pubs.acs.org/Langmuir © 2009 American Chemical Society
In Situ Electrochemical Switching of Wetting State of Oil Droplet on Conducting Polymer Films Mingjie Liu,‡,§ Fu-Qiang Nie,† Zhixiang Wei,*,‡ Yanlin Song,† and Lei Jiang*,† †
Beijing National Laboratory for Molecular Sciences (BNLMS), Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China, ‡National Centre for NanoScience and Technology, Beijing 100190, P. R. China, and §Graduate University of Chinese Academy of Sciences, Beijing 100049, P. R. China Received September 9, 2009. Revised Manuscript Received September 29, 2009 Switching of wettability is achieved in situ, which is a challenge of materials science. Generally, changing liquid droplet is required to ex situ study the wettability response before and after the surface given a treatment, in the sense that the liquid impregnation in the surface structures is irreversible. Herein, an in situ wettability switch is achieved by utilizing the same liquid droplet to characterize the dynamic wettability when the conducting polymer is being stimulated. The oil droplet is facilitated to escape from the nanoscale traps through electrochemically tuning surface composition and surface micro/nanostructures, permitting a reversible and rapid transition between partly wetting and superantiwetting state. This in situ switch is promising for integration into a microfluidic system for the control of the liquid droplet’s motion.
Introduction Switching of surface wettability by external stimuli has raised broad scientific interest due to its great importance in microfluidic systems, materials science, biotechnology and sensor devices.1-8 Recently, many efforts have been focused on the stimuli-responsive surfaces. Several thermally,9 pH,10,11 optically,12,13 solvent,14,15 electrically,16 or mechanical force17 responsive smart surfaces that can be switched between superwetting and superantiwetting have been reported. However, in all the cases, changing liquid droplet is required to ex-situ study the wettability response before and after the surface given a treatment, in the sense that the liquid impregnation in the surface structures is irreversible.18 Therefore, the achievement of in situ wettability switch remains a challenge. *Corresponding authors. E-mail: (Z.W.)
[email protected]; (L.J.) jianglei@ iccas.ac.cn. (1) Feng, X. J.; Jiang, L. Adv. Mater. 2006, 18, 3063–3078. (2) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431–432. (3) Ichimura, K.; Oh, S. K.; Nakagawa, M. Science 2000, 288, 1624–1626. (4) Blossey, R. Nat. Mater. 2003, 2, 301–306. (5) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 2125–2127. (6) Sun, T. L.; Feng, L.; Gao, X. F.; Jiang, L. Acc. Chem. Res. 2005, 38, 644–652. (7) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z. Q.; Jiang, L.; Li, X. Y. J. Am. Chem. Soc. 2004, 126, 3064–3065. (8) Zhai, L.; Cebeci, F. C.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349–1353. (9) Sun, T. L.; Wang, G. J.; Feng, L.; Liu, B. Q.; Ma, Y. M.; Jiang, L.; Zhu, D. B. Angew. Chem., Int. Ed. 2004, 43, 357–360. (10) Yu, X.; Wang, Z. Q.; Jiang, Y. G.; Shi, F.; Zhang, X. Adv. Mater. 2005, 17, 1289–1293. (11) Xia, F.; Feng, L.; Wang, S. T.; Sun, T. L.; Song, W. L.; Jiang, W. H.; Jiang, L. Adv. Mater. 2006, 18, 432–436. (12) Feng, X. J.; Feng, L.; Jin, M. H.; Zhai, J.; Jiang, L.; Zhu, D. B. J. Am. Chem. Soc. 2004, 126, 62–63. (13) Wang, S. T.; Feng, X. J.; Yao, J. N.; Jiang, L. Angew. Chem., Int. Ed. 2006, 45, 1264–1267. (14) Zhao, B.; Brittain, W. J. Macromolecules 2000, 33, 8813–8820. (15) Azzaroni, O.; Moya, S.; Farhan, T.; Brown, A. A.; Huck, W. T. S. Macromolecules 2005, 38, 10192–10199. (16) Isaksson, J.; Tengstedt, C.; Fahlman, M.; Robinson, N.; Berggren, M. Adv. Mater. 2004, 16, 316–320. (17) Zhang, J. L.; Lu, X. Y.; Huang, W. H.; Han, Y. C. Macromol. Rapid Commun. 2005, 26, 477–480. (18) Callies, M.; Quere, D. Soft Matter 2005, 1, 55–61.
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It will be a great breakthrough that if the same liquid droplet could be utilized to in situ characterizes the dynamic and changeable wettability when the surface is being stimulated. More importantly, if the wetting state of liquid droplet is capable of in situ reversible switch, the liquid droplet’s motion might be effectively controlled. This will develop a new strategy of driving liquid droplets, conquering conventional ways that highly depend on an imbalance of surface tension force inducing by a thermal,19 chemical,20-22 structural,23,24 or electrical gradient25 along the flow direction. In this paper, we develop a simple and efficient strategy to in situ electrochemical switch the wetting state of oil droplet on polypyrrole (PPy) films. Through electrochemically tuning surface composition and surface micro-/nanostructures, the wetting state of the oil droplet can be reversible switched between partly wetting and superantiwetting state. Furthermore, this reversibly switched PPy film is applied for smart control of oil droplet’s motion; i.e., an oil droplet can be selectively governed at a adhering or rolling state. Being a typical conducting polymer with unique electrochemical properties, PPy possessed broad range of high technological applications, such as artificial muscle,26,27 biosensor,28 and energy storage apparatus.29 Recently, how to control PPy wettability has attracted more and more attentions. PPy usually contains a (19) Chaudhury, M. K.; Whitesides, G. M. Science 1992, 256, 1539–1541. (20) Sumino, Y.; Magome, N.; Hamada, T.; Yoshikawa, K. Phys. Rev. Lett. 2005, 94, 06301–4. (21) Daniel, S.; Chaudhury, M. K.; Chen, J. C. Science 2001, 291, 633–636. (22) Bain, C. D. Chemphyschem 2001, 2, 580–582. (23) Linke, H.; Aleman, B. J.; Melling, L. D.; Taormina, M. J.; Francis, M. J.; Dow-Hygelund, C. C.; Narayanan, V.; Taylor, R. P.; Stout, A. Phys. Rev. Lett. 2006, 96, 154502. (24) Prakash, M.; Quere, D.; Bush, J. W. M. Science 2008, 320, 931–934. (25) Abdelgawad, M.; Wheeler, A. R. Adv. Mater. 2009, 21, 920–925. (26) Baughman, R. H. Science 2005, 308, 63–65. (27) Smela, E. Adv. Mater. 2003, 15, 481–494. (28) Li, Y. L.; Neoh, K. G.; Cen, L.; Kang, E. T. Langmuir 2005, 21, 10702– 10709. (29) Goward, G. R.; Leroux, F.; Nazar, L. F. Electrochim. Acta 1998, 43, 1307– 1313.
Published on Web 11/02/2009
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positively charged conjugated backbone and negatively charged counterions. By tuning the types and the doping level of the counterions, surface wettability of PPy could be controlled.30 eq 1 and 2 show two typically reversible switching processes during the PPy being redoxed, respectively. If the polymer is doped with a small and mobile anion, the influx and outflux of anionic counterions from the polymer matrix dominate during the redox process (eq 1). On the other hand, if the polymer is doped with a large and immobilized anion, influx and outflux of the cations in the electrolyte dominate during the redox process (eq 2). By choosing the perfluorooctanesulfonate as counterions, a reversible conversion of conducting polymer film from superhydrophobic to superhydrophilic state has been reported.31 In fact, the electrochemical reaction of PPy usually happens in the electrolyte, while the measurement of wettability is always carried out in the ambient atmosphere. To the best of our knowledge, the dynamic change of the wettability of PPy films during the redox process is still a puzzle. Herein, by using an oil droplet as a detective probe, we could realize in situ reversible control of wettability on a PPy films.
Results and Discussion The PPy films used in this work were synthesized via the electrochemical polymerization method. The Au-coated silicon wafer was used as a working electrode to prepare PPy films by a constant potential (0.7 V vs Ag/AgCl) polymerization from an aqueous solution containing 0.14 M pyrrole and 0.015 M dodecyl benzenesulfonic acid (HDBS). A cyclic voltammogram (CV) measurement of the PPy film was carried out in the 0.1 M lithium perchlorate electrolyte, where an oxidation peak at ∼-0.5 V and reduction peak at ∼-1.0 V could be observed (see Supporting Information, Figure S1). The morphology of the as-prepared dodecyl benzenesulfonic anion (DBS) doped PPy film was investigated by using atom force microscope (AFM) (Figure 1a), which revealed that numbers of PPy particles packed densely on the film forming a roughness structure. The wettability of PPy films was then evaluated by the contact angle measurement by using an oil droplet as a detecting probe. An optical microscope lens and a charge-coupled device (CCD) camera system combined with the electricity workstation were used to in situ monitor the contact angle of an oil droplet during adjusting the electrochemical potentials (see Supporting Information, Figure S2). In the in situ contact angle measurement, 1,2dichloroethane, and the 0.1 M lithium perchlorate aqueous were used as the detective oil droplet and electrolyte, respectively. The density of 1,2-dichloroethane is about 1.2 g/mL, which is larger than the electrolyte. As a result, the wettability of PPy film changed during it was redoxed, so the dynamic change of the contact angle could be in situ observed (Figure 1b). This wettability switch could be repeated more than ten times, and a good reversibility was observed (Figure 1b). Figure 1c is contact angle (30) Azioune, A.; Chehimi, M. M.; Miksa, B.; Basinska, T.; Slomkowski, S. Langmuir 2002, 18, 1150–1156. (31) Xu, L. B.; Chen, W.; Mulchandani, A.; Yan, Y. S. Angew. Chem., Int. Ed. 2005, 44, 6009–6012.
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Figure 1. PPy surface morphology and in situ characterize the wettability of the PPy surface change when the film was applied with different voltages. (a) AFM image of the PPy surface. (b) Reversible wettability switching by applying with negative and positive voltage. The inserted figure is an oil droplet profile for transferring from low contact angle state to high contact angle state on the PPy surface when the voltage being switched to 0.55 V and -0.9 V, respectively. c) Contact angle vs time for an oil droplet in electrolyte at different voltage proving real-time reversibility of the wettability switching. The reduced voltage is -0.9 V and the oxidized voltage is 0.55 V.
vs time for an oil droplet in electrolyte at different voltage, demonstrating real-time reversibility of the wettability switching. When a negative voltage (-0.9 V vs Ag/AgCl) was applied on the PPy film, it showed superoleophobic with a contact angle of ∼149.1° in the lithium perchlorate electrolyte. When the voltage was switched at a positive voltage (0.55 V vs Ag/AgCl) applied on the PPy film, the contact angle of oil droplet dramatically decreased to 116.7° in half a minute. Interestingly, when the negative voltage (-0.9 V vs Ag/AgCl) was applied on the PPy film again, the contact angle of the same oil droplet reversibly increased. About 1 min later, the PPy film behaved superoleophobic with a contact angle of ∼145.6° in the reduced state. Undoubtedly, it would be a great breakthrough that the contact angle of the same oil droplet could be reversibly transform from a low state to a higher state by tuning the electrochemical potential, which was attributed to the change of PPy films from the oxidized to the reduced state. In general, surfaces with different contact angles show different adhesive force, so we also investigated the adhesive force changing during electrochemically process. In our experiments, the adhesive force preventing oil droplet being drawn away from the surface can be assessed by a high-sensitivity micromechanical balance system. Dynamic force change was recorded by the above balance system during the PPy film was controlled to contact and leave an oil droplet. In the in situ adhesive force measurements, the PPy film applied with different voltages displayed different adhesive behavior for oil droplet, indicated in the change of the shape of oil droplet. When the PPy film was oxidized at a positive Langmuir 2010, 26(6), 3993–3997
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Figure 2. Adhesive force between oil droplet and the surface when the PPy film in different redox state, the inside pictures are the shapes of oil droplet taken at different stages during the force measurements.
voltage (0.55 V vs Ag/AgCl) and controlled to leave oil droplet, the shape of oil droplet was drawn from the spherical to elliptical, and then the shape changed back to the spherical after oil droplet left the surface (Figure 2). The adhesive force measured was 8.7 ( 1.8 μN. In contrast, when the PPy film was reduced at a negative voltage (-0.9 V vs Ag/AgCl), it displayed a lower adhesive force of 1.6 ( 0.9 μN. Interestingly, the shape of oil drop kept spherical during the whole measurement. The wetting state of the oil droplet could be in situ switched on the PPy films, so this property could be utilized to study smart control of the liquid droplet’s motion. By switching the electrochemical potential of the film, it has been realized that the oil droplet could selectively adhere onto or roll down the surface. In this study, oil droplet was also 1,2-dichloroethane. In Figure 3, the PPy film was tilted about 4 degrees. An oil droplet was “parking” on the PPy film by adhering at a positive voltage (0.55 V vs Ag/ AgCl) (Figure 3a). When the PPy film was reduced (-0.9 V vs Ag/ AgCl), the contact angle of the oil droplet increased with the shape changing (Figure 3b). After the PPy film was reduced for 53s, it became superoleopobic, and the oil droplet started to roll down (Figure 3c, d), which indicated the PPy film became superoleophobic, and the adhesive force between oil droplet and the surface greatly decreased. After the oil droplet had run a distance, a positive voltage (0.55 V vs Ag/AgCl) was applied. Oil droplet could “brake” and adhere on the film again. When the negative voltage (-0.9 V vs Ag/AgCl) was applied again, it restarted to roll down the film half a minute later (Figure 3e-g). Therefore, the motion of the liquid droplet could be controlled by in situ tuning the wettability of the PPy film. This strategy was different from most traditional ways that highly depended on an imbalance of surface tension force.22 To thoroughly understand what caused the in situ reversible switching of wetting state of the oil droplet when the PPy film was redoxed, surface composition and morphology as two main factors governing the surface wettability were considered. Our previous studies indicated that the contact angle of the oil droplet on the solid substrate in the presence of the water phase could be calculated from eq 3,32 cos θ3 ¼
γl1 -g cos θ1 - γl2 -g cos θ2 γl1 -l2
ð3Þ
where γl1-g is the oil-air interface tension, θ1 is the contact angle of oil droplet on the substrate in air phase, γl2-g is the water-air interface tension, θ2 is the contact angle of water droplet on the substrate in air phase, γl1-l2 is the oil-water interface tension, and θ3 is the contact angle of oil droplet on the substrate in water phase. (32) Liu, M. J.; Wang, S. T.; Wei, Z. X.; Song, Y. L.; Jiang, L. Adv. Mater. 2009, 21, 665–669.
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Figure 3. The smart control of an oil droplet motion on the PPy surface by adjusting the electrochemical potentials. (a) An oil droplet was parking on the PPy surface by adhesion. (b) The contact angle of oil droplet increased when the PPy surface was reduced. (c) The oil droplet began to run on the surface. (d) The oil droplet braked and adhered on the PPy surface again. (e-g) The oil droplet came back to the ball state and rolled down.
The experimental results mentioned above could be explained by eq 3. The PPy films synthesized were doped with DBS-, which is a large and immobile anion that cannot be expelled upon reduction. The influx and outflux of lithium cations dominates in the redox process. When the oxidized PPy film was changed to reduced state, the lithium cations were influxed to the PPy matrix, strengthening the hydrophilicity of the surface, which meant water contact angle in air phase (θ2 in eq 3.) would decrease (see Supporting Information, Figure S3). As γl1-g, γl2-g, γl1-l2, and θ1 kept constant (see Supporting Information, Figure S3), oil contact angle θ3 would increase according to eq 3. Therefore, oil contact angle became larger than that in the oxidized state. When the reduced PPy film was changed to oxidized state, the lithium cations were outfluxed from the PPy matrix, DBS- anions were coupled to the PPy chain via an ionic bond to the sulfonic acid group, resulting in the dodecyl chains pointing out from the polymer backbone. Thus, the dodecyl weakened the hydrophilicity of the surface, which meant the water contact angle in air phase (θ2 in eq 3 would increase. Since γl1-g, γl2-g, γl1-l2, and θ1 kept constant, the contact angle θ3 would decrease according to eq 3. As reflected in our experiments, the contact angle of the same oil droplet became smaller than that in the reduced state. XPS results revealed that little lithium element existed in the oxidized state while certain amounts of lithium were detected in the reduced state (see Supporting Information, Table S1, Figure S4). The peak-fitted N 1s core-line spectra of the PPy surface in oxidized and reduced state are shown in Figure S5, respectively. The high resolution spectrum revealed that two kinds of nitrogen environments are present in the oxidized state. The stronger peak at 400.4 eV could be assigned to neutral -NH- whereas the higher binding energy peak at 402.4 eV was assigned to the oxidized -Nþ- moieties (see Supporting DOI: 10.1021/la903392n
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Figure 4. In situ electrochemical atomic force microscopy (EAFM) of the PPy surface morphologies during the redox process. (a, b) In situ EAFM images of PPy surface in oxidized state and reduced state at the same area, respectively. (c, d) Height traces across the regions indicated by the black line in parts a and b, respectively. It could be seen that the morphology is different between two states, and the reduced state is rougher.
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according to the EAFM results. Microscale papillae formed because of the stress effect between the PPy films and the gold layers. The ripples and papillae in the reduced state further increased the roughness of the surface, while no obvious microstructure was observed in the oxidized state (see Supporting Information, Figure S6). The roughness in the reduced state was propitious to enhance the oleophobicity of the PPy films. According to the change of surface composition and morphology, a mechanism scheme was proposed to explain the in situ wettability switch, as shown in Figure 5. Because the PPy films possessed a micro-/nanostructure in the reduced state, the electrolyte would be trapped in the PPy films. This effect significantly decreased the contact area between the oil droplet and the rough surface in contrast to the oxidized state. The micro-/nanostructures would greatly enhance the superoleophobic effect and decrease the adhesive force between oil droplet and the surface. More importantly, the tuning microstructures facilitated the oil drop to escape from the surface nanoscale traps, permitting a reversible and rapid transition between partly wetting and superantiwetting states.
Conclusion In summary, a new strategy was presented for the in situ reversible switching of the wetting state of an oil droplet on PPy films by adjusting the electrochemical potentials. As the droplet was switched between the oxidized and reduced states, the contact angles of the same oil droplet on PPy films could be reversibly tuned. This property was utilized for manipulating liquid droplet, so that the states of oil droplet like adhering or rolling down the surface could be selectively governed. XPS and EAFM results indicated that the in situ switching of wettability was ascribed to the tunable surface composition and surface micro-/nanostructures. In situ wettability switch would be promising for a wide range of applications, such as microfluidic devices, biotechnologies, and liquid transport.
Experimental Section Surface Morphology Characterization. The in situ EAFM Figure 5. Scheme of proposed mechanism for the in situ electrochemical switching of the wetting state of an oil droplet on the PPy films.
Information, Figure S5a). In contrast, only one nitrogen environment was present in the reduced state with the peak at 399.8 eV assigned to the neutral -NH- (see Supporting Information, Figure S5b).33 These results further confirmed that the change of surface composition caused the switching of contact angles. Besides surface composition changing, the surface morphology influence should also be taken into account to explain the in situ wettability switch. In our study, the morphology of the PPy films would be changed during the redox process because of influx and outflux of lithium ions. For this aim, an in situ electrochemical atomic force microscopy (EAFM) study of the PPy film that could provide a real-time evidence for the morphology changing during the electrochemical process was utilized. As shown in Figure 4, it was found that microstructure formed in the reduced state, which originated from the volume expansion of the PPy films due to the influx of lithium ions. The roughness factors were 11.58 nm in the oxidized state and 12.31 nm in the reduced state (33) Tan, K. L.; Tan, B. T. G.; Kang, E. T.; Neoh, K. G. J. Chem. Phys. 1991, 94, 5382–5388.
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images were obtained by an atomic force microscopy (SPA 300). The sample was placed in a special designed electrochemical cell which contained a counter electrode and reference electrode. The AFM possessed an electrochemical control system which would enable the sample go through the electrochemical reaction during the scanning process. In Situ Contact Angle Measurements. The contact angles of oil droplet were measured by using a CCD camera system combined with a special designed electrochemical cell (see Supporting Information, Figure S2). The platinum plate was used as a counter electrode and the Ag/AgCl was used as the reference electrode. The electrochemical process was controlled by the electrochemical station. In Situ Adhesive Force Measurements. The adhesive force that prevented the oil droplet being drawn away from the surface was measured by using a high-sensitivity microelectromechanical balance system (Data-Physics DCAT 11, Germany). The substrate was a work electrode controlled by an electrochemical station. The dynamic force change was measured under water environment. An oil droplet (about 2 μL) was suspended with a metal ring first, and the substrate was placed in an electrochemical cell. The substrate was moved upward at a constant speed of 0.05 mms-1 until the substrate contacted with the oil droplet. When the substrate was moved down, the force increased and the shape of the oil droplet changed from spherical to elliptical. The contact force sharply reduced and the shape of the oil changed back to spherical, after the oil droplet left the substrate. Langmuir 2010, 26(6), 3993–3997
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Control of Liquid Droplet’s Motion. The control of the liquid droplet’s motion experiment was also carried out by the contact angle measurement system. An oil droplet was placed on the tilted PPy film. The voltage applied on the PPy film was controlled by the electrochemical station. The CCD was used to record the whole process. Acknowledgment. This work was supported by a grant of Major State Basic Research Development Program (2007-
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Article CB936403) and by the National Nature Science Foundation of China (20571077). We thank L. Y. Liu for technical support and Y. Zhao, J. Wang, H. Meng, and H. Bai for helpful discussion.
Supporting Information Available: Figures showing the cyclic voltammogram measurement, scheme of the experiment setup, contact angle measurements, and XPS results and a table of XPS data. This material is available free of charge via the Internet at http://pubs.acs.org.
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