The Effect of AC Frequency on the Electrowetting Behavior of Ionic Liquids Yasith S. Nanayakkara, † Sirantha Perera, † Shreyas Bindiganavale, ‡ Eranda Wanigasekara, † Hyejin Moon,* ,‡ and Daniel W. Armstrong* ,† Department of Chemistry and Biochemistry, and Department of Mechanical and Aerospace Engineering, The University of Texas at Arlington, Arlington, Texas 76019 This paper presents a study of electrowetting of ionic liquids (ILs) under AC voltages, where nine different ILs (including mono-, di-, and tricationic varieties) with three different AC frequencies (60 Hz, 1 kHz, 10 kHz) were experimentally investigated. The main foci of this study are (i) an investigation of AC frequency dependence on the electrowetting of ILs; (ii) obtaining theoretical relation- ships between the relevant factors that explain the experi- mentally achieved frequency dependence; and (iii) a systematic comparison of electrowetting of ILs using AC vs DC voltage fields. The frequency of the AC voltage was found to be directly related to the apparent contact angle change (Δθ) of the ILs. This relationship was further analyzed and explained theoretically. The electrowetting properties of ILs under AC voltages were compared to that under DC voltages. All tested ILs showed greater apparent contact angle changes with AC voltage conditions than with DC voltage conditions. The effect of structure and charge density also was examined. Electrowetting revers- ibility under AC voltage conditions was studied for few ILs. Finally, the physical properties and AC electrowetting properties of ILs were measured and tabulated. Electrowetting is a well-known phenomenon in which a liquid drop spreads on a solid surface upon application of an electric field across the liquid/solid interface. The spreading of a liquid drop by an electric field is often observed on dielectric coated surfaces as well as conductive solid surfaces. In either case, one of the conventional methods to measure the extent of electrowet- ting is measuring the change in apparent (macroscopic) contact angle. 1 When a liquid drop on a solid surface spreads due to electric field, its apparent contact angle decreases. Since our measurements are apparent contact angles (not intrinsic contact angles), herein we will use “contact angle” to refer the “apparent contact angle.” If electrowetting is performed on a smooth dielectric solid (e.g., Teflon), when the voltage is removed from the system, the contact angle will tend to approach its original value. 2,3 Electrowetting on dielectric (EWOD) is popular because of its usefulness in droplet based microfluidic devices or digital microfluidics. 4,5 Drop actuation using EWOD in laboratory-on-a- chip platforms is relatively trouble-free and inexpensive compared to traditional pump and microchannel based laboratory-on-a-chip platforms. Indeed they have been used successfully in the preparation of biological samples as well as for some analytical processes. 6-11 Other than microfluidics, electrowetting is used in fluid focal lenses, 12 electrowetting displays, 13 programmable optical filters, 14 paint drying, 14 micromotors, 15 electronic microre- actors, 16 and to control fluids in multichannel structures. 17 There are some significant advantages of ILs as compared to water and other aqueous electrolytes. ILs have greater stability at elevated temperatures, are liquid over a much wider temper- ature range, have negligible evaporation, have tunable physical properties and have selective solubility and selective extractability for various organic compounds, metal ions and biological molecules. 2,18 Also corrosion inhibition is achievable for some ILs (however diluted/hygroscopic ILs can facilitate corrosion). 19 In recent studies the electrowetting behavior of ILs was evaluated * To whom correspondence should be addressed. Phone: (817) 272-0632. Fax: (817) 272-0619. E-mail: hyejin.moon@uta.edu (H.M.); sec4dwa@uta.edu (D.W.A.). † Department of Chemistry and Biochemistry. ‡ Department of Mechanical and Aerospace Engineering. (1) Mugele, F.; Buehrle, J. J. Phys.: Condens.Matter 2007, 19, 375112/1– 375112/20. (2) Nanayakkara, Y. S.; Moon, H.; Payagala, T.; Wijeratne, A. B.; Crank, J. A.; Sharma, P. S.; Armstrong, D. W. Anal. Chem. 2008, 80, 7690–7698. (3) Millefiorini, S.; Tkaczyk, A. 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