reserved in SHSs highly resisted the heat transfer. While infusing silicon oil to replace air layer, the LISs favored droplet growth and movement due to the higher thermal conductivity of lubricant than gas and negligible CA hysteresis.35, 47 Compared to droplets on the SHSs mostly “nudge” intermittently, the droplets on the LISs displayed gross continual motion.35 Maintaining sustained high-speed motion on LISs was very beneficial because 15



it triggered a comprehensive impact on the condensing surface, which paved the way for new nucleation droplets. The condensation heat transfer is a strong function of droplet mobility,48 which was significant for fog collection. Thereby, the silicone oil impregnated surface with superior heat transfer performance had better ability to collect fog. Displayed in Figure 4b, the weight of collected water presented an approximately linear trend during 4 h, which indicated that the WCR of four kinds of surface was approximately constant. After measurement, the oil content in the water was 43mg/L (COD is 43 mg/L), which was harmless to the human body. And silicone oils were registered on the FDA’s approved list and can be added to foods as deforming agents.49 Therefore, the collected water of LISs can be drunk.

Figure 5. (a)The schematic illustration of the change in lubricant layer at various spin rate. (b) Contact angle and sliding angle of water and oleic acid after placed on a spin-coater for 60 s at various speeds. Water contact angle (WCA), Waster sliding angle (WSA), oil contact angle (OCA), oil sliding angle (OSA). (c) WCR of LISs that placed on a spin-coater for 60 s at various speeds. The thickness of lubricant layer had important effect on water collection. A uniform layer of lubricant was formed on the surface of sample after high-speed centrifugation. In this work, the thickness of lubricant layer was controlled by adjusting spin rate from 1000 rpm to 7000 rpm. As shown in Figure 5b, the quality of lubricant decreased with increases the spin rate, indicating that the thickness of lubricant layer was thinning that changed from 57.5μm to 8.4μm. However, the WCA, WSA, OCA and OSA had no obviously changed. Because the hierarchical surfaces possessed 16


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superior ability to lock lubricant when suffering high shear stress.50 As displayed in Figure 5c, the WCR of LISs continues to increase during 1000rpm to 4000rpm (the thickness of lubricant is thinning) and reached to 0.643g h-1 cm-2 at 4000rpm. Because a thin lubricant layer is beneficial to facilitate the droplet coalescence. A thicker layer of lubricating oil tends to form droplets with significant wrapping layers, so that forming non-agglomerated droplets.51 However, when the spin rate increased from 5000rpm to 7000rpm, the WCR first decreased and then remained the constant. Since the lubricant layer thinned further, the droplets mobility got slow and nucleation efficiency became worse. As displayed in Figure 5a, the thickness of lubricant layer decreased with the increase of spin rate. Therefore, the reasonable thickness of lubricant layer had important effect on water collection.

3.4 Durable slippery surface for hot vapor collection, resisting heating/cooling cycles and enduring long-time irradiation Due to high water sliding ability and favorable heat-transfer performance, the liquid-infused surfaces have been developed and researched for many years. However, after long-term use, the condensed water droplets will eventually be fixed, reducing their heat transfer performance, because the rough underlayer is exposed as the injected liquid is lost. Especially suffering high temperature and hot water, the lubricant was easily to lost to reduce heat transfer performance, which was not conducive to condensation of water droplets. In this work, the durable LISs were prepared to prevent lubricant layer losing and improve condensation efficiency. So that suffering the multiple heating/cooling cycles, long-time irradiation and hot vapor, the LISs still remain excellent water collection. Because the lubricant layer can be fixed on micro-nanostructure by strong intermolecular forces between silicone oil molecules and grafted PDMS brush. And high viscosity of silicone oil played an important role in fixing lubricant layer. At a water vapor of 105 ℃, the water collection capacity of slippery was explored. Small hot water condensed, grew and removed on slippery surfaces, so that hot vapor can be collected successfully. After measurement, the WCR of LISs was 0.24 g h-1 cm-2, while the WCR of original Si wafer only was 0.085 g h-1 cm-2. Therefore, the LISs had outstanding water collection ability. After heating with hot vapor for many times, the water contact angle on LISs still was 98° that was 17



as big as the original value. Although suffering high temperature vapor, the lubricant of LISs evaporated was only reduced by 0.0011 g in 30 min that the thickness of lubricant layer reduced by 2.7 μm. The calculation of lubricant layer thickness refers to formula 1. Hence, the LISs had better ability to resist hot water and high temperature as well as had outstanding hot water vapor collection efficiency.

Figure 6. The experiment of resisting heating/cooling. (a) The variation of contact angle and sliding angle with heating/cooling cycles increase. The downward green arrow indicated the change in the sliding angle and contact angle after re-oiling. (b) The variation of water collection rates with heating/cooling cycles increased. The optical photo of condensation after heating/cooling for 25 times. (Herein, the water collected samples were centrifuged at 3000 rpm.) Although water-collecting materials have been developed for many years, their geometric topography and surface chemistry can easily be destroyed in harsh environments, losing their fog harvesting ability. Therefore, it is expected to fabricate durable water-collecting materials to endure 18


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various damages. Due to the needs of the working environment such as desert, water collection materials need to constantly face changes in high temperature and low temperature. To explore the ability of resisting cycles of heating and cooling, the LISs was first placed into 150 ℃ for 10 min and placed in water (20 ℃) immediately. As shown in Figure 6a, with the heating/cooling cycles increased, the water contact angle had tendency to increase since the lubricant layer was thinning. Water sliding angle changed obviously and increased to 8.7°. Because the lubricant layer became thinner with multiple heating, and small droplets covered on the lubricant layer hindered the movement during the cooling process. However, after the slippery surface was reoiled, the sliding angle returned to its original value, seeing downward arrow displayed in Figure 6a. As displayed in Figure 6b, the WCR of slippery surface had no obviously change and reduced less than 10% compared to original LISs at 25 heating/cooling cycles. Seeing optical photograph in Figure 6b, the fog was easily to condense in LISs. Therefore, although heated and cooled for many times, the silicone oil could be stored in grafted PDMS brush firmly by strong intermolecular forces, which was meaning that a uniform lubricant layer still existed in slippery surface. The increase in the SA did not affect the condensate droplets removed from the vertical surface. It was obvious that the LISs had superior ability to resist multiple heating and cooling.

19



Figure 7. An experiment of resisting sunlight for 7 days at April 2019 in Lanzhou, China. (a) Schematic illustration of irradiating by sunlight and re-oiling. (b) The variation of contact angle and sliding angle with times (days). The downward green arrow indicated the change in the sliding angle and contact angle after re-oiling. (c) The variation of WCR and ∆M with times. The downward green arrow indicated the change in the sliding angle and contact angle after re-oiling. (Herein, the water collected samples were centrifuged at 3000 rpm.) In addition, in views of the working environment of water collection containing sunshine and wind that will speed up the evaporation of the lubricant, the slippery surfaces must have good stability. (see Figure 7a.) The LISs fabricated in this work maintained high water collection 20


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efficiency and could be reused many times after refilling when placed in outdoor. As shown in Figure 7b, relative to WCA that did not change appreciably, the SA of water increased a lot with times increasing. On the seventh day, the SA increased to 12.3 ° and WCA changed into 97.5 °. Because the lubricant was continuously decreased as the placement time increased, as displayed in Figure 7c. The 10 µL water slid from the 15° tilt surface that was shown in Movie S5. On the third day, the lubricant was reduced by 0.0039 g (the thickness of lubricant layer reduced by 9.89 μm), and the amount of lubricant was reduced slowly in the following four days. At the seventh day, the thickness of lubricant layer was still 8.7μm. And the WCR of LISs decreased from 0.544 g h-1 cm2(original

value) to 0.3798 g h-1 cm-2(on seventh day). As the resistance increases, the removal speed

of the droplets from the slippery surface became slower, resulting in a slower nucleation rate of the fog. However, the WCA and SA of water on LISs recovered to original value after refilling the oil and centrifuged, as shown by the green arrow in Figure 7b. And the WCR increased to 0.538 g h-1 cm-2 that was close to the initial value. By reason of the stability of lubricant layer and reusability, the LISs could be used for hot vapor collection and resisted heating/cooling cycles as well as resisted sun exposure, which was of great significance for the use of LISs for water collection.

4. Conclusion LISs have been reported to be a promising approach to improve liquid omniphobicity and sliding ability. In addition, LISs enhanced condensation heat transfer surfaces and lower nucleation energy barrier, leading to high water collection efficiency. However, the traditional LISs had problems in lubricant storage, hot water repellency and resistant of high temperature. In this work, a durable LISs were produced by grafting PDMS onto ZnO nanorods (metal oxide photocatalysts) under UV light and residual unbound silicone oil as a lubricant. Because of the strong intermolecular force and high viscosity of silicone oil, the lubricant can be firmly fixed on the slippery surface with low sliding angle (less than 2°). Immersed in hot water for 20 min, the LISs still exhibited omniphobicity for water and oleic acid. Although the sliding speed of water and oleic acid decreased by 30% approximately, the sliding speed recovered to original value after refilling silicone oil. Based on the comparison of four surfaces (original Si wafer, micro-pyramids Si wafer, SHS and LISs), the LISs 21



with micro-nanostructures were superior to the other three surfaces in water capture, water supply and water removal. Due to pinned condensed water droplets within the rough structure and a high thermodynamic energy barrier for nucleation, SHSs had poor ability to collect water. While, the LISs possessed high heat-transfer performance and lower nucleation energy, which was beneficial to enhance fog collection efficiency. Furthermore, under a self-made hot vapor collection device, the surface could maintain good water collection capacity and there is no obvious change in the lubrication layer. After prolonged exposure to sunlight for 7 days and 25 times heating/cooling cycles, the LISs exhibited excellent water collection efficiency and repairability, resulting from the durable lubricant layer formed by strong intermolecular force between silicone oil and PDMS brush connected by covalent bonds. Based on the simple green reaction, the low cost and environmentfriendly chemicals involved, the durable LISS is expected to be used in large-scale water collection and production.

Supporting Information The innovations of this paper, additional performance tests and optical photographs of water collection, and five movies of droplet sliding under different conditions Movie 1 Movie 2 Movie 3 Movie 4 Movie 5

Conflicts of interest There are no conflicts to declare.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (no. 51675513 and 51735013).

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By grafting PDMS onto ZnO nanowires (via bonds of Zn-O-Si) under UV light irradiation and remaining nonbound silicone oil as a lubricant, the lubricant impregnated surfaces with superior sliding ability were prepared to resist boiling water, collect hot vapor and remain high collection efficiency over longtime irradiation and multiple heating/cooling cycles. 178x121mm (96 x 96 DPI)


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Durable Lubricant Impregnated Surfaces for Water Collection under

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