Laser Direct Writing of Tree-Shaped Hierarchical Cones on a

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Laser Direct Writing of Tree-shaped Hierarchical Cones on a Superhydrophobic Film for High Efficiency Water Collection Meng Wang, Qian Liu, Haoran Zhang, Chuang Wang, Lei Wang, Bingxi Xiang, Yongtao Fan, Chuanfei Guo, and Shuangchen Ruan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08116 • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 15, 2017

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ACS Applied Materials & Interfaces

Laser Direct Writing of Tree-shaped Hierarchical Cones on a Superhydrophobic Film for High Efficiency Water Collection Meng Wang1,2, Qian Liu4, Haoran Zhang4, Chuang Wang4, Lei Wang4, Bingxi Xiang1,2, Yongtao Fan5, Chuan Fei Guo3*, Shuangchen Ruan1,2*

1 Shenzhen Key Laboratory of Laser Engineering, College of Optoelectronic Engineering, Shenzhen University, Shenzhen, 518060, P. R. China 2 Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen, 518060, P. R. China 3 Department of Materials Science and Engineering, South University of Science and Technology, Shenzhen, 518055, P. R. China 4 National Center for Nanoscience and Technology, Beijing, 100190, P. R. China 5 Shanghai Institute of Optics and Fine Mechanics, Shanghai, 201800, P. R. China E-mail: [email protected]; [email protected] Keywords: laser direct writing, water collection, superhydrophobic, hierarchical patterns, microfluidics

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Abstract Directional water collection has stimulated a great deal of interests because of its potential applications in the field of microfluidics, liquid transportation, fog harvesting, and so forth. There have been some bio or bioinspired structures for directional water collection, from onedimensional spider silk to two-dimensional star-like patterns to three-dimensional Nepenthes alata. Here we present a simple way for the accurate design and highly controllable driving of tiny droplets: by laser direct writing of hierarchical patterns with modified wettability and desired geometry on a superhydrophobic film, the patterned film can precisely and directionally drive tiny water droplets, and dramatically improve the efficiency of water collection with a factor of ∼36 compared with the original superhydrophobic film. Such a patterned film might be an ideal platform for water collection from humid air, and for planar microfluidics without tunnels. Introduction Water collection and shortage is a big challenge, which is especially critical in dry and waterladen areas, such as in the desert or in the seaside. In nature, some creatures including desert beetles,1 spider silk,2 cacti3 and pitcher plants4 have developed the amazing ability to collect water with some special micro- and nanostructures on their surfaces. Some current technologies of water collection were inspired by such natural structures.5-10 For example, unprecedented droplet growth and transport have been achieved by designing structures mimicking Namib desert beetles, cacti and pitcher plants. Bioinspired artificial spider silks with controllable spindle-knot geometry have been successfully fabricated and shown outstanding water-collecting


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ability.11-12 Water could be transported from the inner side to the outer side on the superhydrophilic artificial Nepenthes alata surface.4, 13 These bioinspired structures are mostly one-dimensional or three-dimensional, which are relatively difficult to design and fabricate. By contrast,

a

two-dimensional

(2D)

surface

is

well

compatible

with

the

modern

micro/nanofabrication techniques and much commonly used in industry. Bai et al. have also succesfully made 2D patterns with cones (such as stars) for water collection, exhibiting a significant enhancement of efficiency of ∼5 times.14 However, the existing methods exhibit either limited resolution, or limited capacity of large area fabrication. In addition, it is difficult to spin-coat a layer of photoresist on a surperhydrophobic surface such that some lithographic method may not be used for making patterns on a superhydrophobic film. Hierarchical structures are found to be more efficient for water collection than simple patterns,3 it is therefore urgent to develop a method that can generate high resolution and large area hierarchical patterns with controllable surface wettability for high efficient water collection. Laser direct writing (LDW), a laser-beam based technique, can directly make complicated patterns with a high resolution (a few hundred nanometers), high writing speed, large area and high flexibility on a 2D surface.15-19 This technique can not only make patterns on photoresists,20-21 but also on metal22 and ceramic films,23 allowing for micro-nanofabrication or surface modification of a variety of materials. Especially for the patterning on metals and ceramics, LDW can modify the surface properties or morphology by a simple one-step process. Here, we report on that by using LDW to write tree-shaped hierarchical cones on a superhydrophobic TiO2 film with a surface energy gradient and Laplace pressure gradient, the patterned film can precisely and continually drive tiny water droplets toward the target location in the horizontal plane without the help of gravity. As a result, this type of surface is highly


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efficient with a factor of ∼36 in directional water collection than a film without the pattern. Moreover, this study provides an insight into design of novel patterned film with complex wettability and “planar microfluidic”. It can be used to enhance the efficiency in directional water collection or droplets manipulation, and might be extended to more diverse applications. This work further confirms that geometry is a critical factor for water collection design and other possible applications in surface science. Experimental Section Laser direct writing of porous TiO2 films: Ti films with a thickness of 100 nm were deposited on silicon substrates by radio-frequency magnetron sputtering (ULVAC ACS400-C4) with a power of 50 W and working pressure of 0.57 Pa for 3000 s and were then immersed in aerated 10 Μ NaOH at 60 °C for 30 min to prepare the porous TiO2 on the surface. The porous TiO2 films were modified with 1H,1H,2H,2H-Perfluorodecyltriethoxysilane through chemical vapor deposition (CVD) at 120 °C for 4 h to achieve superhydrophobicity. A laser direct writing system (HWN laser direct writing system-1500) was used for writing patterns on the superhydrophobic surface. The laser writer applied a laser with a wavelength of 405 nm, a laser spot size of ∼300 nm, pulse duration of 2000 ns, point spacing of 100 nm, repetition rate of 500 kHz and an energy density of 56.6 J/cm2. The samples were raster scanned at linear speed of 100 mm s–1 with parallel line density of 2500 mm–1. Characterization: Morphology of the superhydrophobic porous TiO2 surface was observed by field emission scanning electron microscopy (FESEM, Hitachi S-4800), surface profiler (Veeco Dektak 150, tip radius 12.5 µm) and laser scanning confocal microscope (LSCM, Olympus, LEXT-OLS 4000). Composition of the films was analyzed by X-ray photoelectron spectroscopy (XPS). The water contact angle (CA) was measured at ambient temperature using an OCA 20


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instrument (Data-physics, Germany). Deionized water (Millipore, 18 ΜΩ cm) was employed as the source for the contact angle measurement. The CA values were the average of five drops at different locations. The processes of water collection were observed by optical microscopy, and recorded by using a charge coupled device camera (CCD). A fog flow generated by an ultrasonic humidifier was used to examine the water collection properties. Results and discussion Controllable wettability of a TiO2 surface using LDW. TiO2 films with micropatterns were fabricated following the procedures in Figure 1. First, a porous TiO2 surface was fabricated with a hydro-thermal method by immersing a metal Ti film in sodium hydroxide solution followed by depositing a layer of 1H,1H,2H,2H-Perfluorodecyltriethoxysilane, showing nonwetting property to fog droplets (Figure 1a). After that, the superhydrophobic TiO2 surface was patterned by laser writing at a high-resolution of 300 nm. The laser beam removes the surface structures, making the film smoother and relatively wettable (Figure 1b). On the surfaces with laser scanned patterns, water droplets were found to directionally aggregate toward more wettable regions (Figure 1c). The laser scanned area was found to have a significant change in surface morphology. As shown in Figure 1d-f. Figure 1d shows an as-made superhydrophobic titanium oxide surface with a highly porous surface.24 Such a film exhibits superhydrophobic property with a CA as high as 160° (the inset of Figure 1d). However, after laser scanning, the film becomes less hydrophobic with a CA of 95.1° (the left inset of Figure 1e) and good adhesion to the substrate (the right inset of Figure 1e). The contact angle hysteresis for both chemically etched and laser textured patterns also reflects the different wetting properties (see Table S1 and S2 in Supporting Information). This is because that the laser beam melts the porous film and significantly reduces the roughness of the scanned areas (Figure 1e),

25-26

while the


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surrounding unscanned areas still keep the porosity and the superhydrophobicity. As a result, there is a significant difference in surface wettability at the edge of the laser processed area (Figure 1f). Figure 1h shows a 3D surface of the stripe structure fabricated by laser direct writing. The surface roughness of the laser scanned area is about 60 nm lower than the surrounding untreated area (Figure 1h). We should point out that there is still 1H,1H,2H,2HPerfluorodecyltriethoxysilane on the surface inevitably after laser scanning due to a self-healing process.27-29 After laser beam scans, 1H,1H,2H,2H-Perfluorodecyltriethoxysilane reabsorbs on laser treated surface. This is evidenced by the result showing that there is no obvious change in the composition or chemical state of the surface before and after laser processing. Due to the charge effect caused by the laser beam, Ti (2p) on the surface moves to a lower energy state (458.76 eV), but is still in the form of titanium dioxide.30 O 1s exists mainly in the Ti-O bond (530.64 eV) from TiO2,31 and Si-O bond (532.26 eV) is from fluorosilane.32 After laser processing, the proportion of Ti-O bond is reduced, which is mainly because that Ti is partly sublimated in the laser processing. These results show that the change of CA after laser processing mainly comes from the change of roughness rather than the composition. This can also explain that the laser scanned film has not yet become fully hydrophilic.


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Figure 1. (a–c) Schematic illustration for the fabrication process of bioinspired surfaces with anisotropic micropatterns. (a) Superhydrophobic porous TiO2 surface showing non-wetting


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property to fog droplets. (b) Laser writing of a hierarchical cone structures on the TiO2 film. (c) The directional driving of tiny droplets with a smart surface. (d–e) SEM image of the superhydrophobic porous titanium oxide surface (d) before and (e) after LDW process. (f) A boundary between laser processed and unscanned areas. (g) 3D surface profile of stripes fabricated by LDW. (h) The surface profile along y-direction of the surface in panel (i) XPS spectra of the laser treated and untreated area.

The controllable anisotropy of adhesion of the laser written surface. Patterned superhydrophobic surfaces with a changed wettability are known to provide adhesion anisotropy.14, 33, 31 The degree of anisotropy depends on the line width, titling direction, and surface homogeneity. Using adhesion anisotropy, the transportation of water droplets in the intended direction is feasible.9, 34 Figure 2a and b show the effect of laser written dot, line size, as well as water drop volume on the mobility of water droplets on superhydrophobic surface. As expected, for the dot (Figure 2a), the sliding angle decreases upon increasing the volume of the droplet, regardless of the titling direction. At a constant drop volume, the sliding angle increases upon increasing the dot diameter. Lines have a similar behavior with dots, for each line (Figure 2b), the sliding angle decreases upon increasing the volume of the droplet, but on the other hand, has a direction-controlled sliding angle, and the droplet is easier to slide along the line than along the orthogonal direction. The strong anisotropy of the sliding angle and droplet distortion for laser-patterned surfaces is attributed to the difference in the energy barrier of wetting between the two directions. At smaller drop, droplets slide easier along the line than that in the vertical direction. With the increasing of drop volume, the sliding angle contrast is lower due to the larger gravitational force. We can also see that for the anisotropy properties in Figure 2d,


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compared to the dot, the line can obviously cause anisotropic movement of the droplet. It indicates that surface with anisotropic patterns can transport droplets in designed direction. We further fabricated a triangle array on superhydrophobic surfaces to test the unidirectional wetting properties, as shown in Figure 2e. The triangle arrays integrate a shape gradient at solid-liquid interfaces along the tips of the triangle, and thus generate the release or the pinning of the solidliquid contact lines. For a single triangle, there is a maximum contact width in direction 1 and a minimum contact width in direction 2, the contact width (w) can be gradiently dependent on the direction along the tips of the triangle, causing the release of liquid in direction 1 and pinning of liquid in direction 2. These features induce a special retention force, based on the tips of triangles. The directional shedding-off property is examined by measuring the drop rolling-off angle in the direction 1 to 2 on the patterned film. Figure 2f shows the roll-off angle of water drops with volumes from 3 to 15 µL. The anisotropy is enlarged with the increasing of drop volume. Especially, for droplet volumes of 10–12 µL, the droplet is still pinning in direction 2, while it is very easy to roll off in direction 1. These investigations indicate that we can control the mobile behavior of droplets with patterns by integrating both surface energy gradient and Laplace pressure gradient, and it will be helpful for designing a novel droplet-controlling surface that can be extended to applications such as water collection, liquid-transport and cell-directed projects.


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Figure 2. (a–b) Sliding angles of droplets on patterned superhydrophobic surface with respect to droplet volume, patterning size, and titling direction for (a) dots and (b) lines fabricated by using LDW. (c) Top view (left), front view (middle) and right view (right) optical images of a droplet on a dot with a radius of 0.3 mm. (d) Top view (left), orthogonal direction (middle), and parallel direction (right) cross-sectional images of a droplet on a 0.8 mm-wide-line. (e) Optical image of the triangle arrays and illustration of directional shedding-off of droplets on the triangular array


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surface. (f) Anisotropic rolling-off properties in contrasting directions for drops of different volumes. The droplet releases in direction 1 and pins in direction 2, respectively.

Direction controlled driving of tiny water drops. LDW is an effective way to change the surface energy and to make surface patterns. Geometry has been considered to have an important impact on the movement and the collection of water, and a few simple geometries have been proven to effectively direct droplet motion. 2, 12, 35 In the following, we show that by taking advantage of the surface energy gradient and Laplace pressure gradient, tree-shaped hierarchical cone patterns can drive tiny drops directionally and finally collect water in a target location. As shown in Figure 3, we first test the directional driving of the droplets by a branched hierarchical cone in the horizontal plane (Figure 3a). The fog droplets are initially captured homogeneously on the patterned surface, since there is a surface energy gradient, tiny droplets on the superhydrophobic region are easily pushed into the patterned region to form larger droplets, and finally gather into one big drop. After the droplets are collected, a new cycle of collection begins immediately (see Movie S1 in Supporting Information). In fact, the tips of the branch generate a Laplace pressure gradient due to the shape gradient, which further enhances this the directional movement of water droplets.2, 12, 36 In more details, because of the confined “wedge” (shapegradient), the water droplets cannot reach balance CAs at the gradient start because of the asymmetric confinement of the tip. Therefore, the droplet has an imbalance apparent CA temporarily14, 37, i.e., an apparent CA toward the center ( ) and an apparent CA toward the ′ tip ( ), resulting an imbalance force. Thus, a driving force ( ) arises from

the shape-gradient and wettability gradient to move the droplet directionally toward the center of the pattern, that is


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′

≈ cos − cos + (cos − cos ) (1),

where is the apparent CA outside of the pattern. In other words, cones have a high water collection efficiency, because a large driving force pushes the tiny droplets toward the collecting site. For droplets that grow up during the collection process, when the droplet size is much larger than the wedge structure, as shown in Figure 3a at 12.1 s, F is not large enough to guide the flow of droplets, the movement of the droplets mainly depends on the coalescence between neighboring droplets, that is, the droplets are easily coalesced to move toward the larger cones. As observed in Figure 3a, the droplets finally move and spread toward the center of the water storage areas owing to the restriction of the cones. The surface with such cones is more efficient in collecting water. Based on the above discussion, we introduce a tree-shaped pattern with hierarchical cones for effective water collection. The tree-shaped pattern we designed in Figure 3b consists of a hierarchy of cone branches, ranging from micron size to millimeter size. Such a size scale well matches that of the water drop size: a water droplet often has an initial size of a few microns and grows to millimeters. The film is fixed on a sample stage in the horizontal plane, and exposed under a fog flow (∼75 mg·s–1) (Figure 3c), we use pipes to collect water located at points 1, 2, 3 as indicated in Figure 3d. All surfaces are 10 mm×10 mm in size. Figure 3d shows the geometry of the pattern and water collection on the film. The droplets were expected to be driven from branches to the crotch and finally to the trunk, and eventually flows to the specified position 1 along the direction of the red arrow. In the collecting process, water drops were driven along the designed direction and the driving force causes the anisotropic shape change of the droplet (Figure 3e). We further tested the directional properties of the tree-shaped film and


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compared the efficiency of the cases with a superhydrophobic surface and a fully written film (Figure 3f). As expected, the tree-shaped hierarchical cones capture and directionally drive the majority of water to position 1, while the other two samples do not have the function of directional collecting water. The hierarchical cones dramatically improved the efficiency of water collection with a factor of ∼36 compared to the superhydrophobic surface, which is a quite high efficiency for water collection. While the ability to drive water droplet directionally in a controllable manner is desired for water harvesting, it can also be utilized to perform interesting tasks such as “planar microfluidic”. Compared to the traditional microfluidic technology, this method does not need pipelines or tunnels, but can achieve the directional transport of liquid, similar to microfluidic tunnels. Meanwhile, fabrication of a 2D pattern is more flexible and simpler than making tunnels. We can also manipulate tiny droplets on patterned surface with a higher resolution, such as the control of direction or amount of liquid drops. It is interesting that the water collection process of the tree-shaped pattern is very similar to the body's blood circulation system, all the water droplets captured can be driven along the designed direction and finally flow into the roots of the tree-pattern, just like the fact that blood in the body flows into the heart. The tree-shaped pattern is expected to simulate the human blood system for biological research.


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Figure 3. (a) Driving tiny water droplets with controllable direction on a branch of tree-shaped hierarchical cones. (b) Optical images of hierarchical cones fabricated on a superhydrophobic surface. (c) Schematic illustration of the method used to quantitatively measure the fog collection efficiency of different surfaces. (d) The design of the tree-pattern. Points 1, 2 and 3 represent the location where the water pipes were placed, the red arrow represents the driving direction of the


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water droplets. (e) A patterned film in the process of collecting water. (f) Fog-collection efficiency of different surfaces at locations of 1, 2, and 3.

Conclusions In conclusion, we have successfully designed and fabricated tree-shaped hierarchical cones on a superhydrophobic titanium oxide film by using LDW, which is a technique capable of fabricating high resolution, large area, and complicated surface patterns. Such a two-dimensional pattern can precisely and continually drive tiny water droplets toward the root of tree-shaped patterns without the help of gravity, and exhibit efficient water collection property with an enhancement factor of ∼36 compared with superhydrophobic films. The results indicate that geometry is a key factor for the design of surface properties of thin films. The tree-shaped hierarchical cones might also open new paths of 2D microfluidics and for the simulation of circulation system in human body.

Supporting Information Table S1: The contact angle hysteresis for chemically etched surface. Table S2: The contact angle hysteresis for laser textured surface. Movie S1: Driving tiny water droplets with controllable direction on a branch of tree-shaped hierarchical cones

Corresponding Author Chuan Fei Guo*, Shuangchen Ruan*


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E-mail: [email protected]; [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources The work performed at SUSTC was supported by the funding of the National Natural Science Foundation of China (No. U1613204), the Program for Guangdong Introducing Innnovative and Entrepreneurial Team (No. 2016ZT06G587), and “The Recruitment Program of Global Youth Experts of China” (No. K16251101). The work performed at NCNST was supported by the National Key Research and Development Program of China (2016YFA0200403), the CAS Strategy Pilot Program (XDA 09020300), National Natural Science Foundation of China (10974037, 61505038). The work performed at Shenzhen University was supported by the Shenzhen science and Technology Planning (No. JCYJ20160422142912923). This work was partly supported by National Nature Science Foundation of China (No.61405223).

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Figure 1. (a–c) Schematic illustration for the fabrication process of bioinspired surfaces with anisotropic micropatterns. (a) Superhydrophobic porous TiO2 surface showing non-wetting property to fog droplets. (b) Laser writing of a hierarchical cone structures on the TiO2 film. (c) The directional driving of tiny droplets with a smart surface. (d–e) SEM image of the superhydrophobic porous titanium oxide surface (d) before and (e) after LDW process. (f) A boundary between laser processed and unscanned areas. (g) 3D surface profile of stripes fabricated by LDW. (h) The surface profile along y-direction of the surface in panel (i) XPS spectra of the laser treated and untreated area. 199x237mm (300 x 300 DPI)


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Figure 2. (a–b) Sliding angles of droplets on patterned superhydrophobic surface with respect to droplet volume, patterning size, and titling direction for (a) dots and (b) lines fabricated by using LDW. (c) Top view (left), front view (middle) and right view (right) optical images of a droplet on a dot with a radius of 0.3 mm. (d) Top view (left), orthogonal direction (middle), and parallel direction (right) cross-sectional images of a droplet on a 0.8 mm-wide-line. (e) Optical image of the triangle arrays and illustration of directional shedding-off of droplets on the triangular array surface. (f) Anisotropic rolling-off properties in contrasting directions for drops of different volumes. The droplet releases in direction 1 and pins in direction 2, respectively. 170x160mm (300 x 300 DPI)



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Figure 3. (a) Driving tiny water droplets with controllable direction on a branch of tree-shaped hierarchical cones. (b) Optical images of hierarchical cones fabricated on a superhydrophobic surface. (c) Schematic illustration of the method used to quantitatively measure the fog collection efficiency of different surfaces. (d) The design of the tree-pattern. Points 1, 2 and 3 represent the location where the water pipes were placed, the red arrow represents the driving direction of the water droplets. (e) A patterned film in the process of collecting water. (f) Fog-collection efficiency of different surfaces at locations of 1, 2, and 3. 184x200mm (300 x 300 DPI)


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35x15mm (300 x 300 DPI)


Laser Direct Writing of Tree-Shaped Hierarchical Cones on a

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