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Electrically Induced Changes in Dynamic Wettability Matthew Schneemilch,*,† Wim J. J. Welters,‡ Robert A. Hayes,†,§ and John Ralston† Ian Wark Research Institute, University of South Australia, The Levels, South Australia 5095, Australia, and Philips Research Laboratories Eindhoven, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands Received April 29, 1999. In Final Form: November 29, 1999
The wettability of water on a solid fluoropolymer surface was modified through the application of an electrical potential between the aqueous phase and a conductive substrate underlying the insulating polymer layer. A high-resolution optical technique was used to acquire contact angle-velocity data for different applied dc potentials. The experimental data were reconciled with the molecular kinetic model of contact line motion.
Introduction It has long been recognized that wetting behavior, for example, the contact angle of a liquid drop on a solid surface, can be modified by application of a potential difference across the solid-liquid interface. The investigations by Lippmann1 into electrocapillarity demonstrated the influence of an applied potential on the interfacial energy of the phase boundary. Exploitation of the ideal properties of the mercury electrolyte interface continued,2 but it was not until Froumkine3 investigated the effect of applied potential on the static contact angle of an electrolyte solution on a metallic surface that quantitative measurements of electrowetting4 began. Sondag-Huethorst and Fokkink5,6 investigated electrowetting on thiol-modified gold electrodes, using tensiometry. The effect proved to be both moderate and irreversible, due to the comparatively low dielectric strength of the adsorbed monolayers. Hato7 also used tensiometry to investigate electrowetting, on a thin (0.1 µm) polyaniline surface. The contact angle variation over a range of 0.8 V was dramatic (∼40°). The behavior was found to be reversible, but during the course of the experiment, the liquid level was slowly advanced continuously exposing fresh surface, even if changes in the surface properties were in fact occurring. Recently, Vallet, Berge, and Vovelle8 achieved significant contact angle variation with drops of electrolyte resting on polymer surfaces. The effect was irreversible, however, due to changes in the surface properties of both the PET (poly(ethylene terephthalate)) and PTFE (poly(tetrafluoroethylene)) films in the region * Current address of corresponding author: Physique Matiere Condensee, College de France, 11 Pl. Marcelin Berthelot, 75231 Paris Cedex 05, France. † Ian Wark Research Institute. ‡ Philips Research. § Current address: Philips Research. (1) Lippmann, G. Anal. Chim. 1875, 5, 494. (2) Grahame, D. C. Chem. Rev. 1947, 41, 441. (3) Froumkine, A. Act. Sci. 1936, 373, 5. (4) Beni, G.; Hackwood, S. Appl. Phys. Lett. 1981, 38, 207. (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 1992, 8, 2560. (7) Hato, M. Chem. Lett. 1988, 1960. (8) Vallet, M.; Berge, B.; Vovelle, L. Polymer 1996, 37, 2465.
of the drop perimeter. Welters and Fokkink9 were eventually able to achieve significant and switchable electrowetting by using polymers of high dielectric strength, parylene and polyimide, as the insulating layer. A low-energy fluoropolymer layer was used on the surface of the insulator to enhance the electrowetting effect by increasing the hydrophobicity and minimizing contact angle hysteresis. These surfaces were manufactured inside capillaries, enabling the capillary height of the electrolyte solution to be controlled by electrical means with possible applications to programmable optical filters, for example. All reported investigations on electrowetting have been restricted to studies of static wettability. Recently, we developed an optical technique for measuring dynamic contact angles on thin-coated silica rods.10 This was used to investigate the dynamic wetting behavior of a series of liquids on fluoropolymer-coated silica rods. The observed variation in dynamic contact angle was modeled using the molecular kinetic theory of Blake and Haynes11 as well as the combined molecular hydrodynamic model of Petrov and Petrov.12 Attempts were made to correlate the fitting parameters with the liquid structure at the solid-liquid interface. The dramatic, switchable electrowetting behavior achievable with the coating system identified by Welters and Fokkink offers the exciting possibility of altering the solid-liquid interaction without varying the bulk liquid-liquid molecular interaction, enabling one to isolate the effect of the solid-liquid interaction on the dynamic wetting behavior. To this end, the surfaces of silica rods were prepared for electrowetting studies and were then subjected to a series of forced wetting measurements under different applied potentials. Theory Under an applied voltage, the conducting liquid and insulator between the metallic electrodes form a capacitor. It is assumed that the potential gradient occurs only through the insulating layer; i.e., the potential drop (9) Welters, W. J. J.; Fokkink, L. G. J. Langmuir 1998, 14, 1535. (10) Schneemilch, M. Ph.D. Thesis. (11) Blake, T. D.; Haynes, J. M. J. Colloid Interface Sci. 1969, 30, 421. (12) Petrov, P. G.; Petrov, J. G. Langmuir 1992, 8, 1762.
10.1021/la990524g CCC: $19.00 © 2000 American Chemical Society Published on Web 02/16/2000
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through the thin AF1600 coating and the double layer are neglected. The thickness of the insulating layer compared to the AF1600 layer and the double layer justifies this assumption, which has been shown to be valid for insulator coatings greater than 0.1 µm.5,9,13 In this case, the capacitance, C, is determined by the dielectric constant, R, of the insulating material. For a cylindrical capacitor of length L, inner radius R1, and outer radius R2, the capacitance is given by14
C)
2π0RL
(1)
ln(R2/R1)
where 0 is the permittivity of free space. When the potential, E, is applied, there is a buildup of equal and opposite charge, Q, at the surfaces of the capacitor. From the consideration of simple electrostatics, the following expression for the surface charge density, σ, at the solidliquid interface, may be written
σ)
0RE CE Q ) ) A 2πR2L R2 ln(R2/R1)
(2)
The contribution of the stored electrostatic energy to the interfacial energy, γSL(E), is derived by combining eq 2 with the well-known Lippmann equation1
γSL(E) ) γSL(0) -
0R 2R2 ln(R2/R1)
E2
(3)
the velocity of the contact line is written
where the electrode potential is chosen in relation to the potential at which the interface is uncharged (point of zero charge (pzc)). Substituting this expression into the Young15 equation yields the following expression for the static contact angle θ0
cos θ0(E) ) cos θ0(0) +
0R
E2 2R2 ln(R2/R1)γLV
Figure 1. Dynamic contact angle dependence on velocity in the absence of applied potential (upper data set) and results from a previous investigation (lower data set). The main difference between the surface preparation is that the fluoropolymer surface was deposited on parylene N in this investigation and on clean silica in our other investigations.10,16,17 The solid lines show the best fit to the molecular kinetic theory in the upper data set and to the combined theory in the lower data set. The respective theories were applied to the advancing and receding data separately.
(4)
assuming that the interfacial energy of both the solidvapor and the liquid-vapor interfaces are unaffected by the application of a potential.5,9,13 To characterize the dynamic contact angle dependence on velocity, the molecular kinetic model11 was employed. Developed by Blake and Haynes, the model assumes that the contact line motion is governed by the rate of the molecular displacement processes occurring in the contact line region, the three-phase zone (TPZ). At equilibrium, when the contact line is at rest, the TPZ fluctuates about its mean position, as molecules of each fluid interchange on the adsorption sites. When in motion, there is an imbalance in force on the contact line, arising from the difference between the static and dynamic contact angle. The work expended by this force alters the energy barriers of the two displacement processes, allowing the contact line to move in the appropriate direction. The distance between the centers of the adsorption sites is λ, and the molecular rate constant for the adsorption/desorption process is KW. The equation relating the contact angle to (13) Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1994, 10, 4380. (14) Giancoli, D. C. Physics for Scientists and Engineers with Modern Physics, 2nd ed.; Prentice Hall International: Eaglewood Cliffs, NJ, 1988; p 568. (15) Young, T. Miscellaneous works; Peacock, G., Ed.; Murray: London, 1855; p 418.
(
θD ) arccos cos θ0 -
( ))
2nkT ν arcsinh γLV 2KWλ
(5)
where k is the Boltzmann constant, T is the temperature, v is the velocity of the wetting line relative to the solid surface, and n is the adsorption site density. If the site distribution is assumed to be isotropic, eq 5 can be rewritten, substituting n ) λ-2. Usually the value of KW is found to be of order 106 s-1, lower than the rate of molecular displacements in bulk liquids (∼1011 s-1). The value of λ is often taken to be determined in some way by the properties of the surface, for example, the size of the component molecules or atoms or the distance between isolated lyophilic sites on heterogeneous surfaces. λ is generally found to be somewhat larger than the diameter of the liquid molecule. The surface used in this study, however, was both homogeneous and amorphous. The adsorption site density is therefore more likely to be determined by the liquid structure in the threephase zone, which may well differ significantly from that of bulk liquid. Materials and Methods The conducting layer was formed by vapor deposition of gold on silica rods 1 mm in diameter. An insulating parylene N (poly(di-p-xylene)) layer was vapor deposited to a thickness of 30 µm (confirmed by optical measurement) at room temperature and a pressure of 0.05 mbar, using a Labcoater 1 deposition chamber (Specialty Coating Systems). Finally an amorphous fluoropolymer layer was created by immersing the rod to a depth of 4 cm for 1 min in a 1% (w/w) solution of AF1600 (Dupont) dissolved in the fluorocarbon solvent FC75 (3M). The rod was allowed to dry for 1 h in a laminar flow cabinet before the dipcoating procedure was repeated. Wilhelmy balance traces revealed that repetition of the coating step resulted in a smoother surface. Imaging with atomic force microscopy confirmed that after the repeated dipcoating the surface was smooth (peak to valley roughness ,1
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Table 1. Fitting Parameters for Water on AF1600 AF1600 on parylene N site separation, λ (nm) frequency, Kw (s-1) measured static contact angle, θ0 (deg) fitted static contact angle, θ0 (deg)
AF1600 on silica
advancing
receding
advancing
receding
3.14 ( 0.07 1400 ( 400 123.6 ( 0.3 123.8 ( 0.2
1.2 ( 0.6 (1.7 ( 4.5) × 106 116.6 ( 0.2 116.4 ( 3.3
2.49 ( 0.09 (5.0 ( 1.1) × 105 116.7 ( 0.1 118.5 ( 0.2
1.36 ( 0.04 (1.6 ( 0.2) × 106 112.7 ( 0.1 111.6 ( 0.1
Figure 2. Static contact angle dependence on applied potential. The error bars show the standard deviation of measurements made over a length of 2 cm of the surface. The solid line is the solution to eq 3, with R ) 3, R1 ) 500 µm, R2 ) 530 µm, and γLV ) 72.8 mN m-1. nm over a scan area of 10 × 10 nm and
