Electrowetting Of Ionic Liquids Research Articles

Electrowetting of Ionic Liquids on Teflon AF1600 in Ambient Hexadecane

A droplet of ionic liquid is immersed in an immiscible liquid (n-hexadecane) and electrowetted on a flat electrode insulated with Teflon AF1600. Two series of ionic liquids were studied: 1-alkyl-3-methylimidazolium tetrafluoroborates (Rmim.BF4) and 1-butyl-3-methylimidazolium with various anions (bmim.X). When voltage is applied between the droplet and the electrode, the static contact angle decreases from about 150° down to 50° (with DC voltage) and 15° (with AC voltage). The electrowetting curves (contact angle vs. voltage) are symmetric with respect to zero voltage and obey the Young-Lippmann equation below saturation. Reversibility is excellent and contact angle hysteresis is minimal (≈2°). The saturation contact angle cannot be predicted with the zero-interfacial tension theory most probably due to the presence of a hexadecane film between the droplet and the solid surface. Spreading of the ionic liquid (under DC voltage) is about twice as fast as its retraction (when voltage is switched off). The base area of the droplet varies exponentially during wetting (exponential saturation) and dewetting (exponential decay). The characteristic time is related to the viscosity of the ionic liquid. The spreading kinetics (dynamic contact angle vs. contact line speed) can be described by the hydrodynamic model (Voinov’s equation) for small contact angles and by the molecularkinetic model (Blake’s equation) for large contact angles. The role of viscous and molecular dissipation follows the scheme outlined by Brochard-Wyart and de Gennes.

On the performance of thermostable electrowetting agents

Electrowetting of ionic liquids (ILs) in different alkane ambient and at different temperatures were studied under different AC voltages. The performance of IL‐based electrowetting at elevated temperatures and that at room temperature was compared. It turns out that the electrowetting of ILs at elevated temperatures resulted in faster response and lower operation voltage. Furthermore, the relationship between the structure of the ion and the contact angle change was specifically investigated and theoretically explained. Finally, the properties of IL‐based electrowetting in different alkane ambient were studied. It was demonstrated that the long length of the alkyl chain guaranteed a smaller contact angle under the same applied voltage. All the experimental results showed attractive features involving wide operating temperatures and in particular high stability, fast response, and good reversibility at elevated temperatures. The performance of IL‐based electrowetting could be further improved through an optimal choice of an ambient phase or a rational design and synthesis of ionic liquids. Copyright © 2011 John Wiley & Sons, Ltd.

Electrowetting of Ionic Liquids: Contact Angle Saturation and Irreversibility

In this work, electrowetting of several room temperature ionic liquids having in common the cation or the anion, [EMIM][EtSO4], [EMPy][EtSO4], [EMIM][NTf2], [BMIM][BF4], [BMIM][TFA], and [OMIM][BF4], on poly(tetrafluoroethylene) (PTFE) was investigated. The typical behavior of aqueous salt solutions was found: symmetric parabolic curves of contact angle versus (positive and negative) applied voltage, at low voltages, and contact angle saturation after a threshold voltage. Furthermore, the contact angle did not recover its initial value when the voltage decreased, and this irreversibility was found even at low voltages. The dependence of the contact angle with the applied voltage is generally described by the Young−Lippmann equation before contact angle saturation. In contrast, the Langevin function as well as a modified form of the Young−Lippmann equation were found to fit the experimental electrowetting curves in the whole range of voltages. A correlation between the electrowetting behavior and the surfa...

Electrowetting of Ionic Liquids on Low Surface Energy Electrodes

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Electrowetting of Ionic Liquids under High Vacuum Conditions

Electrowetting of two imidazolium based ionic liquids, [C4mim][PF6] and [C6mim][NTf2], on different dielectric substrates, SiO2-Si and epoxide-Cu, has been studied at various temperatures and droplet radii R = 1–3 mm. To avoid the effect of adsorbed thin liquid films on the contact angle measurements and to resolve possible decomposition of the ionic liquids at high applied voltages we have performed the electrowetting experiments under ultra high vacuum conditions which is reported here for the first time. The electrowetting curves of both ionic liquids on both substrates exhibit a symmetrical, parabolic shape as a function of the applied dc-voltage. However, in comparison with aqueous and organic electrolytes saturation is reached at a lower variation of the contact angle. At saturation a clear decomposition of the ionic liquids sets in which is observed by the formation of large bubbles. Presumably, this is caused by electrical breakdown and the corresponding discharging. The decomposition products can strongly affect the interfacial tensions of the droplets what explains a pronounced contact angle hysteresis of ∼15°. The measured temperature dependence of the electrowetting curves is essentially determined by that of the liquid/vapor interfacial tension in accordance with the classical Young–Lippmann equation. Interpretation of the droplet size dependence of the electrowetting curves by the modified Young equation yields an apparent line tension of ∼10-5 N, which is comparable in magnitude to organic liquids.