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“Dual-Parallel-Channel” Shape-Gradient Surfaces: Toward Oriented and Reversible Movement of Water Droplets† Jilin Zhang and Yanchun Han* State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate University of the Chinese Academy of Sciences, Changchun 130022, P. R. China Received April 27, 2009. Revised Manuscript Received June 26, 2009 In this paper, we prepared “dual-parallel-channel” shape-gradient surfaces, on which water droplets can reversibly and orientedly move between two adjacent pools under the guidance of an external voltage. Furthermore, it is found that the motion speed is governed by several parameters, including bath condition, gradient angle, and the working voltage. In this self-transportation process of water droplets, the external voltage works like a traffic light, which can give “moving”, “stopping”, “turning” and “straight-going” signals to the water droplets.
1. Introduction The self-motion of a liquid droplet on a solid surface has attracted great interest in relation to exchange of material and energy transduction by surface materials for achieving liquid selftransportation and smart microfluidic devices. Since a gradient surface displays a gradual change in the chemical or physical property along its length, i.e., a gradual change in surface tension,1-3 water droplets tend to move on it because of having asymmetry surface tensions along the gradient direction. In 1978, Greenspan4 first predicted that the water droplet would self-move on a surface tension gradient, and then Chaudhury and Whitesides5 first achieved the movement in experiments. They prepared a gradient self-assembled monolayer (SAM) of decyltrichlorosilane molecules on the silicon surface, on which water droplets could self-run uphill at a velocity of 1 to 2 mm/s. Recently, the surfaces with gradient surface tension to drive droplet motion were realized by several different approaches,5-21 i.e., wetting † Part of the “Langmuir 25th Year: Wetting and superhydrophobicity” special issue. *Corresponding author. Tel.: þ86-431-85262175. Fax: þ86-431-85262126. E-mail:
[email protected].
(1) Ruardy, T. G.; Schakenraad, J. M.; van der Mei, H. C.; Busscher, H. J. Surf. Sci. Rep. 1997, 29, 1. (2) Genzer, J.; Bhat, R. R. Langmuir 2008, 24, 2294. (3) Kim, M. S.; Khang, G.; Lee, H. B. Prog. Polym. Sci. 2008, 33, 138–164. (4) Greenspan, H. P. J. Fluid Mech. 1978, 84, 125. (5) Chaudhury, M. K.; Whiteside, G. M. Science 1992, 256, 1539. (6) Bain, C. D.; Burnett-Hall, G. D.; Montgomerie, R. R. Nature 1994, 372, 414. (7) Lee, W.; Laibinis, P .E. J. Am. Chem. Soc. 2000, 122, 5395. (8) Sumino, Y.; Kitahata, H.; Yashikawa, K.; Nagayama, M.; Nomura, S. M.; Magome, N.; Mori, Y. Phys. Rev. E 2005, 72, 041603. (9) Sumino, Y.; Magome, N.; Hamada, T.; Yashikawa, K. Phys. Rev. Lett. 2005, 94, 068301. (10) Nagai, K.; Sumino, Y.; Kitahata, H.; Yashikawa, K. Phys. Rev. E 2005, 71, 065301(R). (11) Cazabat, A. M.; Heslot, F.; Troian, S. M.; Carles, P. Nature 1990, 346, 824. (12) Gallardo, B. S.; Gupta, V. K.; Eagerton, F. D.; Jong, L. I.; Craig, V. S.; Shah, R. R.; Abbott, N. L. Science 1999, 283, 57. (13) Yamada, R.; Tada, H. Langmuir 2005, 21, 4254. (14) Ichimura, K.; Oh, S. K.; Nakagawa, M. Science 2000, 288, 1624. (15) Oh, S. K.; Nakagawa, M.; Ichimura, K. J. Mater. Chem. 2002, 12, 2262. (16) Abbott, N. L.; Ralston, J.; Reynolds, G.; Hayes, R. Langmuir 1999, 15, 8923. (17) Ito, Y.; Heydari, M.; Hashimoto, A.; Konno, T.; Hirasawa, A.; Hori, S.; Kurita, K.; Nakajima, A. Langmuir 2007, 23, 184. (18) Bico, J.; Quere, D. Europhys. Lett. 2000, 51, 546. (19) Daniel, S.; Chuadhury, M. K.; Chen, J. C. Science 2001, 291, 633. (20) Daniel, S.; Chuadhury, M. K. Langmuir 2002, 18, 3404. (21) Zhang, J.; Han, Y. Langmuir 2007, 23, 1845.
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gradient surfaces are prepared by chemical,5-10 thermal,11 electrochemical,12,13 and photochemical14-17 methods and so on. For example, Lee et al.7 used noncovalent molecular adsorption to achieve the movement of drops on the patterned surfaces. Sumino et al.8,9 used chemosensitive running droplets, which can self-run periodically and circularly by a chemical Marangoni effect.23,24 Ito et al.17 prepared a gradient SAM surface by photodegradation technique, which could drive water droplets motion at an average speed of 0.5 to 6 mm/s. In our previous work, we21 prepared a trapeziform mica/wax composite surface, which we called shapegradient composite surface. This surface contains three regions, the gradient start, the transportation region (“road”), and the storage region (“pool”) (Figure 1). It was found that water droplets could spontaneously move (including (1) spreading from the hydrophobic wax/low-density polyethylene (LDPE) surfaces to the hydrophilic mica surfaces; (2) elongating along the hydrophilic transportation region (“road”) because the gradient start is in high space limitation and the droplet cannot reach a balance contact angle at the gradient start.; (3) shrinking from the hydrophilic transportation region to the hydrophilic storage region (“pool”) because of the lower surface energy at the storage region21) horizontally and uphill on the surface along the gradient direction in a very high speed (>10 mm/s). To indicate the shapechange degree of the trapeziform transportation region, a parameter, gradient angle (R)21 is defined and can be calculated by eq 1, where W1 and W2 are the width of the gradient start and the transportation region end, respectively, and L is the length of the shape-gradient (Figure 1). We found that the motion speed improved with the increase of the gradient angle (R).21 R ¼ 2 tan -1 ½ðW 2 - W 1 Þ=2L
ð1Þ
Even with such a high transportation speed, however, in most cases, the achieved motion is only in one-way, not reversible. Thus, how to make the movement reversible and oriented is still an issue to limit their applications. Washizu22 used a size-matched big electrode array to achieve the droplet motion, deflection, and coalescence. However, this motion is step by step along the electrode array, and the motion speed is quite low (∼0.4 mm/s) (22) Washizu, M. IEEE Trans. Ind. Appl. 1998, 34, 732. (23) Magome, N.; Yoshikawa, K. J. Phys. Chem. 1996, 100, 19102. (24) Shioi, A.; Katano, K.; Onodera, Y. J. Colloid Interface Sci. 2003, 266, 415.
Published on Web 08/17/2009
DOI: 10.1021/la9014898
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Figure 2. Schematic representation of the fabrication process of the dual-parallel-channel shape gradient surfaces. Figure 1. Schematic representation of a shape-gradient surface which contains the gradient start, the transportation region (“road”), and the storage region (“pool”). The gradient angle (R) indicates the shape-change degree of the transportation region.
because of the deep restriction from the switching speed of the energized electrode. Recently, Yamada et al.13 fabricated a ferrocenyl alkanethiol monolayer, on which water droplets can self-run reversibly by using electrochemical reactions. To achieve both reversible and high-speed self-transportation of droplets, herein, we prepared “dual-parallel-channel” shapegradient surfaces based on our previous work,21 which also contain three regions, the gradient start, the transportation region (“road”), and the storage region (“pool”). On these surfaces, water droplets could be triggered to reversibly and orientedly move between any two adjacent “pools” under the guidance of an external voltage. Furthermore, in contrast to the previously reported surfaces, on which water droplets could reversibly selfmove,13,22 the self-motion speed on the “dual-parallel-channel” shape-gradient surface is much higher and without any restriction of the switching speed of the energized electrode. It is also found that the transportation speed is well controlled by the applied voltage and the gradient angle (R). For example, when the applied voltage is 25 V and the gradient angle is 16°, the average transportation speed is as high as ∼6.2 mm/s. Here, the applied voltage works like a traffic light, which can give “moving”, “stopping”, “turning”, and “straight-going” signals to water droplets. Having such intelligence and controllability, the “dual-parallel-channel” shape gradients may be applied in designing controllable chemical reactors, biochips, and so forth to govern chemical and bioreactions.
2. Experimental Section Preparation of Samples. Figure 2 is the schematic representation of the whole fabrication processes of the “dual-parallelchannel” shape gradient surfaces. Herein, commercial In2O3/ SnO2 (ITO) glass slides (ITO thickness: 100 nm) were used as substrates. Before use, all substrates were cleaned by acetone, ethanol, and deionized (DI) water, and dried by N2. Then ∼5 μm thick positive photoresist (PR) (Shipley 1818) was spin-coated onto the ITO layer, where the spin-speed was 3000 rpm, and the acceleration was 300 for 45 s. Then all samples were baked at 100 °C oven for 1 min, and exposed by UV (∼365 nm) light (10 mW/cm2) under the mask for 20 s. Subsequently, the samples were developed in Microposit 351/H2O solution (50:250 in weight) for 5 min, and rinsed by DI water. Then 20 wt % HCl solution was used as an etchant to remove ITO, and the etching time was 15 min. After that, the remanent PR was removed by acetone, and cleaned by DI water, and dried by N2. Then, 100 nm thick Al2O3 film was coated by atomic layer deposition. After that, 50 nm of Cytop (Cytop CTL-809 M (Asahi) at 1% in CTsolv-180 (Asahi)) was spin-coated (1000 rpm/100 Acc. for 30 s.) and thermally cured at 180 °C for 20 min to form a solid hydrophobic film. Finally, one corner (∼1 1 mm) of each pool 14196 DOI: 10.1021/la9014898
was gently scratched by a silicon cutter to remove Cytop and Al2O3 layers, connected with a Tungsten wire by Pelco Colloidal Silver Paste, and sealed by epoxy. Contact Angle Test. Contact angles were measured via an AST VCA-Optima system, which was integrated with a custom LabVIEW program to automatically step the voltage and capture a photograph every 1 s. The contact angles were calculated in the VCA-Optima software. Contact angle data was plotted from the average of three curves. Reversible and Oriented Motion Test. The samples were immerged in n-dodecane or air bath, and then 10 drops (∼0.5 mL) DI water were deposited. To obvious view the movement, 1 ppm KMnO4 was also added. The motion was performed by connecting the positive electrode to the target pool and placing the negative electrode to the aqueous solution. After the motion from original pool to the target one finished, the adding voltage (25 V, 1 kHz square wave) was released. The motion process was captured by Kodak Easyshare M7.1 digital camera.
3. Results and Discussion 3.1. Switching Surface Wettability. At equilibrium, the three phases (aqueous/air/dielectric or aqueous/oil/dielectric) rest at Young’s contact angle for the water (θY).25 As voltage is applied, an electromechanical force26 reduces the aqueous phase contact angle (θV) according to the so-called electrowetting equation (eq 2): cos θV ¼ cos θY þ CV 2 =2γAO
ð2Þ
where C is capacitance per unit area of the hydrophobic dielectric, and γAO is the interfacial surface tension between the aqueous and air (or oil) phases. This equation means hydrophobic surfaces can reversibly switch bewteen hydrophobicity and hydrophilicity with releasing or adding an external voltage. Figure 3 is the contact angle versus voltage curves of a water droplet on the prepared sample in an n-dodecane bath and an air bath. In an air bath, the contact angles of the water droplet decrease from ∼116 ( 2° at 0 V to 66 ( 4° at 25 V. However, after releasing the voltage, the contact angle of the water droplet can switch back to 109 ( 3° at 0 V. On the other hand, in an n-dodecane bath, the water contact angle is 162 ( 2° without the voltage being on. Upon adding voltage, the contact angles of the water droplet decrease gradually, from 162 ( 2° at 0 V, to 146 ( 2° at 7 V, 113 ( 4° at 14 V, 82 ( 4° at 19 V, and 57 ( 3° at 25 V (Figure 3 inset pictures), which means the wettability of the surface changes from “(super)hydrophobic” to “hydrophilic”. However, after releasing the voltage, the contact angle of the water droplet can switch back to 159 ( 3° at 0 V. Thus, the surface changes back from “hydrophilic” to “(super)hydrophobic”, and the value of (25) Young, T. Philos. Trans. R. Soc. London 1805, 95, 65. (26) Jones, T. B.; J. Micromech. Microeng. 2005, 15, 1184.
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Figure 3. Contact angle versus working voltage curves (1 kHz square wave) of DI water droplets in oil and air bath, respectively.
Figure 4. Reversible motion of water droplets on the dual-parallel-channel shape gradient surface with double pools. (a) Schematic representation of the dual-parallel-channel shape gradient surface with double pools. (b) A water droplet (∼0.5 mL) moves from pool A to pool B along road B via adding 25 V on pool B. (c) The droplet moves back from pool B to pool A along road A via adding 25 V on pool A.
capacitance (C) is around 2.5 10-6 F/m2, which can be calculated by the electrowetting equation (eq 2).26,27 3.2. Reversible and Oriented Motion of Water Droplets. On the basis of our previous work21 and the reversible switching wettability in electrowetting, herein we prepared a “dual-parallelchannel” shape gradient surface with double “pools”, which is schematically shown as Figure 4a. Here, two 1 1 cm square pools are designed to store water droplets, and each of them is connected to a shape gradient “road” with the size of 0.5 (width) 3 (width) 18 mm (length) (i.e., the gradient angle21 is 8°) (eq 1). (27) Berry, S.; Kedzierski, J.; Abedian, B. Langmuir 2007, 23, 12429.
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Furthermore, each “road” is extended into the unconnected pool 3 mm, and the distance between two parallel “roads” is 0.5 mm. Figure 4b shows the motion of water droplets on this “dual-parallel-channel” shape gradient surface from the bottom pool (pool A) to the upper pool (pool B). A big water droplet (∼0.5 mL) was first placed in pool A (Figure 4b(I)). After adding 25 V on pool B, both pool B and road B switched from (super)hydrophobic to hydrophilic, which drove the water droplet spreading from pool A onto road B (Figure 4b(II)). Then, the droplet elongated along road B gradually because of the unbalanced contact angle at the high-space limited gradient start21 (Figure 4b(III)). When the droplet reached pool B (Figure 4b(IV)), it started to spread on pool B (Figure 4b(V)). After the water droplet moved from pool A onto road B completely (Figure 4b(VI)), it started to shrink from road B to pool B as a result of the lower surface tension of the water droplet21 (Figure 4b(VII)). After releasing the voltage, the water droplet could shrink from road B to pool B completely (Figure 4b(VIII)). The whole motion process is 4.7 s, and the average speed is 3.2 mm/s (or 0.11 mL/s). Subsequently (Figure 4c(I)), we added 25 V on pool A instead of pool B to switch both pool A and road A from (super)hydophobic to hydrophilic. Driven by the asymmetry surface tensions, the droplet started to spread from pool B onto road A (Figure 4c(II)), and elongated along road A gradually (Figure 4c(III)). When the droplet reached pool A (Figure 4c(IV)), it started to spread on pool A as well (Figure 4c(V)). At the time the droplet departed from pool B onto road A completely (Figure 4b (VI)), it tended to shrink from road A to pool A gradually (Figure 4b(VII)). After releasing the voltage, the water droplet could shrink from road A to pool A completely (Figure 4b(VIII)). The whole motion process is 4.6 s, and the average speed is 3.3 mm/s (or 0.11 mL/s), which is almost same to the motion from pool A to pool B. Furthermore, the whole movement process from pool B to pool A (Figure 4b) is completely same to the motion from pool A to pool B (Figure 4c), both of which included three stages: (1) spreading from the original pool to the target road; (2) elongating along the target road; (3) shrinking from the road to the target pool (see the Supporting Information, movie S1). Both the same motion process and the equal transportation speed prove that water droplets can reversibly move on this “dual-parallel-channel” shape gradient surface by applied an external voltage. We also prepared four-pool samples to achieve reversible and oriented motion of water droplets. In this sample, four pools were fabricated end to end in a circle, and connected by dual-parallelroads (Figure 5a). Furthermore, each pool was connected with two shape gradient roads which were extended into the adjacent pools over 3 mm. In this sample, all the sizes of pools (1 1 cm), roads (0.5 (width) 3 (width) 18 mm (length) (i.e., the gradient angle is 8°)) and the distance between roads (0.5 mm) are same to the above-mentioned double-pool sample. Figure 5b shows the clockwise motion of a water droplet on this four-pool sample, and the route is from pool A to pool B, to pool C, to pool D, and finally back to pool A. First, a water droplet (∼0.5 mL) was placed in pool A (Figure 5b(I)). After adding 25 V on pool B, both pool B and roads B switched from (super)hydrophobic to hydrophilic, from which the water droplet was driven to spread from pool A onto road B, and elongated along road B gradually (Figure 5b(II)). After the water droplet moved from the pool A onto the road B completely, it started to shrink from road B to pool B (Figure 5b(III)). Finally, with the release of the external voltage, the droplet could shrink from road B to pool B completely (Figure 5b(IV)). Subsequently, adding DOI: 10.1021/la9014898
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Figure 5. Reversible and oriented motion of water droplets on the dual-parallel-channel shape gradient surface with four pools. (a) Schematic representation of the motion route on the four-pool sample. (b) The clockwise motion of a water droplet (∼0.5 mL) from pool A, to pool B, to pool C, to pool D, and finally back to pool A. (c) The anticlockwise motion of a water droplet (∼0.5 mL) from pool A, to pool D, to pool C, to pool B, and finally back to pool A.
25 V on pool C instead of pool B, the droplet started to move from pool B onto road C, and elongated along road C gradually too (Figure 5b(V)). After moving from pool B onto road C completely, the droplet started to shrink from road C to pool C gradually (Figure 5b(VI)), and would completely enter pool C after releasing the voltage (Figure 5b(VII)). Then, switching 25 V on pool D, same to the above-mentioned process, the droplet began to move onto road D and elongated along it (Figure 5b(VIII)), and shrank along road D (Figure 5b(IX)) to pool D (Figure 5b(X)). Finally, adding 25 V on pool A, the droplet could move onto road A, and elongated (Figure 5b(XI)) and shrank (Figure 5b(XII)) along road A back to pool A completely (Figure 5b(I)), which just finished a clockwise circle motion. (See the Supporting Information, movie S2). On the other hand, water droplets can also achieve anticlockwise motion on this four-pool sample, i.e., the new motion route is from pool A to pool D, to pool C, to pool B, and finally back to pool A (Figure 5c). Adding 25 V on pool D, the droplet on pool A (Figure 5c(I)) started to move onto road D (Figure 5c(II)), then elongated and shrank along road D (Figure 5b(III)) to pool D completely after releasing the voltage (Figure 5c(IV)). Subsequently, adding 25 V on pool C, the droplet could repeat moving, elongating (Figure 5c(V)), and shrinking (Figure 5c(VI)) processes along road C, and finally enter pool C with the release of the external voltage (Figure 5c(VII)). Then adding 25 V on pool B, the droplet motion process from pool C to pool B was repeated, including moving, elongating (Figure 5c(VIII)), and shrinking (Figure 5c(IX)) processes along road B, and finally entered pool B completely after releasing the voltage (Figure 5c(X)). Finally, adding 25 V on pool A, the droplet could move from pool B back 14198 DOI: 10.1021/la9014898
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to pool A (Figure 5c(I)) according to the moving, elongating (Figure 5c(XI)), and shrinking (Figure 5c(XII)) processes, respectively, which just finished an anticlockwise circle motion. (See the Supporting Information, movie S3) In total, the clockwise and anticlockwise motions are completely reversible, and the average motion speed between any two adjacent pools is same, around 3.3 ( 0.2 mm/s (or 0.11 ( 0.01 mL/s). Moreover, although each pool has been connected by two shape gradient roads, water droplets can recognize which one is the “right” way they should go under the guidance of the external voltage, which works like a traffic light herein. 3.3. The Effects of Oil Bath, Gradient Angle, and External Voltage on the Motion Speed. We have discussed that water droplets could reversibly and orientedly move on the “dualparallel-channel” shape gradient surfaces under the guidance of an external voltage. However, the average motion speed depends on several parameters, including with/without oil bath, gradient angle, and the working voltage. It was found that, in air, water droplets could also reversibly move between two adjacent pools; however, the average motion speed is ∼1.6 ( 0.4 mm/s (or 0.05 ( 0.01 mL/s), much lower than that in an n-dodecane bath. According to the eq 3,21 which is used to estimate the driving force, we believe the lower motion speed in air is due to the lower driving force (∼61.5 mN/m) in contrast to the one (∼78.1 mN/m) in an n-dodecane bath. Herein, γw-x is the interfacial tension between water and the bath material (γw-air =72.8 mN/m, and γw-dodecane = 52.3 mN/m),28 and θ25V and θ0V are the contact angles at 25 and 0 V, respectively. δF ¼ γw -x ðcos θ25V - cos θ0V Þ
ð3Þ
According to the eq 3,21 the driving force to the water droplet increases as the difference of asymmetry contact angles increases. In our experiment, the water contact angle on the “hydrophilic” area (charged area) of the sample decreases from 162° to 57° with the voltage increasing from 0 to 25 V (Figure 3). Therefore, the driving force will increase with the improvement of the working voltage. Figure 6 shows the relationship between the average motion speed and the external voltage in an n-dodecane bath with an 8° gradient angle. With the working voltage at 0, 7, 14, 19, and 25 V, respectively, correspondingly, the average motion speed increases from 0 to 0.3, 1.2, 2.6, and 3.3 mm/s gradually ((0.2° error) (Figure 6). Furthermore, the motion speed is also governed by the gradient angle (R) (Figure 1), which indicates the space limitation of the droplet elongation. The bigger the gradient angle is, the less space limitation the droplet has when it elongates along the shape gradient road.21 In our experiments, the motion speed was tested on the samples with different gradient angles, 0°, 4°, 8°, 12°, and 16° ((0.2° error), respectively. In an n-dodecane bath, the average motion speed increased from 0.9 ( 0.1 to 6.2 ( 0.3 mm/s with the increase of the gradient angle from 0° to 16° at 25 V (Figure 6). We believe that, at low gradient angles, especially at zero gradient angle (R = 0), the space limitation of the transportation region is big, which deeply restricts the elongating speed of the droplets.21 With the gradient angle increasing, the space limitation of the transportation region releases gradually, correspondingly, the motion speed of droplets increases gradually too. Moreover, it should be noted here that, in our experiments, we find that during the sudden release of the applied voltage (25 V), the droplet sometimes breaks into two droplets (a main droplet at the “pool” (28) The data were obtained from the database “The Liquid Data Base (SFT) Drop Shape Analysis System G10/DSA10”; Kruess, Germany.
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acceleration, the narrow-neck phenomenon may occur on the droplet on the “road” because of the big space-limited (or narrow) rectangular “road”, and finally the droplet breaks into two after releasing the voltage. However, on shape-gradient “roads” with high gradient angles (R g 4°), this phenomenon is not found. Therefore, the shape-gradient “road” is the essential requirement to assist the droplet to completely shrink into the “pool” from the “road”.
Figure 6. Average speeds of ∼0.5 mL water droplet moving on the samples with different gradient angles (0, 4, 8, 12, and 16°) (the gradient start width of all samples is 0.5 mm) or under different working voltages (0, 7, 14, 19, and 25 V) (the gradient angle of the testing sample is 8°), respectively. (All tests were performed in an n-dodecane bath).
and a tiny droplet on the “road”) when the “road” is rectangular (R = 0) (see Supporting Information). We believe this phenomenon is due to two factors: (1) After releasing the voltage, the hydrophilic “road” and the “pool” suddenly turn (super)hydrophobic, which gives the droplet a sudden big acceleration to shrink toward the “pool”. (2) With such a big
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Conclusion In summary, based on the switching wettability of solid surfaces in electrowetting, we prepared “dual-parallel-channel” shape-gradient surfaces, on which water droplets can reversibly and orientedly move between two adjacent pools under the guidance of an external voltage. Furthermore, the motion speed is governed by several parameters, including bath condition, gradient angle, and working voltage. It is found that an n-dodecane bath, high gradient angle, and high working voltage all benefit the motion. In this self-transportation process of water droplets, the external voltage works like a traffic light, which can give “moving”, “stopping”, “turning” and “straight-going” signals to the water droplets. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (20621401) and the Ministry of Science and Technology of China (2009CB930603, 2009CB623604). Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.
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