Langmuir 1998, 14, 1535-1538
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Fast Electrically Switchable Capillary Effects Wim J. J. Welters* and Lambertus G. J. Fokkink Philips Research Laboratories Eindhoven, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands Received October 22, 1997. In Final Form: February 11, 1998 Through the application of a potential between a water droplet on a hydrophobic coating and the underlying conductive substrate, we have shown control over the contact angle of the droplet. The same phenomenon can be used to control capillary rise of a liquid in a coated capillary over several centimeters in a short time. Interesting applications such as a temporary improvement of adhesion (drying of paint), electrical control of liquid lenses, and patternwise and dynamic filling of a capillary array are in sight.
Introduction Owing to fundamental thermodynamical relationships1 a water droplet on a hydrophobic surface changes shape when a voltage is applied between the liquid and a conducting layer underlying the hydrophobic surface. An example is shown in Figure 1, where an electrolyte droplet spreads over a surface when a potential is applied. This enables the wetting of otherwise not wettable surfaces, which offers interesting applications in the temporary improvement of adhesion (for instance during the drying of paint), the electrical control of the shape of a liquid lens, or the pattern wise and dynamic filling of a capillary array to be used as a programmable optical filter. The potential-induced spreading of a droplet is an electrostatic effect, caused by the formation of a diffuse ionic layer of charge in the liquid adjacent to the solidliquid interface. According to Gibbs2 the energy of the solid-liquid interface is at its maximum in the absence of accumulated charge at this interface,1 i.e. at the socalled potential of zero charge (pzc). At increasing potential differences across this interface, charge builds up both at the solid electrode underlying the dielectric film and at the liquid side, causing a decrease in interfacial energy. As a result, the contact angle between the liquidvapor surface and the solid-liquid interface measured through the liquid phase decreases (see Figure 1). This effect is called electrowetting.3,4 In recent years, electrowetting has been receiving increasing interest in surface chemistry. Electrowetting of aqueous solutions on thiol-modified gold electrodes has been the subject of a detailed study by Sondag-Huethorst and Fokkink.5,6 The electrowetting effects obtained on such electrodes are small and irreversible. Much greater and reversible effects can be obtained with electro adsorption and -desorption.7,8 In this case, an increase in wetting is obtained through electrodesorption of hydro* Corresponding author: tel, +31-40-2742691; e-mail, welters@ natlab.research.philips.com. (1) Lippman, G. Ann. Chim. Phys. 1875, 5, 494. (2) Gibbs, J. W. Collected works of J. W. Gibbs; Longmans, Green: New York, 1931; Vol. I. (3) Berge, B. C. R. Hebd. Seances Acad. Sci., Ser. B 1993, 317, 157. (4) Vallet, M.; Berge, B.; Vovelle, L. Polymer 1996, 37, 2465. (5) Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. Electroanal. Chem. 1994, 367, 49. (6) Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1994, 10, 4380. (7) Abbott, N. L.; Gorman, C. B.; Whitesides, G. M. Langmuir 1995, 11, 16. (8) Gorman, C. B.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1995, 11, 2242.
Figure 1. A 10-4 M KNO3 droplet on a AF1600/Parylene C surface, at 0 (a) and 200 V (b).
phobic molecules from a gold electrode. Minnema et al.9 were the first to observe a decrease in surface energy on a polymer surface at increasing potentials across a polyethylene film. Berge et al.3,4 started to study electrowetting systematically using a droplet of an aqueous solution on electrically insulating polymer films. A decrease in contact angle of up to 50° was obtained for poly(ethylene terephthalate) and poly(tetrafluoroethylene) foils. However, these results show a strong degree of irreversibility in contact angle change due to electrochemical alterations of the polymer surface under the high voltage conditions required.3,4 In theory, the electrowetting effect can be used to control the liquid level in a capillary. Except for mercury penetration in a glass capillary as employed in the Lippman electrocapillary meter,1 capillary rise as a result of a potential-induced change of the contact angle has never been demonstrated. We will show that electrowetting can be used to control capillary rise in a polymercoated capillary. A fast and reversible rise can be achieved by switching the potential. (9) Minnema, L.; Barneveld, H. A.; Rinkel, P. D. IEEE Trans. Electr. Insul. 1980, IE-15, 461.
S0743-7463(97)01153-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/10/1998
1536 Langmuir, Vol. 14, No. 7, 1998
Letters
Theory equation10
the contact angle According to the Young is dependent on the interfacial tensions γLV, γSL, and γSV between the solid (S), liquid (L), and vapor (V) phases:
γLV cos θ ) γSV - γSL
(1)
The solid-liquid interfacial energy γSL (and thus θ) is a function of the electrode potential. Both γLV and γSV may in a first approximation be assumed to remain unaffected by the electrode potential.5,6,11 γSL has contributions from an electrical component (γSLel) and from a chemical (potential-independent) component (γSL0). The electrical contribution can be derived to equal the double integral of the capacitance with respect to the potential. A more elaborate derivation is given by Sondag-Huethorst and Fokkink.5,6
γSL(V) ) γSL0 + γSLel ) γSL0 -
∫∫C dV dV
2γLV cos θ ∆Fgd
(4)
In this equation g is the acceleration due to gravity. The contact angle has a strong effect on the liquid height. Combining eqs 3 and 4 gives the liquid rise between two plates due to potential V:
∆h )
V2 ∆Fgdδ
(5)
From this relationship it follows that the absolute meniscus rise ∆h is proportional to the square of the applied potential and is not dependent on the surface tension of the liquid. The surface tension of the liquid will only determine the initial liquid level, not the extent of the electrically induced meniscus rise. Experimental Section
(2)
The electrode potential V is chosen in relation to the potential at which the interface is uncharged (pzc). C is the capacitance of the solid-liquid interface, i.e., when a potential difference is applied between the solid and the liquid phase, opposite charges build up on both sides of the interface. In the case of dielectric coatings thicker than 0.1 µm, the capacitance is determined mainly by the capacitance of the electrically insulating coating,5,6,12 and the solid-liquid region can be modeled as a parallel plate capacitor with the coating as the medium between the plates. In that case, eq 2 can be integrated, and the capacitance is only dependent on the thickness of the coating δ and its dielectric constant (C ) /δ). Combination with the Young equation yields a relationship between the contact angle and the potential:
V2 cos θ(V) ) cos θ(0) + 1/2 δγLV
h)
(3)
The capacitance of the coating influences the change in contact angle via the dielectric constant and the thickness. At sufficiently high potentials cos θ(V) will become 1, indicating complete wetting (θ(V) ) 0°). From the Laplace equation13 it can be derived that the equilibrium height h of a liquid level between two plates relative to the liquid level outside the plates is determined by the surface tension of the liquid γLV, the difference in density between the liquid and the surroundings (usually air) ∆F, the distance between the plates d, and the contact angle θ: (10) Young, T. Miscellaneous works, Peacock, G., Ed.; Murray: London, 1855; p 418. (11) Kabanov, B. N. In Electrochemistry of metals and adsorption; Frued Publishing House: Holon, Israel, 1969; p 18. (12) In our case the solid-liquid interface consisted of two layers: an electrically insulating coating, in which the potential dropped linearly with the thickness, and the diffuse ionic layer in the liquid. This system behaves as a pair of capacitors in series. However, when the capacitance of the diffuse ionic layer becomes much greater than the capacitance of the coating, the total capacitance equals the capacitance of the coating. From the studies of thiol-modified gold electrodes by Sondag-Huethorst and Fokkink,5 it can be concluded that this condition applies only if the capacitance of the coating is lower than 1 µF cm-2. On the basis of the thickness and the dielectric constant, the capacitances of the coatings used in our study can be calculated to be in the order of 0.1 nF cm-2, so the total capacitance will be equal to the capacitance of the coating. (13) de Laplace, P. S. Mechanique Celeste; Supplement to book 10, 1806.
Glass plates covered on one side with a conductive coating (indium tin oxide, ITO) are used as substrates. Parylene C or polyimide is used as insulating layer. Parylene C (poly(dichlorodi-p-xylylene)) is deposited at room temperature by gas-phase deposition of dichlorodi-p-xylylene (Nova Tran) at a pressure of 0.03 mbar, using a Labcoter 1 deposition chamber (Specialty Coating Systems). Polyimide is deposited by spin-coating of a polyamic acid solution (Probimide 115A, OCG) followed by curing in air at 575 K. A very thin (about 0.1 µm) Teflon-like coating is used as a top layer to obtain the desired hydrophobic properties. This Teflon AF1600 (DuPont) layer is deposited on the insulating coating by dipping in a 1 wt % solution of AF1600 in FC726 (3M). The substrates are withdrawn from the solution at a constant speed of 0.5 mm s-1. The AF1600 layer does not have sufficient insulating properties to be used without the Parylene C or polyimide coating. Contact angles are measured on water droplets by means of the sessile drop method using a camera connected to a video recorder, a TV monitor, and a printer. A 10-µL aqueous droplet is deposited on a coated substrate. A voltage is applied between the conductive layer and the droplet (a 0.1-mm Pt wire is used as the electrode in the droplet). The droplet is recorded, and the contact angles are measured as a function of the potential. Switchable capillary rise was measured between two coated glass plates. A transparent layer of ITO was used as the conductive coating. After the deposition of the coatings, the glass plates were clamped together with a 0.25-mm spacer in between. With this setup a transparent capillary slit was obtained. With the same voltage applied to the conductive layers of both plates, a capillary rise can be achieved. The height of the liquid between the plates is measured visually. For speed measurements again a video setup is used.
Results and Discussion Electrowetting can only be exploited reversibly when the solid surface remains unchanged during the experiments; i.e., no surface reactions should take place. This is only possible when little or no charge is flowing through the liquid-solid interface. Therefore, an insulating coating is needed. Additionally, the surface should be sufficiently hydrophobic so that a substantial decrease in contact angle can be obtained. Polymer layers with thicknesses of a few micrometers seem to have the best properties for these measurements. Potentials of up to several hundreds of volts can be used.3,4 The only limitation is the dielectric strength of the polymer coating in an aqueous environment. We found vacuum-deposited Parylene C to be a very useful coating material because of its high dielectric
Letters
Figure 2. Contact angle of a 10-4 M KNO3 solution as a function of the applied potential and the coating thickness: The Parylene C and polyimide coatings are covered with a thin Teflon AF1600 top layer to obtain a hydrophobic surface. The lines give the theoretical contact angles based on the coating thickness and dielectric constant. b, 6 µm Parylene C; O, 10 µm Parylene C; +, 18 µm polyimde; 0, 35 µm polyimide.
strength of about 200 V µm-1.14-17 Parylene can form conformal homogeneous coatings on surfaces of different shapes and compositions. Polyimide coatings on flat substrates also appear to have sufficient dielectric strength in water. A thin layer (less than 100 nm) of Teflon AF1600 is deposited on top of the insulating coating in order to obtain a hydrophobic surface with a low contact angle hysteresis. With these duplex coatings we were able to induce electrically a dramatic and reversible change in contact angle as well as a fast and substantial rise and depression in capillary slits. Figure 1 shows the change in shape of a droplet before and after the potential was increased from 0 to 200 V. The droplet spreads significantly over the hydrophobic surface. The contact angle first changes from 110° to 63°. When switched back to 0 V the surface becomes hydrophobic again, and the contact angle increases to 102°. The original 110° can no longer be reached due to contact angle hysteresis of the hydrophobic surface; i.e., due to ever present imperfections (roughness, chemical heterogeneities) of the surface, the contact angle for an advancing three-phase line is always larger than for a receding threephase line for practical systems. In subsequent switches between 0 and 200 V, the contact angle changes from 102° to 63° and back. When the insulating coating is thicker, a higher potential is needed to affect the same decrease in contact angle (eq 3). This trend is demonstrated in Figure 2. The parylene and polyimide coatings used here do not differ much in relative dielectric constants (2.7 vs 3.1). Therefore, the decreasing effect of the potential on the contact angle is almost exclusively due to the increasing insulator thickness. For the same decrease in contact angle a 6-fold increase in coating thickness causes a 2.3 (≈x6) fold increase in potential, which is in good agreement with eq 3. The lines in Figure 2 are the theoretical curves based on the thickness and the permittivity of the coatings (eq 3, 0 V taken as the pzc). At higher contact angles good (14) Sharma, A. K.; Hahn, A. W.; Nichols, M. F. J. Appl. Polym. Sci. 1988, 36. (15) Beach, W. F.; Austin, T. M. Proc. Int. SAMPE Electron. Conf., 3rd 1989, 78. (16) Dabral, S.; et al. J. Vac. Sci. Technol. B 1993, 11, 1825. (17) Quade, R. Soc. Vacuum Coaters, Annu. Tech. Conf., Proc., 38th 1995, 449.
Langmuir, Vol. 14, No. 7, 1998 1537
Figure 3. Capillary rise of a 10-4 M KNO3 solution between two plates (plate distance 0.25 mm) coated with a 10-µm Parylene C layer and a AF1600 finish (as described).
Figure 4. Capillary rise and depression between two plates (plate distance 0.25 mm) coated with a 10 µm thick Parylene C layer and a AF1600 finish as a function of time for switching between 0 and 200 V with a 10-4 M KNO3 solution.
agreement between the experimental results and the theoretical curves was obtained. At lower angles (e.g., below 75°), the theoretical contact angles were lower than the measured values, which hardly change anymore. Similar results are reported by Berge et al.3,4 for polymer foils. This effect may be due to a modification of the coatings properties at the solid-liquid interface in the high electric field. Further research is performed in order to explain this effect. Switchable capillary rise was measured between two glass plates coated with the same duplex coatings. From 0 V, the meniscus rose in proportion to the square of the potential, as can be seen in Figure 3. The measured liquid heights correlate well with the contact angles measured on the 10-µm parylene-coated sample shown in Figure 2. The line in Figure 3 represents the theoretical meniscus height (eq 5) based on the coating properties and the measured plate distance. At voltages higher (lower) than +150 V (-150 V) the capillary rise was found to be smaller than the rise predicted by theory. As for the contact angles shown in Figure 2, at higher voltages the experimental results no longer agreed with the theoretical values. When after the capillary rise the potential returns to 0 V, the liquid meniscus recedes to a level corresponding to the receding contact angle: -13 mm relative to the liquid level outside the capillary slit. Both the rise and the fall of the liquid between the plates occurred quickly (Figure 4). The voltage was switched from 0 to 200 V and back to 0 V at 5-s intervals. The initial rise over 27 mm starting
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at -24 mm occurred within 1.5 s. The speed of the liquid over the first few millimeters meniscus rise is estimated to be 0.2 m s-1. This speed is mainly dependent on the viscosity of the liquid and the dynamic contact angle during the rise.18 The liquid level recedes 15 mm again in about 1.5 s. The subsequent switching cycles show the reproducibility of the liquid movement. To summarize, we have identified a coating system suitable for exploiting the electrowetting effect. Elec(18) Blake, T. R. In Wettability; Berg, J. C., Ed.; Marcel Dekker, Inc.: New York, 1993; p 251-309 and references therein.
Letters
trowetting can be used to control the interfacial energy of the solid-liquid interface. The contact angle can be reversibly switched over a wide range (110-60°), and capillaries can be filled and emptied quickly. The results show promising features for future technical electrowetting applications. Acknowledgment. The authors thank M. R. Bo¨hmer and M. W. J. Prins for their support and the useful discussions on this work. LA971153B
