J. Phys. Chem. B 2003, 107, 1163-1169
1163
Influence of the Electrical Double Layer in Electrowetting Anthony Quinn, Rossen Sedev, and John Ralston* Ian Wark Research Institute, UniVersity of South Australia, Mawson Lakes, SA 5095, Australia ReceiVed: July 16, 2002
Electrowetting (wetting under the influence of an applied electric field) of three fluoropolymer surfaces (amorphous Teflon, DuPont) by electrolyte solutions was studied with the sessile drop method. The electrowetting curve (contact angle/potential) is analogous to the electrocapillary curve (surface tension/ potential) and may be described by a combination of the Young and Lippmann equations. The influence of the electrical double layer at the polymer/solution interface has been neglected in the past because the overall interfacial capacitance is mainly determined by the capacitance of the insulating polymer layer. We demonstrate that for some surfaces a systematic deviation occurs at positive potentials. This departure from the constant capacitance regime is attributed to double layer effects, namely, the adsorption of hydroxide and halide anions. The pH, ionic strength, and polymer composition can all influence electrowetting behavior.
Introduction The equilibrium contact angle, θ, at the three-phase contact line is given by the Young equation:
γ cos θ ) γSV - γSL
(1)
where γ, γSV, and γSL are the interfacial tensions associated with the liquid/vapor, solid/vapor, and solid/liquid interfaces. If an external voltage is applied across the solid/liquid interface, the contact angle diminishes. The most common experimental configuration consists of a drop of electrolyte solution resting on top of an electrode insulated with a thin (thickness d about a micrometer) polymer layer (Figure 1). The effect is referred to as electrowetting and has attracted significant interest in the past decade.1 The earliest reported use of the term electrowetting appears in a patent entitled “Electrowetting reproduction process”,2 which describes the use of an external electric potential (30800 V) for improving the wettability of insulating hydrophobic surfaces with respect to ionically conductive liquids (typically from θ0 ) 120°-145° to θ ) 45°-85°). The influence of electric charge on the interfacial tension of the mercury/aqueous solution interface is well-known and harks back to the classic work of Lippmann.3 Electrocapillary measurements on solid electrodes have been pursued by many authors (e.g., refs 4 and 5 and references therein). Frumkin and co-workers recognized that a potential originating from an external source will influence the solid/liquid interface only (because the gas phase is a good insulator) and therefore changes in γSL can be followed by monitoring the contact angle. Experiments confirmed that γSL(V) and θ(V) for the mercury electrode follow a similar trend and the electrocapillary maximum practically coincided with the electrowetting maximum.6 On solid metals, only θ(V) measurements are feasible, but they turned out to be irreproducible and interest faded away (since then alternative methods have been devised5). Decades later, Morcos revived the approach by successfully employing the * To whom correspondence should be addressed. Phone: +61 8 8302 3066. Fax: +61 8 8302 3683. E-mail:
[email protected].
Figure 1. Sessile drop setup for electrowetting measurements.
capillary rise method. He obtained consistent results for metals, semiconductors, and insulating electrodes.4 Sondag-Huethorst and Fokkink carried out detailed studies of gold electrodes modified with thiol self-assembled monolayers.7-9 The agreement between wettability and electrochemical measurements was good as long as the thiol layer was stable. The capacitance of the system was practically constant and determined mainly by the thickness and dielectric constant of the self-assembled layer.8 Sparnaay10 seems to be the first to use the arrangement depicted in Figure 1. He studied electrolyte droplets on the flat (111) surface of a Ge crystal. The contact angle decreased as the voltage was applied, but the values were scattered excessively. The earliest determination of contact angles on an insulating surface at different voltages is that of Chudleigh.11 He reported angles (captive bubble technique) of water (at normal, acidic, and basic pH) and ethanol on Teflon FEP (fluorinated ethylene propylene) surfaces. At 1000 V negative voltage, all contact angles decreased by 10-20% from their initial values (the FEP samples were 25 µm thick). Berge and co-workers12,13 studied the electrowetting of poly(tetrafluoroethylene) (PTFE) and poly(ethylene terephthalate) (PET) by drops of aqueous NaCl. These systems, as distinct from earlier examples, permitted larger voltages (several hundred volts) to be applied while maintaining a negligible current flow. Therefore larger reversible changes of the contact angle (up to 70°) could be realized without inducing electrochemical reactions. Significant advances in the field followed rapidly with respect to instrumentation,14 type of insulating coating,15,16 electrowetting in solid/water/oil systems17 and under dynamic conditions,14,18-20 and theoretical interpretations.21-23 This renewed interest was and is stimulated by the possible use of
10.1021/jp0216326 CCC: $25.00 © 2003 American Chemical Society Published on Web 01/08/2003
1164 J. Phys. Chem. B, Vol. 107, No. 5, 2003
Quinn et al.
electrowetting for applications such as liquid micromanipulation,24-26 liquid movement in capillary arrays,27 and the variation of the focal length of a liquid lens.28 Current electrowetting theory assumes that γSL is composed of two contributions10,29schemical, γ0SL, and electrical (potential-dependent), γVSL:
1 γSL ) γ0SL + γVSL ) γ0SL - CV2 2
Figure 2. Structure of the two monomers of Teflon AF.
(2)
where V is the applied DC voltage. The quadratic term is obtained by integrating Lippmann’s equation at constant C, capacitance per unit area of the insulating polymer layer. The latter can be modeled as a parallel-plate capacitor, and by combining eqs 1 and 2, we arrive at
cos θ ) cos θ0 +
1 0 2 V 2 γd
(3)
where is the dielectric constant of the polymer and 0 is the permittivity of a vacuum. It is largely accepted that eq 3 provides a good description of electrowetting data as long as the voltage does not exceed a certain critical limit. Beyond this threshold, the contact angle remains constant (unaffected by the increasing voltage). Different explanations have been proposed for this saturation effect,21-23 but no definite agreement on the causes has yet been reached. Blake et al.19 have argued that the correct numerical factor in eq 3 is 1/4 rather than 1/2, while Janocha et al.17 have reported values between 0.5 and 0.01. The weight of experimental evidence points to 0.5, in agreement with eq 3. Peculiar behavior at the three-phase contact line, as opposed to the whole solid area occupied by the drop, has been observed repeatedly (e.g., local polymer hydrophilization,13 reduced coalescence between adjacent drops,1 air ionization,21 small droplet expulsion13,21,30). It has been suggested that the electrowetting effect is entirely due to line (rather than surface) tension effects;31 however, this idea has gained little acceptance1. An electrical double layer is formed at the surface of a neutral polymer when it is immersed in water,32 and therefore, the capacitance of the solid/liquid interface (Figure 1) consists of both polymer and double layer contributions.8 The latter however, is usually much larger, and because the capacitors are in series, only the polymer contribution appears in eq 3; conventional opinion dictates that double layer effects are negligible. Indeed it has been reported that electrowetting is not affected by salt type or concentration.12,14,22 Deviations from the behavior prescribed by eq 3 for a range of solids have been noted; however, only rarely has it been speculated that a material deficiency may be the cause.16 In this investigation, our focus is fixed firmly on how fluoropolymer composition and electrical double layer contributions can influence electrowetting. It is important to note that thin fluoropolymer films with excellent dielectric strength quality may be prepared in a straightforward fashion. We report results for three different grades of amorphous Teflon (Teflon AF). The study is limited to advancing angles and voltages below the saturation threshold. Deviations at positive potentials are found experimentally and attributed to double layer effects. Materials and Methods The insulating materials studied are amorphous fluoropolymers belonging to the Teflon AF line developed by DuPont.33,34 Teflon AF is a random copolymer of 4,5-difluoro-2,2-bis-
TABLE 1: Glass Transition Temperature, Tg, PDD Content, Dielectric Constant, E, and Critical Surface Tension, γC, of the Teflon AF Variants Used in This Study grade T1 T2 T3e
Tg (°C)
PDD (mol %)
γCa (mN/m)
36b 133b 160f
