Uphill Water Transport on a Wettability-Patterned Surface

Apr 19, 2017 - (11, 12) Its gills need a certain amount of water to breathe, but it cannot .... were wetted, and the trapped water droplets again form...
0 downloads 0 Views 7MB Size
Download PDF
Research Article www.acsami.org

Uphill Water Transport on a Wettability-Patterned Surface: Experimental and Theoretical Results Yuji Hirai,*,† Hiroyuki Mayama,‡ Yasutaka Matsuo,§ and Masatsugu Shimomura† †

Department of Applied Chemistry and Bioscience, Chitose Institute of Science and Technology (CIST), Bibi 758-65, Chitose 066-8655, Japan ‡ Asahikawa Medical University, E2-1-1-1, Midorigaoka, Asahikawa 078-8510, Japan § Nanotechnology Research Center, Research Institute for Electronic Science (RIES), Hokkaido University, N21W10, Kita-ku, Sapporo 011-0021, Japan S Supporting Information *

ABSTRACT: In nature, there exist many functional watercontrolling surfaces, such as the water-repellent surface of lotus leaves, the superhydrophobic water-adhesive surface of rose petals, the water-harvesting surface of a beetle’s back, and the water-transporting surface of the legs of Ligia exotica. These natural surfaces suggest that surface chemistry and hierarchical structures are essential for controlling the water behavior. We have reported the preparation of superhydrophobic and antireflection silicon nanospike-array structures using selforganized honeycomb-patterned films as three-dimensional dry-etching masks. Moreover, the surface wettability of the silicon nanospike-array structures can be easily transformed from superhydrophobic to superhydrophilic by changes in the surface chemistry. In this report, we show the preparation of water-controlling surfaces, such as water-harvesting and watertransporting surfaces, by the wettability patterning of silicon nanostructured surfaces. We prepared honeycomb-patterned films for dry-etching masks made from polystyrene and an amphiphilic polymer by casting a chloroform solution. After the fixation of the top layer of the honeycomb-patterned films on a single-crystal silicon substrate, reactive ion etching was performed. The asprepared silicon nanospike-array structure showed superhydrophobicity, and the water contact angles were over 170°. After UVO3 treatment with photomasks, only the UV-irradiated surfaces showed superhydrophilicity, suggesting that we can obtain superhydrophobic- and superhydrophilic-patterned surfaces for which the patterns are the same as those of the photomasks. On the basis of these wettability-patterned surfaces, we demonstrated water harvesting by superhydrophilic dot-patterned surfaces and water transportation against gravity by superhydrophilic triangular-patterned surfaces. In particular, we investigated uphill water transport through the motion of droplets on tilting slopes based on the equation of motion. These results suggested that we can obtain superior microfluidic devices suitable for various applications through the use of optional wettability patterns. KEYWORDS: water transport, water harvesting, wettability pattern, superhydrophobicity, superhydrophilicity, biomimetics, self-organization

■

INTRODUCTION

One well-known and quite simple water-controlling surface is the superhydrophobic surface of the lotus leaf.9 A lotus leaf has a superhydrophobic water-repellent surface, generated by surface hydrophobic waxy compounds and surface roughness, which acts as a self-cleaning surface via small open capillary bundles. Another well-known example of water control in nature is the Namib desert beetle, which possesses a watercollecting surface, patterned with superhydrophobic and superhydrophilic surfaces, on their back for harvesting drinking water from fog-laden wind.10 Further, the Ligia exotica, which

In recent years, biomimetics have attracted a good deal of attention because of the potential for the creation of superior, new environmental load-reducing functional materials, which could lead to the generation of further technical innovations.1−4 The control of water by the modification of surface chemistry and topology, in particular, is one such area of research, and there are many reports describing water-controlling techniques, such as electrowetting5,6 and digital microfluidic devices.7,8 These techniques can be used to control water; however, they require external energy, such as electricity, mechanical energy, or thermal energy. On the contrary, there is a lot of watercontrolling surfaces that can work without external energy, using only surface tension, in nature. © XXXX American Chemical Society

Received: January 17, 2017 Accepted: April 19, 2017 Published: April 19, 2017 A

DOI: 10.1021/acsami.7b00806 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

a volume ratio of PSt:1 = 15:1 on glass substrates. After UV-O3 treatment of the honeycomb-patterned films for 1 min (OCA-150L-D, Iwasaki Electric Co., Ltd., 10 Japan), 1 wt % poly(vinyl alcohol) (PVA, Chart 1c, purchased from Wako Pure Chemical Industries, Ltd., Japan) aqueous solution was spin-coated (1000 rpm, 120−150 s) on the honeycomb-patterned films, and the honeycomb-patterned films were fixed upside down on single-crystal silicon substrates (n-type semiconductor with 100 facets and a resistivity ≤0.02 Ω cm, purchased from The Nilaco Corporation, Japan). The silicon substrates with the honeycomb-patterned films were annealed at 90 °C for 10 min to completely dry the PVA aqueous solution. After peeling off the dimpled bottom layer of the honeycomb-patterned films using a commercial adhesive tape (Scotch Tape, 3M, USA), the excess PVA adhesive on the bare silicon surface was washed out with deionized water. The masked silicon substrates were etched using an inductively coupled plasma dry-etching equipment (SPM-200, Sumitomo Precision Products Co., Ltd., Japan) with SF6, etching gas, and C4F8, passivating gas. The etching and passivation processes were repeatedly performed for a period of several tens of minutes. For the etching process, the gas flow ratio of SF6 and C4F8 was 50 and 90 sccm, respectively, and the processing time was 5.5 s. For the passivation process, the gas flow ratio of C4F8 was 140 sccm and the processing time was 5.0 s. To pattern the surface wettability of the nanostructured silicon substrate, the silicon substrates were then treated by UV-O3 for 15 min with photomasks. The surface structures of the silicon substrates were observed using a field emission scanning electron microscope (FE-SEM, Hitachi HighTechnologies Corporation, Japan; Vacc: 7 kV and Ie: 10 μA), and the surface chemical compositions were analyzed using an X-ray photoelectron spectroscope (XPS, JPS-9200, JEOL, Japan) with Al Kα X-rays (1486.6 eV) with a pass energy of 10 eV. Water contact angles (WCAs) on the etched silicon surfaces were measured using a contact angle analyzer (FAMAS, Drop Master, Kyowa Interface Science, Japan). The volume of the ultrapure water droplets was 3.0 μL for WCA measurement and 2.0, 5.0, and 8.0 μL for the dynamic analysis of the uphill water transport phenomena. The dynamic behavior of the water droplets was examined using a high-speed camera (Exilim, EXFH20, Casio, Japan) operated at a frame rate of 210 and using the contact angle analyzer.

lives on rocky seashores and displays branchial respiration, has intelligent water-controlling surfaces on its legs.11,12 Its gills need a certain amount of water to breathe, but it cannot enter the sea without drowning. To transport a suitable amount of water from tidal pools to its gills, it has evolved watercontrolling structures on its legs. Its sixth and seventh legs possess linearly arranged hairlike structures. These structures enhance wettability, with water transported solely through the gaps between the linearly arranged hairlike structures despite the surface chemistry of the legs being homogeneous in nature. On the basis of these surfaces, patterning of surface wettability and surface nanostructures and microstructures was speculated to allow the preparation of superior microfluidic and fluid transportation devices, including energy-free transportation systems. The preparation of wettability-patterned surfaces has been achieved using techniques such as microcontact printing,13 chemical vapor deposition,14 and photolithography.15 The preparation of water-harvesting surfaces16 and water-controlling, uphill surfaces17 has also been demonstrated. We have previously reported the formation of self-organized honeycomb-patterned porous polymer films by casting polymer solutions containing amphiphilic copolymers under humid conditions using water droplet arrays as templates.18−22 We have also demonstrated the preparation of biomimetic bifunctional silicon surfaces with antireflective and superhydrophobic properties using self-organized honeycombpatterned polymer films as three-dimensional (3-D) reactive ion etching (RIE) masks.23 This biomimetic bifunctional surface has a large surface area of hierarchic nanospike arrays and hydrophobic fluorocarbons. We also demonstrated that the surface wettability of silicon nanospike-array structures was dramatically changed from superhydrophobic to superhydrophilic by O2 plasma treatment through the modification of the surface chemistry from fluorocarbons to silicon dioxide. We, therefore, focused on this wettability inversion as the surface wettability that can be controlled by O2 plasma treatment or similar processes. In this report, we demonstrate the preparation of water-harvesting and water-transporting surfaces using wettability-patterned silicon nanospike-array structures. The water transportation was also expressed theoretically using an equation of motion and experimental data. These experiments and theoretical discussions lead to generate useful waterhandling devices without external energy, which contribute for sustainable society.

■

■

RESULTS AND DISCUSSION Surface Observations of Wettability-Patterned Surfaces. Figure 1a shows a schematic diagram of the preparation of the wettability-patterned silicon nanostructured surfaces using a self-organization process, a RIE process and UV-O3 treatment. Honeycomb-patterned porous polymer films possessing hexagonally arranged 1.1 μm micropores were obtained by the simple casting method (Figure 1b). Figure 1c shows a FE-SEM image of the porous polymer mask on a single-crystal silicon substrate. Hexagonally arranged micropores and pillar structures were fixed within the porous polymer mask. After RIE for 30 min, a silicon nanospike-array structure was formed, in which the 3-D structures were reflected on the polymer pillars of the porous masks (Figure 1d). This silicon nanospike-array structure has bifunctional antireflective25,26 (Figure 1g) and superhydrophobic properties generated by the surface nanostructures and microstructures and fluorocarbons that formed on the surface during RIE (details of the generation mechanisms of these functions were described previously23). After UV-O3 treatment with photomasks for 15 min, the nanospike-array structures remained (Figure 1e,f), but the UV-O3-irradiated areas showed superhydrophilicity, whereas the areas covered by the photomasks maintained their superhydrophobicity (Figure 1e,f inset images). Detailed analysis of the surface chemical changes before and after UV-O3 treatment is described in Figure S1. Figure 1g,h shows the photographs of the silicon nanospikearray structures before and after UV-O3 treatment with the

EXPERIMENTAL SECTION

The details of the preparation procedure and the surface analysis of the biomimetic bifunctional surface of the silicon nanospike-array structures were described previously.23 The honeycomb-patterned films were prepared by casting 16 mg/mL chloroform solutions of polystyrene (PSt, Chart 1a, Mw ≈ 280 000 g/mol, purchased from Aldrich, USA) and amphiphilic copolymer 1 (Chart 1b, m/n = 4:1, synthesized by free-radical polymerization as described elsewhere24) at

Chart 1. Chemical Structures of (a) PSt, (b) Amphiphilic Copolymer (1), and (c) PVA

B

DOI: 10.1021/acsami.7b00806 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic diagram of the preparation procedures of the wettability-patterned silicon nanospike-array structures. (b−f) FE-SEM images of the surfaces correspond to the illustrations of (a). Bars are 2.0 μm. (g−j) Photographs of the silicon nanospike-array structures.

photomask. Before UV-O3 treatment, the silicon nanospikearray structure was black because of the antireflective property caused by the moth-eye effect and optical well structures. After UV-O3 treatment with a photomask for 15 min, no change in the appearance was noted. When this nanospike-array structure was immersed in water, this substrate showed an angle of total reflection, and the same pattern as that of the photomask was observed (Figure 1i). After the removal from the water, water remained only on the UV-O3-irradiated superhydrophilic surfaces (Figure 1j). This phenomenon can be explained as follows: In water, the superhydrophobic surfaces of the silicon nanospike-array structures were not wetted, so air was trapped at the interface between the water and the silicon surface. Under this condition, light could be reflected at the interface between the water and the trapped air. The superhydrophobic surfaces appeared silver because of the reflection of light. By contrast, the superhydrophilic surfaces were completely wetted. As a result, there was no obvious optical interface between the water and the superhydrophilic silicon surfaces, and light was not reflected so that they remained black in appearance. These results indicated that the surface wettability could be patterned by UV-O3 treatment with photomasks. Mimicking of the Namib Desert Beetle’s Back. We were able to mimic the water collection surface of the Namib desert beetle’s back. Figure 2 shows the continuous photographs of water collection using the silicon nanospike-array structures. The substrate (a) in Figure 2 is the superhydrophobic surface, substrate (b) is the wettability-patterned surface obtained using a dot-array photomask (Figure S2), and substrate (c) is the superhydrophilic surface. When water mist was sprayed onto these surfaces fixed vertically using a spray gun, each surface showed different results as follows: The superhydrophobic surface was never wetted during this demonstration. By contrast, the superhydrophilic surface was immediately wetted (Figure 2, 0.45 s, white arrows) and became completely wetted with time. In the case of the wettability-patterned surface, the surface was selectively wetted over the superhydrophilic areas. After this, water droplets formed on the superhydrophilic areas and grew larger with time. When the weight of the trapped water droplets exceeded the water adhesion force of the superhydrophilic surface, the water droplets trickled downward (Figure 2, white arrows in 9.27−9.39 s). After this, downward trickling of the large water droplets, only the superhydrophilic surfaces were wetted, and the trapped water droplets again

Figure 2. Continuous photographs of the silicon nanospike-array structures during water mist supply: (a) Superhydrophobic surface, (b) wettability-patterned surface, and (c) superhydrophilic surface.

formed and grew larger as more water mist was applied. This phenomenon occurred repeatedly until the application of water mist was discontinued. This mechanism is the same as that observed on the desert beetle. Uphill Water Transport on the Wettability-Patterned Surface. We also demonstrated uphill water transportation using the wettability-patterned silicon nanospike-array structures. For water transportation, we designed a handmade photomask. Figure 3a shows a photograph of a handmade photomask prepared using a commercial aluminum tape and a quartz substrate. In this photomask, the triangular shape is very important for water transport as the interfacial energy increases with increases in the superhydrophilic area, and the angle of this triangle was 10°. Figure 3b shows the silicon nanospikearray structures after UV-O3 treatment with the handmade photomask and immersion in water. Only the UV-O3-irradiated area (triangular shape) was wetted. Figure 3c shows a photograph of the experimental setup, and Figure 3d−f shows the continuous photographs of the demonstration. The substrate was tilted, and a hydrophilic glass slide was placed C

DOI: 10.1021/acsami.7b00806 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 3. Photographs of (a) handmade photomask, (b) wettabilitypatterned silicon nanospike-array structure, and (c) experimental conditions. (d−f) Continuous photographs of water behavior on the wettability-patterned substrate.

upon it. When water droplets were dropped on the tilted wettability-patterned surface, we observed the following phenomenon: When the first water droplet was dropped onto the wettability-patterned surface, the water droplet adhered to the surface and climbed slowly as the proportion of the superhydrophilic surface increases toward the upper part (Figure 3d). After that, the water droplet remained on the upper part, where the interfacial force seems to be balanced with the effect of gravity along the slope. When a second water droplet was dropped onto the surface, the second water droplet rapidly fused with the first water droplet and the position of the center mass moved upward. As the water droplet increased in size, it eventually reached the hydrophilic glass slide, where it spread over the surface of the glass slide (Figure 3e). From the third water droplet, the water rapidly climbed the tilted surface and spread over the surface of the glass slide (Figure 3f). To investigate this uphill water transport phenomenon, we measured the changes in WCA and the wetting distance of the leading edge of the applied water. Figure 4a−c shows the WCAs and the wetting distances of the right-side water droplet edges on the wettability-patterned surface obtained from the continuous photographs (Figure S3). The tilting angles of the substrates were 0°, 15°, and 30°, and the water volume was 2.0, 5.0, and 8.0 μL, respectively. On the basis of these experimental

Figure 4. (a−c) Graphs of WCAs and wetting distances of leading edge of the water. Tilting angles of the substrates were (a) 0°, (b) 15°, and (c) 30°.

data, we were able to express the uphill motion of the water as shown below. Theory Underlying Uphill Water Transport on the Wettability-Patterned Surface. Here, we describe a theoretical scenario to explain the uphill motion of the water droplet on the superhydrophilic slope. First, we consider the equation of motion of the droplet climbing the superhydrophilic slope, including the driving force at the leading edge of the climbing droplet as well as the suppression forces due to gravity, pinning, and energy dissipation due to viscosity. Second, we discuss the pinning effect based on the experimental results. This works on the tail of the climbing droplet at the boundary between the superhydrophilic and superhydrophobic areas. Finally, we discuss the time dependD

DOI: 10.1021/acsami.7b00806 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 5. Schematic representation of a climbing droplet on the slope: (a) force balance on the climbing droplet on the slope; (b) definition of notations; (c) origin of pinning force on the tail of the climbing droplet; and (d) relationship among pinning force per unit length, central angle, and surface tension.

Table 1. Obtained Values from the Final States of the Climbing Droplets V (μL) 2.0 2.0 2.0 5.0 5.0 8.0

m (kg) 2.0 2.0 2.0 5.0 5.0 8.0

× × × × × ×

−6

10 10−6 10−6 10−6 10−6 10−6

ϕ (deg) 0 15 30 15 30 30

mg sin ϕ (N)

l (mm)

0 2.0 × 10−6 9.8 × 10−6 1.27 × 10−5 2.45 × 10−5 3.92 × 10−5

11.2 10.3 8.9 14.2 13.4 15.9

1.95 1.80 1.55 2.48 2.34 2.78

m

1.31 1.22 1.05 1.64 1.55 1.82

× × × × × ×

−4

10 10−4 10−4 10−4 10−4 10−4

Freal pin (N) 1.31 1.17 9.51 1.51 1.30 1.42

× × × × × ×

Freal pin /2l (N/m)

−4

10 10−4 10−5 10−4 10−4 10−4

5.86 5.68 5.34 5.33 4.87 4.48

× × × × × ×

10−3 10−3 10−3 10−3 10−3 10−3

d2x = Fdriv + Fsup dt 2 = γLw(t ) cos θ(t ) − Fpin(l) − mg sin ϕ

(3)

where x is the position of the center of mass of the droplet along the slope. The relationship between x(t) and l(t) is roughly x(t ) ≈ l(t )

(4)

On the other hand, w(t) is also proportional to the length as w(t) = φl(t), where φ is the central angle (φ = 0.17 rad). Therefore, eq 3 becomes m

d2l = γLφl(t ) cos θ(t ) − Fpin(l) − mg sin ϕ dt 2

(5)

Here, we put Fpin(t) ≈ 2f pinl(t) as discussed below. As shown later, the time dependence of the front position can be derived from eq 5. Pinning Force: Origin of Fpin. Here, we explain the pinning effect in general terms. The pinning effect is caused by continuous contact-angle change at physical and chemical defects. An edge on a physical defect creates a conflict between two flat surfaces; that is, it is not only a part of one flat surface but also a part of the other flat surface.27,28 Therefore, continuous contact-angle change occurs for both surfaces at the edge. On the other hand, chemical defects are boundaries between two flat areas with different surface tensions. At such a boundary, the contact angle can switch between the two

(1)

On the other hand, the resisting forces Fsup include three factors: the first is the effect of gravity (−mg sin ϕ), the second is the pinning force Fpin(l), and the third is the viscosity of the water in spreading (−γLw(t)[1 − cos θ(t)]). Therefore, Fsup is given by Fsup = −mg sin ϕ − Fpin(l) − γLw(t )[1 − cos θ(t )]

21.2 19.4 20.3 23.1 23.1 24.6

γLw cos θ (N)

Therefore, we get the equation of motion of a climbing droplet on a slope as

ence of the position of the climbing droplet on the slope to explain the experimental results. Situation. Figure 5a,b shows the schematic representations of a water droplet climbing an inverted triangular shape in a superhydrophilic area, where forward and backward forces are working along the slope. On the basis of the experimental findings, we assume that there is a wicking film in front of the macroscopic water droplet as it climbs while the tail of the climbing droplet is stacked at the edge of the superhydrophilic area. Here, we assume that the mass of the droplet is m, the surface tension of the liquid is γL, the contact angle at the front line is θ(t) and that at the edge is θedge(t), the position of the front is l(t), the width of the front is w(t) at time t, the tilting angle of the slope is ϕ, and the pinning force working at the edge is Fpin(l). The reason why Fpin(l) is considered is explained below. Equation of Motion of a Climbing Droplet on a Slope. Both driving and resisting forces are working on the climbing droplet. The driving force working on the front of the water droplet Fdriv is Fdriv = γLw(t )

θ (deg)

w (mm)

(2)

where g is the gravitational acceleration. E

DOI: 10.1021/acsami.7b00806 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

where θ̅edge is the average contact angle over the edge. φ theor Fpin = 2fpin l = 2γLl cos θedge ̅ sin 2

contact angles. The magnitude of the pinning force is proportional to the length of the edge and the boundary. Real Pinning Force Freal pin Based on the Experimental Results. From eq 5 for the final state of the climbing droplet (md2l/dt2 = 0) and the experimental results, it is possible to evaluate Freal pin . real Fpin = γLφl(t ) cos θ(t ) − mg sin ϕ

(8)

sin φ ≈ φ when φ < 1. Therefore, eqs 7 and 8 are modified as fpin =

(6)

The obtained values are summarized in Table 1, with the order −4 of Freal N. Because Freal pin being 10 pin works on the boundary, it depends on the length of the edge, as shown in Figure 6a,

φγL cos θedge ̅ (9)

2

theor Fpin = φγLl cos θedge ̅

(10)

If θ̅edge = 30° and φ = 0.17 rad, f pin = 5.3 mN/m, which is consistent with Freal pin /2l. This is valid because the droplet on the hydrophilic part looks like a teardrop from the side view. f pin, Fpin, and θ̅edge as Driving and Resisting Forces. Here, we discuss the fact that f pin and Fpin sometimes work as a driving force. It is natural to consider the change in θ̅edge throughout the whole process. Figure 7 shows the schematic representations of the process. Figure 7a shows θ̅edge > π/2. This emerges when we apply a droplet (the early stage). In this case, f pin and Fpin work as a driving force. In the intermediate stage, θ̅edge ≈ π/2, as shown in Figure 7b. The magnitude of f pin and Fpin is small. In the final stage, θ̅edge < π/2, as shown in Figure 7c. These factors work as a resisting force. Thus, θ̅edge determines the role of f pin and Ftheor pin . This allows us to understand the reason why the large acceleration is obtained when the droplet contacts with the substrate. Time Dependence of the Displacement. Here, we show the essence of the time dependence of displacement of an uphill droplet. Detailed discussion is shown in Figure S4. From eqs 5 and 10, we obtain the following equation of motion. m

d2l = γLφl(t ) cos θ(t ) − φγLl cos θedge ̅ − mg sin ϕ dt 2 (11)

Although we skip the details on how to solve this, we obtain the following solution. Figure 6. (a) Relationship between Fpin vs 2l and (b) relationship between Fpin/2l vs 2l. The average is 5.3 × 10−3 N/m.

0.7

0.7

l(t ) = l1e α(t + t0) + l 2e−α(t + t0) −

where the total length of the edge is 2l in the final state. Obviously, Freal pin is proportional to 2l. Figure 6b shows the pinning force per unit length Freal pin /2l, which is almost constant at approximately 5.3 mN/m. The validity of this value is discussed below. On the other hand, the magnitude of mg sin ϕ is one order smaller than those of the other factors. Theoretical Pinning Force Ftheor pin . Here, we discuss the theoretical treatment of Ftheor pin . As shown in Figure 5c,d, the tail of the climbing droplet is pinned at the edge. On the basis of these geometries, the theoretical pinning force per unit length along the downward direction f pin is φ fpin = γL cos θedge ̅ sin (7) 2

+

1 g (t + t0)2 sin ϕ 2

1 δg (t + t0)3 sin ϕ − l3(t + t0) − l4 6

(12)

where t0, δ, l3, and l4 are added to introduce the experimental errors in the observation of the uphill droplets because it is very difficult to determine the initial wetting state at t = 0 precisely. On the other hand, α is ⎡ γ φ(B − A) ⎤1/2 ⎥ α=⎢ L ⎣ 0.56m ⎦

(13)

where A and B reflect the initial conditions of the contact angles at the front and edge, respectively, as shown in the Supporting Information.

Figure 7. Schematic representation of the contact angle at the boundary between non-superhydrophilic and superhydrophilic areas: (a) θ̅edge > π/2 at t ≈ 0, (b) θ̅edge ≈ π/2 at t > 0, and (c) θ̅edge < π/2 at t ≫ 0. F

DOI: 10.1021/acsami.7b00806 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces The fitting results of l(t) under the condition of ϕ = 0°, 15°, and 30° are shown in Figure 8. The fitting parameters are summarized in Table 2. The experimental results are explained very well.

transport water without the need for external energy, using only interfacial energies similar to those observed for the Namib desert beetle’s back and the legs of L. exotica. In particular, we examined the uphill water transport phenomenon by the equation of motion, and the driving force for the uphill motion and the pinning force were expressed. This is just one example of the functionality of these surfaces, as the functions are dependent on the design of the wettability patterns. Therefore, these surfaces possess enormous potential for the generation of various external energy-free, water-controlling functional surfaces: water transport devices against gravity, water manipulation surfaces for chemical or medical analysis chips, water fluidic devices without high-pressure pump, and so on, based on the abundant superior surface designs in nature.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b00806. XPS analysis of before and after UV-O3 treatment, photograph of the photomask used for water-harvesting demonstration, continuous photographs of water behavior on the wettability-patterned substrate for analysis of the uphill water transport phenomenon (the tilting angle of the substrate is 30°), and detailed discussion of the equation of uphill motion of a water droplet on the slope (PDF) High-speed movie of the water-harvesting demonstration (m4v movie file, flame rate of 210 fps) (MPG) High-speed movie of the uphill water transport phenomenon (m4v movie file, flame rate of 210 fps) (MPG)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone & Fax: +81 123 27 6068.

Figure 8. Fitting results for the change in l(t) on the slope of ϕ = 0° (a), 15° (b), and 30° (c). The red-dashed, dash-dot-dashed, and dashdot-dot-dashed lines are fitting results for the 2, 5, and 8 μL, respectively.

ORCID

Yuji Hirai: 0000-0001-9850-6461 Notes

■

The authors declare no competing financial interest.

■

CONCLUSIONS In conclusion, we demonstrated water collection and water transportation by wettability-patterned hierarchically structured silicon surfaces. The wettability-patterned silicon nanospikearray structures were prepared by RIE of silicon with selforganized porous polymer masks and UV-O3 treatment with photomasks. This wettability-patterned surface can collect and

ACKNOWLEDGMENTS This work was partially supported by JSPS KAKENHI grant number JP24120001 (Y.H. and M.S.), JP24120003 (Y.H.), JP24120004 (M.S.), and JP24120005 (Y.M.) in Scientific Research on Innovative Areas “Innovative Materials Engineering Based on Biological Diversity” and JP26400424 (H.M.) in

Table 2. Fitting Parameters ϕ (deg)

V (μL)

l1

l2

l3

0

2 5 8 2 5 8 2 5 8

−2.85 × 10−4 −8.15 × 10−5 6.29 × 10−4 3.59 × 10−3 1.12 × 10−2 1.62 × 10−2 −0.131 −0.112 −3.27 × 10−3

−0.591 0.240 −3.98 × 10−4 −5.10 × 10−2 0 −0.125 0 0.974 1.49

−0.121 −0.205 0.118 −0.420 −0.498 −0.534 −2.85 −2.69 −2.56

15

30

G

B−A

l4 5.42 × 5.20 × 2.57 × 5.98 × 0.101 0.153 6.17 × 0.655 1.20

10−5 10−2 10−2 10−2

10−2

2.58 2.33 4.44 2.11 3.64 4.43 1.58 1.31 1.40

× × × × × × × × ×

10−3 10−2 10−4 10−3 10−3 10−3 10−3 10−3 10−3

t0

δ

0.694 0.257 0.221 0.112 0.237 0.227 0.347 0.217 0.270

0 0 0 2.07 0.976 1.451 9.212 1.98 1.11

DOI: 10.1021/acsami.7b00806 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

pared by using Self-Organized Honeycomb Templates and Reactive Ion Etching. J. Mater. Chem. 2010, 20, 10804−10808. (24) Yabu, H.; Hirai, Y.; Shimomura, M. Electroless Plating of Honeycomb and Pincushion Polymer Films Prepared by SelfOrganization. Langmuir 2006, 22, 9760−9764. (25) Clapham, P. B.; Hutley, M. C. Reduction of Lens Reflexion by the “Moth Eye” Principle. Nature 1973, 244, 281−282. (26) Stavenga, D. G.; Foletti, S.; Palasantzas, G.; Arikawa, K. Light on the Moth-Eye Corneal Nipple Array of Butterflies. Proc. R. Soc. B 2006, 273, 661−667. (27) de Gennes, P.-G.; Brochard-Wyart, F.; Quere, D. Capillarity and Wetting Phenomena; Springer, 2004. (28) Mayama, H.; Nonomura, Y. Theoretical Consideration of Wetting on a Cylindrical Pillar Defect: Pinning Energy and Penetrating Phenomena. Langmuir 2011, 27, 3550−3560.

Scientific Research (C). A part of this work was conducted at Hokkaido University, supported by “Nanotechnology Platform” Program of MEXT, Japan.

■

REFERENCES

(1) Vincent, J. F. V.; Bogatyreva, O. A.; Bogatyrev, N. R.; Bowyer, A.; Pahl, A.-K. Biomimetics: Its Practice and Theory. J. R. Soc., Interface 2006, 3, 471−482. (2) Bhushan, B. Biomimetics: Lessons From NatureAn Overview. Philos. Trans. R. Soc., A 2009, 367, 1445−1486. (3) Jin, M.; Feng, X.; Feng, L.; Sun, T.; Zhai, J.; Li, T.; Jiang, L. Superhydrophobic Aligned Polystyrene Nanotube Films with High Adhesive Force. Adv. Mater. 2005, 17, 1977−1981. (4) Ball, P. Engineering Shark Skin and Other Solutions. Nature 1999, 400, 507−509. (5) Velev, O. D.; Prevo, B. G.; Bhatt, K. H. On-Chip Manipulation of Free Droplets. Nature 2003, 426, 515−516. (6) Mugele, F.; Baret, J.-C. Electrowetting: From Basics to Applications. J. Phys.: Condens. Matter 2005, 17, R705−R774. (7) Fair, R. B. Digital Microfluidics: Is a True Lab-on-a-Chip Possible? Microfluid. Nanofluid. 2007, 3, 245−281. (8) Teh, S.-Y.; Lin, R.; Hung, L.-H.; Lee, A. P. Droplet Microfluidics. Lab Chip 2008, 8, 198−220. (9) Barthlott, W.; Neinhuis, C. Purity of the Sacred Lotus, or Escape from Contamination in Biological Surfaces. Planta 1997, 202, 1−8. (10) Parker, A. R.; Lawrence, C. R. Water Capture by a Desert Beetle. Nature 2001, 414, 33−34. (11) Horiguchi, H.; Hironaka, M.; Meyer-Rochow, V. B.; Hariyama, T. Water Uptake via Two Pairs of Specialized Legs in Ligia exotica (Crustacea, Isopoda). Biol. Bull. 2007, 213, 196−203. (12) Ishii, D.; Horiguchi, H.; Hirai, Y.; Yabu, H.; Matsuo, Y.; Ijiro, K.; Tsujii, K.; Shimozawa, T.; Hariyama, T.; Shimomura, M. Water Transport Mechanism Through Open Capillaries Analyzed by Direct Surface Modifications on Biological Surfaces. Sci. Rep. 2013, 3, 3024. (13) Drelich, J.; Miller, J. D.; Kumar, A.; Whitesides, G. M. Wetting Characteristics of Liquid Drops at Heterogeneous Surfaces. Colloids Surf., A 1994, 93, 1−13. (14) Sun, T.; Wang, G.; Liu, H.; Feng, L.; Jiang, L.; Zhu, D. Control Over the Wettability of an Aligned Carbon Nanotube Film. J. Am. Chem. Soc. 2003, 125, 14996−14997. (15) Tadanaga, K.; Morinaga, J.; Matsuda, A.; Minami, T. Superhydrophobic−Superhydrophilic Micropatterning on Flowerlike Alumina Coating Film by the Sol−Gel Method. Chem. Mater. 2000, 12, 590−592. (16) Zhai, L.; Berg, M. C.; Cebeci, F. Ç .; Kim, Y.; Milwid, J. M.; Rubner, M. F.; Cohen, R. E. Patterned Superhydrophobic Surfaces: Toward a Synthetic Mimic of the Namib Desert Beetle. Nano Lett. 2006, 6, 1213−1217. (17) Chaudhury, M. K.; Whitesides, G. M. How to Make Water Run Uphill. Science 1992, 256, 1539−1541. (18) Widawski, G.; Rawiso, M.; François, B. Self-Organized Honeycomb Morphology of Star-Polymer Polystyrene Films. Nature 1994, 369, 387−389. (19) Nygard, A.; Davis, T. P.; Barner-Kowollik, C.; Stenzel, M. H. A Simple Approach to Micro-Patterned Surfaces by Breath Figures with Internal Structure using Thermoresponsive Amphiphilic Block Copolymers. Aust. J. Chem. 2005, 58, 595−599. (20) Bunz, U. H. F. Breath Figures as a Dynamic Templating Method for Polymers and Nanomaterials. Adv. Mater. 2006, 18, 973−989. (21) Park, J. S.; Lee, S. H.; Han, T. H.; Kim, S. O. Hierarchically Ordered Polymer Films by Templated Organization of Aqueous Droplets. Adv. Funct. Mater. 2007, 17, 2315−2320. (22) Maruyama, N.; Koito, T.; Nishida, J.; Sawadaishi, T.; Cieren, X.; Ijiro, K.; Karthaus, O.; Shimomura, M. Mesoscopic Patterns of Molecular Aggregates on Solid Substrates. Thin Solid Films 1998, 327−329, 854−856. (23) Hirai, Y.; Yabu, H.; Matsuo, Y.; Ijiro, K.; Shimomura, M. Biomimetic Bi-Functional Silicon Nanospike-Array Structures PreH

DOI: 10.1021/acsami.7b00806 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX