pubs.acs.org/Langmuir © 2009 American Chemical Society
Water Droplets’ Internal Fluidity during Horizontal Motion on a Superhydrophobic Surface with an External Electric Field Munetoshi Sakai,† Hiroki Kono,†,‡ Akira Nakajima,*,†,§ Hideki Sakai,‡ Masahiko Abe,‡ and Akira Fujishima† † Kanagawa Academy of Science and Technology, 308 East, Kanagawa Science Park, 3-2-1 Sakado, Takatsu-ku, Kawasaki-shi, Kanagawa 213-0012, Japan, ‡Department of Pure and Applied Chemistry, Science University of Tokyo, 2641 Yamazaki, Noda-shi, Chiba 278-8510, Japan, and §Department of Metallurgy and Ceramics Science, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan
Received October 1, 2009. Revised Manuscript Received November 11, 2009 On a superhydrophobic surface, the internal fluidity of water droplets with different volumes (15, 30 μL) and their horizontal motion in an external electric field were evaluated using particle image velocimetry (PIV). For driving of water droplets on a superhydrophobic coating between parallel electrodes, it was important to place them at appropriate positions. Droplets moved with slipping. Small droplets showed deformation that is more remarkable. Results show that the dielectrophoretic force induced the initial droplet motion and that the surface potential gradient drove the droplets after reaching the middle point between electrodes.
I. Introduction Surfaces with a water contact angle greater than 150° (i.e., superhydrophobic surfaces) are currently a subject of great interest and intensive study. The small contact area between water and a solid surface of this type is expected to mitigate various phenomena such as friction drag and the adherence of snow or water droplets. Hydrophobic properties are known to be enhanced by increased surface roughness.1,2 Superhydrophobic surfaces require appropriate surface roughness and low surface energy. Numerous methods to accomplish this have been reported in the literature to date.3 Water droplets on superhydrophobic surfaces typically slide down with a large sliding velocity that is attributable to the large contribution of the slipping motion.4,5 The surfaces are almost nonresistant to the liquid’s flow. For that reason, small external forces can move a water droplet on such a surface.6,7 Reportedly, a water droplet slides down a tilted superhydrophobic surface with almost no deformation.4 In addition to gravity, the motion and wettability of a water droplet are controllable by an electric field on the superhydrophobic surface.8-11 This phenomenon results from electrification9,10 or polarization12 that occurs by contact with a solid under a strong *To whom correspondence should be addressed. E-mail: anakajim@ ceram.titech.ac.jp. Telephone: þ81-3-5734-2525. Fax: þ81-3-5734-3355.
(1) Wenzel, R. N. J. Phys. Colloid Chem. 1949, 53, 1466. (2) Cassie, A. B. D. Discuss. Faraday Soc. 1948, 3, 11. (3) Nakajima, A.; Hashimoto, K.; Watanabe, T. Monatsh. Chem. 2001, 132, 31. (4) Sakai, M.; Song, J.-H.; Yoshida, N.; Suzuki, S.; Kameshima, Y.; Nakajima, A. Langmuir 2006, 22, 4906. (5) Gogte, S.; Vorobieff, P.; Tresdell, R.; Mammoli, A.; van Swol, F.; Shah, P.; Brinker, C. J. Phys. Fluids 2005, 17, 51701. (6) Miwa, M.; Fujishima, A.; Nakajima, A.; Hashimoto, K.; Watanabe, T. Langmuir 2000, 16, 5754. (7) Quere, D. Rep. Prog. Phys. 2005, 68, 2495. (8) Beni, G.; Hackwood, S. Appl. Phys. Lett. 1981, 38(4), 207. (9) Higashiyama, Y.; Yanase, S.; Sugimoto, T. IAS Annu. Meet., IEEE Ind. Appl. Conf. 1998, 3, 1808. (10) Takeda, K.; Nakajima, A.; Murata, Y.; Hashimoto, K.; Watanabe, T. Jpn. J. Appl. Phys. 2002, 41, 287. (11) Lee, J.; Kim, C.-J. J. Microelectromech. Syst. 2000, 9(2), 171. (12) Mugele, F.; Baret, J.-C. J. Phys.: Condens. Matter 2005, 17, R705.
Langmuir 2010, 26(3), 1493–1495
electric field. Tuning of the wettability, perhaps by decreasing a contact angle of liquid by the electric charge, is commonly designated as electrowetting. The dynamic aspects of electrowetting have supported various applications: lab-on-a-chip, optical instruments, display technology, and microelectromechanical systems.11-13 However, detailed analyses of shape deformation, including internal fluidity on the droplet moving on a horizontal superhydrophobic surface by an electric field, have not been well conducted to date. Recently, we developed a particle image velocimetry (PIV) system.14 In the current study, using a high-speed camera with the PIV system, we evaluated the internal fluidity during the motion of water droplets by an electric field on a superhydrophobic surface.
II. Experimental Section The superhydrophobic coating was prepared by spraying a commercial paint (HIREC450; NTT Advanced Technology Corp., Tokyo, Japan) onto glass plates (25 mm 50 mm 1 mm). This coating, which comprises poly(tetrafluoroethylene) (PTFE) particles and fluoropolymer and which has low surface energy and low electric permittivity, is described in detail in the Supporting Information. To minimize the effect of gravity, the experimental setup depicted in Figure 1a was used to induce horizontal motion. Glass with the superhydrophobic coating was laid down horizontally, and a pair of parallel copper electrodes (interval: 10 mm) connected to a power supply (HVS 448 6000 D; Lab Smith Ltd.) was placed on the bottom side of the glass. To show the internal fluidity and the droplet silhouette, the water droplets contained 0.06 wt % fluorescent particles (3 μm diameter, 1.05 cm3/g density, 542 nm excitation wavelength, 612 nm emission wavelength, R0300; Duke Scientific Corp., CA). Preliminary experiments revealed that the additional amount of the indicator particles only slightly affected the water viscosity and the practical moving behavior under an electric field. (13) Cho, S. K.; Moon, H.; Kim, C-J J. Microelectromech. Syst. 2003, 12(1), 70. (14) Sakai, M.; Hashimoto, A.; Yoshida, N.; Suzuki, S.; Kameshima, Y.; Nakajima, A. Rev. Sci. Instrum. 2007, 78, 045103.
Published on Web 11/19/2009
DOI: 10.1021/la903730k
1493
Letter
Sakai et al.
Figure 2. Images showing the distribution of the velocity vector of the internal fluidity in moving water droplets of (a) 15 μL and (b) 30 μL. The P-number of the figure denotes a droplet position between electrodes (Figure 1b).
Figure 1. Schematic illustrations of the (a) alignment of the experimental setup and (b) position number on the capacitor. Suspension droplets of 15 and 30 μL were placed onto the coating at a point between the electrodes. The position of placement was as depicted in Figure 1b. The droplet was dropped from a height of around 1 mm above the coating. The droplet’s central section was imaged using a sheet-like Ar ion laser (1000 mW intensity, 200 μm sheet width, 488 and 514 nm wavelengths; Seika Corp., Tokyo). The gradually applied voltage had a voltage difference between electrodes of 4000 V DC. Voltages for positive and negative electrodes were, respectively, þ2000 and -2000 V. Sequential images depicting the central section of the sliding droplet were obtained using a high-speed camera (1024 PCI; Photron Ltd., Tokyo, Japan). The movement distance was estimated by measuring the sliding distance of the front edge of the contact line between the droplet and sample surface from the initial starting point. Simultaneously, the advancing and receding contact angles were obtained. Detailed experimental conditions for the evaluation of internal fluidity are described in the Supporting Information. Moreover, to investigate the wettability between electrodes under the influence of the electric field, contact angle measurements were conducted using the sessile drop method with a contact angle meter (Dropmaster 500; Kyowa Interface Science Co. Ltd., Saitama, Japan). Water and an ionic liquid [1-butyl-3methyl-imidazolium hexafluorophosphate (1.38 g/mL density, 198 mPa 3 s viscosity; Aldrich Chemical Co. Inc.)] were used for this measurement, as was hexadecane. The droplet volumes of ionic liquid and water were 5 and 10 μL, respectively. The droplets remained at their positions of placement under the influence of an applied voltage. Contact angles were measured at five different points for the coating. The surface was blown with ionized air before each measurement. To stabilize the contact angles, measurements were performed 200 s after placing the droplet.
III. Results and Discussion The droplet moved smoothly from the positive electrode side to the negative electrode side when the droplet was placed at point 2; half the mass of the droplet was above the positive electrode (Figure 1). In this case, it was necessary to apply voltage to the negative electrode after working the positive electrode. Similarly, 1494 DOI: 10.1021/la903730k
Figure 3. Advancing contact angle, receding contact angle, and moved distance during elapsed time for water droplets: (a) 15 μL and (b) 30 μL.
a droplet moved smoothly from the negative electrode side to the positive electrode side when the droplet was placed at position 6 and the electrodes were turned in a reverse direction. However, the droplet did not move while maintaining the contact angle, when the droplet was placed completely above an electrode, such as at position 1 or 7. The droplet moved toward the nearer running electrode when the droplet was placed between the positive electrode and negative electrode, for example, at positions 3-5. These results suggest that the main driving force for the droplet motion in this case is not the Coulomb force but the dielectrophoretic force, which results from the electric field gradient (see the Supporting Information). For driving of a water droplet on the superhydrophobic coating between the electrodes, it was important to place it at an appropriate position. Figure 2 presents sequential photographs of the moving behaviors of a water droplet on the superhydrophobic coating. The vector implies the velocity of the internal fluidity. The speed is increased according to the color transition from blue to red. For the 15 μL droplet, the velocity vector of internal flow expressed the slipping motion with the velocity vector of similar colors in the elapsed time pointed in the same direction toward the negative electrode (Figure 2a). A similar slipping trend was also observed for the 30 μL droplet. Moreover, the curved array of the velocity vector in the tail of the droplet resembled a trailing vortex (Figure 2b). This vortex is remarkable in the early stage of sliding. Figure 3 displays the advancing contact angle, the receding contact angle, and the distance of movement of the droplets according to elapsed time. The distance of 0 mm in the figure corresponds to position 2. Under the electric fields, the 15 and 30 μL water droplets moved by nearly constant acceleration motion, although the initial acceleration seems quite small. Langmuir 2010, 26(3), 1493–1495
Sakai et al.
Letter
Figure 4. Contact angles at each position under an electric field with 4000 V: (a) water, (b) ionic liquid, and (c) hexadecane.
Immediately after applying high voltage, the receding contact angle decreased and became smaller than the advancing contact angle, as is typically shown with sliding on a normal hydrophobic surface. Subsequently, the advancing contact angles decreased once the droplet passed about position 3 and became smaller than the receding contact angle. This deformation is a reverse trend to the sliding that occurs on a normal hydrophobic surface against the direction of motion. The deformation seems to pull the triple line of the droplet to the negative electrode. The 30 μL droplet exhibited a similar trend, although the degree of the contact angle change is less remarkable. When a water droplet slides down a tilted superhydrophobic surface, the difference between advancing and receding contact angles is not remarkable4 because of the small sliding angle. In this work, however, some differences are apparent between the advancing and receding contact angles. Moreover, their magnitude relation was switched once the droplets passed around the center position between electrodes. Discussion of the additional reverse of advancing and receding contact angles after position 6 is presented in the Supporting Information. Figure 4 presents contact angles of water, hexadecane, and an ionic liquid obtained at each position between electrodes. We used smaller droplets for this measurement (see the Supporting Information). The water droplet contact angles were distributed symmetrically on either side of a central position between the positive and negative electrodes (Figure 4a). They decreased with the approach of each electrode. This trend suggests that the solid surface becomes hydrophilic because of the increase of electric charges affecting the solid-liquid interface8,12,15-17 from the electric field: a leakage field generated through the glass by the electrodes. A similar trend is observable on an ionic liquid (Figure 4b). Based on the electrode alignment and contact angle value distribution, it is inferred that there exists a charge gradient against the distance from the electrode at the solid-liquid interface. The high contact angle at position 4 in Figure 4 is attributable to the long distance and resultant dilution of the electric field from both electrodes. The advancing contact angles temporarily increased at position 3 (Figure 3a) after originating in the contact angle at position 4 (Figure 4a). The line in Figure 4 shows the expected trend of the contact angle change. Based on the contact angle value of the ionic liquid, we believe that the minimal contact angle positions are both immediately after position 2 and immediately before position 6. However, the contact angle of the hexadecane was independent of the distance from the electrodes (Figure 4c). Additional experiments revealed that hexadecane droplets did not move on the superhydrophobic surface under the same conditions as those of (15) (16) (17) 461. (18)
Vallet, M.; Berge, B.; Vovelle, L. Polymer 1996, 37(12), 2465. Yoshida, H. Microfluid. Nanofluid. 2005, 1, 289. Minnema, L.; Barneveld, H. A. IEEE Trans. Electr. Insul. 1980, EI-15(6), Kaelble, D. H.; Uy, K. C. J. Adhes. 1970, 2, 50.
Langmuir 2010, 26(3), 1493–1495
water droplets. Water18 and ionic liquids are polar; hexadecane19 is a nonpolar liquid. These results also suggest that polarization at the solid-liquid interface and the resultant increase of hydrophilicity plays an important role in the motion of water droplets. Based on those results, it was deduced that the initial motion from positions 2 and 6 was triggered by the dielectrophoretic force. Because of the large dielectrophoretic force, the droplet can move to about position 4 by this motion; once the droplet passes the middle point between electrodes, the wettability gradient by the electric field becomes the driving force and carries the droplet to the opposite electrode. Therefore, during the initial movement duration from positions 2 and 6 to around the center position between electrodes, the droplet receives resistance at the three-phase contact line. Consequently, large contact angle hysteresis appears, resembling the sliding of a water droplet on a normal hydrophobic surface, and a trailing vortex is generated in the droplet tail. The droplet motion toward the near running electrode, when the droplet was on the place between the positive electrode and negative electrode such as positions 3-5, supports this inference. The actual droplet shape also supports this discussion, as presented in Figure 2. Results revealed that water droplets slip with a sliding motion on a tilted superhydrophobic surface. However, that deformation trend is not similar to sliding behavior. It changes during this motion under these experimental conditions. Detailed analyses of the contribution of surface roughness, droplet viscosity, and the voltage increase ratio shall be addressed in future studies.
IV. Conclusion In this study, we evaluated water droplets’ internal fluidity using different droplet volumes on a superhydrophobic surface in an electric field induced by DC 4000 V. The droplet motion depends strongly on its initial position. Subsequently, PIV analysis revealed that droplets move almost with a slipping motion; their deformation shape against the moving direction changes during movement. This trend was more remarkable for smaller droplets. Results show that the dielectrophoretic force triggered the initial motion. Once the droplet almost passed the center position between electrodes, the droplet was driven by the wettability gradient provided by the electric field. A liquid that is a polar molecule having a large dielectric constant, such as water, will be affected more strongly, inducing this motion. Supporting Information Available: SEM micrographs showing the structure and hydrophobicity of the sample surface; conditions of PIV analysis; dielectrophoretic force acting on droplet; additional reversal of advancing and receding contact angles after position 6; size dependence of droplet motion, contact angle measurement, and ionic liquid behavior. This material is available free of charge via the Internet at http://pubs.acs.org. (19) Kaelble, D. H. J. Adhes. 1970, 2, 66.
DOI: 10.1021/la903730k
1495
