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A High-Capacitance and Spin-Coatable Ion Gel Dielectric for Stable Electrowetting on Dielectric (Ewod) V

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A High-Capacitance and Spin-Coatable Ion Gel Dielectric for Stable Electrowetting on Dielectric (Ewod) V A HIGH-CAPACITANCE AND SPIN-COATABLE ION GEL DIELECTRIC FOR STABLE ELECTROWETTING ON DIELECTRIC (EWOD) V. Narasimhan and S.-Y. Park* Department of Mechanical Engineering, National University of Singapore, SINGAPORE ABSTRACT For many practical electrowetting-on-dielectric (EWOD) applications, the use of high-capacitance dielectric materials is critically demanded to reduce operating voltage. Several high-κ dielectrics such as SiO2, Al2O3, and Ta2O5, which are typically fabricated by conventional IC processes (e.g. PECVD, ALD, and sputtering), have been studied toward this application. However, these IC fabrication processes require complex and expensive laboratory setups such as high vacuum facilities for thin-film layer deposition. Here, we present a novel ion gel dielectric that offers 2 or 3 orders higher capacitance (~ 10 μF/cm2) than that of conventional dielectrics, while being fabricated through a simple low-cost spin-coating process. Our experimental results indicates that the ion gel’s capacitance is thickness-independent. Theoretical and experimental studies are conducted to gauge the suitability of the ion gel for low-cost EWOD applications. KEYWORDS: Electrowetting-on-dielectric, High capacitance dielectric, Ion gel INTRODUCTION Electrowetting-on-dielectric (EWOD) has been widely used for liquid handling due to its force domi- nance over body forces in small scale. The contact angle modulation of a liquid droplet can be mathemati- cally estimated with the applied electric potential (V) by using the popular Young-Lippmann equation 1 2 cos cos cV , where θ0 is the droplet contact angle with zero potential application, is the surface 0 2 tension between two immiscible fluids, and c is the capacitance per unit area which is expressed as ct 0 , where ε0 is the permittivity of free space, κ is the dielectric constant, and t is the thickness of the capacitor formed between a liquid and a solid electrode. The contact angle modulation is determined by electrostatic energy ( cV 2 2 ) stored across the capacitor without breakdown. To meet such requirements, extensive studies have been conducted on high-κ dielectrics materials such as silicon dioxide (SiO2, 3.9 ), aluminum oxide (Al2O3, 9.5 ), and tantalum pentoxide (Ta2O5, 23 ). However, SiO2 Al23 O Ta25 O these dielectric materials are fabricated by conventional integrated circuit (IC) processes which are typi- cally time-consuming and require complex and expensive laboratory setups such as high vacuum facilities. In this paper, we propose a novel the use of an ion gel material that is not only very simple to fabricate using a spin-coating process, but also offers a high capacitance for low-cost EWOD applications. We dis- cuss theoretical modelling of an ion gel capacitor, which is supported by our experimental EWOD studies. EWOD performance of the ion gel samples was compared with other conventional dielectric materials and displayed an improvement in contact angle modulation. THEORY Ionic liquids, known as room temperature molten organic salts, have been very attractive due to their favourable properties such as a wide electrochemical window, high thermal stability, negligible vapour pressure, and large ionic capacitance. For practical applications of ionic liquids, it is desired to utilize them in the form of a solid film. This thin-film form known as ion gels has been fabricated either by chemical or physical cross-linking of structuring polymers with ionic liquids. Figure 1(a) shows an equivalent circuit model of the ion gel dielectric for EWOD, consisting of a resistor from the bulk electrolyte sandwiched between two electrical double layer (EDL) capacitors formed by its constituent ionic liquid. At electrified interfaces, counter-ions from the ion gel compactly accumulate at the interface and form a nanometer-level EDL. Such a thin (1 ~ 4 nm) EDL formed by the ion gel allows for exceptionally large layer capacitances [1]. The bulk of the ion gel layer exhibits a resistance emerging from the constituent ionic liquid. th 978-0-9798064-8-3/µTAS 2015/$20©15CBMS-0001 1196 19 International Conference on Miniaturized Systems for Chemistry and Life Sciences October 25-29, 2015, Gyeongju, KOREA Figure 1: (a) Schematic of the ion gel layer sandwiched between two electrified surfaces of opposite polarity (left) and its equivalent circuit model (right). (b) Chemical structures of the P(VDF-HFP) copolymer and the [EMIM][TFSI] ionic liquid which are cured to obtain a free standing ion gel layer on a flexible ITO substrate EXPERIMENTAL The ion gel films were successfully fabricated through a simple spin-coating process. The thin-film gel layer was made through the gelation of a P(VDF-HFP) copolymer dissolved in [EMIM][TFSI] ionic liquid. It was then spin-coated on ITO substrates and cured in an oven. The gelation process occurs when the crystals are brought together by polymer chains dissolved in the solution. Figure 1(b) illustrates the chem- ical structures of the copolymer and the ionic liquid. Subsequently, fluoropolymer (Teflon AF) layers were spin-coated on the ion gel films to provide hydrophobicity RESULTS AND DISCUSSION Our first study was to experimentally verify the effect of the ion gel thickness on EWOD performance. For the experiment, the thickness of the Teflon layer was maintained at 550 nm throughout, while the thickness of the gel was varied from 5 µm to 18 µm. Figure 2(a) shows the experimental results for the contact angle with the applied voltage. Despite fairly large variations in the dielectric thickness, it was observed that the contact angle change behaved very similarly for the all the cases and the angle is very close even at the saturation voltage around 70 V. Interestingly, no electrolysis or dielectric breakdown was observed at larger voltages beyond saturation. The experimental results agree with the previous reports that indicate the capacitance of the ion gel does not rely on the layer, but on the EDL layer formed by its constituent ionic liquid. The second study was to understand the thickness effect of the hydrophobic layer on EWOD performance. While the thickness of the ion gel layer was maintained at 10 µm, the thickness of the Teflon layer was first varied from 40 nm to 650 nm. Figure 2(b) presents the effect of the Teflon thickness on EWOD performance. The results obey the Young-Lippmann theory. EWOD performance of the gel layer was benchmarked against standard EWOD dielectric materials such as SiO2, Al2O3, and PDMS. As expected, better EWOD performance is shown in the order from ion Figure 2: (a) The thickness of the Teflon layer was held fixed at 550 nm, while that of the ion gel layer was varied from 5 µm to 18 µm. Negligible variations in contact angle are observed even for large variations in ion gel layer thickness. (b) The thickness of the ion gel layer was held fixed at 10 µm, while that of the Teflon layer was varied from 40 nm to 650 nm. Significant variations in contact angle are observed for variations in Teflon layer thickness. (c) The EWOD performance of the ion gel was compared with other dielectric materials. Insets (a) and (b) display images of a 5 μL water droplet over the Teflon / ion gel stack at 0 V and 40 V respectively. Inset (c) displays an image of the water droplet over the Teflon / Al2O3 stack at 55 V where electrolysis is clearly observed (circled in red). 1197 Figure 3: A 2 μL droplet is actuated over a series of 1.6 x 1.6 mm finger-shaped electrodes in a sandwiched structure. The movement speed was 1.7 cm/s under 30 V. A 10 μm ion gel with a 400 nm Teflon layer was used for the study. gel to PDMS, which is the same order of their capacitance magnitudes (Figure 2(c)). One interesting observation was that electrolysis occurs on the Al2O3 sample at 55 V (see inset (c) in Figure 2(c)), while no such phenomena occurs for the ion gel (see insets (b)) even at the saturation voltage around 50 V. This implies that, due to reliability issues, a thin layer of conventional dielectrics while providing high capacitance are not practical for EWOD-based microfluidic applications. In contrast, the ion gel dielectric layer can provide a thickness-independent capacitance, which serves as a benefit for robust EWOD usage. Additionally, being a high capacitance material, the ion gel induces large storage energy within the dielectric and hydrophobic stack which is in turn crucial for large surface tension modulation. Figure 3 shows droplet transportation on an EWOD device coated with an ion gel, demonstrating the ion gel’s potential for low-cost EWOD applications with a high-capacitance dielectric property. CONCLUSION We present a novel ion gel dielectric that offers two or three orders higher capacitance (~ 10 μF/cm2) than that of conventional dielectrics, while being fabricated through a simple low-cost spin-coating process. We successfully fabricated the ion gel films, which consist of a [P(VDF-HFP)] copolymer and an [EMIM][TFSI] ionic liquid using a simple spin-coating method. The use of the ion gel dielectric allows large electric storage energy responsible for large contact angle modulation at a given voltage input. Our experimental studies interestingly indicate that the ion gel’s capacitance is thickness-independent, which was explained by the theoretical circuit modeling the ion gel dielectric, consisting of a resistor from the bulk electrolyte sandwiched between two electrical double layer (EDL) capacitors formed by its constituent ionic liquid. This thickness-independent capacitance provides a great benefit in improving device reliability issues such as electrolysis by using a micrometer thick dielectric layer. Our comparative EWOD studies indicate that the thick (~ 10 μm) and high-capacitance ion gel layer enables stable and improved EWOD performance without dielectric breakdown, while a thin-film Al2O3 layer experiences electrolysis even be- fore the saturation voltage.
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