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Manipulation of Droplets by Dynamically Controlled Wetting Gradients Ryo Yamada*,† and Hirokazu Tada‡ Molecular-scale Electronics Division, Research Center for Molecular-scale Nanoscience, Institute for Molecular Science, Higashiyama, Okazaki 444-8787, Japan, The Graduate University for Advanced Studies, Higashiyamai Okazaki, 444-8787, Japan, and CREST, Japan Science and Technology Agency, Honcho, Kawaguchi 332-0012, Japan Received December 8, 2004. In Final Form: March 2, 2005 The reversible transportation of droplets was realized by spatiotemporal control of the wetting gradient. The surface wetting was reversibly regulated by using electrochemical reactions of the ferrocenyl (Fc) alkanethiol monolayer, and the wetting gradient was generated by the application of the in-plane bias voltage to the substrate. The back-and-forth motion of the wetting boundary, where the surface changed from wetting to repulsive, sequentially caused a droplet unidirectional spreading and shrinking on the surface. These unidirectional deformations resulted in the net transport of the droplet in an inchwormlike manner. The droplet moved backward when the direction of the in-plane bias voltage was reversed.
Introduction The manipulation of a droplet on a solid surface is of growing interest because it is a key technology to construct lab-on-a-chip systems.1 The imbalance of surface tensions is known to play an important role in the movement of droplets on surfaces.2 The droplet moves on the gradient surface because of the difference in advancing and receding contact angles at the front and rear ends of the droplet. The wetting gradient causing liquid motion has been prepared by chemical,2 thermal,3 electrochemical,4 and photochemical5,6 methods. The mechanical deformation of the droplet on the gradient surface was also reported to cause a ratchetlike motion when hysteresis of the contact angle was large.7 In the methods reported so far, the static nature of the wetting gradient was employed to move the droplet. Here, we report that a reversible and stepwise manipulation of a droplet was realized by a dynamically controlled wetting gradient on a thin-film metal electrode. The wetting boundary, where the surface changed from wetting to repulsive, was generated and reversibly moved on the surface by applying the in-plane slope of the electrochemical potential to the substrate modified with redox-active monolayer. The one side of the droplet spread and the other side shrunk when the wetting area approached and left the droplet, respectively. These sequential deformations resulted in the net transport of the droplet in inchworm motion. The liquid-liquid interface * Corresponding author. E-mail:
[email protected]. Fax: 81564-59-5519. Phone: 81-564-59-5522. † Institute for Molecular Science and The Graduate University for Advanced Studies. ‡ Institute for Molecular Science, The Graduate University for Advanced Studies, and CREST. (1) For a recent review, see: Reyes, D. R.; Iossifidis, D.; Auroux, P.-A.; Manz, A. Anal. Chem. 2002, 74, 2623. Velev, O. D.; Prevo, B. G.; Bhatt, K. H. Nature 2003, 426, 515 and references therein. (2) Chaudhury, M. K.; Whitesides, G. M. Science 1992, 256, 1539. (3) Cazabat, A. M.; Heslot, F.; Troian, S. M.; Carles, P. Nature 1990, 346, 824. (4) 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. (5) Ichimura, K.; Oh, S.-K.; Nakagawa, M. Science 2000, 288, 1624. (6) Abbott, S.; Ralston, J.; Reynolds, G.; Hayes, R. Langmuir 1999, 15, 8923. (7) Daniel, S.; Sircar, S.; Gliem, J.; Chaudhury, M. K. Langmuir 2004, 20, 4085.
Figure 1. (a) Schematic drawing of the experimental configuration. (b) Potential profile and wetting distribution on the substrate under the biased condition.
between the droplet and the solution was found to act as a barrier for small particles on the surface, which enabled a direct transport of them. Results and Discussion Figure 1 shows principles for the generation and control of the wetting gradient. The gold thin film covered with ferrocenyl (Fc) alkanethiol monolayer was immersed in the electrolyte solution, and the potential of the gold substrate, Eoffset, was controlled with respect to the reference electrode in the solution. Lateral bias voltage, Vbias, was applied to the gold substrate to generate inplane gradients in the electrochemical potential.8 The electrochemical potential between “A” and “B” denoted in Figure 1a is expected to change linearly when only a negligible current flows into the counter electrode in the electrochemical cell. Figure 1b schematically shows the relationships between these electrochemical parameters and the concentration of Fc+ in the monolayer as a function of the position on the gold substrate, x. When the electrochemical potential in the gold substrate and the redox potential of Fc+/Fc in the monolayer have an intersection, we obtain gradients in the Fc+ concentration around the point P denoted in Figure 1b. Since Fc- and (8) Terrill, R. H.; Balss, K. M.; Zhang, Y.; Bohn, P. W.; J. Am. Chem. Soc. 2000, 122, 988.
10.1021/la046982t CCC: $30.25 © 2005 American Chemical Society Published on Web 03/31/2005
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Figure 2. Photographs of nitrobenzene droplets. Vbias was fixed at 0.35 V, and Eoffset was shifted (a-d). The line in the photograph represents the position of the wetting boundary estimated from the shape of the droplets. Note that the current peak for the Fc+/Fc reaction was observed at ∼ -0.5 V in the cyclic voltammogram.
Fc+-covered surfaces are known to be hydrophobic and hydrophilic, respectively,14,15 the wetting gradient is formed around P. We call P the wetting boundary, where the surface changes from hydrophilic to hydrophobic. The position of the wetting boundary and the magnitude of the wetting gradient can be reversibly controlled as functions of Eoffset and Vbias/l, respectively, where l is the length of the substrate in the biased direction. Figure 2 shows the pictures of the nitrobenzene droplets on the gold electrode covered with Fc monolayer in aqueous solution when Vbias was applied. The local potentials of the substrate were measured by contacting the wires on the substrate, and linear potential distribution was confirmed. Figure 2a clearly shows that the wetting changes as a function of the position of the substrate. The position of the wetting boundary corresponds to a potential of ∼ -0.5 V where the current peaks of Fc+/Fc reactions were observed in the cyclic voltammogram. Figure 2b-d shows that the boundary shifts as a function of Eoffset. Figure 3 shows motions of the droplet caused by the shift of the wetting boundary. Initially (Figure 3a), Eoffset and Ebias were set to -0.3 and -0.5 V, respectively. The potentials of the left and right edges of the substrate were expected to be ∼ -0.8 and ∼ -0.3 V, respectively, under these conditions.16 The left-hand side of the substrate in (9) Chidsey, C. E. D. Science 1991, 251, 919. (10) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991, 7, 1510. (11) Collard, D. M.; Fox, M. A. Langmuir 1991, 7, 1192. (12) Rowe, G. K.; Greager, S. E. Langmuir 1991, 7, 2307. (13) Popenoe, D. D.; Denhammer, R. S.; Porter, M. D. Langmuir 1992, 8, 2521. (14) Abbott, N. L.; Whitesides, G. M. Langmuir 1994, 10, 1493. (15) Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1994, 10, 4380. (16) Contact and cable resistances were not considered in this estimation.
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Figure 3. Photographs of inchworm motion of the droplet in the solution. See text for details. Ebias ) 0.5 V. Eoffset ) -300 mV (a), -340 mV (b), and -300 mV (c). (d) Trace of the multistep inchworm motions of the droplet. Six photographs were superimposed.
Figure 3 was more wetting to nitrobenzene than the right side of it under this bias direction. The initial appearance of the droplet shown in Figure 3a indicated the droplet was on the repulsive side of the wetting boundary at this stage. When Eoffset was changed to more negative values to move the wetting boundary to the droplet, the droplet gradually spread for wetting areas. Figure 3b shows the droplet when Eoffset was -0.34 V, where the droplet was almost completely wet. The net transport of the droplet was not clearly observed at this stage while the center of the droplet moved. When Eoffset was changed to the initial value to move the wetting boundary to the reverse direction, the droplet gradually shrunk from the repulsive side and returned to its original appearance, as shown in Figure 3c. The unidirectional spreading-and-shrinking cycle resulted in the net transport of the droplet in an inchwormlike manner. The positions of the contact lines moved as a result of the imbalance between the forces acting on the droplet edges during the deformation of the droplet. Since the surface had a wetting gradient, the forces acting on the contact lines at the more wetting side were always larger than those acting on the other side during the deformation of the droplet. As a result, the rear and front ends were pinned when the droplet spread and shrank, respectively. It should be noted that the directional spreading of the droplet was caused by the imbalance of advancing contact angles acting on the opposite sides of the droplet, while the directional shrinking was due to the imbalance of receding contact angles. These mechanisms differentiate the motion of the droplet presented here from the motion driven by the static wetting gradient originating from the difference in the contact angles between advancing and receding sides of the droplet. A single inchworm step shown in Figure 3 was completed in 35 s. The wetting transition accompanied by electrochemical reaction took place in less than 1 s, and thus, the
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for the droplets of dichloromethane and hexadecane in aqueous solutions. The motion of droplets in solutions was used to manipulate microparticles on surfaces.17 Figure 4a shows a picture of the dichloromethane droplet that was moved on the gold substrate covered with Fc alkanethiol monolayer in the presence of hydrophilic glass beads of 40 µm in diameter. The droplet was moved from the upper right of the picture for several steps. The beads located in the path of the droplet were swept away because the hydrophilic glass beads could not get into the oil droplet. In contrast, the hydrophobic beads initially dispersed randomly in the dichloromethane droplet (Figure 4b) were gathered at the receding end of the droplet (Figure 4c) when the droplet was moved several times because the hydrophobic beads could not get out of the droplet. In both cases, the liquid-liquid and/or solid-liquid-liquid interfaces acted as barriers for the particles. In conclusion, we have shown that the dynamically controlled wetting gradient can be used for the stepwise and reversible manipulation of the droplet in solutions. The in-plane regulation of the electrode potential coupled with the electrochemical reaction of the monolayer realized the reproducible and reversible control of the wetting gradient as a function of the electrode potential. The proposed method would lead a new transportation mechanism for small droplets and particles on the surface in small spaces. Experimental Section The substrates were thin gold films of 50 nm thickness formed by thermal vacuum evaporation on glass slides with a 5 nm Ti underlayer and were modified in 1 mM 11-ferrocenyl-1-undecanethiol (Dojindo chemicals) in methanol for 2 h. Electrochemical measurement was carried out with a potentiostat (PS-06, Toho Technical Research) in 1 M HClO4 aqueous solution. AuOx and Pt foil electrodes were used as reference and counter electrodes, respectively. In-plane bias voltage was applied to the gold substrate with a voltage source (LX010, Takasago). Hydrophobic glass beads were prepared by modifying hydrophilic glass beads with hexamethyldisilazane. All electrochemical potentials in the figures are presented with respect to the AuOx reference electrode. In the present experiment, the potential window was limited by hydrogen evolution and oxidative desorption of the monolayer, starting around -1.5 and 0.25 V, respectively. The reductive desorption of the monolayer would limit the negative potential range in the solution of higher pH.18
Figure 4. Transportation of glass beads by the droplet. (a) Hydrophilic glass beads were pushed by the oil droplet. The droplet moved from the upper right of the figure. A magnified image of the squared region is shown in the inset. Hydrophobic beads in the droplet (b) were carried with the motion of the oil droplet (c). The droplet shown in part c was moved for several millimeters in the direction shown by the arrow in part b.
rate of the droplet motion was limited by the viscous flow of the liquid. The pinning of the droplet sometimes took place at the defect of the surface. In most cases, we could move the pinned droplet by making the wetting gradient stronger or changing the limits of potential cycles. The inchworm motion could be repeated many times, as shown in Figure 3d, and the reverse motion was possible by changing the bias direction. The same motion was observed
Acknowledgment. This work was partly supported by Grant-in-Aid for Young Scientists (no. 16750018) of Japan Society for the Promotion of Science. Supporting Information Available: The droplet deformations measured with the cyclic voltammogram and movies of the droplet motions. This material is available free of charge via the Internet at http://pubs.acs.org. LA046982T (17) In these experiments, Ebias was ∼0.8 V and Eoffset was cycled between -0.8 and -0.3 V at a rate of 100 mV/s. Exact potential values slightly differ in every experiment because of the different contact resistance, size, and position of the droplet. In practice, the cycle window of Eoffset was adjusted manually so that the droplet began to move. (18) Sato, Y.; Ye S.; Haba, T.; Uosaki, K. Langmuir 1996, 12, 2726. Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335.
