Low Voltage Electrowetting on Ferroelectric PVDF-HFP Insulator with

Aug 24, 2016 - (21) It is worth mentioning that the introduction of new materials requires a modification of the basic Young–Lippmann equation to mo...
0 downloads 10 Views 2MB Size
Download PDF
Research Article www.acsami.org

Low Voltage Electrowetting on Ferroelectric PVDF-HFP Insulator with Highly Tunable Contact Angle Range Yogesh B. Sawane,† Satishchandra B. Ogale,‡ and Arun G. Banpurkar*,† †

Centre for Advanced Studies in Condensed Matter and Solid State Physics, Department of Physics, Savitribai Phule Pune University, Pune-411007, India ‡ Department of Physics and Centre for Energy Science, Indian Institute of Science Education and Research, Dr. Homi Bhabha Road, Pashan, Pune-411008, India S Supporting Information *

ABSTRACT: We demonstrate a consistent electrowetting response on ferroelectric poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) insulator covered with a thin Teflon AF layer. This bilayer exhibits a factor of 3 enhancement in the contact angle modulation compared to that of conventional single-layered Teflon AF dielectric. On the basis of the proposed model the enhancement is attributed to the high value of effective dielectric constant (εeff ≈ 6) of the bilayer. Furthermore, the bilayer dielectric exhibits a hysteresis-free contact angle modulation over many AC voltage cycles. But the contact angle modulation for DC voltage shows a hysteresis because of the field-induced residual polarization in the ferroelectric layer. Finally, we show that a thin bilayer exhibits contact angle modulation of Δθ (U) ≈ 60° at merely 15 V amplitude of AC voltage indicating a potential dielectric for practical low voltage electrowetting applications. A proof of concept confirms electrowetting based rapid mixing of a fluorescent dye in aqueous glycerol solution for 15 V AC signal. KEYWORDS: ferroelectric PVDF-HFP insulator, bilayer dielectric, low voltage electrowetting, contact angle saturation, droplet mixing

1. INTRODUCTION The controlled manipulation of small liquid volume has gained a special attention because of its usefulness in diversified applications, such as liquid lenses,1,2 electronic displays,3−5 and liquid transport in microfluidic devices,6 physical characterization,7−9 etc. In these cases, a tiny liquid volume is actively manipulated by means of controlled alteration in wettability of the dielectric surface using external electric field, so-called as electrowetting on dielectric (EWOD).10 This phenomenon has a number of advantages such as real time actuation, fast response, long-term reliability and stability that make it a versatile technique in handling microliter to nanoliter volume of liquid.11 The external electric field at the liquid−solid interface builds up electrostatic energy per unit area which changes the intrinsic surface energy at the liquid−solid interface. Therefore, a controlled motion of liquid along the surface energy gradient is possible. To illustrate this generic electrowetting (EW) phenomenon, a schematic setup with planar electrode geometry is shown in Figure 1. In this case, a planar conductive electrode is covered with a dielectric and the voltage is applied between the liquid drop and the bottom electrode as shown. It is assumed that the whole drop remains at a constant voltage due to finite conductivity. The applied voltage (U) increases the charge density at solid−liquid interface that changes interfacial surface energy given by the equation: γSL(U) = γSL(0) − [(ε0ε)/2d]U2. This interfacial © 2016 American Chemical Society

Figure 1. Schematic shows electrowetting setup using bilayer dielectric: PVDF-HFP as an insulator with a top layer of Teflon AF.

surface energy gives rise to change in the equilibrium contact angle which is given by Young−Lippmann equation10 εε cos θ(U ) = cos θY + 0 U 2 2dγLV (1) where γSL(0) is solid−liquid interfacial energy, θ(U) is contact angle at applied voltage U, θY is Young’s contact angle without Received: May 18, 2016 Accepted: August 24, 2016 Published: August 24, 2016 24049

DOI: 10.1021/acsami.6b05958 ACS Appl. Mater. Interfaces 2016, 8, 24049−24056

Research Article

ACS Applied Materials & Interfaces voltage bias, ε0 is permittivity of free space, ε is dielectric constant, d is dielectric thickness, and γLV is interfacial surface energy of the liquid drop with respect to ambient air or immiscible oil medium. The above equation clearly shows that for a given voltage magnitude the contact angle change is determined by the value of the dielectric constant (ε) of the insulating layer. Thus, a large contact angle modulation at relatively small voltage amplitude can be realized on insulators with high dielectric constant. Also practical EWOD application demands an insulator with high Young’s contact angle (>100°) and minimal contact angle hysteresis (CAH). Therefore, many researchers are exploring a combination of several types of dielectric layers covered with a thin layer of fluoropolymer.12,13 The insulating layers have ranged from polymers, inorganic oxides, ceramics, etc. J. B. Chae et al. have used polyimide dielectric with a top hydrophobic coating of Teflon AF or a coating of Cytop.12 V. Srinivasan et al. have used Parylene C as a dielectric and top layer from Teflon AF.14 They studied biocompatibility of EW with high reproducibility. P. Mach et al. have achieved dynamic tuning of optical waveguides by recirculating fluidic channels actuated by electrowetting on Cytop coated polyimide dielectric.15 Verheijen et al. have used Payrlene N insulator with a top Teflon AF layer.16 S. K. Cho et al. have used silicone dioxide (SiO2) as a dielectric and Teflon as a hydrophobic layer.17 Moon et al. have used SiO2 and barium strontium titanate (BST) as dielectric with Teflon AF as a top layer for low voltage EW.18 The increase in EW performance was seen on high dielectric constant insulators like SiO2 and BST. There are alternative strategies to EWOD, which use other materials than conductive-electrode and insulators, for example, ferroelectric materials19 and semiconductors,20 which enable advantages such as improved performance and other effects such as photoactuation.21 It is worth mentioning that the introduction of new materials requires a modification of the basic Young−Lippmann equation to model the electrowetting behavior.20 Recently H. You et al.22 have observed EW on nonfluorinated hydrophobic surfaces with low CAH. Further, a low voltage (AC and DC) EW response has been studied by various researchers using combination of metal oxide and nitride bilayer. Berry et al. reported irreversible contact angle modulation on Cytop-SiO2 bilayer;23 Kim et al. showed movement of droplet using bismuth zinc niobate (BZN) for 14 V DC;24 and Y. Li et al. demonstrated low voltage EW on anodic Ta2O5 with top layer of fluoropolymer.25 Kim et al. have found reliable EW for both AC and DC voltages on a thermally grown SiO2 dielectric. Also sputtered-anodized Ta2O5 dielectric was found suitable only for DC voltages.26 Chang et al. have shown EW response at comparatively low voltages on atomic layer deposited (ALD) Al2O3 dielectric with a top layer of Teflon AF.27 Raj et al. have shown low voltage EW on the composite dielectric of SiO2/Al2O3 and SiO2/Si3N4 with a top layer of Cytop.28 Also, low voltage EW was studied using various dielectrics, such as Parylene HT,29 plasma-deposited FC,30 and BaTiO3 nanoparticle dispersion.19 The successes in these efforts are limited because of the following: The deposition of oxide and nitride insulators requires specialized techniques like atomic layer deposition (ALD) and sputtering and chemical vapor deposition (CVD), along with hightemperature processing. In some dielectric systems processed at low temperature, the EW response shows inconsistency over a large number of voltage cycles.19 On the contrary ferroelectric polymers have promising properties and are likely to be the

most suitable dielectric for practical EWOD based devices. Polymeric insulators are normally coated on conductive electrodes using solution based techniques. Also, in comparison to oxide and nitride dielectrics, polymers offer several advantages; for instance, low temperature processing, economical, large area deposition etc. Despite these advantages, there is only a single attempt in this direction thus far. Janocha et al. examined EW on ferroelectric poly(vinylidene fluoride) (PVDF); however, the EW response was completely irreversible and inconsistent with cyclic changes in voltage magnitude.31 Recently, ion-gel dielectric covered with Teflon AF layer was employed as a dielectric for enhanced contact angle modulation but did not demonstrate a consistency in the contact angle modulation with a large number of repetitive voltage cycles.32 Thus, more efforts in experimentation and in theoretical modeling are required to make use of ferroelectric polymer as a suitable dielectric for EWOD applications. In this Research Article, we present bilayer dielectric made from ferroelectric fluoropolymer, poly(vinylidene fluoride-cohexafluoropropylene) (PVDF-HFP) insulator with a thin layer of Teflon AF. This top layer is essentially used for boosting up Young’s contact angle along with minimal CAH in the ambient of immiscible oil. The schematic of the experimental setup for bilayer insulator is shown in Figure 1. The electrowetting of a conductive aqueous droplet is examined for both AC and DC voltages and the contact angle modulation shows a factor of 3 enhancement as compared to the modulation seen on a conventional single layered Teflon AF insulator. Upon reducing thickness of this bilayer, the modulation in contact angle Δθ ≈ 60° is observed for 15 V AC signal. In the following sections, the experimental methods are discussed in detail. In the subsequent sections the results and discussion are presented. The results related to low voltage electrowetting is presented followed by electrowetting response for DC voltage. Finally, we demonstrate proof of principle based on the low voltage electrowetting for a rapid mixing of dye in aqueous glycerol droplet. The results are concluded at the end.

2. EXPERIMENTAL SECTION The ferroelectric layer of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) was deposited on a conductive glass surface using solution based dip-coating technique. The solution of PVDF-HFP (average molecular wt. Mw ≈ 400 000 g/mol) at 10 wt % concentration in acetone (both from Sigma-Aldrich, USA) was prepared at room temperature under constant stirring. The stock solution was filtered using PTFE syringe filter (1 μm pore size) to remove the particulates. Prior to the dielectric coating, Indium Tin Oxide (ITO) conductive glass (Sigma-Aldrich) substrates were cleaned in ultrasonic bath containing acetone followed by isopropyl alcohol and dionized (DI) water for 15 min each. The substrates were dried in nitrogen gas flow and then subjected to dip-coating in PVDFHFP solution maintaining the constant speed of 7 cm/min during dipping and withdrawal process. The coating process was repeated twice to achieve a desired film thickness of PVDF-HFP layer. The substrates were heat treated at 100 °C for 12 h in a vacuum oven for removal of the solvent traces. Then a thin layer of Teflon AF 1600 (DuPont, USA) was applied using 1 wt % Teflon AF diluted solution in Fluorinert FC 40 (Sigma-Aldrich) at constant dipping and withdrawal speed of 7 cm/min. Finally the substrates were heat treated in a vacuum oven at 100 °C for 12 h. The thickness values of each insulating layer were measured using profilometer (Veeco) which were 1.8 ± 0.01 μm (PVDF-HFP) and 700 ± 10 nm (Teflon AF) giving total bilayer thickness d = 2.5 ± 0.01 μm. Also a single layer of Teflon AF dielectric was applied on ITO by dip-coating technique 24050

DOI: 10.1021/acsami.6b05958 ACS Appl. Mater. Interfaces 2016, 8, 24049−24056

Research Article

ACS Applied Materials & Interfaces

Figure 2. (a) Contact angle modulation on the bilayer (PVDF−HFP−Teflon AF) dielectric for AC voltage cycle from 0 to 150 V. The inset shows EW for many voltage cycles in Young−Lippmann regime. (b) The cosine of contact angle modulation is plotted against voltage amplitude. The open symbols are used for increasing (circle) and decreasing (square) voltage magnitude. The continuous line indicates the theoretical curve for CA modulation determined from eq 1. using viscous solution of Teflon AF (6 wt % in Fluorinert FC 40). The above-mentioned cleaning procedures were followed and Teflon AF coated substrate were heated in a vacuum oven at 200 °C for 24 h followed by slow cooling to room temperature. The thickness of Teflon AF layer was 1.5 ± 0.02 μm. Then EW response was extensively tested using AC and DC voltages on both the types of dielectrics viz. bilayer (PVDF-HFP-Teflon AF) and on a single layered Teflon AF substrates. The aqueous drop of conductivity 1 mS/cm due to addition of 0.01 M KCl was dispensed on the dielectric surface in ambient of silicone oil (Sigma-Aldrich, coefficient of kinematic viscosity ν = 5 cSt). The oil ambient prevents evaporation of aqueous drop and also provides thin lubricating film between the drop and the insulator surface yielding minimal CAH. The electrical connections to the droplet were made using thin copper wires with indium solder contact on ITO surface. Keithley-2400 source meter was employed as a voltage source for DC electrowetting study. A DC voltage ramp with a constant rate of changing magnitude dU/dt = 4 V/s was generated using custom mode program. Also for AC electrowetting experiments, the amplitude modulated AC voltage signal was generated through a function generator (HP 33120 A) and further amplified using a home build differential OPAMP (MSK 130) based high voltage amplifier. The amplitude of carrier frequency (5 kHz) AC voltage was modulated using frequency of 0.02 Hz triangular waveform for the linear variation in the voltage amplitude. The voltage magnitudes referred in this paper were measured on aqueous droplet with respect to the grounded ITO electrode using true RMS digital multimeter (Agilent U1242B). The contact angle change was simultaneously followed with automated optical contact angle (OCA) goniometer and analyzed using software SCA 20 (DataPhysics Instruments, GmbH). The effective dielectric constant of bilayer (PVDF-HFP-Teflon AF) was determined using the model proposed by Bharat Bhushan et al.33 Thus, the equivalent capacitance Ceq of PVDF-HFP-Teflon AF bilayer dielectric is given as Ceq =

Ceq

C TC PVD C T + C PVD

(2)

( )( ) = ( )+( ) ε0εPVD dPVD

(3)

ε0εTεPVD εTdPVD + εPVDd T

(4)

Ceq =

ε0εT dT

ε0εT dT

both AC and DC voltage. Figure 2a shows contact angle modulation for AC voltage cycles. A large modulation can be seen in contact angle from 164° to 54° (Δθ > 100°) for moderate change in voltage value. The contact angle saturation is seen above 72°. Also Young’s contact angle on this bilayer is 165° due to silicone oil ambient. The high value of Young’s contact angle provides a large contact angle tuning range essential for the practical EWOD applications. Notably, the contact angle recovery is clearly observed when external voltage bias is removed. The contact angle modulation below the saturation regime (Young−Lippmann region) was realized for many voltage cycles as shown in the Figure 2a (inset). Here contact angle shows a consistent variation over a large number of voltage cycles, which is important for many EWOD applications. Also contact angle change is reversible with voltage cycles essentially due to the presence of top layer of Teflon AF. In the absence of top Teflon AF layer, that is, single layered PVDF-HFP insulator shows a large hysteresis in EW contact angle modulation regardless of the presence of immiscible ambient oil (see the Figure S1). The contact angle modulation on the bilayer dielectric film is plotted in Figure 2b for further analysis. The two regimes are clearly seen; first Young−Lippmann regime in which EW response matches with eq 1 and second the contact angle saturation regime. The contact angle saturation phenomenon is still an unresolved issue11,34 and a detailed discussion on this topic is not the scope of the present study. In the following we focus our experimental studies in Young−Lippmann regime. Now we analyze the key role of PVDF−HFP layer in enhancing contact angle change at relatively moderate voltage. Before this analysis, we discussed the CA modulation observed on the single layered Teflon AF dielectric for identical experimental conditions. The quantitative changes in the contact angle for bilayer and single layered Teflon AF are presented in Figure 3. The contact angle change on both the dielectric surfaces was rescaled using the insulator thickness (d) values to normalize the effect for comparison. The graph of Φ (= d cos θ(U)) versus U2 is plotted for the bilayer and for single layered Teflon AF dielectric. A straight line fitting to the curve of Φ against voltage square validates Young−Lippmann eq 1 for both the insulators. Moreover, it yields a definite slope value [(ε0εeff)/(2γLV)] where εeff is the effective dielectric constant of the bilayer. Here the slope value of the straight line for bilayer dielectric is clearly higher than slope value observed for a single layered Teflon AF dielectric system. It is clear from eq 1 that the change in contact angle modulation is linearly proportional

ε0εPVD dPVD

where εPVD and εT are the dielectric constants of the PVDF-HFP and Teflon AF layers, respectively.

3. RESULTS AND DISCUSSION 3.1. AC Electrowetting. The EW response on the bilayer (PVDF−HFP−Teflon AF) surface was extensively studied for 24051

DOI: 10.1021/acsami.6b05958 ACS Appl. Mater. Interfaces 2016, 8, 24049−24056

Research Article

ACS Applied Materials & Interfaces ⎡ 2Δ cos θ(U )γ ⎤ LV ⎥d U2 = ⎢ ε0ε ⎣ ⎦

(5)

We plot this function in the Figure 4 for Teflon AF (ε = 1.9) and for PVDF-HFP ferroelectric insulators (ε = 10). Usually

Figure 3. Graph of Φ (= d cos θ(U)) against voltage square for AC voltages on bilayer (circle) and single-layer Teflon AF (square) dielectric. The solid lines represent least-squares linear fit to the given data set. The arrow indicates the enhancement in contact angle modulation on the bilayer dielectric. Inset shows variation in the contact angle change Δθ (= θY − θ(U)) with respect to the applied electric field. Figure 4. Plot shows variation in voltage against dielectric thickness (d) for contact angle change of Δθ (U) = 60° on Teflon AF (blue line) and on (PVDF−HFP) (red line) surfaces. The breakdown voltage is indicated by green line. The region of successful EW on PVDF−HFP is clearly indicated.

to the value of [(ε0εeff)/(2γLV)]. Therefore, it is clear that enhancement in contact angle modulation is mainly due to incorporation of ferroelectric PVDF-HFP in the bilayer dielectric. We note that the interfacial surface energy for water−oil system in both the cases was constant. The top Teflon AF layer has a low value of dielectric constant that could affect the effective dielectric constant of the bilayer. This effective dielectric constant of the bilayer can be estimated from the EW response using known value of interfacial tension, γLV = 38 mN/m for the water-silicone oil interface. eq 4 was used for estimating the effective dielectric constant of the bilayer, which was found to be εeff = 6.1. This value is three times high as compared to the dielectric constant ε = 1.9 for commonly used Teflon AF insulator. In the present analysis the contribution of the capacitance due to the electrical double layer (EDL), arising because of the salt concentration in droplet, to the overall capacitance of the system is negligibly small. Hence it is not considered.35,36 Also a quantitative change in contact angle Δθ (= θY − θ (U)) with respect to the applied electric field is shown in Figure 3 (inset). These plots clearly indicate enhancement in contact angle modulation on the bilayer dielectric as compared to conventional single layered Teflon AF dielectric. This indicates that the ferroelectric embedded bilayer is a potential dielectric for low voltage EW applications. We first discuss the model for successful EW on both type of insulators namely Teflon AF and PVDF-HFP before the results for low voltage EW on the bilayer dielectric. The amorphous fluoropolymers like Teflon AF and Cytop both have surface energy of 18 mJ/m2 thus they are commonly preferred as a top layer on other insulating layers or applied as a single dielectric layer. These fluoropolymer layers are coated on solid substrates using solution based techniques followed by heat treatment for curing and the evaporation of solvent. Such surfaces demonstrate excellent hydrophobicity with water contact angle of 110° in air and 165° in the immiscible oil ambient. However, the dielectric constant or the breakdown field value is a critical material parameter that limits the minimum thickness of the insulating layer.18 The voltage required to achieve a desired variation in contact angle (Δcos θ(U)) of a liquid droplet (γLV) on the insulating surface of dielectric permittivity (ε0ε) is given by

the dielectric breakdown occurs above the breakdown voltage UBD. The competition between the electric field and the dielectric breakdown for Teflon AF and PVDF-HFP (UBD ≈ 200 V/μm) is shown in Figure 4. The intersection between the straight line (green) and the square root functions depicted in red and blue lines determines the minimal thickness of PVDFHFP and Teflon AF insulator required for contact angle change by Δθ (U)= 60°. Figure 4 clearly shows that smallest voltage required for a successful EW on PVDF-HFP insulator can be as low as 4 V. However, this voltage value for Teflon AF is 17 V. Thus, PVDF-HFP insulator of reduced thickness could be successfully used for EW in low voltage regime. A thin coat of Teflon AF on insulating layer is necessary to achieve high Young’s contact angle and low CAH. 3.2. Low Voltage AC Electrowetting. Above analysis shows that the bilayer (PVDF−HFP−Teflon AF) dielectric possesses high dielectric constant and hence could be a promising candidate for EWOD applications. To validate the above model a bilayer is prepared with the PVDF-HFP and Teflon AF with thickness values of dPVD= 270 nm and dT = 50 nm, respectively. The contact angle modulation on this thin bilayer was tested for KCl water droplet (conductivity = 1 mS/ cm) in silicone oil ambient at 15 V AC (see Figure 5). We observed considerably large contact angle modulation and complete recovery to Young’s contact angle upon reducing the bias voltage to zero. The consistency in contact angle modulation is confirmed on identically prepared samples for a large number of voltage cycles37 (∼3600) and for the extended period of time of 3 h. Here voltage ramp time period was reduced to T = 3 s. (see the Figure S2). Also the stability of the dielectric was confirmed by giving an extended number of voltage cycles (∼3600) using KCl added water:ethylene glycol (EG) solution (70:30 wt %). (See Figure S3.) The electrowetting response remains stable for such a large number of voltage cycles for both KCl water and aqueous EG solution. The consistency in contact angle modulation is observed possibly due to reduced defect density in the bilayer dielectric (PVDF−HFP−Teflon AF).38 When the dielectric thickness is reduced for low voltage electrowetting, the defects or pores in 24052

DOI: 10.1021/acsami.6b05958 ACS Appl. Mater. Interfaces 2016, 8, 24049−24056

Research Article

ACS Applied Materials & Interfaces

Figure 5. Graph shows CA change as a function of applied AC voltage magnitude. The arrows indicate paths of contact angle modulation for increasing (red) and decreasing (black) voltage magnitude. The inset graph shows CA modulation and voltage amplitude as a function of time advance.

Figure 6. Plot highlights contact angle modulation against applied AC voltage amplitude for 1 wt % SDS aqueous droplet. The arrows indicate increasing (red) and decreasing (black) voltage magnitude paths. The inset graph shows contact angle modulation and voltage magnitude as a function of time.

the dielectric significantly affect electrowetting leading to unforeseen electrical breakdown. Schultz et al. showed that when the multilayer dielectric approach is employed, the defect density in the dielectric is considerably reduced giving improved dielectric performance in low voltage regime.38 In the present study the dielectric, PVDF−HFP−Teflon AF shows consistent contact angle modulation without breakdown for a large number of voltage cycles that supports the hypothesis of lower defect density due to use of a bilayer. In the case of a multilayer dielectric, the probability of overlapping identical pore/defect at the same position from multiple layers gets reduced with increasing numbers of layers. Thus, a reduced defect density in the bilayer dielectric is leading to a consistent electrowetting for large number of voltage cycles. During the dip-coating, it is commonly practiced that individual layer of same material is coated twice or thrice by using diluted solution or by controlling a speed of withdrawal of substrate from solution bath. This scheme is also commonly employed in spincoated dielectric. The high quality dielectric can be prepared in specialized laboratory ambient. Sayrat and Hayes used clean room technology for Teflon AF dielectric coating which give highest breakdown field of 200 V/μm.39 The inset of Figure 5 shows change in voltage values and corresponding contact angle modulation as a function of time. The consistent contact angle change clearly represents robustness of the bilayer even at lower thickness values. In the past, some efforts were made to achieve low voltage EW using drop of aqueous solution of sodium-dodecyl sulfate (SDS) surrounded by immiscible dodecane oil. Shaun Berry et al.40 used SiO2 covered Cytop insulator in dodecane oil ambient. The addition of a surfactant in the aqueous drop decreases the effective oil−water interfacial energy hence there is increase in range of contact angle modulation for a given voltage amplitude. However, their low voltage EW response using aqueous SDS was not reliable over many voltage cycles.40 We also tested EW on the thin bilayer using 1 wt % SDS in distilled water. This surfactant concentration is well above the critical micelle concentration (CMC ≈ 8 mM) so as to maintain equilibration concentration of surfactant molecule at the oil−water interface. Figure 6 shows electrowetting response on a thin bilayer for 1 wt % aqueous SDS droplet in silicone oil ambient. The enhancement in contact angle modulation clearly indicates a large tuning range in contact angle Δθ ≈ 65°. The graph depicts that the desired voltage to bring similar

change in contact angle Δθ is considerably reduced for surfactant laden droplet as compared to KCl added water droplet. The reduction in oil−water interfacial tension of SDS drop is responsible for increase in contact angle modulation compared with normal KCl water. The inset of Figure 6 shows a consistent electrowetting for a number of applied AC voltage cycles proving the usefulness of the bilayer dielectric in low voltage EW applications. Also the EW response is consistent for many voltage cycles that demonstrate the potential use of the bilayer for EW-based device fabrication even at low voltage regime. The enhancement in the contact angle modulation is clearly seen in the case of ferroelectric (PVDF−HFP)−Teflon AF bilayer. However, in the case of ferroelectric material, a remnant polarization due to external field may lead to the pseudowetting state reported before in case of nanocomposite fluoropolymerdielectric system.19 We therefore explored EW using DC voltages on the bilayer (PVFD−HFP−Teflon AF) system. Interestingly, such ferroelectric effects are not seen in AC electrowetting likely due to rapid changes in the electric field polarity. 3.3. DC Electrowetting. Figure 7a and b shows the electrowetting response on bilayer and single layered Teflon AF surface for consecutive positive and negative voltage cycles. The response remains stable for many voltage cycles for both types of dielectric. The extensive data is provided in the Figure S4. Figure 7a shows that, the contact angle modulation on the bilayer dielectric decreases for positive voltage biased droplet. But the contact angle remains pinned along the decreasing voltage path. This effect is also seen in the case of negative voltage biased droplet. To illustrate the possible dependence of this hysteretic nature on the rate of the change in DC voltage amplitude, the EW was observed at varied rate of the voltage magnitude (dU/dt) from 2 to 32 V/s. However, the nature of hysteresis and corresponding offset voltage ΔU ≈ 17 V remains unchanged with dU/dt (see the Figure S5). This confirms that the observed hysteresis is not because of surface inhomogeneity or surface roughness. During a positive voltage cycle, the contact angle modulation follows distinct paths and when voltage amplitude reduced to zero it does not reach to previous Young’s contact angle value. Similar behavior is observed during negative voltage cycle. This EW response is mainly attributed to the electric field induced remnant polarization in the ferroelectric PVDF−HFP layer. It is well-known that ferroelectric materials including PVDFHFP polarize and depolarize giving different polarization24053

DOI: 10.1021/acsami.6b05958 ACS Appl. Mater. Interfaces 2016, 8, 24049−24056

Research Article

ACS Applied Materials & Interfaces

Figure 7. Contact angle modulation is shown on (a) bilayer and (b) single-layered Teflon AF when positive followed by negative voltage cycle is applied to the droplet. (The arrows indicate paths for increasing (red) and decreasing (black) voltage magnitude.)

Young’s angle θY = 155° was changed to θ(U) = 110° for Umax = 15 V AC. The application of AC signal of linearly changing voltage magnitude brings internal flows in the droplet due to electrowetting induced mechanical oscillations in the droplet that help to carry out rapid mixing within a drop compared to the diffusion process.43 Figure 8 shows the advancement of

electric field (P−E) paths under the external DC electric field. Upon removal of the external electric field, this polarization relaxes with different relaxation time giving a hysteresis.41,42 The prevailing hysteresis in contact angle modulation for DC field on ferroelectric bilayer can be attributed to the slow relaxation in remnant polarization in PVDF-HFP layer. Particularly, when positive voltage bias to the droplet is reduced to zero, the polarization and associated charge density in ferroelectric PVDF-HFP does not reduce to zero but stays at some finite value.41 Because of this remanent charge density, the Young’s contact angle is held at 5° below the initial Young’s contact angle of 165°. If a negative voltage bias is continued on same droplet, particularly in the range of 0 to −10 V the remanent polarization decreases and becomes zero at −10 V.41 In this voltage regime, contact angle increases with the magnitude of negative voltage and attains a maximum value at −10 V. The corresponding field is known as coercive field. With further increase in the negative voltage magnitude, the bilayer polarizes in the reverse direction causing decrease in contact angle. Thus, the contact angle change follows polarization path demonstrating hysteresis, as observed in case of positive voltage cycle. Such a behavior is not seen on a single layered Teflon AF dielectric (Figure 7b) for which increasing and decreasing in contact angle modulation follow the identical paths for positive as well as negative voltage cycles leading to negligible contact angle hysteresis. This clearly shows that the ferroelectric properties introduce nonlinear polarization-field effects which influence DC electrowetting. 3.4. Application of Bilayer for EW-Based Reagent Mixing. Now we present a case of effectiveness of the bilayer for practical EWOD application like rapid mixing on droplet scale. In the past the mixing of the dye on a droplet scale has been carried at voltage magnitude greater than 50 V.43,44 We explicitly demonstrate the bilayer (PVFD−HFP−Teflon AF) as a potential dielectric in low voltage electrowetting application. The rapid mixing of fluorescent dye in a small droplet using 15 V AC is demonstrated on a thin bilayer. A mixture of 70% water (with KCl added for conductivity of 1 mS/cm) and glycerol (30%) was prepared for EW experiment. The generic EW setup with thin bilayer dielectric was employed for homogeneous mixing of dye (fluorescein sodium salt, Sigma-Aldrich) in aqueous glycerol drop. An aqueous glycerol drop of 3 μL volume was dispensed on the bilayer surface and a similar solution containing fluorescent dye was gently pipetted on the aqueous glycerol drop. The AC voltage with sinusoidal carrier frequency of 10 kHz with linear ramp frequency of 70 Hz was applied between the drop and the counter electrode. The

Figure 8. Figure shows the advancement in mixing of the fluorescent dye when AC voltage ramp between 0 and 15 V is applied. The carrier frequency was kept at 10 kHz and ramp frequency was held at 70 Hz. The observed CA change during the EW was Δθ(U) = 45°. The complete mixing was observed with the time span of 50 s.

mixing of the florescent dye in aqueous glycerol drop. A complete mixing is seen after 50 s. The real time video of the droplet mixing has been provided as Supporting Information. This demonstrates that the bilayer from PVFD−HFP−Teflon AF has a superior dielectric property giving enhanced contact angle modulation for AC voltages and thus suitable for electrowetting based applications in low voltage regime.

4. CONCLUSIONS A ferroelectric insulator (PVDF−HFP) with a top layer of Teflon AF shows a factor of 3 enhancement in EW contact angle modulation as compared to that on the conventional single-layered Teflon AF dielectric. The stable and hysteresis free EW is observed over thousands of voltage cycles that confirm the robustness of the ferroelectric insulator for EWOD applications. It has been demonstrated that by virtue of high dielectric constant, a thin bilayer is used in low voltage (∼15 V AC) EW giving stable contact angle modulation for a large number of voltage cycles exceeding 3600. This bilayer is employed for EW based rapid mixing of a fluorescent dye in aqueous glycerol drop for 15 V AC signal. Thus, the bilayer from ferroelectric polymer, PVFD-HFP, with a thin layer of Teflon AF has a high potential to be a promising dielectric for AC electrowetting. However, DC electrowetting shows hysteresis in contact angle modulation due to remnant polarization in the ferroelectric PVDF−HFP layer. 24054

DOI: 10.1021/acsami.6b05958 ACS Appl. Mater. Interfaces 2016, 8, 24049−24056

Research Article

ACS Applied Materials & Interfaces

■

(10) Mugele, F.; Baret, J. C. Electrowetting: From Basics to Applications. J. Phys.: Condens. Matter 2005, 17 (28), R705−R774. (11) Chen, L.; Bonaccurso, E. Electrowetting – from Statics to Dynamics. Adv. Colloid Interface Sci. 2014, 210, 2−12. (12) Chae, J. B.; Kwon, J. O.; Yang, J. S.; Kim, D.; Rhee, K.; Chung, S. K. Optimum Thickness of Hydrophobic Layer for Operating Voltage Reduction in EWOD Systems. Sens. Actuators, A 2014, 215, 8−16. (13) Nelson, W. C.; Kim, C. C. J. Journal of Adhesion Science and Droplet Actuation by (EWOD): A Review. J. Adhes. Sci. Technol. 2012, 26, 1747−1771. (14) Srinivasan, V.; Pamula, V. K.; Fair, R. B. An Integrated Digital Microfluidic Lab-on-a-Chip for Clinical Diagnostics on Human Physiological Fluids. Lab Chip 2004, 4 (4), 310−315. (15) Mach, P.; Krupenkin, T.; Yang, S.; Rogers, J. a. Dynamic Tuning of Optical Waveguides with Electrowetting Pumps and Recirculating Fluid Channels. Appl. Phys. Lett. 2002, 81 (2), 202−204. (16) Verheijen, H. J. J.; Prins, M. W. J. Reversible Electrowetting and Trapping of Charge: Model and Experiments. Langmuir 1999, 15, 6616−6620. (17) Cho, S. K.; Moon, H. J.; Kim, C. J. Creating, Transporting, Cutting, and Merging Liquid Droplets by Electrowetting-Based Actuation for Digital Microfluidic Circuits. J. Microelectromech. Syst. 2003, 12 (1), 70−80. (18) Moon, H.; Cho, S. K.; Garrell, R. L.; Kim, C. J. Low Voltage Electrowetting-on-Dielectric. J. Appl. Phys. 2002, 92 (7), 4080−4087. (19) Kilaru, M. K.; Heikenfeld, J.; Lin, G.; Mark, J. E. Strong Charge Trapping and Bistable Electrowetting on Nanocomposite Fluoropolymer: BaTi O3 Dielectrics. Appl. Phys. Lett. 2007, 90, 212906. (20) Arscott, S. Electrowetting and Semiconductors. RSC Adv. 2014, 4 (55), 29223. (21) Palma, C.; Deegan, R. Electrowetting on Semiconductors. Appl. Phys. Lett. 2015, 106 (1), 014106. (22) You, H.; Steckl, A. J. Electrowetting on Non-Fluorinated Hydrophobic Surfaces. J. Soc. Inf. Disp. 2013, 21, 411−416. (23) Berry, S.; Kedzierski, J.; Abedian, B. Irreversible Electrowetting on Thin Fluoropolymer Films. Langmuir 2007, 23 (8), 12429−12435. (24) Kim, N.-Y.; Hong, S.-M.; Park, S.-S.; Hong, Y.-P. The Movement of Micro Droplet with the Effects of Dielectric Layer and Hydrophobic Surface Treatment with R.F. Atmospheric Plasma in EWOD Structure. J. Phys.: Conf. Ser. 2006, 34, 650−655. (25) Li, Y.; Parkes, W.; Haworth, L. I.; Ross, A. W. S.; Stevenson, J. T. M.; Walton, A. J. Room-Temperature Fabrication of Anodic Tantalum Pentoxide for Low-Voltage Electrowetting on Dielectric (EWOD). J. Microelectromech. Syst. 2008, 17 (6), 1481−1488. (26) Huang, L.-X.; Koo, B.; Kim, C.-J. Sputtered-Anodized Ta2O5 as the Dielectric Layer for Electrowetting-on-Dielectric. J. Microelectromech. Syst. 2013, 22, 253−255. (27) 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. (28) Raj, B.; Smith, N. R.; Christy, L.; Dhindsa, M.; Heikenfeld, J. Composite Dielectrics and Surfactants for Low Voltage Electrowetting Devices. Bienn. Univ. Microelectron. Symp. - Proc. 2008, 1, 187−190. (29) 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 (1), 465−470. (30) Papageorgiou, D. P.; Tserepi, A.; Boudouvis, A. G.; Papathanasiou, A. G. Superior Performance of Multilayered Fluoropolymer Films in Low Voltage Electrowetting. J. Colloid Interface Sci. 2012, 368 (1), 592−598. (31) Janocha, B.; Bauser, H.; Oehr, C.; Brunner, H.; Gopel, W. Competitive Electrowetting of Polymer Surfaces by Water and Decane. Langmuir 2000, 16 (7), 3349−3354. (32) Narasimhan, V.; Park, S.-Y. An Ion Gel as a Low-Cost, SpinCoatable, High-Capacitance Dielectric for Electrowetting-on-Dielectric (EWOD). Langmuir 2015, 31 (30), 8512−8518.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b05958. Electrowetting response on planar PVDF-HFP (Figure S1); low voltage EW response on bilayer dielectric for extended number of cycles ∼3600 using KCl−water (Figure S2); low voltage EW response on bilayer dielectric for extended number of cycles ∼3600 using KCl−water: ethylene glycol (70:30 wt %) (Figure S3); EW response for DC voltage on bilayer and single layered Teflon AF (Figure S4); and variation of CA when the applied voltage ramp rate dU/dt is varied (Figure S5) (PDF) Real time video of EW-based mixing of a dye in the small liquid droplet (AVI)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS A.G.B. would like to acknowledge Physics of Complex Fluids (PCF), University of Twente, The Netherlands, for Visiting Scientist Program also a financial support from Board of College and University Development (BCUD), Savitribai Phule Pune University, Pune (India). Y.B.S. would like to acknowledge University Grant Commission (UGC), Govt. of India for JRF fellowship.

■

REFERENCES

(1) Berge, B.; Peseux, J. Variable Focal Lens Controlled by an External Voltage: An Application of Electrowetting. Eur. Phys. J. E: Soft Matter Biol. Phys. 2000, 3 (2), 159−163. (2) Kuiper, S.; Hendriks, B. H. W. Variable-Focus Liquid Lens for Miniature Cameras. Appl. Phys. Lett. 2004, 85 (7), 1128−1130. (3) Hayes, R. A.; Feenstra, B. J. Video-Speed Electronic Paper Based on Electrowetting. Nature 2003, 425, 383−385. (4) Heikenfeld, J.; Zhou, K.; Kreit, E.; Raj, B.; Yang, S.; Sun, B.; Milarcik, A.; Clapp, L.; Schwartz, R. Electrofluidic Displays Using Young−Laplace Transposition of Brilliant Pigment Dispersions. Nat. Photonics 2009, 3, 292−296. (5) Lee, P. T. C.; Chiu, C. W.; Lee, T. M.; Chang, T. Y.; Wu, M. T.; Cheng, W. Y.; Kuo, S. W.; Lin, J. J. First Fabrication of Electrowetting Display by Using Pigment-in-Oil Driving Pixels. ACS Appl. Mater. Interfaces 2013, 5 (13), 5914−5920. (6) Satoh, W.; Hosono, H.; Suzuki, H. On-Chip Microfluidic Transport and Mixing Using Electrowetting and Incorporation of Sensing Functions. Anal. Chem. 2005, 77 (21), 6857−6863. (7) Banpurkar, A. G.; Nichols, K. P.; Mugele, F. Electrowetting-Based Microdrop Tensiometer. Langmuir 2008, 24 (19), 10549−10551. (8) Banpurkar, A. G.; Duits, M. H. G.; van den Ende, D.; Mugele, F. Electrowetting of Comple Fluids: Perspectives for Rheometry on Chip. Langmuir 2009, 25 (2), 1245−1252. (9) De Ruiter, R.; Wennink, P.; Banpurkar, A. G.; Duits, M. H. G.; Mugele, F. Use of Electrowetting to Measure Dynamic Interfacial Tensions of a Microdrop. Lab Chip 2012, 12, 2832. 24055

DOI: 10.1021/acsami.6b05958 ACS Appl. Mater. Interfaces 2016, 8, 24049−24056

Research Article

ACS Applied Materials & Interfaces (33) Bhushan, B.; Pan, Y. Role of Electric Field on Surface Wetting of Polystyrene Surface. Langmuir 2011, 27, 9425−9429. (34) Mugele, F. Fundamental Challenges in Electrowetting: From Equilibrium Shapes to Contact Angle Saturation and Drop Dynamics. Soft Matter 2009, 5, 3377. (35) Sawane, Y. B.; Wadhai, S. M.; Limaye, A. V.; Banpurkar, A. G. Electrolyte Concentration Effects on DC Voltage Electrowetting. Sens. Actuators, A 2016, 240, 126−130. (36) Arscott, S. Continuous Electrowetting at the Low Concentration Electrolyte-Insulator-Semiconductor Junction. Appl. Phys. Lett. 2014, 105 (23), 231604. (37) Koo, B.; Kim, C.-J. Evaluation of Repeated Electrowetting on Three Different Fluoropolymer Top Coatings. J. Micromech. Microeng. 2013, 23, 067002. (38) Schultz, A.; Chevalliot, S.; Kuiper, S.; Heikenfeld, J. Detailed Analysis of Defect Reduction in Electrowetting Dielectrics through a Two-Layer “barrier” Approach. Thin Solid Films 2013, 534, 348−355. (39) Seyrat, E.; Hayes, R. A. Amorphous Fluoropolymers as Insulators for Reversible Low-Voltage Electrowetting. J. Appl. Phys. 2001, 90 (3), 1383. (40) Berry, S.; Kedzierski, J.; Abedian, B. Low Voltage Electrowetting Using Thin Fluoroploymer Films. J. Colloid Interface Sci. 2006, 303 (2), 517−524. (41) Wegener, M.; Künstler, W.; Richter, K.; Gerhard-Multhaupt, R. Ferroelectric Polarization in Stretched Piezo- and Pyroelectric Poly(vinylidene Fluoride-Hexafluoropropylene) Copolymer Films. J. Appl. Phys. 2002, 92 (12), 7442−7447. (42) He, X.; Yao, K.; Gan, B. K. Phase Transition and Properties of a Ferroelectric Poly(vinylidene Fluoride-Hexafluoropropylene) Copolymer. J. Appl. Phys. 2005, 97, 084101. (43) Mugele, F.; Baret, J. C.; Steinhauser, D. Microfluidic Mixing through Electrowetting-Induced Droplet Oscillations. Appl. Phys. Lett. 2006, 88, 204106. (44) Mugele, F.; Staicu, A.; Bakker, R.; van den Ende, D. Capillary Stokes Drift: A New Driving Mechanism for Mixing in ACElectrowetting. Lab Chip 2011, 11, 2011−2016.

24056

DOI: 10.1021/acsami.6b05958 ACS Appl. Mater. Interfaces 2016, 8, 24049−24056