Article pubs.acs.org/Langmuir
An Ion Gel as a Low-Cost, Spin-Coatable, High-Capacitance Dielectric for Electrowetting-on-Dielectric (EWOD) Vinayak Narasimhan and Sung-Yong Park*
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Department of Mechanical Engineering, National University of Singapore, Block EA, #07-08, 9 Engineering Drive 1, Singapore 117576 ABSTRACT: For many practical electrowetting-on-dielectric (EWOD) applications, the use of high-capacitance dielectric materials is critically demanded to induce a large surface tension modulation. Thin-film dielectric layers such as Parylene C, silicon dioxide (SiO2), and aluminum oxide (Al2O3) have been commonly used for EWOD. However, these dielectric materials are fabricated by conventional integrated circuit (IC) processes which are typically timeconsuming and require complex and expensive laboratory setups such as high-vacuum facilities. In this article, a novel ion gel material was demonstrated as a spin-coatable and highcapacitance dielectric for low-cost EWOD applications. The ion gel offers a 2 or 3 order higher capacitance (c ≈ 10 μF/cm2) than conventional dielectrics commonly used for EWOD while being fabricated through a simple low-cost spin-coating process. We discuss the fundamentals of an ion gel dielectric, its fabrication process of spin coating, and the interaction with a hydrophobic layer for practical EWOD applications. The ion gel films, which consist of a copolymer, poly(vinylidene fluoride-cohexafluoropropylene) [P(VDF-HFP)], and an ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [EMIM][TFSI], were successfully deposited on ITO substrates by using a simple spin-coating process. The experimental demonstrations validated the theoretical modeling of the ion gel layer as a high-capacitance dielectric. The EWOD performance of the ion gel samples was compared to that of other conventional dielectric materials to show the performance improvement.
1. INTRODUCTION Liquid handling and actuation by means of controlling surface tension has proven to have many advantages in small-scale applications due to the surface tension force dominance over body forces. In 1875, Lippmann first explored the phenomenon of the surface tension of liquids modulated with an electric field, which is called an electrowetting or electrocapillary effect.1 When an electric potential is applied between a liquid and a solid electrode, the charge redistribution modifies the surface tension at the liquid−solid interface where the like-charge repulsion decreases the work by expanding the surface area. The resulting contact angle (θ) of a liquid droplet can be mathematically estimated with the applied electric potential (V) by using the popular Young−Lippmann equation1−3 cos θ = cos θ0 +
1 2 cV 2γ
Conventional electrowetting (EW) utilizes electrolytes directly contacting a solid electrode. With an applied electric potential, the storage energy is sustained across the electric double layer (EDL) capacitor by recombining the charges at the interface. However, the EDL thickness is generally only about 1 to 10 nm, and such a thin layer cannot hold large voltages (i.e., electric breakdown occurs even at very small voltages). Consequently, the modulation of contact angle is very limited by conventional EW, and the process cannot be reversible after breakdown. In recent years, the principle of electrowetting-on-dielectric (EWOD) was discovered to mitigate this issue prevalent in conventional EW.2−4 A thin dielectric layer is inserted between the liquid and the electrode and replaces the EDL capacitor in conventional EW. A dielectric layer ideally blocks electron transfer and is therefore able to hold large electrostatic energy across the dielectric capacitor, which in turn is responsible for large contact angle modulation. In addition, a hydrophobic coating on the dielectric layer makes the initial surface energy large and provides excellent reversibility. With the benefits of large forces in micro/mesoscales, a fast response time in the range of milliseconds, and easy implementation, the EWOD technology has been used for numerous applications, including lab-on-a-
(1)
where θ0 is the droplet contact angle with zero potential application, γ is the surface tension between two immiscible fluids, and c is the capacitance per unit area which is expressed as c = ε0κ /t
(2)
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. © 2015 American Chemical Society
Received: May 13, 2015 Revised: July 6, 2015 Published: July 8, 2015 8512
DOI: 10.1021/acs.langmuir.5b01745 Langmuir 2015, 31, 8512−8518
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Langmuir chip,5,6 optical devices,7,8 thermal management,9,10 solar concentration,11 and surface science.12 As indicated in eq 1, the contact angle modulation is determined by how much electrostatic energy (cV2/2) can be stored across the capacitor without breakdown. To meet such requirements, extensive studies have been conducted on dielectric materials such as silicon dioxide (SiO2,κSiO2 ≈ 3.9),4 aluminum oxide (Al2O3, κAl2O3 ≈ 9.5),13 and tantalum pentoxide (Ta2O5, κTa2O5 ≈ 23).14 A thin-film layer of these materials is fabricated by conventional integrated circuit (IC) processes such as plasma-enhanced chemical vapor deposition (PECVD), atomic-layer deposition (ALD), and sputtering. However, these fabrication methods are typically time-consuming processes requiring complex and expensive laboratory setups such as high-vacuum facilities to provide thin-film dielectric layers. Some high-κ dielectrics such as barium strontium titanate (BST, [(Ba,Sr)TiO3], κBST ≈ 180) have been utilized to enhance the capacitance of a dielectric material but by employing complex and rarely used processes such as metalorganic chemical vapor deposition (MOCVD).4 Recent studies for three-dimensional (3D) flexible EWOD applications have used polymer-based spin-coatable dielectric materials such as SU-8 and PDMS to avoid the need for conventional IC fabrication processes.15,16 However, these dielectric materials offer a dielectric constant even lower than that of SiO2, and when deposited via a spin-coating process result in a thick layer in the order of micrometers, leading to a very low capacitance. Hence, there is a strong motivation to develop a dielectric material that is not only very simple to fabricate but also offers a high capacitance for more practical EWOD applications. We have for the first time demonstrated the use of an ion gel material as a spin-coatable, high-capacitance dielectric for lowcost EWOD applications. The ion gel offers 2 or 3 order higher capacitance than that of conventional dielectrics such as SiO2 and Al2O3 while being fabricated through a simple low-cost spin-coating process. In this article, we discuss the theoretical modeling of an ion gel capacitor, its fabrication method, and the interaction with a hydrophobic layer for practical EWOD applications. Experimental demonstrations support our ion gel modeling as a high-capacitance dielectric. EWOD performance of the ion gel samples was compared to that of other conventional dielectric materials and displayed an improvement in contact angle modulation.
liquids provides a large specific capacitance. Compared to conventional aqueous and organic electrolytes, ionic liquids can realize significantly larger (by a few orders) surface charge densities which allows for wide electrochemical windows without the breakdown of the EDL.25−27 Using the Gouy− Chapman theory, the specific capacitance of the EDL is modeled as cEDL = ε0κ/LD. It can be calculated by characterizing the thickness (LD) of the EDL, known as the Debye length.20,27 One accurate approach was through measuring electrostatic forces normal to the liquid−electrode interface thereby estimating the effective Debye length of 1−4 nm for most ionic liquids,24 which closely matches the EDL thickness values reported in other works.27,28 This approach provided an approximate EDL capacitance value in the range of 3−11 μF/ cm2 for most commonly used ionic liquids.20 2.2. Theoretical Modeling of an Ion Gel Dielectric. 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. On the basis of previous studies,17,18,20,27,29 the ion gel layer can be modeled for EWOD using an equivalent circuit consisting of capacitors and a resistor in series, as presented in Figure 1.
Figure 1. Schematic of the ion gel layer sandwiched between two electrified surfaces of opposite polarity (left) and its equivalent circuit model (right). Counterions from the gel pack themselves against their respective electrode surfaces, forming nanometer-thick EDL capacitors. The bulk of the ion gel layer exhibits a resistance emerging from the constituent ionic liquid. An ion gel layer can be simply modeled in a series connection of a resistor (Rbulk) sandwiched by two capacitors (cEDL) for EWOD studies.
Following its constituent ionic liquid, the ion gel also forms two compact EDL capacitors at electrified interfaces while there is a bulk electrolyte resistance also emerging from the constituent ionic liquid. In this article, we selected an [EMIM][TFSI] ionic liquid (1-ethyl-3-methylimidazolium bis(triuoromethylsulfonyl)imide) and a P(VDF-HFP) copolymer (poly(vinylidene fluoride-co-hexafluoropropylene)) to provide an ion gel dielectric for experimental EWOD studies. Our theoretical modeling and fundamental studies of an ion gel capacitor are based on these materials. To characterize the electrical properties of ion gel films (P(VDF-HFP)/[EMIM][TFSI]), one experimental approach utilized impedance spectroscopy and measured the frequencydependent impedance (Z), which is in the form of Z = Z′ + iZ″, where i is an imaginary unit and Z′ and Z″ constitute the real and imaginary parts of the impedance, respectively.17,18 The capacitance value of the gel was calculated from Z″ data to be cEDL ≈ 10 μF/cm2 at a low frequency of around 1 Hz. Another approach using a constant phase element (CPE) allowed a frequency-independent capacitance of the ion gel to be computed to ∼4 μF/cm2.17,20 The experimentally measured
2. FUNDAMENTALS OF IONIC LIQUIDS AND ION GELS 2.1. Ionic Liquids. The ion gel is a recently developed material in the form of a thin film made of an ionic liquid and a structuring polymer. Because the ionic liquid is an essential constituent of the ion gel, it also features similar electrical properties to that of the ionic liquid.17−20 Therefore, it is helpful to first understand the electrochemical characteristics of ionic liquids to address the theoretical modeling of an ion gel dielectric for EWOD. Ionic liquids, known as room-temperature molten organic salts, have been very attractive due to their favorable properties such as a wide electrochemical window, high thermal stability, negligible vapor pressure, and large ionic capacitance.19,21−24 Under an applied electric field, free counterions inside ionic liquids compactly accumulate at the liquid−electrode interface and form the nanometer-thick electric double layer (EDL) which functions as a capacitor. Such a thin EDL of the ionic 8513
DOI: 10.1021/acs.langmuir.5b01745 Langmuir 2015, 31, 8512−8518
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Langmuir
75 °C to ensure that any remaining solvent was fully purged, enabling a cured ion gel layer to be formed. Figure 3 represents the thickness measurement of the gel layers that were fabricated under various spin conditions. The
capacitance values of the P(VDF-HFP)/[EMIM][TFSI] ion gel fall within the proposed range of the EDL capacitance while using the [EMIM][TFSI] ionic liquid.17,18,20 They also investigated the thickness effect of the ion gel and interestingly found that the structuring polymers do not contribute to the capacitance of the ion gel layer.18 This indicates that the high capacitance of the gel layer is purely obtained as a result of the thin EDL formed by its constituent ionic liquid and is therefore not influenced by the type of structuring polymers used. This explains the reason that two different ABA block copolymers based on [SMS]18 and P(VDF-HFP)17 exhibit similar capacitance values when both are structured around the same ionic liquid of [EMIM][TFSI]. The term Z′ represents the specific resistance (Rbulk) of the gel, which again originates from its constituent ionic liquid. Z′ is generally computed through the impedance at high frequencies where the gel resistance is dominant. For common ionic liquids such as [EMIM][TFSI], the conductivity ranges from 10−4 to 10−2 S/cm.18,19 For example, a 10 μm thick ion gel based on [EMIM][TFSI], which corresponds to our experimental condition, would exhibit a specific resistance of Rbulk = 0.1−10 Ω cm2. However, the voltage drop across this resistance of the ion gel layer can be neglected because the capacitive impedance (Z″) is mostly dominant for EWOD operated typically at dc or very low frequency ac.
Figure 3. Various thicknesses of an ion gel layer fabricated by varying the spin speed and relative wt % of the mixtures. The legend indicates the relative wt % ratio of copolymer P(VDF-HFP), ionic liquid [EMIM][TFSI], and acetone solvent.
3. FABRICATION OF ION GEL LAYERS Using a simple and low-cost spin-coating method without conventional IC fabrication processes such as PECVD, ALD, and sputtering, the layers of an ion gel dielectric were successfully fabricated on ITO substrates. Subsequently, Teflon AF layers were coated on the top to provide a hydrophobic surface. Figure 2 illustrates the chemical structures of the
thickness was characterized with the dry film after curing using an optical profilometer (ZYGO NewView 5032). By varying the spin speed as well as the wt % ratio of the copolymer, ionic liquid, and solvent, we were able to vary the ion gel thickness from 4 to 18 μm. Following this, we attempted to perform EWOD directly on the ion gel. However, it was observed that the initial contact angle, while remaining thickness-independent, was around 70°, which is impractical for typical EWOD applications. Therefore, a Teflon AF solution (DuPont) was spin coated on various ion gel layers to provide a hydrophobic surface. The thickness of the Teflon AF was similarly controlled by several spin speeds over the cured ion gel layer. The effective thin-film stack was cured at 105 °C for 12 h in an oven.
4. EXPERIMENTAL RESULTS Several experiments for static EWOD studies were conducted to verify the understanding of ion gels discussed in section 2. An experimental setup for the studies is shown in Figure 4. A water droplet was placed on the spin-coated ion gel and Teflon layers, forming an initial contact angle of around 116°. The thickness of the gel layer and the Teflon layer were individually varied to study the thickness effect on the EWOD performance. An electric potential, V, was applied between the water droplet through a platinum probe and an ITO electrode. The equivalent circuit is modeled for EWOD studies as the impedances of three layers in a series connection. The contact angle was then measured at incremental voltages, which was controlled by LabVIEW. 4.1. Thickness Effect of the Ion Gel Layer. It is generally understood from eqs 1 and 2 that lowering the thickness of the dielectric layer increases its capacitance; therefore, a larger contact angle change is expected at a given applied voltage. However, as discussed in section 2.2, the capacitance of the ion gel layer was expected to be thickness-independent. 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 to 18 μm.
Figure 2. Chemical structures of the P(VDF-HFP) copolymer (top) and the [EMIM][TFSI] ionic liquid (bottom) which are cured to obtain free-standing ion gels. The right-side image shows layers of an ion gel and Teflon fabricated on a flexible ITO substrate, which can be bent with ease.
copolymer and the ionic liquid. The ion gel was first prepared by selective dissolution of the PHFP copolymer in the ionic liquid while the PVDF crystals are insoluble.17,29 The gelation process occurs when these crystals are brought together by polymer chains dissolved in the solution. P(VDF-HFP) copolymer in the form of a pellet was added to the [EMIM][TFSI] ionic liquid to form a 1:4 mixture by weight. Acetone was added to further dilute the mixture. The polymer was then dissolved completely over 3 h at 60 °C. The solution was spin coated on an ITO substrate at various spin speeds ranging from 1000−5000 rpm for 30 s, immediately forming a gel layer. The substrate was then placed in an oven for 24 h at 8514
DOI: 10.1021/acs.langmuir.5b01745 Langmuir 2015, 31, 8512−8518
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4.2. Thickness Effect of the Hydrophobic Layer. Our 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 650 to 170 nm. Figure 6 presents the effect of the Teflon
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Figure 4. Experimental setup for a static EWOD study. A 5 μL water droplet is loaded on top of the hydrophobic (Teflon AF) and dielectric (ion gel) layers (not to scale). The equivalent circuit diagram consists of the three-layer impedances (water, a hydrophobic, and a dielectric layer). Figure 5 shows the experimental results for the contact angle with the applied voltage. Despite fairly large variations in the dielectric
Figure 6. Contact angle modulation with applied voltage. The thickness of the ion gel layer was held fixed at 10 μm while that of the Teflon layer was varied from 170 to 650 nm. Significant variations in contact angle are observed for variations in the Teflon layer thickness. thickness on EWOD performance, indicated as solid lines. The measured contact angle data for each different experimental condition was collected from four different tests, and the mean value is presented in Figure 6. The maximum percentage error was around ±4%. Unlike the previous results for varying the ion gel thickness, the trend observed was that the EWOD performance continually improved as the Teflon thickness was reduced. The saturation voltage also drops accordingly. In other words, large contact angle modulations were possible for a given applied voltage because the thinner the Teflon layer, the larger the capacitance. 4.3. Interaction of the Ion Gel with the Hydrophobic Layer. Because the experimental results presented in Figure 6 match the Young−Lippmann theory in eqs 1 and 2, the following experiments were repeated with the further reduced Teflon thickness while maintaining the same ion gel layer of 10 μm. A 6% Teflon solution was diluted to 1% by adding FC-72 to fabricate Teflon layers thinner than 100 nm. Additional EWOD test data with hydrophobic thicknesses of 40 and 100 nm are also included in Figure 6, indicated as dotted lines. While initial contact angle modulation in both cases was considerable, electrolysis was observed at relatively low voltages applied at around 10 V for the case of the 40 nm Teflon and at around 25 V for the 100 nm Teflon layer. Upon the onset of electrolysis, the modulation steadily saturated. Although the hydrophobic layer in the range of tens of nanometers in thickness has been often used for many EWOD studies,4,14,30 in our case, the interaction of the high-capacitance gel layer with the Teflon layer limits the lowest possible voltage repeatable for EWOD actuation. Our next study focused on how the highcapacitance ion gel dielectric interacts with the hydrophobic layer spin coated on top of it. Table 1 presents information on various dielectrics used in conjunction with a thin-film Teflon layer at 20 nm for the EWOD studies reported in earlier works. It also offers a comparison between conventional dielectrics and the ion gel material in terms of capacitance, voltage consumption, and electrostatic energy stored across the capacitors. The ion gel layer is assumed to be 10 μm thick, which corresponds to our experimental condition. The total potential drop across the dielectric and hydrophobic layers is standardized to 10
Figure 5. Contact angle modulation with applied voltage. The thickness of the Teflon layer was held fixed at 550 nm, while that of the ion gel layer was varied from 0 to 18 μm. Negligible variations in contact angle are observed even for large variations in ion gel layer thickness. For comparison, data from tests involving pure 550 nm Teflon over ITO is added to highlight the contribution of the ion gel.
thickness, it was observed that the contact angle change behaved very similarly for all cases, and the angle is very close even at the saturation voltage of 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 the capacitance of the ion gel does not rely on the thickness change of the structuring polymer layer but on the EDL layer formed by its constituent ionic liquid.18,20 To clearly understand the contribution of the ion gel to EWOD performance, another experiment was conducted with a pure 550-nm-thick Teflon layer over ITO. The thickness of a Teflon layer is consistently maintained with all of the previous cases that have variable ion gel thickness. It can be experimentally seen that the mean contact angle at saturation when using the ion gel is roughly 20° lower than the mean contact angle for Teflon over bare ITO. This comparative study clearly indicates that the performance of the Teflon and ion gel stack is superior to that of pure Teflon. 8515
DOI: 10.1021/acs.langmuir.5b01745 Langmuir 2015, 31, 8512−8518
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Table 1. Theoretical Estimations and Comparisons of Capacitance and Electric Energy Stored Across Various Hydrophobic and Dielectric Layer Stacks Used for EWOD
The specific capacitance is calculated by c = ε0κ/t except for the ion gel. 2The total potential drop across both dielectric and hydrophobic layers is assumed to be Vtotal = Vh + Vd = 10 V in all cases. 3The electric storage energy is calculated by E = cV2/2 for each layer and Etotal = Eh + Ed. 4The ion gel thickness is assumed to be 10 μm, which corresponds to our experimental condition. 5This value is selected from refs 33 and 34. 6The capacitance (cd) of the ion gel dielectric is dominantly contributed by two EDL capacitors (cEDL = 10 μF/cm2) formed at electrified interfaces.17−20 Thus, it is estimated as 1/cd = 1/cEDL + 1/cEDL. 1
in Figure 5. Thus, many EWOD studies have typically used it as a thin hydrophobic layer with another dielectric material.5,14,35,36 In addition, it is interestingly noted that complete dielectric breakdown occurs only when the breakdown voltage of the higher-dielectric-strength material is exceeded.37,38 For our case using the high-capacitance ion gel, although the voltage distributed across the hydrophobic layer exceeds its breakdown voltage, the device can continuously perform while being slowly degraded. This behavior was experimentally observed, as shown in Figure 6. For the 40- and 100-nm-thick Teflon layers, the theoretical breakdown voltages are estimated to be 8 and 20 V at the given thickness of the layers, respectively.4 Even at voltages exceeding this breakdown threshold, the contact angle for the droplet was able to be further modulated. Note that the angle saturation was observed at around 10 V for the 40 nm Teflon and at around 25 V for the 100 nm Teflon. 4.4. Comparison to Other Dielectrics for EWOD. Finally, we attempted to benchmark the performance of the ion gel against certain standard dielectrics commonly used in EWOD. This would help gauge the practicality of its use in various EWOD applications. Al2O3 and SiO2 were chosen to represent standard thin-film dielectric materials deposited through conventional IC processes that typically require complex and expensive fabrication facilities. In contrast, PDMS was chosen to represent dielectric materials fabricated by a simple spincoating procedure. The thickness of the ion gel was 10 μm, while those of Al2O3, SiO2, and PDMS were 100 nm, 100 nm, and 10 μm, respectively. A Teflon layer of the same thickness (170 nm) was spin coated on the above-mentioned dielectrics as well as for the pure Teflon case for comparison. Similar to previous tests, the mean value of the contact angle measurements was selected from four different tests with the maximum percentage error around ±3%. Figure 7 shows that the gel layer performs much better than other benchmarked dielectrics with the same given thickness of the Teflon layer. As expected, EWOD performance is shown in the order of Al2O3, SiO2, and PDMS, which is the same order of their capacitance magnitudes. To highlight the beneficial effects of using the Teflon and ion gel stack, a pure Teflon layer with the same thickness of 170 nm was compared to those with different dielectric layers. It was observed that pure Teflon performs poorer than both Al2O3 and SiO2 but better than PDMS. One interesting observation was that electrolysis occurs on the Al2O3 sample at 55 V (inset (c)), while no such phenomena occurs in the case of the ion gel (insets (b)) even at the saturation
V in all cases. For comparison, the information on a pure 20-nm-thick Teflon layer was also included. On the basis of the equivalent circuit model presented in Figure 1, the total capacitance, ctotal, is estimated by considering a series connection of the capacitances of a hydrophobic (ch) and a dielectric (cd) layer. It can be expressed as
1 1 1 = + ctotal ch cd
(3)
As the capacitance (cd) of a dielectric layer increases, several interesting phenomena are observed from Table 1 as follows: (a) ctotal increases, (b) ctotal is close to ch when a high-capacitance dielectric such as the ion gel is used, (c) most of the voltage drop occurs over the hydrophobic layer, (d) correspondingly, electric energy (Eh = chVh2/2) stored across the hydrophobic layer increases, although its thickness is kept constant in all the cases, (e) total electric storage energy, Etotal = Eh + Ed, also increases, and (f) particularly for the cases of high-capacitance materials such as ion gels, the total electric energy stored across both layers is more than 2-fold that of conventional dielectric materials such as SiO2 and Al2O3. The studies shown in Table 1 represent the fact that the use of highcapacitance dielectrics such as ion gels can offer a significant increase in the total electric storage energy at a given voltage, which is responsible for large contact angle modulation as indicated in eq 1. This is the reason due to which numerous EWOD studies have attempted either to increase the dielectric constant, κ, and/or reduce the thickness, t, of the layers. Another interesting observation is that a very large portion of the total electric energy is stored across the hydrophobic layer when high-capacitance dielectric materials are used. In the case with the ion gel layer of 10 μm thickness, more than 98% of the total potential drop is sustained across the Teflon layer of 20 nm, as presented in Table 1. This indicates that the total electric storage energy of the Teflon and ion gel stack is close to that of pure Teflon because ctotal ≈ ch and Vtotal ≈ Vh. However, it is practically known that Teflon does not provide a strong dielectric characteristic when it is used alone as a dielectric layer. This is supported by the prior experimental results4,14 as well as our experimental observations shown 8516
DOI: 10.1021/acs.langmuir.5b01745 Langmuir 2015, 31, 8512−8518
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Fundamentals of the ion gel dielectric were discussed for EWOD studies. Our understanding of the ion gel indicates that it can be equivalently circuit-modeled as a series connection of an electrolyte resistor in the bulk layer and two nanometerthick EDL capacitors at the interfaces. By using a simple spincoating method, we successfully fabricated the ion gel films, which consist of a [P(VDF-HFP)] copolymer and an [EMIM][TFSI] ionic liquid. With the fabricated ion gel layer, the thickness effect of both dielectric and hydrophobic layers on EWOD performance was investigated. The use of the ion gel dielectric allows large electric storage energy responsible for large contact angle modulation while inducing a very large portion of the storage energy to be sustained across the hydrophobic layer. We also benchmarked the performance of the ion gel against several dielectrics typically used in EWOD. The comparison results clearly showed that the ion gel dielectric offers a better performance without dielectric breakdown, while thin-film dielectrics such as Al2O3 in the experiment are impractical for regular use in microfluidics because of reliability issues. Additionally, while providing similar high-storage-energy characteristics as pure Teflon over ITO, the ion gel provides far better and more reliable EWOD performance in comparison. The ion gel studied in this article would be potentially very useful for low-cost EWOD-based microfluidic applications by offering a capacitance as high as c ≈ 10 μF/cm2 and while being fabricated through a simple spin-coating method.
Figure 7. Comparative study of the EWOD performance of the ion gel versus Al2O3, SiO2, and PDMS. Points (a) and (b) represented by insets display 5 μL water droplets over the Teflon and ion gel stack at 0 and 50 V, respectively. Point (c) also represented by an inset displays a droplet over the Teflon and Al2O3 stack at 55 V where bubbles caused by electrolysis are clearly observed inside the droplet (indicated by a red arrow). Point (d), similar to (c), corresponds to the voltage (50 V) at which breakdown occurs for Teflon over bare ITO. The thickness of the ion gel was 10 μm, while those of Al2O3, SiO2, and PDMS were 100 nm, 100 nm, and 10 μm, respectively. A Teflon layer of the same thickness (170 nm) was spin coated on the abovementioned dielectrics as well as for the pure Teflon case for comparison.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
voltage around 50 V. Similarly, electrolysis is observed for pure Teflon at a lower voltage of 50 V. This implies that, because of reliability issues, thin layers of conventional dielectrics, while providing high capacitance, are not practical for EWOD-based microfluidic applications such as droplet dispensing or splitting that typically require more force and hence higher voltages than droplet transportation.14 In contrast, the ion gel dielectric layer can provide a thickness-independent capacitance, which serves as a benefit for robust use in microfluidic applications. Additionally, being a highcapacitance material, the ion gel induces a large amount of storage energy within the dielectric and hydrophobic stack, which is in turn crucial for large surface tension modulation. This again is evident from the theoretical study shown in Table 1, which is verified by our experimental results in Figure 7.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the grants (R-265-000-470-133 and R-265-000-528-112) from the Ministry of Education, Singapore.
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REFERENCES
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5. CONCLUSIONS Ionic liquids, known as room-temperature molten organic salts, have been demonstrated to have a large ionic capacitance (c ≈ 10 μF/cm2) due to the nanometer-thick EDL formed at electrified interfaces. Ionic liquids, when combined with structuring polymers, are gelated upon spin coating to form the thin films called ion gels. Because the ionic liquid is an essential constituent of the ion gel, it also offers a highcapacitance property similar to that of the ionic liquid. In this article, we have demonstrated such a high-capacitance property of the ion gel for EWOD applications. Unlike conventional thin-film dielectric materials typically requiring complex and expensive laboratory setups such as high-vacuum facilities, the layer of ion gels can be simply fabricated by a spin-coating process that is cost efficient and time saving. Although being fabricated via easy fabrication methods, the ion gel offers a few orders of magnitude higher capacitance than conventional dielectrics commonly used for EWOD. 8517
DOI: 10.1021/acs.langmuir.5b01745 Langmuir 2015, 31, 8512−8518
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Langmuir (9) Mohseni, K.; Baird, E. Digitized Heat Transfer Using Electrowetting on Dielectric. Nanoscale and Microscale Thermophysical Engineering 2007, 11, 90−108. (10) Park, S.-Y.; Cheng, J.; Chen, C.-L. Active Hot Spot Cooling driven by Single-Side Electrowetting-on-Dielectric (SEWOD). Proceeding of the ASME 2012 Summer Heat Transfer Conference (ASME HT 2012); Puerto Rico, 2012. (11) Cheng, J.; Park, S.-Y.; Chen, C.-L. Optofluidic Solar Concentrators using Electrowetting Tracking: Concept, Design, and Characterization. Sol. Energy 2013, 89, 152−167. (12) Barberoglou, M.; Zorba, V.; Pagozidis, A.; Fotakis, C.; Stratakis, E. Electrowetting Properties of Micro/Nanostructured Black Silicon. Langmuir 2010, 26 (15), 13007−13014. (13) Chang, J.-h.; Choi, D. Y.; Han, S.; Pak, J. J. Driving characteristics of the electrowetting-on-dielectric device using atomic-layer-deposited aluminum oxide as the dielectric. Microfluid. Nanofluid. 2010, 8, 269−273. (14) Lin, Y.-Y.; Evans, R. D.; Welch, E.; Hsu, B.-N.; Madison, A. C.; Fair, R. B. Low voltage electrowetting-on-dielectric platform using multi-layer insulators. Sens. Actuators, B 2010, 150, 465−470. (15) Abdelgawad, M.; Freire, S. L. S.; Yang, H.; Wheeler, A. R. Allterrain droplet actuation. Lab Chip 2008, 8, 672−677. (16) Fan, S.-K.; Yang, H.; Hsu, W. Droplet-on-a-wristband: Chip-tochip digital microfluidic interfaces between replaceable and flexible electrowetting modules. Lab Chip 2011, 11, 343−347. (17) Lee, K. H.; Kang, M. S.; Zhang, S.; Gu, Y.; Lodge, T. P.; Frisbie, C. D. Cut and stick” rubbery ion gels as high capacitance gate dielectrics. Adv. Mater. 2012, 24, 4457−4462. (18) Lee, K. H.; Zhang, S.; Lodge, T. P.; Frisbie, C. D. Electrical impedance of spin-coatable ion gel films. J. Phys. Chem. B 2011, 115, 3315−3321. (19) Galiński, M.; Lewandowski, A.; Stępniak, I. Ionic liquids as electrolytes. Electrochim. Acta 2006, 51, 5567−5580. (20) Zhang, S.; Lee, K. H.; Frisbie, D.; Lodge, T. P. Ionic conductivity, capacitance, and viscoelastic properties of block copolymer-based ion gels. Macromolecules 2011, 44, 940−949. (21) Balducci, A.; Bardi, U.; Caporali, S.; Mastragostino, M.; Soavia, F. Ionic liquids for hybrid supercapacitors. Electrochem. Commun. 2004, 6, 566−570. (22) Ue, M.; Takeda, M.; Takahashi, T.; Takehara, M. Ionic liquids with low melting points and their application to double-layer capacitor electrolytes. Electrochem. Solid-State Lett. 2002, 5, A119−A121. (23) Yuan, H.; Liu, H.; Shimotani, H.; Guo, H.; Chen, M.; Xue, Q.; Iwasa, Y. Liquid-gated ambipolar transport in ultrathin films of a topological insulator Bi2Te3. Nano Lett. 2011, 11, 2601−2605. (24) Min, Y.; Akbulut, M.; Sangoro, J. R.; Kremer, F.; Prud’homme, R. K.; Israelachvili, J. Measurement of Forces across Room Temperature Ionic Liquids between Mica Surfaces. J. Phys. Chem. C 2009, 113, 16445−16449. (25) Lynden-Bell, R. M.; Frolov, A. I.; Fedorov, M. V. Electrode screening by ionic liquids. Phys. Chem. Chem. Phys. 2012, 14, 2693− 2701. (26) Chaplin, M. Theory vs Experiment: What is the Surface Charge of Water? Water 2009, 1, 1−28. (27) Fedorov, M. V.; Kornyshev, A. A. Ionic liquids at electrified interfaces. Chem. Rev. 2014, 114, 2978−3036. (28) Kornyshev, A. A. Double-Layer in Ionic Liquids: Paradigm Change? J. Phys. Chem. B 2007, 111, 5545−5557. (29) Jansen, J. C.; Friess, K.; Clarizia, G.; Schauer, J.; Izák, P. High ionic liquid content polymeric gel membranes: preparation and performance. Macromolecules 2011, 44, 39−45. (30) Cho, S. K.; Moon, H.; Kim, C.-J. Creating, transporting, cutting, and merging liquid droplets by electrowetting-based actuation for digital microfluidic circuits. J. Microelectromech. Syst. 2003, 12, 70−80. (31) Klein, N.; Gafni, H. The maximum dielectric strength of thin silicon oxide films. IEEE Trans. Electron Devices 1966, ED-13, 281− 289.
(32) Wu, Y. Q.; Lin, H. C.; Ye, P. D.; Wilk, G. D. Current transport and maximum dielectric strength of atomic-layer-deposited ultrathin Al2O3 on GaAs. Appl. Phys. Lett. 2007, 90, 072105. (33) Izgorodina, E. I.; Forsyth, M.; MacFarlane, D. R. On the components of dielectric constants of ionic liquids: in search of ionic polarization. Phys. Chem. Chem. Phys. 2009, 11, 2452−2458. (34) Krossing, I.; Slattery, J. M.; Daguenet, C.; Dyson, P. J.; Oleinikova, A.; Weingärtner, H. Why Are Ionic Liquids Liquid? A Simple Explanation Based on Lattice and Solvation Energies. J. Am. Chem. Soc. 2006, 128, 13427−13434. (35) Gong, J.; Kim, C.-J. All-electronic droplet generation on-chip with real-time feedback control for EWOD digital microfluidics. Lab Chip 2008, 8, 898−906. (36) Mugele, F.; Baret, J.-C. Electrowetting: from basics to applications. J. Phys.: Condens. Matter 2005, 17, 705−774. (37) Berry, S.; Kedzierski, J.; Abedian, B. Low voltage electrowetting using thin fluoroploymer films. J. Colloid Interface Sci. 2006, 303.51710.1016/j.jcis.2006.08.004 (38) Berry, S.; Kedzierski, J.; Abedian, B. Irreversible electrowetting on thin fluoropolymer films. Langmuir 2007, 23, 12429−12435.
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DOI: 10.1021/acs.langmuir.5b01745 Langmuir 2015, 31, 8512−8518
An Ion Gel as a Low-Cost, Spin-Coatable, High-Capacitance Dielectric for Electrowetting-on-Dielectric (EWOD) Vinayak Narasimhan and Sung-Yong Park*
Downloaded by WEST VIRGINIA UNIV on September 11, 2015 | http://pubs.acs.org Publication Date (Web): July 22, 2015 | doi: 10.1021/acs.langmuir.5b01745
Department of Mechanical Engineering, National University of Singapore, Block EA, #07-08, 9 Engineering Drive 1, Singapore 117576 ABSTRACT: For many practical electrowetting-on-dielectric (EWOD) applications, the use of high-capacitance dielectric materials is critically demanded to induce a large surface tension modulation. Thin-film dielectric layers such as Parylene C, silicon dioxide (SiO2), and aluminum oxide (Al2O3) have been commonly used for EWOD. However, these dielectric materials are fabricated by conventional integrated circuit (IC) processes which are typically timeconsuming and require complex and expensive laboratory setups such as high-vacuum facilities. In this article, a novel ion gel material was demonstrated as a spin-coatable and highcapacitance dielectric for low-cost EWOD applications. The ion gel offers a 2 or 3 order higher capacitance (c ≈ 10 μF/cm2) than conventional dielectrics commonly used for EWOD while being fabricated through a simple low-cost spin-coating process. We discuss the fundamentals of an ion gel dielectric, its fabrication process of spin coating, and the interaction with a hydrophobic layer for practical EWOD applications. The ion gel films, which consist of a copolymer, poly(vinylidene fluoride-cohexafluoropropylene) [P(VDF-HFP)], and an ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [EMIM][TFSI], were successfully deposited on ITO substrates by using a simple spin-coating process. The experimental demonstrations validated the theoretical modeling of the ion gel layer as a high-capacitance dielectric. The EWOD performance of the ion gel samples was compared to that of other conventional dielectric materials to show the performance improvement.
1. INTRODUCTION Liquid handling and actuation by means of controlling surface tension has proven to have many advantages in small-scale applications due to the surface tension force dominance over body forces. In 1875, Lippmann first explored the phenomenon of the surface tension of liquids modulated with an electric field, which is called an electrowetting or electrocapillary effect.1 When an electric potential is applied between a liquid and a solid electrode, the charge redistribution modifies the surface tension at the liquid−solid interface where the like-charge repulsion decreases the work by expanding the surface area. The resulting contact angle (θ) of a liquid droplet can be mathematically estimated with the applied electric potential (V) by using the popular Young−Lippmann equation1−3 cos θ = cos θ0 +
1 2 cV 2γ
Conventional electrowetting (EW) utilizes electrolytes directly contacting a solid electrode. With an applied electric potential, the storage energy is sustained across the electric double layer (EDL) capacitor by recombining the charges at the interface. However, the EDL thickness is generally only about 1 to 10 nm, and such a thin layer cannot hold large voltages (i.e., electric breakdown occurs even at very small voltages). Consequently, the modulation of contact angle is very limited by conventional EW, and the process cannot be reversible after breakdown. In recent years, the principle of electrowetting-on-dielectric (EWOD) was discovered to mitigate this issue prevalent in conventional EW.2−4 A thin dielectric layer is inserted between the liquid and the electrode and replaces the EDL capacitor in conventional EW. A dielectric layer ideally blocks electron transfer and is therefore able to hold large electrostatic energy across the dielectric capacitor, which in turn is responsible for large contact angle modulation. In addition, a hydrophobic coating on the dielectric layer makes the initial surface energy large and provides excellent reversibility. With the benefits of large forces in micro/mesoscales, a fast response time in the range of milliseconds, and easy implementation, the EWOD technology has been used for numerous applications, including lab-on-a-
(1)
where θ0 is the droplet contact angle with zero potential application, γ is the surface tension between two immiscible fluids, and c is the capacitance per unit area which is expressed as c = ε0κ /t
(2)
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. © 2015 American Chemical Society
Received: May 13, 2015 Revised: July 6, 2015 Published: July 8, 2015 8512
DOI: 10.1021/acs.langmuir.5b01745 Langmuir 2015, 31, 8512−8518
Article
Downloaded by WEST VIRGINIA UNIV on September 11, 2015 | http://pubs.acs.org Publication Date (Web): July 22, 2015 | doi: 10.1021/acs.langmuir.5b01745
Langmuir chip,5,6 optical devices,7,8 thermal management,9,10 solar concentration,11 and surface science.12 As indicated in eq 1, the contact angle modulation is determined by how much electrostatic energy (cV2/2) can be stored across the capacitor without breakdown. To meet such requirements, extensive studies have been conducted on dielectric materials such as silicon dioxide (SiO2,κSiO2 ≈ 3.9),4 aluminum oxide (Al2O3, κAl2O3 ≈ 9.5),13 and tantalum pentoxide (Ta2O5, κTa2O5 ≈ 23).14 A thin-film layer of these materials is fabricated by conventional integrated circuit (IC) processes such as plasma-enhanced chemical vapor deposition (PECVD), atomic-layer deposition (ALD), and sputtering. However, these fabrication methods are typically time-consuming processes requiring complex and expensive laboratory setups such as high-vacuum facilities to provide thin-film dielectric layers. Some high-κ dielectrics such as barium strontium titanate (BST, [(Ba,Sr)TiO3], κBST ≈ 180) have been utilized to enhance the capacitance of a dielectric material but by employing complex and rarely used processes such as metalorganic chemical vapor deposition (MOCVD).4 Recent studies for three-dimensional (3D) flexible EWOD applications have used polymer-based spin-coatable dielectric materials such as SU-8 and PDMS to avoid the need for conventional IC fabrication processes.15,16 However, these dielectric materials offer a dielectric constant even lower than that of SiO2, and when deposited via a spin-coating process result in a thick layer in the order of micrometers, leading to a very low capacitance. Hence, there is a strong motivation to develop a dielectric material that is not only very simple to fabricate but also offers a high capacitance for more practical EWOD applications. We have for the first time demonstrated the use of an ion gel material as a spin-coatable, high-capacitance dielectric for lowcost EWOD applications. The ion gel offers 2 or 3 order higher capacitance than that of conventional dielectrics such as SiO2 and Al2O3 while being fabricated through a simple low-cost spin-coating process. In this article, we discuss the theoretical modeling of an ion gel capacitor, its fabrication method, and the interaction with a hydrophobic layer for practical EWOD applications. Experimental demonstrations support our ion gel modeling as a high-capacitance dielectric. EWOD performance of the ion gel samples was compared to that of other conventional dielectric materials and displayed an improvement in contact angle modulation.
liquids provides a large specific capacitance. Compared to conventional aqueous and organic electrolytes, ionic liquids can realize significantly larger (by a few orders) surface charge densities which allows for wide electrochemical windows without the breakdown of the EDL.25−27 Using the Gouy− Chapman theory, the specific capacitance of the EDL is modeled as cEDL = ε0κ/LD. It can be calculated by characterizing the thickness (LD) of the EDL, known as the Debye length.20,27 One accurate approach was through measuring electrostatic forces normal to the liquid−electrode interface thereby estimating the effective Debye length of 1−4 nm for most ionic liquids,24 which closely matches the EDL thickness values reported in other works.27,28 This approach provided an approximate EDL capacitance value in the range of 3−11 μF/ cm2 for most commonly used ionic liquids.20 2.2. Theoretical Modeling of an Ion Gel Dielectric. 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. On the basis of previous studies,17,18,20,27,29 the ion gel layer can be modeled for EWOD using an equivalent circuit consisting of capacitors and a resistor in series, as presented in Figure 1.
Figure 1. Schematic of the ion gel layer sandwiched between two electrified surfaces of opposite polarity (left) and its equivalent circuit model (right). Counterions from the gel pack themselves against their respective electrode surfaces, forming nanometer-thick EDL capacitors. The bulk of the ion gel layer exhibits a resistance emerging from the constituent ionic liquid. An ion gel layer can be simply modeled in a series connection of a resistor (Rbulk) sandwiched by two capacitors (cEDL) for EWOD studies.
Following its constituent ionic liquid, the ion gel also forms two compact EDL capacitors at electrified interfaces while there is a bulk electrolyte resistance also emerging from the constituent ionic liquid. In this article, we selected an [EMIM][TFSI] ionic liquid (1-ethyl-3-methylimidazolium bis(triuoromethylsulfonyl)imide) and a P(VDF-HFP) copolymer (poly(vinylidene fluoride-co-hexafluoropropylene)) to provide an ion gel dielectric for experimental EWOD studies. Our theoretical modeling and fundamental studies of an ion gel capacitor are based on these materials. To characterize the electrical properties of ion gel films (P(VDF-HFP)/[EMIM][TFSI]), one experimental approach utilized impedance spectroscopy and measured the frequencydependent impedance (Z), which is in the form of Z = Z′ + iZ″, where i is an imaginary unit and Z′ and Z″ constitute the real and imaginary parts of the impedance, respectively.17,18 The capacitance value of the gel was calculated from Z″ data to be cEDL ≈ 10 μF/cm2 at a low frequency of around 1 Hz. Another approach using a constant phase element (CPE) allowed a frequency-independent capacitance of the ion gel to be computed to ∼4 μF/cm2.17,20 The experimentally measured
2. FUNDAMENTALS OF IONIC LIQUIDS AND ION GELS 2.1. Ionic Liquids. The ion gel is a recently developed material in the form of a thin film made of an ionic liquid and a structuring polymer. Because the ionic liquid is an essential constituent of the ion gel, it also features similar electrical properties to that of the ionic liquid.17−20 Therefore, it is helpful to first understand the electrochemical characteristics of ionic liquids to address the theoretical modeling of an ion gel dielectric for EWOD. Ionic liquids, known as room-temperature molten organic salts, have been very attractive due to their favorable properties such as a wide electrochemical window, high thermal stability, negligible vapor pressure, and large ionic capacitance.19,21−24 Under an applied electric field, free counterions inside ionic liquids compactly accumulate at the liquid−electrode interface and form the nanometer-thick electric double layer (EDL) which functions as a capacitor. Such a thin EDL of the ionic 8513
DOI: 10.1021/acs.langmuir.5b01745 Langmuir 2015, 31, 8512−8518
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Langmuir
75 °C to ensure that any remaining solvent was fully purged, enabling a cured ion gel layer to be formed. Figure 3 represents the thickness measurement of the gel layers that were fabricated under various spin conditions. The
capacitance values of the P(VDF-HFP)/[EMIM][TFSI] ion gel fall within the proposed range of the EDL capacitance while using the [EMIM][TFSI] ionic liquid.17,18,20 They also investigated the thickness effect of the ion gel and interestingly found that the structuring polymers do not contribute to the capacitance of the ion gel layer.18 This indicates that the high capacitance of the gel layer is purely obtained as a result of the thin EDL formed by its constituent ionic liquid and is therefore not influenced by the type of structuring polymers used. This explains the reason that two different ABA block copolymers based on [SMS]18 and P(VDF-HFP)17 exhibit similar capacitance values when both are structured around the same ionic liquid of [EMIM][TFSI]. The term Z′ represents the specific resistance (Rbulk) of the gel, which again originates from its constituent ionic liquid. Z′ is generally computed through the impedance at high frequencies where the gel resistance is dominant. For common ionic liquids such as [EMIM][TFSI], the conductivity ranges from 10−4 to 10−2 S/cm.18,19 For example, a 10 μm thick ion gel based on [EMIM][TFSI], which corresponds to our experimental condition, would exhibit a specific resistance of Rbulk = 0.1−10 Ω cm2. However, the voltage drop across this resistance of the ion gel layer can be neglected because the capacitive impedance (Z″) is mostly dominant for EWOD operated typically at dc or very low frequency ac.
Figure 3. Various thicknesses of an ion gel layer fabricated by varying the spin speed and relative wt % of the mixtures. The legend indicates the relative wt % ratio of copolymer P(VDF-HFP), ionic liquid [EMIM][TFSI], and acetone solvent.
3. FABRICATION OF ION GEL LAYERS Using a simple and low-cost spin-coating method without conventional IC fabrication processes such as PECVD, ALD, and sputtering, the layers of an ion gel dielectric were successfully fabricated on ITO substrates. Subsequently, Teflon AF layers were coated on the top to provide a hydrophobic surface. Figure 2 illustrates the chemical structures of the
thickness was characterized with the dry film after curing using an optical profilometer (ZYGO NewView 5032). By varying the spin speed as well as the wt % ratio of the copolymer, ionic liquid, and solvent, we were able to vary the ion gel thickness from 4 to 18 μm. Following this, we attempted to perform EWOD directly on the ion gel. However, it was observed that the initial contact angle, while remaining thickness-independent, was around 70°, which is impractical for typical EWOD applications. Therefore, a Teflon AF solution (DuPont) was spin coated on various ion gel layers to provide a hydrophobic surface. The thickness of the Teflon AF was similarly controlled by several spin speeds over the cured ion gel layer. The effective thin-film stack was cured at 105 °C for 12 h in an oven.
4. EXPERIMENTAL RESULTS Several experiments for static EWOD studies were conducted to verify the understanding of ion gels discussed in section 2. An experimental setup for the studies is shown in Figure 4. A water droplet was placed on the spin-coated ion gel and Teflon layers, forming an initial contact angle of around 116°. The thickness of the gel layer and the Teflon layer were individually varied to study the thickness effect on the EWOD performance. An electric potential, V, was applied between the water droplet through a platinum probe and an ITO electrode. The equivalent circuit is modeled for EWOD studies as the impedances of three layers in a series connection. The contact angle was then measured at incremental voltages, which was controlled by LabVIEW. 4.1. Thickness Effect of the Ion Gel Layer. It is generally understood from eqs 1 and 2 that lowering the thickness of the dielectric layer increases its capacitance; therefore, a larger contact angle change is expected at a given applied voltage. However, as discussed in section 2.2, the capacitance of the ion gel layer was expected to be thickness-independent. 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 to 18 μm.
Figure 2. Chemical structures of the P(VDF-HFP) copolymer (top) and the [EMIM][TFSI] ionic liquid (bottom) which are cured to obtain free-standing ion gels. The right-side image shows layers of an ion gel and Teflon fabricated on a flexible ITO substrate, which can be bent with ease.
copolymer and the ionic liquid. The ion gel was first prepared by selective dissolution of the PHFP copolymer in the ionic liquid while the PVDF crystals are insoluble.17,29 The gelation process occurs when these crystals are brought together by polymer chains dissolved in the solution. P(VDF-HFP) copolymer in the form of a pellet was added to the [EMIM][TFSI] ionic liquid to form a 1:4 mixture by weight. Acetone was added to further dilute the mixture. The polymer was then dissolved completely over 3 h at 60 °C. The solution was spin coated on an ITO substrate at various spin speeds ranging from 1000−5000 rpm for 30 s, immediately forming a gel layer. The substrate was then placed in an oven for 24 h at 8514
DOI: 10.1021/acs.langmuir.5b01745 Langmuir 2015, 31, 8512−8518
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4.2. Thickness Effect of the Hydrophobic Layer. Our 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 650 to 170 nm. Figure 6 presents the effect of the Teflon
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Figure 4. Experimental setup for a static EWOD study. A 5 μL water droplet is loaded on top of the hydrophobic (Teflon AF) and dielectric (ion gel) layers (not to scale). The equivalent circuit diagram consists of the three-layer impedances (water, a hydrophobic, and a dielectric layer). Figure 5 shows the experimental results for the contact angle with the applied voltage. Despite fairly large variations in the dielectric
Figure 6. Contact angle modulation with applied voltage. The thickness of the ion gel layer was held fixed at 10 μm while that of the Teflon layer was varied from 170 to 650 nm. Significant variations in contact angle are observed for variations in the Teflon layer thickness. thickness on EWOD performance, indicated as solid lines. The measured contact angle data for each different experimental condition was collected from four different tests, and the mean value is presented in Figure 6. The maximum percentage error was around ±4%. Unlike the previous results for varying the ion gel thickness, the trend observed was that the EWOD performance continually improved as the Teflon thickness was reduced. The saturation voltage also drops accordingly. In other words, large contact angle modulations were possible for a given applied voltage because the thinner the Teflon layer, the larger the capacitance. 4.3. Interaction of the Ion Gel with the Hydrophobic Layer. Because the experimental results presented in Figure 6 match the Young−Lippmann theory in eqs 1 and 2, the following experiments were repeated with the further reduced Teflon thickness while maintaining the same ion gel layer of 10 μm. A 6% Teflon solution was diluted to 1% by adding FC-72 to fabricate Teflon layers thinner than 100 nm. Additional EWOD test data with hydrophobic thicknesses of 40 and 100 nm are also included in Figure 6, indicated as dotted lines. While initial contact angle modulation in both cases was considerable, electrolysis was observed at relatively low voltages applied at around 10 V for the case of the 40 nm Teflon and at around 25 V for the 100 nm Teflon layer. Upon the onset of electrolysis, the modulation steadily saturated. Although the hydrophobic layer in the range of tens of nanometers in thickness has been often used for many EWOD studies,4,14,30 in our case, the interaction of the high-capacitance gel layer with the Teflon layer limits the lowest possible voltage repeatable for EWOD actuation. Our next study focused on how the highcapacitance ion gel dielectric interacts with the hydrophobic layer spin coated on top of it. Table 1 presents information on various dielectrics used in conjunction with a thin-film Teflon layer at 20 nm for the EWOD studies reported in earlier works. It also offers a comparison between conventional dielectrics and the ion gel material in terms of capacitance, voltage consumption, and electrostatic energy stored across the capacitors. The ion gel layer is assumed to be 10 μm thick, which corresponds to our experimental condition. The total potential drop across the dielectric and hydrophobic layers is standardized to 10
Figure 5. Contact angle modulation with applied voltage. The thickness of the Teflon layer was held fixed at 550 nm, while that of the ion gel layer was varied from 0 to 18 μm. Negligible variations in contact angle are observed even for large variations in ion gel layer thickness. For comparison, data from tests involving pure 550 nm Teflon over ITO is added to highlight the contribution of the ion gel.
thickness, it was observed that the contact angle change behaved very similarly for all cases, and the angle is very close even at the saturation voltage of 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 the capacitance of the ion gel does not rely on the thickness change of the structuring polymer layer but on the EDL layer formed by its constituent ionic liquid.18,20 To clearly understand the contribution of the ion gel to EWOD performance, another experiment was conducted with a pure 550-nm-thick Teflon layer over ITO. The thickness of a Teflon layer is consistently maintained with all of the previous cases that have variable ion gel thickness. It can be experimentally seen that the mean contact angle at saturation when using the ion gel is roughly 20° lower than the mean contact angle for Teflon over bare ITO. This comparative study clearly indicates that the performance of the Teflon and ion gel stack is superior to that of pure Teflon. 8515
DOI: 10.1021/acs.langmuir.5b01745 Langmuir 2015, 31, 8512−8518
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Langmuir
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Table 1. Theoretical Estimations and Comparisons of Capacitance and Electric Energy Stored Across Various Hydrophobic and Dielectric Layer Stacks Used for EWOD
The specific capacitance is calculated by c = ε0κ/t except for the ion gel. 2The total potential drop across both dielectric and hydrophobic layers is assumed to be Vtotal = Vh + Vd = 10 V in all cases. 3The electric storage energy is calculated by E = cV2/2 for each layer and Etotal = Eh + Ed. 4The ion gel thickness is assumed to be 10 μm, which corresponds to our experimental condition. 5This value is selected from refs 33 and 34. 6The capacitance (cd) of the ion gel dielectric is dominantly contributed by two EDL capacitors (cEDL = 10 μF/cm2) formed at electrified interfaces.17−20 Thus, it is estimated as 1/cd = 1/cEDL + 1/cEDL. 1
in Figure 5. Thus, many EWOD studies have typically used it as a thin hydrophobic layer with another dielectric material.5,14,35,36 In addition, it is interestingly noted that complete dielectric breakdown occurs only when the breakdown voltage of the higher-dielectric-strength material is exceeded.37,38 For our case using the high-capacitance ion gel, although the voltage distributed across the hydrophobic layer exceeds its breakdown voltage, the device can continuously perform while being slowly degraded. This behavior was experimentally observed, as shown in Figure 6. For the 40- and 100-nm-thick Teflon layers, the theoretical breakdown voltages are estimated to be 8 and 20 V at the given thickness of the layers, respectively.4 Even at voltages exceeding this breakdown threshold, the contact angle for the droplet was able to be further modulated. Note that the angle saturation was observed at around 10 V for the 40 nm Teflon and at around 25 V for the 100 nm Teflon. 4.4. Comparison to Other Dielectrics for EWOD. Finally, we attempted to benchmark the performance of the ion gel against certain standard dielectrics commonly used in EWOD. This would help gauge the practicality of its use in various EWOD applications. Al2O3 and SiO2 were chosen to represent standard thin-film dielectric materials deposited through conventional IC processes that typically require complex and expensive fabrication facilities. In contrast, PDMS was chosen to represent dielectric materials fabricated by a simple spincoating procedure. The thickness of the ion gel was 10 μm, while those of Al2O3, SiO2, and PDMS were 100 nm, 100 nm, and 10 μm, respectively. A Teflon layer of the same thickness (170 nm) was spin coated on the above-mentioned dielectrics as well as for the pure Teflon case for comparison. Similar to previous tests, the mean value of the contact angle measurements was selected from four different tests with the maximum percentage error around ±3%. Figure 7 shows that the gel layer performs much better than other benchmarked dielectrics with the same given thickness of the Teflon layer. As expected, EWOD performance is shown in the order of Al2O3, SiO2, and PDMS, which is the same order of their capacitance magnitudes. To highlight the beneficial effects of using the Teflon and ion gel stack, a pure Teflon layer with the same thickness of 170 nm was compared to those with different dielectric layers. It was observed that pure Teflon performs poorer than both Al2O3 and SiO2 but better than PDMS. One interesting observation was that electrolysis occurs on the Al2O3 sample at 55 V (inset (c)), while no such phenomena occurs in the case of the ion gel (insets (b)) even at the saturation
V in all cases. For comparison, the information on a pure 20-nm-thick Teflon layer was also included. On the basis of the equivalent circuit model presented in Figure 1, the total capacitance, ctotal, is estimated by considering a series connection of the capacitances of a hydrophobic (ch) and a dielectric (cd) layer. It can be expressed as
1 1 1 = + ctotal ch cd
(3)
As the capacitance (cd) of a dielectric layer increases, several interesting phenomena are observed from Table 1 as follows: (a) ctotal increases, (b) ctotal is close to ch when a high-capacitance dielectric such as the ion gel is used, (c) most of the voltage drop occurs over the hydrophobic layer, (d) correspondingly, electric energy (Eh = chVh2/2) stored across the hydrophobic layer increases, although its thickness is kept constant in all the cases, (e) total electric storage energy, Etotal = Eh + Ed, also increases, and (f) particularly for the cases of high-capacitance materials such as ion gels, the total electric energy stored across both layers is more than 2-fold that of conventional dielectric materials such as SiO2 and Al2O3. The studies shown in Table 1 represent the fact that the use of highcapacitance dielectrics such as ion gels can offer a significant increase in the total electric storage energy at a given voltage, which is responsible for large contact angle modulation as indicated in eq 1. This is the reason due to which numerous EWOD studies have attempted either to increase the dielectric constant, κ, and/or reduce the thickness, t, of the layers. Another interesting observation is that a very large portion of the total electric energy is stored across the hydrophobic layer when high-capacitance dielectric materials are used. In the case with the ion gel layer of 10 μm thickness, more than 98% of the total potential drop is sustained across the Teflon layer of 20 nm, as presented in Table 1. This indicates that the total electric storage energy of the Teflon and ion gel stack is close to that of pure Teflon because ctotal ≈ ch and Vtotal ≈ Vh. However, it is practically known that Teflon does not provide a strong dielectric characteristic when it is used alone as a dielectric layer. This is supported by the prior experimental results4,14 as well as our experimental observations shown 8516
DOI: 10.1021/acs.langmuir.5b01745 Langmuir 2015, 31, 8512−8518
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Langmuir
Fundamentals of the ion gel dielectric were discussed for EWOD studies. Our understanding of the ion gel indicates that it can be equivalently circuit-modeled as a series connection of an electrolyte resistor in the bulk layer and two nanometerthick EDL capacitors at the interfaces. By using a simple spincoating method, we successfully fabricated the ion gel films, which consist of a [P(VDF-HFP)] copolymer and an [EMIM][TFSI] ionic liquid. With the fabricated ion gel layer, the thickness effect of both dielectric and hydrophobic layers on EWOD performance was investigated. The use of the ion gel dielectric allows large electric storage energy responsible for large contact angle modulation while inducing a very large portion of the storage energy to be sustained across the hydrophobic layer. We also benchmarked the performance of the ion gel against several dielectrics typically used in EWOD. The comparison results clearly showed that the ion gel dielectric offers a better performance without dielectric breakdown, while thin-film dielectrics such as Al2O3 in the experiment are impractical for regular use in microfluidics because of reliability issues. Additionally, while providing similar high-storage-energy characteristics as pure Teflon over ITO, the ion gel provides far better and more reliable EWOD performance in comparison. The ion gel studied in this article would be potentially very useful for low-cost EWOD-based microfluidic applications by offering a capacitance as high as c ≈ 10 μF/cm2 and while being fabricated through a simple spin-coating method.
Figure 7. Comparative study of the EWOD performance of the ion gel versus Al2O3, SiO2, and PDMS. Points (a) and (b) represented by insets display 5 μL water droplets over the Teflon and ion gel stack at 0 and 50 V, respectively. Point (c) also represented by an inset displays a droplet over the Teflon and Al2O3 stack at 55 V where bubbles caused by electrolysis are clearly observed inside the droplet (indicated by a red arrow). Point (d), similar to (c), corresponds to the voltage (50 V) at which breakdown occurs for Teflon over bare ITO. The thickness of the ion gel was 10 μm, while those of Al2O3, SiO2, and PDMS were 100 nm, 100 nm, and 10 μm, respectively. A Teflon layer of the same thickness (170 nm) was spin coated on the abovementioned dielectrics as well as for the pure Teflon case for comparison.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
voltage around 50 V. Similarly, electrolysis is observed for pure Teflon at a lower voltage of 50 V. This implies that, because of reliability issues, thin layers of conventional dielectrics, while providing high capacitance, are not practical for EWOD-based microfluidic applications such as droplet dispensing or splitting that typically require more force and hence higher voltages than droplet transportation.14 In contrast, the ion gel dielectric layer can provide a thickness-independent capacitance, which serves as a benefit for robust use in microfluidic applications. Additionally, being a highcapacitance material, the ion gel induces a large amount of storage energy within the dielectric and hydrophobic stack, which is in turn crucial for large surface tension modulation. This again is evident from the theoretical study shown in Table 1, which is verified by our experimental results in Figure 7.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the grants (R-265-000-470-133 and R-265-000-528-112) from the Ministry of Education, Singapore.
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REFERENCES
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5. CONCLUSIONS Ionic liquids, known as room-temperature molten organic salts, have been demonstrated to have a large ionic capacitance (c ≈ 10 μF/cm2) due to the nanometer-thick EDL formed at electrified interfaces. Ionic liquids, when combined with structuring polymers, are gelated upon spin coating to form the thin films called ion gels. Because the ionic liquid is an essential constituent of the ion gel, it also offers a highcapacitance property similar to that of the ionic liquid. In this article, we have demonstrated such a high-capacitance property of the ion gel for EWOD applications. Unlike conventional thin-film dielectric materials typically requiring complex and expensive laboratory setups such as high-vacuum facilities, the layer of ion gels can be simply fabricated by a spin-coating process that is cost efficient and time saving. Although being fabricated via easy fabrication methods, the ion gel offers a few orders of magnitude higher capacitance than conventional dielectrics commonly used for EWOD. 8517
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