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Water harvesting method via a hybrid superwettable coating with superhydrophobic and superhydrophilic nanoparticles
Accepted Manuscript Full Length Article Water Harvesting Method via a Hybrid Superwettable Coating with Superhydrophobic and Superhydrophilic Nanoparticles Xikui Wang, Jia Zeng, Xinquan Yu, Caihua Liang, Youfa Zhang PII: DOI: Reference:
S0169-4332(18)32643-6 https://doi.org/10.1016/j.apsusc.2018.09.210 APSUSC 40514
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
26 June 2018 21 August 2018 25 September 2018
Please cite this article as: X. Wang, J. Zeng, X. Yu, C. Liang, Y. Zhang, Water Harvesting Method via a Hybrid Superwettable Coating with Superhydrophobic and Superhydrophilic Nanoparticles, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.09.210
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Water Harvesting Method via a Hybrid Superwettable Coating with Superhydrophobic and Superhydrophilic Nanoparticles Xikui Wanga, Jia Zenga, Xinquan Yua*, Caihua Liangb, Youfa Zhanga* a
Jiangsu Key Laboratory of Advanced Metallic Materials, School of Materials Science and Engineering, Southeast University, Nanjing 211189, P. R. China b
School of Energy and Environment, Southeast University, Nanjing 210096, P. R. China *E-mail: [email protected]; [email protected]
Abstract Water collection gains increasing attention for harvesting atmospheric water in semi-arid deserts and inland areas. A new biomimetic method, inspired by the fog harvesting ability of hydrophobic-hydrophilic surfaces of Namib desert beetles, is presented in this study. It envisages a low-cost preparation (by the photocatalysis of titanium dioxide through ultraviolet irradiation) of a titanium dioxide-silicon dioxide hybrid superwettable surface combining superhydrophobic and superhydrophilic nanoparticles and exhibiting excellent water/fog collection properties. The surface wettability, condensation properties, water collection rate, and wetting stability of the product samples are investigated in detail. The results obtained strongly indicate that the proposed hybrid superwettable surface prepared by maskless photocatalysis enhances the water drop condensation and water collection characteristics, with very stable wettability. The best water collection and drop removal efficiency was experimentally determined at the ultraviolet irradiation time of 200 s. This work findings are considered instrumental in the further design and implementation of hybrid superhydrophobic-superhydrophilic surfaces for cost-efficient atmospheric water harvesting. Keywords: water harvesting; superhydrophobicity; superhydrophilicity; photocatalysis; wettability
1. Introduction Two-thirds of the world’s lands are experiencing water scarcity, especially in semi-arid and desert regions. Atmospheric water and fog constitute approximately 10% of all fresh water on Earth [1]. Thus, collecting tiny water droplets or fog could help overcome global water shortages [2-9]. In recent years, controlling dropwise condensation for water collection found extensive application prospects in various industries, including water treatment or recovery in power stations [10], seawater desalination [11,12], heat transfer [12-14], microfluidic devices [15], and others. In practical applications, tiny water particles or fog need to be absorbed and then stripped rapidly. These properties can improve the performance of equipment and reduce the waste of water resources. At present, power stations and factories perform water recovery mainly through the gas membrane separation method and phase change condensation water extraction technology [16], which are very expensive and require quite sophisticate equipment. The main technologies of seawater desalination include multi-stage flash (MSF) [17,18], low-temperature multiple effect distillation (LT-MED) [19], and reverse osmosis method (RO) [20]. However, the above technologies have many disadvantages such as high equipment cost, slow condensation rate, low drop-stripping efficiency, and complex process. Due to the advantages such as strong water-absorbing ability, fast condensation rate, and good anti-frost effect, superhydrophilic surfaces have been explored in detail by many scholars [21-25]. Such surfaces are widely used in oil-water separation [21-24], anti-frost [25] and heat exchange processes [26,27]. However, water on hydrophilic or superhydrophilic surfaces often has a lower removal efficiency. Therefore, the residual water film can sharply increase the thermal resistance of the material surface, and promote the growth of bacteria on this surface. In recent years, superhydrophobic surfaces based on the “lotus leaf effect” also have been widely concerned in the industry [28-38]. In 2002, Feng et al. [39] found that the lotus leaf surface contained many nanostructural mastoids of about 100 nm in diameter and reported the nanostructure played an
essential role in the high contact angle value of the superhydrophobic surface. In 2003, Lau et al. [40] first fabricated superhydrophobic surface with the array of carbon nanotubes with a diameter of about 2 μm and found that the condensation occurred only at the top of the structure, with the formation of spherical water droplets of micron size. Such droplets are hard to nucleate and grow up between the nanometer-wide gap, so they are easily separated from the surface. Nowadays, many studies have also revealed that superhydrophobic surfaces have excellent drop-stripping performance, while adjacent water droplets in the condensation process can merge and bounce out from the surface [31-33], which is instrumental in the prevention of dewing and frosting [34-36]. However, there is usually an ‘air cushion’ in the nanometer gap of the superhydrophobic surface, which increases the thermal resistance of the material surface. Thus, even though the superhydrophobic surface improves the stability of Cassie state, so that dew and frost can be rapidly stripped from it, the surface heat transfer efficiency and water harvesting performance are deteriorated by the lower nucleation rate and higher heat resistance. In nature, Darkling beetle (Stenocara gracilipes of Tenebrionidae family), also known as fog stand beetle, is a species of beetle that is native to the Namib Desert of southern Africa, which absorbs water in the atmosphere through the hydrophilic protrusions of its shell surfaces and then transports it to the mouth through the hydrophobic groove to supply water resources. Inspired by the shell structure of this desert beetle species, a large number of scholars have prepared various biomimetic structure surfaces that combined superhydrophilic/hydrophilic and superhydrophobic/hydrophobic regions. These so-called hybrid superwettable (HSW) surfaces have broad application prospects in water/fog harvesting [41-44], desalination [12], and heat exchange [45,46], due to their high condensation ability and high drop-stripping efficiency. Hence, HSW surfaces are instrumental in water collection from atmoshere[12,41]. At present, there are numerous methods of HSW surface preparation, such as reactive ion etching [12,47-49], laser processing [50-55], photocatalysis [7,56,57], and others. Hou
et al. [12] fabricated
superhydrophobic surfaces with an array of superhydrophilic patterns using lithography and deep
reactive ion etching technology. These surfaces were found to enhance the dropwise condensation effect and to have excellent droplet-stripping effect and heat exchange performance. Mahapatra et al. [55] used carbon dioxide laser-processing techniques to create patterns on superhydrophobic aluminum surfaces, which promoted the condensation of droplets on these surfaces. Bai et al. [7] designed superhydrophilic star-shaped patterns using the mask photocatalysis technology for the titanium dioxide (TiO2) superhydrophobic surface, which patterns enhanced the water collection effect of the surface. Zhu and Guo [14] fabricated a titanium dioxide-copper (TiO2-Cu) composite surface modified with thiol and produced an HSW surface with excellent water collection efficiency and outstanding abrasion resistance. According to recent studies, hybrid HSW surfaces with superhydrophilic and superhydrophobic parts have considerable advantages regarding condensation, water harvesting, and heat exchange over more conventional surface structures. Therefore, it is also a vital issue in the field of bionic materials to develop a low-cost, readily prepared HSW surface. The surface of TiO2 modified by a low-surface-energy substance such as fluoro-alkyl silanes (FAS) can become superhydrophobic [7]. Due to excellent photocatalytic properties of TiO2, FAS monolayer deposited on the surface of TiO2 can be catalytically decomposed by UV irradiation [56-58]. This leads to a wetting transformation of the TiO2 surface. On the contrary, the decomposition of FAS monolayer modified on the surface of silicon dioxide (SiO2) nanoparticles is hard to achieve by UV irradiation. Given this, an HSW surface with mixed superhydrophilic TiO2 and superhydrophobic SiO2 nanoparticles can be produced by the maskless UV irradiation. To the best of the authors’ knowledge, this is the first attempt to apply the maskless photocatalysis to the fabrication of HSW surfaces for water harvesting. Herein,
we
fabricated
a
titanium
dioxide-silicon
dioxide
(TiO2-SiO2)
composite
superhydrophobic surface modified with FAS. Then, this surface was irradiated by UV light, and an HSW surface with superhydrophobic and superhydrophilic properties was prepared. Water collection rate was optimized by controlling the UV irradiation time, and the surface exhibited excellent condensation property, high efficiency water harvesting performance, good wetting stability and
durability. Thus, an innovative method is proposed, which seems to quite lucrative for cost-effective water harvesting.
2. Experiment 2.1. Sample preparation The preparation of samples involved the following procedure: (1) 10 ml nitric acidic solution (prepared by deionized water and nitric acid that is provided by Sinopharm Chemical Reagent Co., Ltd. China) with pH value of 2～3 was prepared, and 1.2 g silica sol (chain-type SiO2 nanoparticles, Suzhou Dongxing Surface Technology Co., Ltd., China) was added into the solution and subjected to ultrasonic stirring for 5 min. (2) The acidic solution was added to 50 ml ethanol and stirred for 5 min to form SiO2 nanoparticle solution. (3) 50 ml of ethanol was ultrasonically treated for 5 min, and 0.5 ml of tetrabutyl titanate (TBT, 98%, Shanghai Ling Feng Chemical Reagent Co., Ltd., China) was added to it and stirred for 5 min. (4) The TBT solution was added to SiO2 nanoparticle solution and subjected to
stirring in a water bath at 50 ºC for 24 h. Then, the mixed solution was fluorinated by
FAS (Quanzhou SICONG New Material Development Co., Ltd. China) to form TiO 2-SiO2 superhydrophobic paint. (5) TiO2-SiO2 superhydrophobic paint was sprayed with pressure over the substrate surface (glass sheets,NanJing WanQing Chemical Classware Istrument CO.LTD.) and dried at 80 ºC to form TiO2-SiO2 superhydrophobic surface. Next, these samples were subjected to the maskless UV irradiation of different periods (the UV lamp has a power of 1500 W, and the distance between the UV lamp and the sample surface is about 10.5 cm), and HSW surfaces were fabricated. Illustration of the preparation procedure is shown in Fig. 1. The TiO2-SiO2 coating samples were named according to the UV irradiation time, and abbreviations S0～S300 represent samples irradiated for 0 s～300 s, respectively. Similarly, sample S10M is irradiated for 10 min.
Fig. 1. Illustration of the preparation procedure for the TiO 2-SiO2 HSW surfaces and water harvesting. After UV irradiation, FAS coated on the surface of TiO 2 nanoparticles is decomposed. However, FAS coated on the surface of SiO2 nanoparticles has no noticeable change after UV irradiation, while the variation of chemical composition results in the anisotropic wettability of HSW surfaces.
2.2. Performance characterization The sample’s surface topographies were observed with the field emission scanning electron microscope (SEM, FEI Company, USA). The static contact and roll angles were measured using the contact angle measurement instrument (OCA 15 Pro Contact Angle Meter, Dataphysics Instrument GmbH, Germany). Ultraviolet photocatalytic reaction occurred in the photochemical reaction instrument (Nanjing Pogson Instrument CO., Ltd, China). The dynamic changes in droplets during the condensation process were examined using a stereomicroscope (Navitar, NY, USA). A Photron FASTCAM Mini UX100 type high-speed camera (Photron Ltd, Japan) equipped with Navitar 6000 zoom lens (Navitar, NY, USA) was used to capture the phenomena of self-induced jumping. The environmental temperature was about 23±2 ºC, the cooling stage temperature was 2ºC, and the relative humidity was 80±5%. The schematic diagram of the condensation test system is shown in Fig. S1. During the condensation experiment, the sample was placed at the cooling stage, and the stereomicroscope captured the dynamic pictures of the condensed water droplets. The average drop diameter, drop number density, surface coverage, and drop diameter distribution were counted. All measurements were made for three times, and the average values were calculated and recorded.
Fig. 2. SEM images of different coating surfaces. (a) SiO2 superhydrophobic coating surface. (b) TiO2 superhydrophobic coating surface. (c) TiO2-SiO2 coating surface before UV irradiation. (d) TiO2-SiO2 coating surface after 200 s of UV irradiation. The inserted pictures are magnified versions of the images, respectively. As seen from SEM pictures, the surface topographies of TiO 2-SiO2 coating surface exhibit no apparent change before and after UV irradiation.
2.3. Water harvesting measurements An original test system was designed to evaluate the water harvesting performance of as-prepared samples. A commercial humidifier produced a biomimetic fog flow of approx. 70 cm/s. A sample with a size of 5 cm × 5 cm was fixed on an inclined holder, while the fog flow was perpendicular to the sample surface. A container was placed under the sample, and the collected water was drained from the sample surface into the container. The distance between the fog outlet and sample was 18 cm, and the ambient temperature was 22±3.2ºC. The collected water in the container was weighed with an interval of 0.5 h over an 8 h duration. The schematic diagram of the water
harvesting test system is shown in Fig. 1. 2.4. Stability evaluation The stability of photocatalytic surface is very important. In order to verify the stability of the samples, the photocatalytic samples were divided into two groups, one in the room, and the other one in a clear glass cover and placed outdoor for stability testing, the outdoor samples were exposed to the natural sunlight, and both groups included samples from S0 to S300 (Fig. S2-S3). During the experiment, the samples’ contact and roll angles were measured with an interval of a week. The test time was 10 weeks, and the samples’ contact and roll angles are shown in Fig. S4-S5.
3. Results and discussion The surface morphology and chemical composition are known to have a strong impact on the surface wettability. The surface morphologies of different coating surfaces are shown in Fig. 2 and Fig. S6. It is observed that when the chain-type TiO2 (Fig. S7(a)-(b)) and SiO2 (Fig. S7(c)-(d)) nanoparticles are hybridized, they are entangled with each other to form large particles of 10～25 μm in diameter. According to SEM images (Fig. 2(c)-(d)), the surface morphologies of TiO2-SiO2 coating surface before and after UV irradiation exhibit no obvious differences. This implies that the UV irradiation treatment has no significant effect on the surface structure of TiO 2-SiO2 coating surface. Therefore, the elements’ distribution of F, Si and Ti was analyzed by the energy dispersive spectrum (EDS) method (Fig. S8-S10). The content of F on SiO2 superhydrophobic coating surface has no obvious change before and after UV irradiation (Fig. S11(a)), and the element content of F decreases with UV irradiation time on TiO2 and TiO2-SiO2 coating surfaces (Fig. S11(b)-S11(c)), respectively. This indicates that the decrease of FAS affects the surface wettability of TiO2 coating surface and TiO2-SiO2 coating surfaces.
Fig. 3. Condensation dynamics of different TiO2-SiO2 coating surfaces: (a) before UV irradiation; (b) after UV irradiation for 200 s; (c) after UV irradiation for 300 s; (d) after UV irradiation for 10 min.
For the TiO2-SiO2 coating surface, the surface wettability varies with the UV irradiation time due to the chemical composition variation. Thus, the droplet condensation characteristics are also changed. When the UV irradiation time was less than 200 s, the TiO2-SiO2 HSW surface exhibited perfect superhydrophobicity (Fig. 6(d)). The nucleation, growth, coalescence, and bounce phenomena occur in the condensation process (Fig. 3 and S12), where excellent anti-fogging characteristics (Move S1) and superior drop-stripping efficiency can be observed. However, when the UV irradiation time exceeded 300 s, the surface wettability of TiO2-SiO2 HSW surface was strongly enhanced, with less pronounced drops' jumping phenomenon during the condensation process, and the drop diameter increases with the condensation time (Fig. 3(c)-(d)).
Fig. 4. Condensation data of different sample surfaces: (a) variation of average droplet diameter with time; (b) variation of drop number density with time; (c) variation of surface coverage with time.
Furthermore, the average droplet diameter, drop number density, and surface coverage for different TiO2-SiO2 coating surfaces are depicted in Fig. 4 and S13. During the condensation process, the droplets on the surface of S0 and S200 samples exhibit both coalescence and bounce phenomena (Fig. S12(a) and (c)), which lead to the wave change of droplet diameter and drop number density, while the surface coverage also changes with the extension of condensation time. However, sample S0 is an entirely superhydrophobic surface without any superhydrophilic regions; its droplet nucleation is more difficult, and the drop diameter is smaller than those of S200 and S300. Meanwhile, samples S200 and S300 have HSW surfaces, and the superhydrophilic regions enhanced the droplets’ nucleation and growth. Thus, large drops are more likely to form on these surfaces. Therefore, droplets on S200 have higher nucleation and growth rates than S0, they exhibit more pronounced coalescence and jumping out of the surface. Hence, S200 samples showed more significant fluctuation of drop diameter and drop number density with the extension of condensation time. At the condensation time in the range 8～16 min, S200 has a lower surface coverage than S0. As compared to S0 and S200, the excessive superhydrophilic regions of S300 enhanced the surface wettability and adhesion, which prevents many drops from jumping and removal from the surface. As a result, the average diameter and surface coverage of S300 increased with time, while the drop number density decreased with time, so that many drops retained on the surface and their removal was problematic.
Fig. 5. Drop diameter distribution of S0, S200 and S300.
Moreover, we calculated the diameter distribution of water droplets for different TiO2-SiO2 coating surfaces (Fig.5 and S14). For the surfaces of S0～S200, the share of droplets with d＜30 μm was the most significant, while shares of droplets with d＜10 μm and 10 μm≤ d＜30 μm are fluctuant. At the same time, the share of droplets with d≥30 μm was very low. When the condensation time was 20 min, the share of droplets with d≥30 μm was approx. 1% and 5% for S0 and S200, respectively. However, for the surface of S300, the share of droplets with d≥10 μm decreased with time. After that, the share of droplets with d≥30 μm increased with time, and when the condensation time was 20 min, this share increased to 11.8%, which results were not conducive to the droplets’ removal. Surprisingly, S200 combined excellent nucleation rate with high drop removal efficiency, which combination yielded an outstanding water harvesting efficiency.
Fig. 6. The water collection results on contact and roll angles of different surfaces. (a) The water collection rates of various samples. (b) The dynamic change of water weight during 8 h. (c) The collected water weight for different tilt angles of S200, when the tilt angle is 90°，the water collection effect is the best. (d) The variation of water contact and roll angles for different surfaces with UV irradiation time.
Due to the photocatalysis of TiO2, the wettability of the surface of samples have changed with the catalytic time [56-58]. Water contact and roll angles are used to characterize the wettability of the material surface. According to the results in Fig. 6(d), before UV irradiation, contact angles of the SiO2 superhydrophobic surface, TiO2 superhydrophobic surface, and TiO2-SiO2 superhydrophobic surface exceed 160°, while roll angles are approx. 1°. After UV irradiation, the contact and roll angles of the SiO2 superhydrophobic surface have no visible change. However, for TiO2 and TiO2-SiO2 coating surfaces, both contact and angles decrease with UV irradiation time. For the TiO2-SiO2 coating surface, when the UV irradiation time is less than 300 s, the surfaces still combine excellent superhydrophobicity and self-cleaning properties (Move S2). Then, when the UV irradiation time is
600 s, roll angles of TiO2 surface and TiO2-SiO2 surface exceed 90°. Therefore, the wettability of TiO2-SiO2 HSW surface can be adjusted by controlling the UV irradiation time. The average water weight of unit area per unit time is adopted as the evaluation criterion of the water harvesting effect. Its expression is as follows: Wc
W SH
(1)
where Wc is the water collection rate (g/m2/h) defined as average water weight per unit area collected in 1 hour,
S and H are water harvesting area (m2) and time (h), respectively, while W is the total
weight of collected water (g). To obtain an excellent water harvesting surface, the water collection rates of different surfaces were calculated as shown in Fig. 6(a), and the collected water at different time points during 8 hours is plotted in Fig. 6(b). According to the results obtained, HSW surface of S200 possessed the highest water collection rate of 1742.9 g/m2/h, while those of superhydrophobic surfaces of S0, TiO2 and SiO2 were 741.5, 746.9, and 1064.2 g/m2/h, respectively. For comparison, this study also tested an aluminum sheet (Al) and a superhydrophilic surface of aluminum sheet modified with polyvinyl alcohol (AP). The results implied that the water collection rates of Al and AP were 544.2 and 344.5 g/m2/h, respectively. The water collection rate of S200 was more than 2.3 times higher than that of S0 and 5 times higher than that of AP, respectively. Therefore, an optimized HSW surface of S200 combined excellent condensation effect and drop removal efficiency, which yielded an excellent water harvesting efficiency.
Fig. 7. Self-cleaning properties of HSW surface for S200. (a) The sample of S200 with UV irradiation for 200 s. (b) The sample of S200 after 10 weeks stability test indoor. (c) The sample of S200 after 10 weeks stability test outdoor. Plenty of dry sludge powder is accumulated on S200, and then the sludge powder can be quickly carried away by the rolling water droplets.
The stability evaluation test shows that after 10 weeks’ test, the samples of indoor and outdoor groups exhibited excellent superhydrophobicity (Fig. S4-S5) and self-cleaning properties (Fig.7(b-c), Moves S3 and S4). Therefore, HSW surfaces of both indoor and outdoor samples have superior wetting stability.
From the viewpoint of the photocatalytic mechanism (Fig. 1), when TiO2 and SiO2 nanoparticles are hybridized and modified with FAS, the FAS monolayer is coated on both TiO2 and SiO2 nanoparticles. Therefore, before UV irradiation, the TiO2-SiO2 coating surface is superhydrophobic. After UV irradiation, the FAS monolayer covering the surface of TiO2 nanoparticles is decomposed via the direct dependence with UV irradiation time, which results in the exposure of superhydrophilic TiO2 nanoparticles acting as superhydrophilic regions (Fig. 8). However, FAS monolayers modified on the surface of SiO2 nanoparticles display no noticeable change (Fig. S11(a)). Hence, SiO2 nanoparticles possess excellent superhydrophobicity and act as superhydrophobic regions. After the photocatalytic treatment, the TiO2-SiO2 superhydrophobic surface is transformed into an HSW surface containing both superhydrophobic and superhydrophilic regions. Noteworthy are the test results depicted in Fig. 6(d), were superhydrophilic regions are too scarce to affect the superhydrophobicity of TiO2-SiO2 coating surface, so the samples exhibit the superhydrophobicity after the exposure to UV irradiation for less than 300 s. After UV irradiation, the superhydrophilic TiO2 regions can harvest water vapor from the air, nucleate and proliferate droplets. When the UV irradiation time exceeds 300 s, more FAS regions are decomposed, and the exposed superhydrophilic TiO2 regions are more extensive, which enhances the nucleation and rapid growth of drops. Thus, large drops are readily formed on the surfaces but their removal becomes problematic, which has a negative impact on the water collection rate. Therefore, although HSW surface is beneficial to enhance the drop nucleation and growth, while excessive superhydrophilic regions can improve the wettability and surface adhesion, which, in turn, can reduce the droplet removal efficiency. The condensation images (Fig. 3(c) and 3(d)) and water harvesting results (Fig. 6(a)) have proved this inference. According to this mechanism, insofar as S300 has higher UV irradiation time than S200, so its superhydrophilic regions and surface adhesion are larger than those of S200 (Fig. 8). Herein, these performances have reduced the drop-stripping efficiency and affected the water collection rate. It is well-known that droplets can detach from the surface when their gravitational force exceeds the surface tension, but if the drop adheres to a superhydrophilic surface, its detachment is inhibited
by the strong surface adhesion. With the nucleation and growth of drops on HSW surface, drops can leave the surface through gravity (G) and coalescence-induced jumping [59]. So, the combination of G and jump phenomena enhances the drop detachment [60]. Schematics of the self-induced jumping mechanism of drops on HSW surface is shown in Fig. 8. For S0 and S200, drops are stripped by gravity and coalescence-induced jumping. Then, the jump of drops on S300 is problematic, and they are stripped mainly by gravity. Insofar as superhydrophilic regions enhance the condensation, the diameter of drops removed by jumping is larger at S200 than that of S0, which implies a higher amount of harvested water in the former case. With an increase in UV irradiation time, a higher share of FAS on the surface of TiO 2 nanoparticles is decomposed by catalysis, which enhances the wettability of HSW surface. After then, water vapor prefer to nucleate and form droplets on the exposed TiO2 superhydrophilic areas. When the surface is perpendicular, the coalesced droplet releases the surface energy (Es) due to the reduction of droplet surface area, where one part of the surface energy is dissipated by viscosity (Evis), second part overcomes the work of adhesion (Ew), and the rest is transformed into the kinetic energy (Ek), which drives the droplet to jump [61-64]. Hence, the kinetic energy obtained by the coalescence of two droplets can be expressed as [59]:
Ek Es Evis Ew
(2)
where Ek is the kinetic energy of a jumping microdrop, Es is the released surface energy, Evis is the energy dissipation induced by viscous force, and Ew is the interfacial adhesion-induced energy dissipation. When two drops having different radii (r1 and r2 ) coalesce into a single jumping droplet of a larger radius (R), the relation between the surface energy difference and kinetic energy for jumping [61] can be described as follows: Es lv (2 3 cos cos 3 )(r1 r22 ) 4R 2 lv 2
where γlv is the liquid-vapor surface tension, and θ is water contact angle.
(3)
Fig. 8. Schematics of the self-induced jumping mechanism of drops on different surfaces. (a) Drops' self-induced jumping on S0 (Move S5). (b) Drops' self-induced jumping on S200 (Move S6). (c) Drops growth and removal on S300.
The volume of two drops before coalescence can be expressed as in [62]:
1 1 4 V r13 (2 3 cos cos 3 ) r23 (2 3 cos cos 3 ) R 3 3 3 3
(4)
Then, R can be expressed as follows: （r r ） (2 3 cos cos ) R 4 3 1
3 2
3
1 3
(5)
The viscous dissipation energy of one drop [63] and the work of adhesion [64] can be estimated as:
lv r1or 2 3 Evis 36
(6)
Ew lv R 2 (1 cos ) sin 2
(7)
Therefore, the kinetic energy of the coalescent droplet can be described as: Ek lv (2 3 cos cos 3 )(r1 r22 ) 4R 2 lv - 36 2
lv r13 r3 - 36 lv 2 - lv R 2 (1 cos ) sin 2 (8)
It can be seen from Equation (8) that when Es is larger than the sum of Evis and Ew (Ek>0), the excess surface energy is transformed into Ek with a velocity (v), which drives the droplet to jump and detach from the surface. This can be described as:
3E k 2R 3
(9)
When Ek<0, the coalesced drops cannot jump to leave the surface, this means that they will be removed by gravity. This reduces the drop removal efficiency and affects the water collection rate. For HSW surface, Ew is a crucial influencing factor for Ek. We assume that two coalesced drops have the same radii of 27 μm, γlv=72.8 mN/m, μ=1 mPa.s and ρ= 998 kg/m3. From Equation (7), values of Ew for S200 (θ=163°) and S300 (θ=155°) are estimated as 9.87×10-13 and 4.41×10-12 J/m2, respectively. Besides, for S200 with an initial jump velocity v=0.16 m/s (Equation (9)), Ek=2.16× 10-12 J (Equation (8)), and while for S300, Ek=-4.38×10-12 J (Ek<0). Therefore, drops on the surface of S200 are more prone to jump out of the surface, since a smaller surface adhesion has to be overcome than in the case of S300 (Fig. 8(b)). Therefore, the extension of UV irradiation period can enhance the surface wettability of HSW surface, it increases Ew and decreases Ek to affect the drop stripping efficiency. Hence, the UV irradiation
should be limited to a reasonable range to
prevent the work of adhesion becoming too large, and the optimized UV irradiation
of 200 s is
recommended.
4. Conclusions In this paper, a wettability controllable HSW surface fabrication method with low cost and convenient preparation is proposed. By taking advantage of the TiO2 photocatalytic properties, and using the UV light catalysis technology, the controllable decomposition of FAS monomolecular layer
modified on the surface of TiO2 nanoparticles was promoted. This process turned the TiO2-SiO2 superhydrophobic surface into an HSW surface, which has a combination of superhydrophilic and superhydrophobic regions. Compared to many complicated preparation technologies in previously reported [7-9,42-45], this facile methods offers a mass-production, and inexpensive process makes HSW surface feasible in practical applications. The experimental results show that upon the sample exposure to ultraviolet irradiation for 200 s, the surface condensation properties of the respective sample (S200) are improved due to the presence of superhydrophilic regions, and it demonstrates excellent water harvesting effect and commendable wetting stability. HSW surfaces produced by the proposed method are instrumental in water harvesting and have wide application prospects, including seawater desalination, heat exchange, anti-fogging, anti-frost, and other fields. These issues and the respective refinement of the provided method will be tackled in the follow-up research. Further more, physics of complex systems strategies, such as self-assembly of amphiphilic molecules [65] will be applied to the research in the future.
ACKNOWLEDGMENTS We are grateful for the support of the National Natural Science Foundation of China (Grants 51671055 and 51676033), and the China National Key R&D Program (2016YFC0700304). Notes: The authors declare no competing financial interest.
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Highlights ●
A hybrid superwettable coating surface with superhydrophobic and
superhydrophilic nanoparticles is developed by a simple, controllable and inexpensive maskless photocatalytic technology.
●
Excellent water collection efficiency is achieved by adjusting the optimal
ultraviolet irradiation time, and the hybrid superwettable coating surface both have remarkable superhydrophobicity, self-cleaning capability and Wetting stability.
● Drops condensation nucleation is enhanced and the relationship between the surface wettability and the drop stripping efficiency is clarified.
●
-propelled bouncing phenomenon and surface wetting
theory, the principle of improving water-collection efficiency on hybrid surfaces was explained.
Graphical abstract
S0169-4332(18)32643-6 https://doi.org/10.1016/j.apsusc.2018.09.210 APSUSC 40514
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
26 June 2018 21 August 2018 25 September 2018
Please cite this article as: X. Wang, J. Zeng, X. Yu, C. Liang, Y. Zhang, Water Harvesting Method via a Hybrid Superwettable Coating with Superhydrophobic and Superhydrophilic Nanoparticles, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.09.210
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Water Harvesting Method via a Hybrid Superwettable Coating with Superhydrophobic and Superhydrophilic Nanoparticles Xikui Wanga, Jia Zenga, Xinquan Yua*, Caihua Liangb, Youfa Zhanga* a
Jiangsu Key Laboratory of Advanced Metallic Materials, School of Materials Science and Engineering, Southeast University, Nanjing 211189, P. R. China b
School of Energy and Environment, Southeast University, Nanjing 210096, P. R. China *E-mail: [email protected]; [email protected]
Abstract Water collection gains increasing attention for harvesting atmospheric water in semi-arid deserts and inland areas. A new biomimetic method, inspired by the fog harvesting ability of hydrophobic-hydrophilic surfaces of Namib desert beetles, is presented in this study. It envisages a low-cost preparation (by the photocatalysis of titanium dioxide through ultraviolet irradiation) of a titanium dioxide-silicon dioxide hybrid superwettable surface combining superhydrophobic and superhydrophilic nanoparticles and exhibiting excellent water/fog collection properties. The surface wettability, condensation properties, water collection rate, and wetting stability of the product samples are investigated in detail. The results obtained strongly indicate that the proposed hybrid superwettable surface prepared by maskless photocatalysis enhances the water drop condensation and water collection characteristics, with very stable wettability. The best water collection and drop removal efficiency was experimentally determined at the ultraviolet irradiation time of 200 s. This work findings are considered instrumental in the further design and implementation of hybrid superhydrophobic-superhydrophilic surfaces for cost-efficient atmospheric water harvesting. Keywords: water harvesting; superhydrophobicity; superhydrophilicity; photocatalysis; wettability
1. Introduction Two-thirds of the world’s lands are experiencing water scarcity, especially in semi-arid and desert regions. Atmospheric water and fog constitute approximately 10% of all fresh water on Earth [1]. Thus, collecting tiny water droplets or fog could help overcome global water shortages [2-9]. In recent years, controlling dropwise condensation for water collection found extensive application prospects in various industries, including water treatment or recovery in power stations [10], seawater desalination [11,12], heat transfer [12-14], microfluidic devices [15], and others. In practical applications, tiny water particles or fog need to be absorbed and then stripped rapidly. These properties can improve the performance of equipment and reduce the waste of water resources. At present, power stations and factories perform water recovery mainly through the gas membrane separation method and phase change condensation water extraction technology [16], which are very expensive and require quite sophisticate equipment. The main technologies of seawater desalination include multi-stage flash (MSF) [17,18], low-temperature multiple effect distillation (LT-MED) [19], and reverse osmosis method (RO) [20]. However, the above technologies have many disadvantages such as high equipment cost, slow condensation rate, low drop-stripping efficiency, and complex process. Due to the advantages such as strong water-absorbing ability, fast condensation rate, and good anti-frost effect, superhydrophilic surfaces have been explored in detail by many scholars [21-25]. Such surfaces are widely used in oil-water separation [21-24], anti-frost [25] and heat exchange processes [26,27]. However, water on hydrophilic or superhydrophilic surfaces often has a lower removal efficiency. Therefore, the residual water film can sharply increase the thermal resistance of the material surface, and promote the growth of bacteria on this surface. In recent years, superhydrophobic surfaces based on the “lotus leaf effect” also have been widely concerned in the industry [28-38]. In 2002, Feng et al. [39] found that the lotus leaf surface contained many nanostructural mastoids of about 100 nm in diameter and reported the nanostructure played an
essential role in the high contact angle value of the superhydrophobic surface. In 2003, Lau et al. [40] first fabricated superhydrophobic surface with the array of carbon nanotubes with a diameter of about 2 μm and found that the condensation occurred only at the top of the structure, with the formation of spherical water droplets of micron size. Such droplets are hard to nucleate and grow up between the nanometer-wide gap, so they are easily separated from the surface. Nowadays, many studies have also revealed that superhydrophobic surfaces have excellent drop-stripping performance, while adjacent water droplets in the condensation process can merge and bounce out from the surface [31-33], which is instrumental in the prevention of dewing and frosting [34-36]. However, there is usually an ‘air cushion’ in the nanometer gap of the superhydrophobic surface, which increases the thermal resistance of the material surface. Thus, even though the superhydrophobic surface improves the stability of Cassie state, so that dew and frost can be rapidly stripped from it, the surface heat transfer efficiency and water harvesting performance are deteriorated by the lower nucleation rate and higher heat resistance. In nature, Darkling beetle (Stenocara gracilipes of Tenebrionidae family), also known as fog stand beetle, is a species of beetle that is native to the Namib Desert of southern Africa, which absorbs water in the atmosphere through the hydrophilic protrusions of its shell surfaces and then transports it to the mouth through the hydrophobic groove to supply water resources. Inspired by the shell structure of this desert beetle species, a large number of scholars have prepared various biomimetic structure surfaces that combined superhydrophilic/hydrophilic and superhydrophobic/hydrophobic regions. These so-called hybrid superwettable (HSW) surfaces have broad application prospects in water/fog harvesting [41-44], desalination [12], and heat exchange [45,46], due to their high condensation ability and high drop-stripping efficiency. Hence, HSW surfaces are instrumental in water collection from atmoshere[12,41]. At present, there are numerous methods of HSW surface preparation, such as reactive ion etching [12,47-49], laser processing [50-55], photocatalysis [7,56,57], and others. Hou
et al. [12] fabricated
superhydrophobic surfaces with an array of superhydrophilic patterns using lithography and deep
reactive ion etching technology. These surfaces were found to enhance the dropwise condensation effect and to have excellent droplet-stripping effect and heat exchange performance. Mahapatra et al. [55] used carbon dioxide laser-processing techniques to create patterns on superhydrophobic aluminum surfaces, which promoted the condensation of droplets on these surfaces. Bai et al. [7] designed superhydrophilic star-shaped patterns using the mask photocatalysis technology for the titanium dioxide (TiO2) superhydrophobic surface, which patterns enhanced the water collection effect of the surface. Zhu and Guo [14] fabricated a titanium dioxide-copper (TiO2-Cu) composite surface modified with thiol and produced an HSW surface with excellent water collection efficiency and outstanding abrasion resistance. According to recent studies, hybrid HSW surfaces with superhydrophilic and superhydrophobic parts have considerable advantages regarding condensation, water harvesting, and heat exchange over more conventional surface structures. Therefore, it is also a vital issue in the field of bionic materials to develop a low-cost, readily prepared HSW surface. The surface of TiO2 modified by a low-surface-energy substance such as fluoro-alkyl silanes (FAS) can become superhydrophobic [7]. Due to excellent photocatalytic properties of TiO2, FAS monolayer deposited on the surface of TiO2 can be catalytically decomposed by UV irradiation [56-58]. This leads to a wetting transformation of the TiO2 surface. On the contrary, the decomposition of FAS monolayer modified on the surface of silicon dioxide (SiO2) nanoparticles is hard to achieve by UV irradiation. Given this, an HSW surface with mixed superhydrophilic TiO2 and superhydrophobic SiO2 nanoparticles can be produced by the maskless UV irradiation. To the best of the authors’ knowledge, this is the first attempt to apply the maskless photocatalysis to the fabrication of HSW surfaces for water harvesting. Herein,
we
fabricated
a
titanium
dioxide-silicon
dioxide
(TiO2-SiO2)
composite
superhydrophobic surface modified with FAS. Then, this surface was irradiated by UV light, and an HSW surface with superhydrophobic and superhydrophilic properties was prepared. Water collection rate was optimized by controlling the UV irradiation time, and the surface exhibited excellent condensation property, high efficiency water harvesting performance, good wetting stability and
durability. Thus, an innovative method is proposed, which seems to quite lucrative for cost-effective water harvesting.
2. Experiment 2.1. Sample preparation The preparation of samples involved the following procedure: (1) 10 ml nitric acidic solution (prepared by deionized water and nitric acid that is provided by Sinopharm Chemical Reagent Co., Ltd. China) with pH value of 2～3 was prepared, and 1.2 g silica sol (chain-type SiO2 nanoparticles, Suzhou Dongxing Surface Technology Co., Ltd., China) was added into the solution and subjected to ultrasonic stirring for 5 min. (2) The acidic solution was added to 50 ml ethanol and stirred for 5 min to form SiO2 nanoparticle solution. (3) 50 ml of ethanol was ultrasonically treated for 5 min, and 0.5 ml of tetrabutyl titanate (TBT, 98%, Shanghai Ling Feng Chemical Reagent Co., Ltd., China) was added to it and stirred for 5 min. (4) The TBT solution was added to SiO2 nanoparticle solution and subjected to
stirring in a water bath at 50 ºC for 24 h. Then, the mixed solution was fluorinated by
FAS (Quanzhou SICONG New Material Development Co., Ltd. China) to form TiO 2-SiO2 superhydrophobic paint. (5) TiO2-SiO2 superhydrophobic paint was sprayed with pressure over the substrate surface (glass sheets,NanJing WanQing Chemical Classware Istrument CO.LTD.) and dried at 80 ºC to form TiO2-SiO2 superhydrophobic surface. Next, these samples were subjected to the maskless UV irradiation of different periods (the UV lamp has a power of 1500 W, and the distance between the UV lamp and the sample surface is about 10.5 cm), and HSW surfaces were fabricated. Illustration of the preparation procedure is shown in Fig. 1. The TiO2-SiO2 coating samples were named according to the UV irradiation time, and abbreviations S0～S300 represent samples irradiated for 0 s～300 s, respectively. Similarly, sample S10M is irradiated for 10 min.
Fig. 1. Illustration of the preparation procedure for the TiO 2-SiO2 HSW surfaces and water harvesting. After UV irradiation, FAS coated on the surface of TiO 2 nanoparticles is decomposed. However, FAS coated on the surface of SiO2 nanoparticles has no noticeable change after UV irradiation, while the variation of chemical composition results in the anisotropic wettability of HSW surfaces.
2.2. Performance characterization The sample’s surface topographies were observed with the field emission scanning electron microscope (SEM, FEI Company, USA). The static contact and roll angles were measured using the contact angle measurement instrument (OCA 15 Pro Contact Angle Meter, Dataphysics Instrument GmbH, Germany). Ultraviolet photocatalytic reaction occurred in the photochemical reaction instrument (Nanjing Pogson Instrument CO., Ltd, China). The dynamic changes in droplets during the condensation process were examined using a stereomicroscope (Navitar, NY, USA). A Photron FASTCAM Mini UX100 type high-speed camera (Photron Ltd, Japan) equipped with Navitar 6000 zoom lens (Navitar, NY, USA) was used to capture the phenomena of self-induced jumping. The environmental temperature was about 23±2 ºC, the cooling stage temperature was 2ºC, and the relative humidity was 80±5%. The schematic diagram of the condensation test system is shown in Fig. S1. During the condensation experiment, the sample was placed at the cooling stage, and the stereomicroscope captured the dynamic pictures of the condensed water droplets. The average drop diameter, drop number density, surface coverage, and drop diameter distribution were counted. All measurements were made for three times, and the average values were calculated and recorded.
Fig. 2. SEM images of different coating surfaces. (a) SiO2 superhydrophobic coating surface. (b) TiO2 superhydrophobic coating surface. (c) TiO2-SiO2 coating surface before UV irradiation. (d) TiO2-SiO2 coating surface after 200 s of UV irradiation. The inserted pictures are magnified versions of the images, respectively. As seen from SEM pictures, the surface topographies of TiO 2-SiO2 coating surface exhibit no apparent change before and after UV irradiation.
2.3. Water harvesting measurements An original test system was designed to evaluate the water harvesting performance of as-prepared samples. A commercial humidifier produced a biomimetic fog flow of approx. 70 cm/s. A sample with a size of 5 cm × 5 cm was fixed on an inclined holder, while the fog flow was perpendicular to the sample surface. A container was placed under the sample, and the collected water was drained from the sample surface into the container. The distance between the fog outlet and sample was 18 cm, and the ambient temperature was 22±3.2ºC. The collected water in the container was weighed with an interval of 0.5 h over an 8 h duration. The schematic diagram of the water
harvesting test system is shown in Fig. 1. 2.4. Stability evaluation The stability of photocatalytic surface is very important. In order to verify the stability of the samples, the photocatalytic samples were divided into two groups, one in the room, and the other one in a clear glass cover and placed outdoor for stability testing, the outdoor samples were exposed to the natural sunlight, and both groups included samples from S0 to S300 (Fig. S2-S3). During the experiment, the samples’ contact and roll angles were measured with an interval of a week. The test time was 10 weeks, and the samples’ contact and roll angles are shown in Fig. S4-S5.
3. Results and discussion The surface morphology and chemical composition are known to have a strong impact on the surface wettability. The surface morphologies of different coating surfaces are shown in Fig. 2 and Fig. S6. It is observed that when the chain-type TiO2 (Fig. S7(a)-(b)) and SiO2 (Fig. S7(c)-(d)) nanoparticles are hybridized, they are entangled with each other to form large particles of 10～25 μm in diameter. According to SEM images (Fig. 2(c)-(d)), the surface morphologies of TiO2-SiO2 coating surface before and after UV irradiation exhibit no obvious differences. This implies that the UV irradiation treatment has no significant effect on the surface structure of TiO 2-SiO2 coating surface. Therefore, the elements’ distribution of F, Si and Ti was analyzed by the energy dispersive spectrum (EDS) method (Fig. S8-S10). The content of F on SiO2 superhydrophobic coating surface has no obvious change before and after UV irradiation (Fig. S11(a)), and the element content of F decreases with UV irradiation time on TiO2 and TiO2-SiO2 coating surfaces (Fig. S11(b)-S11(c)), respectively. This indicates that the decrease of FAS affects the surface wettability of TiO2 coating surface and TiO2-SiO2 coating surfaces.
Fig. 3. Condensation dynamics of different TiO2-SiO2 coating surfaces: (a) before UV irradiation; (b) after UV irradiation for 200 s; (c) after UV irradiation for 300 s; (d) after UV irradiation for 10 min.
For the TiO2-SiO2 coating surface, the surface wettability varies with the UV irradiation time due to the chemical composition variation. Thus, the droplet condensation characteristics are also changed. When the UV irradiation time was less than 200 s, the TiO2-SiO2 HSW surface exhibited perfect superhydrophobicity (Fig. 6(d)). The nucleation, growth, coalescence, and bounce phenomena occur in the condensation process (Fig. 3 and S12), where excellent anti-fogging characteristics (Move S1) and superior drop-stripping efficiency can be observed. However, when the UV irradiation time exceeded 300 s, the surface wettability of TiO2-SiO2 HSW surface was strongly enhanced, with less pronounced drops' jumping phenomenon during the condensation process, and the drop diameter increases with the condensation time (Fig. 3(c)-(d)).
Fig. 4. Condensation data of different sample surfaces: (a) variation of average droplet diameter with time; (b) variation of drop number density with time; (c) variation of surface coverage with time.
Furthermore, the average droplet diameter, drop number density, and surface coverage for different TiO2-SiO2 coating surfaces are depicted in Fig. 4 and S13. During the condensation process, the droplets on the surface of S0 and S200 samples exhibit both coalescence and bounce phenomena (Fig. S12(a) and (c)), which lead to the wave change of droplet diameter and drop number density, while the surface coverage also changes with the extension of condensation time. However, sample S0 is an entirely superhydrophobic surface without any superhydrophilic regions; its droplet nucleation is more difficult, and the drop diameter is smaller than those of S200 and S300. Meanwhile, samples S200 and S300 have HSW surfaces, and the superhydrophilic regions enhanced the droplets’ nucleation and growth. Thus, large drops are more likely to form on these surfaces. Therefore, droplets on S200 have higher nucleation and growth rates than S0, they exhibit more pronounced coalescence and jumping out of the surface. Hence, S200 samples showed more significant fluctuation of drop diameter and drop number density with the extension of condensation time. At the condensation time in the range 8～16 min, S200 has a lower surface coverage than S0. As compared to S0 and S200, the excessive superhydrophilic regions of S300 enhanced the surface wettability and adhesion, which prevents many drops from jumping and removal from the surface. As a result, the average diameter and surface coverage of S300 increased with time, while the drop number density decreased with time, so that many drops retained on the surface and their removal was problematic.
Fig. 5. Drop diameter distribution of S0, S200 and S300.
Moreover, we calculated the diameter distribution of water droplets for different TiO2-SiO2 coating surfaces (Fig.5 and S14). For the surfaces of S0～S200, the share of droplets with d＜30 μm was the most significant, while shares of droplets with d＜10 μm and 10 μm≤ d＜30 μm are fluctuant. At the same time, the share of droplets with d≥30 μm was very low. When the condensation time was 20 min, the share of droplets with d≥30 μm was approx. 1% and 5% for S0 and S200, respectively. However, for the surface of S300, the share of droplets with d≥10 μm decreased with time. After that, the share of droplets with d≥30 μm increased with time, and when the condensation time was 20 min, this share increased to 11.8%, which results were not conducive to the droplets’ removal. Surprisingly, S200 combined excellent nucleation rate with high drop removal efficiency, which combination yielded an outstanding water harvesting efficiency.
Fig. 6. The water collection results on contact and roll angles of different surfaces. (a) The water collection rates of various samples. (b) The dynamic change of water weight during 8 h. (c) The collected water weight for different tilt angles of S200, when the tilt angle is 90°，the water collection effect is the best. (d) The variation of water contact and roll angles for different surfaces with UV irradiation time.
Due to the photocatalysis of TiO2, the wettability of the surface of samples have changed with the catalytic time [56-58]. Water contact and roll angles are used to characterize the wettability of the material surface. According to the results in Fig. 6(d), before UV irradiation, contact angles of the SiO2 superhydrophobic surface, TiO2 superhydrophobic surface, and TiO2-SiO2 superhydrophobic surface exceed 160°, while roll angles are approx. 1°. After UV irradiation, the contact and roll angles of the SiO2 superhydrophobic surface have no visible change. However, for TiO2 and TiO2-SiO2 coating surfaces, both contact and angles decrease with UV irradiation time. For the TiO2-SiO2 coating surface, when the UV irradiation time is less than 300 s, the surfaces still combine excellent superhydrophobicity and self-cleaning properties (Move S2). Then, when the UV irradiation time is
600 s, roll angles of TiO2 surface and TiO2-SiO2 surface exceed 90°. Therefore, the wettability of TiO2-SiO2 HSW surface can be adjusted by controlling the UV irradiation time. The average water weight of unit area per unit time is adopted as the evaluation criterion of the water harvesting effect. Its expression is as follows: Wc
W SH
(1)
where Wc is the water collection rate (g/m2/h) defined as average water weight per unit area collected in 1 hour,
S and H are water harvesting area (m2) and time (h), respectively, while W is the total
weight of collected water (g). To obtain an excellent water harvesting surface, the water collection rates of different surfaces were calculated as shown in Fig. 6(a), and the collected water at different time points during 8 hours is plotted in Fig. 6(b). According to the results obtained, HSW surface of S200 possessed the highest water collection rate of 1742.9 g/m2/h, while those of superhydrophobic surfaces of S0, TiO2 and SiO2 were 741.5, 746.9, and 1064.2 g/m2/h, respectively. For comparison, this study also tested an aluminum sheet (Al) and a superhydrophilic surface of aluminum sheet modified with polyvinyl alcohol (AP). The results implied that the water collection rates of Al and AP were 544.2 and 344.5 g/m2/h, respectively. The water collection rate of S200 was more than 2.3 times higher than that of S0 and 5 times higher than that of AP, respectively. Therefore, an optimized HSW surface of S200 combined excellent condensation effect and drop removal efficiency, which yielded an excellent water harvesting efficiency.
Fig. 7. Self-cleaning properties of HSW surface for S200. (a) The sample of S200 with UV irradiation for 200 s. (b) The sample of S200 after 10 weeks stability test indoor. (c) The sample of S200 after 10 weeks stability test outdoor. Plenty of dry sludge powder is accumulated on S200, and then the sludge powder can be quickly carried away by the rolling water droplets.
The stability evaluation test shows that after 10 weeks’ test, the samples of indoor and outdoor groups exhibited excellent superhydrophobicity (Fig. S4-S5) and self-cleaning properties (Fig.7(b-c), Moves S3 and S4). Therefore, HSW surfaces of both indoor and outdoor samples have superior wetting stability.
From the viewpoint of the photocatalytic mechanism (Fig. 1), when TiO2 and SiO2 nanoparticles are hybridized and modified with FAS, the FAS monolayer is coated on both TiO2 and SiO2 nanoparticles. Therefore, before UV irradiation, the TiO2-SiO2 coating surface is superhydrophobic. After UV irradiation, the FAS monolayer covering the surface of TiO2 nanoparticles is decomposed via the direct dependence with UV irradiation time, which results in the exposure of superhydrophilic TiO2 nanoparticles acting as superhydrophilic regions (Fig. 8). However, FAS monolayers modified on the surface of SiO2 nanoparticles display no noticeable change (Fig. S11(a)). Hence, SiO2 nanoparticles possess excellent superhydrophobicity and act as superhydrophobic regions. After the photocatalytic treatment, the TiO2-SiO2 superhydrophobic surface is transformed into an HSW surface containing both superhydrophobic and superhydrophilic regions. Noteworthy are the test results depicted in Fig. 6(d), were superhydrophilic regions are too scarce to affect the superhydrophobicity of TiO2-SiO2 coating surface, so the samples exhibit the superhydrophobicity after the exposure to UV irradiation for less than 300 s. After UV irradiation, the superhydrophilic TiO2 regions can harvest water vapor from the air, nucleate and proliferate droplets. When the UV irradiation time exceeds 300 s, more FAS regions are decomposed, and the exposed superhydrophilic TiO2 regions are more extensive, which enhances the nucleation and rapid growth of drops. Thus, large drops are readily formed on the surfaces but their removal becomes problematic, which has a negative impact on the water collection rate. Therefore, although HSW surface is beneficial to enhance the drop nucleation and growth, while excessive superhydrophilic regions can improve the wettability and surface adhesion, which, in turn, can reduce the droplet removal efficiency. The condensation images (Fig. 3(c) and 3(d)) and water harvesting results (Fig. 6(a)) have proved this inference. According to this mechanism, insofar as S300 has higher UV irradiation time than S200, so its superhydrophilic regions and surface adhesion are larger than those of S200 (Fig. 8). Herein, these performances have reduced the drop-stripping efficiency and affected the water collection rate. It is well-known that droplets can detach from the surface when their gravitational force exceeds the surface tension, but if the drop adheres to a superhydrophilic surface, its detachment is inhibited
by the strong surface adhesion. With the nucleation and growth of drops on HSW surface, drops can leave the surface through gravity (G) and coalescence-induced jumping [59]. So, the combination of G and jump phenomena enhances the drop detachment [60]. Schematics of the self-induced jumping mechanism of drops on HSW surface is shown in Fig. 8. For S0 and S200, drops are stripped by gravity and coalescence-induced jumping. Then, the jump of drops on S300 is problematic, and they are stripped mainly by gravity. Insofar as superhydrophilic regions enhance the condensation, the diameter of drops removed by jumping is larger at S200 than that of S0, which implies a higher amount of harvested water in the former case. With an increase in UV irradiation time, a higher share of FAS on the surface of TiO 2 nanoparticles is decomposed by catalysis, which enhances the wettability of HSW surface. After then, water vapor prefer to nucleate and form droplets on the exposed TiO2 superhydrophilic areas. When the surface is perpendicular, the coalesced droplet releases the surface energy (Es) due to the reduction of droplet surface area, where one part of the surface energy is dissipated by viscosity (Evis), second part overcomes the work of adhesion (Ew), and the rest is transformed into the kinetic energy (Ek), which drives the droplet to jump [61-64]. Hence, the kinetic energy obtained by the coalescence of two droplets can be expressed as [59]:
Ek Es Evis Ew
(2)
where Ek is the kinetic energy of a jumping microdrop, Es is the released surface energy, Evis is the energy dissipation induced by viscous force, and Ew is the interfacial adhesion-induced energy dissipation. When two drops having different radii (r1 and r2 ) coalesce into a single jumping droplet of a larger radius (R), the relation between the surface energy difference and kinetic energy for jumping [61] can be described as follows: Es lv (2 3 cos cos 3 )(r1 r22 ) 4R 2 lv 2
where γlv is the liquid-vapor surface tension, and θ is water contact angle.
(3)
Fig. 8. Schematics of the self-induced jumping mechanism of drops on different surfaces. (a) Drops' self-induced jumping on S0 (Move S5). (b) Drops' self-induced jumping on S200 (Move S6). (c) Drops growth and removal on S300.
The volume of two drops before coalescence can be expressed as in [62]:
1 1 4 V r13 (2 3 cos cos 3 ) r23 (2 3 cos cos 3 ) R 3 3 3 3
(4)
Then, R can be expressed as follows: （r r ） (2 3 cos cos ) R 4 3 1
3 2
3
1 3
(5)
The viscous dissipation energy of one drop [63] and the work of adhesion [64] can be estimated as:
lv r1or 2 3 Evis 36
(6)
Ew lv R 2 (1 cos ) sin 2
(7)
Therefore, the kinetic energy of the coalescent droplet can be described as: Ek lv (2 3 cos cos 3 )(r1 r22 ) 4R 2 lv - 36 2
lv r13 r3 - 36 lv 2 - lv R 2 (1 cos ) sin 2 (8)
It can be seen from Equation (8) that when Es is larger than the sum of Evis and Ew (Ek>0), the excess surface energy is transformed into Ek with a velocity (v), which drives the droplet to jump and detach from the surface. This can be described as:
3E k 2R 3
(9)
When Ek<0, the coalesced drops cannot jump to leave the surface, this means that they will be removed by gravity. This reduces the drop removal efficiency and affects the water collection rate. For HSW surface, Ew is a crucial influencing factor for Ek. We assume that two coalesced drops have the same radii of 27 μm, γlv=72.8 mN/m, μ=1 mPa.s and ρ= 998 kg/m3. From Equation (7), values of Ew for S200 (θ=163°) and S300 (θ=155°) are estimated as 9.87×10-13 and 4.41×10-12 J/m2, respectively. Besides, for S200 with an initial jump velocity v=0.16 m/s (Equation (9)), Ek=2.16× 10-12 J (Equation (8)), and while for S300, Ek=-4.38×10-12 J (Ek<0). Therefore, drops on the surface of S200 are more prone to jump out of the surface, since a smaller surface adhesion has to be overcome than in the case of S300 (Fig. 8(b)). Therefore, the extension of UV irradiation period can enhance the surface wettability of HSW surface, it increases Ew and decreases Ek to affect the drop stripping efficiency. Hence, the UV irradiation
should be limited to a reasonable range to
prevent the work of adhesion becoming too large, and the optimized UV irradiation
of 200 s is
recommended.
4. Conclusions In this paper, a wettability controllable HSW surface fabrication method with low cost and convenient preparation is proposed. By taking advantage of the TiO2 photocatalytic properties, and using the UV light catalysis technology, the controllable decomposition of FAS monomolecular layer
modified on the surface of TiO2 nanoparticles was promoted. This process turned the TiO2-SiO2 superhydrophobic surface into an HSW surface, which has a combination of superhydrophilic and superhydrophobic regions. Compared to many complicated preparation technologies in previously reported [7-9,42-45], this facile methods offers a mass-production, and inexpensive process makes HSW surface feasible in practical applications. The experimental results show that upon the sample exposure to ultraviolet irradiation for 200 s, the surface condensation properties of the respective sample (S200) are improved due to the presence of superhydrophilic regions, and it demonstrates excellent water harvesting effect and commendable wetting stability. HSW surfaces produced by the proposed method are instrumental in water harvesting and have wide application prospects, including seawater desalination, heat exchange, anti-fogging, anti-frost, and other fields. These issues and the respective refinement of the provided method will be tackled in the follow-up research. Further more, physics of complex systems strategies, such as self-assembly of amphiphilic molecules [65] will be applied to the research in the future.
ACKNOWLEDGMENTS We are grateful for the support of the National Natural Science Foundation of China (Grants 51671055 and 51676033), and the China National Key R&D Program (2016YFC0700304). Notes: The authors declare no competing financial interest.
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Highlights ●
A hybrid superwettable coating surface with superhydrophobic and
superhydrophilic nanoparticles is developed by a simple, controllable and inexpensive maskless photocatalytic technology.
●
Excellent water collection efficiency is achieved by adjusting the optimal
ultraviolet irradiation time, and the hybrid superwettable coating surface both have remarkable superhydrophobicity, self-cleaning capability and Wetting stability.
● Drops condensation nucleation is enhanced and the relationship between the surface wettability and the drop stripping efficiency is clarified.
●
-propelled bouncing phenomenon and surface wetting
theory, the principle of improving water-collection efficiency on hybrid surfaces was explained.
Graphical abstract