Frequency Dependence of Low-Voltage Electrowetting Investigated by

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Frequency Dependence of Low-Voltage Electrowetting Investigated by Impedance Spectroscopy Ying-Jia Li, and Brian P. Cahill Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03049 • Publication Date (Web): 17 Oct 2017 Downloaded from http://pubs.acs.org on October 19, 2017

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Frequency Dependence of Low-Voltage Electrowetting Investigated by Impedance Spectroscopy Ying-Jia Li§,‡ and Brian P. Cahill*,§ §

Institute for Bioprocessing and Analytical Measurement Techniques e.V., Rosenhof, 37308 Heilbad Heiligenstadt, Germany

‡

Department of Physical Chemistry, Georg August University of Göttingen, Tammannstraße 6, 37077 Göttingen, Germany

Keywords — electrowetting EWOD, impedance spectroscopy, Young-Lippmann equation, Ta2O5, hydrophobic silane, µL-droplets, entrapped oil layer.

Abstract — An electrowetting-on-dielectric (EWOD) electrode was developed that facilitates the use of low alternating voltages (≤ 5 VAC). This allows on-line investigation of the frequency dependence of electrowetting by means of impedance spectroscopy. The EWOD electrode is based on a dielectric bilayer consisting of an anodic tantalum pentoxide (Ta2O5) thin film (d = 59.35 nm) with a high relative permittivity ( = 26.3) and a self-assembled hydrophobic silane monolayer. The frequency dependence of electrowetting was studied using an aqueous µL-sized sessile droplet on the planar EWOD electrode in oil. Experiments using electrochemical

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impedance spectroscopy and optical imaging indicate the frequency dependence of all three variables in the Young-Lippmann equation: the voltage drop across the dielectric layers, capacitance per unit area and contact angle under voltage. The electrowetting behavior induced by AC voltages is shown to be well described by the Young-Lippmann equation for AC applications below a frequency threshold. Moreover, the dielectric layers act as a capacitor and the stored electrostatic potential energy is revealed to only partially contribute to the electrowetting.

I.

INTRODUCTION

Electrowetting-on-dielectric1 (EWOD) is the voltage induced wettability change of a conductive droplet on a dielectric layer. Based on this principle, droplets of small volumes (~µL) can be definably moved, merged or divided2-3. Moreover, this leads to diverse EWOD applications, such as liquid lenses4-5, displays

6-7

and lab-on-a-chip microsystems8-9 in food-

related or medical analytics. However, electrowetting devices face several obstacles in their improvement, especially their demand for high voltages10. DC voltages around 25 V or AC voltages about 50 V are required at least2,

8, 11

. On the one hand, this requires complicated electronics and materials with high

electrical breakdown strength12 and thus higher costs. It also raises problems, such as irreversible electrowetting13, Ohmic warming and even electrolysis14. On the other hand, high voltage hinders the on-line electrical investigation into electrowetting processes due to the voltage limitation of the electroanalytical devices. Commercial potentiostats have a typical voltage range of around ± 10 V. Most electronic components, such as operational amplifiers, are designed for power supplies within ± 15 V. Moreover, high-voltage applications often result in various signal-


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output-errors, such as limited slew rate and jitter.15 To overcome these drawbacks and limitations low-voltage EWOD electrodes are required. As the fundamental equation of electrowetting theory, the Young-Lippmann equation (1) describes how the droplet contact angle changes with the applied voltage across the dropletelectrode interface as shown below, − =

U 2

(1)

with and , the contact angles of droplet with and without applied voltage respectively, , the vacuum permittivity, , the relative permittivity and d, the thickness of the dielectric coating on electrode, σ, the interfacial tension between the droplet and its surrounding medium and U, the voltage drop across the dielectric layers between electrode and droplet. Based on the Young-Lippmann equation, three approaches can be used to decrease the voltage demand for EWOD. The first approach is to increase the relative permittivity of the dielectric layers ( ). This can be realized by using dielectric coating materials with high relative permittivity. The second approach is to reduce the thickness of the dielectric coatings (d). Meanwhile, the thin dielectric coating should be pin-hole free and have sufficient electrical breakdown strength. The third approach is to enlarge the initial contact angle under zero voltage ( ). This can be achieved by increasing the hydrophobicity of the electrode surface. EWOD electrodes are often constructed as a dielectric bilayer or multilayer stack on an electrode substrate. The stack is usually a combination of a dielectric material with high dielectric strength and a hydrophobic surface coating. Anodic tantalum pentoxide (Ta2O5) is a promising candidate as a dielectric layer in EWOD, because it combines high relative permittivity ( = 27.5)16 with high breakdown strength and low pin-hole density. Due to anodization, it is a self-healing layer as well. For the electrode surface, a silane coating can


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create a smooth electrowetting surface for low resistance to droplet motion and high hydrophobicity. In comparison to the widely used Teflon-AF, silane is less expensive. A selfassembled silane monolayer of a few nanometers is thinner than the spin-coated Teflon-layers that have a typical thickness from sub-µm to a few µm10. For advanced low-voltage EWOD electrodes, Ta2O5 has been combined with an amorphous fluoropolymer coating, such as Teflon-AF17 and CytopTM

18-19

. Li et al. showed that anodic

Ta2O5 forms a smooth and robust layer and when coated with a thin fluoropolymer coating, the EWOD voltage could be reduced to 13 V

17

. Moreover, anodized Ta2O5 was reported to have

better layer quality (e.g. negligible pin-holes) and better EWOD performance under AC actuation (13 VRMS) than sputtered Ta2O518. A bilayer stack with Ta2O5 was revealed to require a lower actuation threshold voltage (6 V) than that with Parylene C (a poly(p-xylylene) polymer with ≅ 3.15)19. Furthermore, EWOD electrodes with a hydrophobic silane integrated with other dielectric materials, such as silicon nitride ( ≅ 7.5) was studied for electrowetting reversibility. In that study, an octadecyltrichlorosilane (OTS) monolayer was shown to have slightly higher hydrophobicity and lower contact angle hysteresis than the Teflon-like fluoropolymers, which makes the droplet motion during electrowetting easier20. In digital microfluidics21, alternating voltages are more widely used and preferred than DC voltages. Li et al.22 have shown that application of alternating voltages instead of DC voltages reduces the contact angle hysteresis in electrowetting. Sticky surfaces can thus become slippery for electrowetting. Despite the advantages of alternating voltages to electrowetting, how the frequency of AC signals affects electrowetting has not yet been thoroughly investigated. In this paper we investigate how the applied frequency affects electrowetting using electrochemical impedance spectroscopy (EIS).


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EIS23 is a non-invasive electrical analysis method for material and process characterization with AC signals. Its application covers versatile fields: on-line monitoring of living cells24 in life science25, control of product quality and production processes26 in food industry and characterization of batteries27 and solar cells28 in energy sector. Based on an EIS sensor, the conductivity of aqueous droplets can be measured on-line in segmented flow29. Combining EIS with electrowetting actuation, the presence and the size of droplets can be determined in active matrix EWOD devices30. Hu et al. have studied impedance spectra for various EWOD arrangements. Through identification of the contact area, droplet configuration can be interrogated during electrowetting31-32. In this work, a low-voltage EWOD electrode was developed as a thin dielectric bilayer stack consisting of an anodic Ta2O5 layer and an octadecyltrichlorosilane (OTS) monolayer. The low voltage electrowetting (≤ 5 VAC) enabled the integration of EIS in the EWOD system by using a common voltage signal for electrowetting and electroanalysis. By means of an on-line investigation of electrowetting with EIS, this contribution studies the frequency dependence of the AC electrowetting. The system under investigation is an aqueous µL-droplet on the EWOD electrode in oil. Between the droplet and the electrode, a thin oil film is entrapped as a dielectric fluid layer, which can be deformed during electrowetting.33 For the impedance analysis, an equivalent circuit model is developed. The study focuses on, how frequency influences the three variables in the Young-Lippmann equation (Equation 1): voltage drop on the dielectric layer in

EWOD electrode (), the capacitance per unit area of the dielectric layer ( =

) and the

contact angle under voltage ( ). Furthermore, the Young-Lippmann equation is modified for AC applications; the frequency influence is analyzed for the correlation of these three variables under various measurement conditions.


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II.

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EXPERIMENTAL SECTION

1. Fabrication and Characterization of EWOD Electrode The EWOD electrode has a multilayer structure (Figure 1): an electrode substrate based on a silicon wafer with tantalum and two dielectric coatings consisting of Ta2O5 and octadecyltrichlorosilane (OTS).

Figure 1. Schematic drawing of the multilayer EWOD electrode construction. Electrode substrate: silicon wafer / tantalum (Ta) (typical thickness 500 ± 50 nm). Dielectric coatings: anodic Ta2O5 layer (typical thickness 59.35 ± 0.55 nm) and hydrophobic octadecyltrichlorosilane (OTS) layer (typical monolayer thickness). The electrode substrate was a silicon wafer with a sputtered tantalum layer covered by a photoresist layer. The wafer was diced into 1 cm x 1.5 cm specimen (IMN MacroNano®, Ilmenau University of Technology, Germany). Each specimen was firstly cleaned with an acid piranha solution (concentrated H2SO4 and 30% H2O2 in a ratio of 3:1), then with a base piranha solution (25% NH4OH, 30% H2O2 and deionized water in a ratio of 1:1:4) at 70°C to remove the photoresist layer. Afterwards the specimen was rinsed with deionized water and dried with nitrogen flow. To form an anodic Ta2O5 thin layer, a round area (0.42 mm2) of the specimen was anodized in 0.1 M citric acid at 30 VDC for 90 min. The anodization was conducted using an electrometer (Keithley 6517, Tektronix Inc., USA) and a graphite counter electrode (Phywe Systeme GmbH


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& Co. KG, Germany). After the anodization, the specimen was cleaned again with the acid piranha solution, rinsed with deionized water and dried with nitrogen flow. The surface morphology of the anodic Ta2O5 was investigated using atomic force microscopy (AFM) (NanoWizard 4, jpk instruments AG, Germany). The Ta2O5 layer thickness was determined with an imaging ellipsometer (EP3, Accurion GmbH, Germany) with an incident angle of 70° and the analyzing programs ep3 and ep4 (Accurion GmbH, Germany). The ellipsometric model was constructed based on the refraction index and the extinction coefficient for Ta (n =2.92, k = 2.13) and for Ta2O5 (n =2.22, k = 0) from the literature34. The capacitance of the anodic Ta2O5 layer was obtained through electrochemical impedance spectroscopy (EIS) at sinusoidal AC signals (frequencies: 10 kHz – 1 mHz, voltage offset: 0.3 V, voltage amplitude: 0.3 V). The EIS measurement was conducted in 0.1 M citric acid in an electrochemical cell with a graphite counter electrode and a silver/silver chloride reference electrode (DRIREF-2, Sensortechnik Meinsberg GmbH, Germany) connected to a potentiostat (SP-300, Bio-Logic Science Instruments SAS, France). The impedance spectra were analyzed with the program EC-lab V10.44 (Bio-Logic Science Instruments SAS, France) and the simplified Randles circuit. The silane coating procedure was performed according to the literature35 with modification. The specimen with the Ta2O5 layer was firstly activated in oxygen plasma under a low pressure of 66.6 Pa for 1 min (Plasma Cleaner, Harrick Inc., USA). It was then immediately immersed in a 0.5 wt% OTS (Abcr GmbH, Germany) solution in dried toluene (Sigma-Aldrich Chemie GmbH, Germany) under argon atmosphere for 3 hours and kept in the dark. Thereafter, the specimen was thoroughly and successively rinsed with chloroform, acetone, deionized water and methanol, blown off with nitrogen gas and dried for 12 hours at 80°C in oven.


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The hydrophobicity of the OTS coating was determined via contact angle measurements of a sessile water droplet (3 µL) (Contact angle system OCA, Dataphysics instruments, Germany) and compared with that of the anodic Ta2O5 surface. The roughness of the OTS surface was investigated through contact angle hysteresis determined by advancing and receding contact angle measurements of the water droplet. The fabricated EWOD electrode was immersed in tetradecane (Sigma-Aldrich Chemie GmbH, Germany) at least for one day before its usage in the experiments to investigate the frequency influence on electrowetting at low alternating voltages. All used chemicals without indications were obtained from Carl Roth GmbH in Germany.

2. Simultaneous Electrowetting, Impedance Spectroscopy and Optical Imaging In this work, a measurement setup (Figure 2) was developed for the simultaneous measurement by means of impedance spectroscopy and optical imaging during the electrowetting of a sessile droplet.

Figure 2. Schematic drawing of the measurement setup for the simultaneous electrowetting, impedance measurements and optical imaging. Amp: front-end amplifier. An aqueous droplet was placed on the EWOD electrode (working electrode) in tetradecane (oil) in the measurement cell. A platinum wire was inserted in the droplet and served as counter


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electrode. Immersion in oil prevents the evaporation of the aqueous droplet, minimizes the resistance to droplet movement on the electrode surface and results in a larger contact angle than that in air. A digital oscilloscope (Handyscope HS3, Tiepie Engineering, Netherlands) combined with a front-end amplifier was used to apply an AC signal to the system under investigation and to measure its response signal. A high-speed camera system (Pike F-032C, Allied Vision Technologies GmbH, Germany) with associated objective (Micro-NIKKOR 105mm f/2.8, Nikon, Japan) and a simple LED lamp was employed to acquire images of the droplet during electrowetting. A Python program was developed to control the digital oscilloscope and camera, to analyze the acquired data, so that impedance spectra, contact angle and contact area could be determined. Measurements were conducted in the frequency range from 1 MHz to 100 Hz at sinusoidal alternating voltages with a DC offset (Uoffset) equal to the amplitude (Upp/2). The amplitudes varied from 0.5 V to 5 V in steps of 0.5 V. The droplet consisted of aqueous KCl solutions in the range from 6.25-200 mM. This concentration range covers the physiologically relevant electrolyte concentrations. All the KCl solutions were prepared in 0.1 mM HCl, so that the influence of the CO2 from air solved in the solutions is negligible. Measurements were conducted with several EWOD electrodes and repeated three times to prove the reproducibility.

III.

ANALYSIS MODEL

1. Equivalent Circuit Model To describe the system under investigation electrically, a circuit model was constructed as shown in Figure 3a. Hereby, the aqueous µL-droplet is depicted as a resistor ( ) and a


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capacitor ( ) in parallel. The droplet resistance depends on its salt concentration. Between the droplet and the EWOD electrode underneath, a thin oil film is entrapped in the interfacial region. This thin oil film acts here as an additional dielectric layer besides the OTS- and the Ta2O5-layer. In a comparable experimental setup, the entrapment of such an oil film was observed by Staicu et al.33, who described how the oil film can be deformed due to the electrostatic pressure induced by an applied voltage. These three dielectric layers are defined in this work as a multilayer dielectric stack (MDS) and are represented electrically by a resistor ( ) in parallel connection with three capacitors in series: !" , #$% and $&' #( . The total capacitance of the multilayer dielectric stack ( ) is given by: )

*+,

=

)

-./

+

)

123

+

)

24'1(

.

(2)

The MDS-capacitance (56% ) and the frequency (7) determine its capacitive reactance (8 56% ): 8 56% =

1 . 2:756%

(3)

In the MDS, each layer can be considered as a plate capacitor. Thus, the capacitance per unit area

is given by:

=

.

(4)


10

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Figure 3. (a) Equivalent circuit model for the system under investigation composed of an aqueous µL-droplet on top of the multilayer dielectric stack (MDS). The MDS composes a thin oil film, the OTS-layer and the Ta2O5-layer. ?@ : droplet resistance; A=>?@ : droplet capacitance; > 8 56% and (ii) OPQ 0.98) and the significance of each linear correlation is confirmed by a Student’s t-test

38

with 99 % confidence. This indicates a significant linear

correlation between the electrowetting number and − in this frequency regime. Furthermore, the right bars in Figure 9b reveal the comparable slopes (h) of the linear correlations in the lower frequency range from 100 Hz to 100 kHz. Their average value is 0.55 ±


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0.02.

The significant linear correlations and the comparable h values

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suggest that the

electrowetting behavior at alternating voltages can be well described by the Young-Lippmann equation (Equation 9) with a stable h value in this frequency region. Moreover, the h value below 1 indicates that the electrostatic potential energy stored in the dielectrics only partially contributes to the electrowetting. The residual energy may be converted to electromechanical work for the oil-layer deformation during the electrowetting. Staicu and Mugele have investigated the deformation of an entrapped oil layer in a comparable measurement setup. The oil film thickness is described by an extension of the Landau-Levich law regarding the electrostatic pressure33.


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Figure 9. (a) Plot of the Young-Lippmann equation (Equation 9) for the electrowetting at frequencies from 100 Hz to 1 MHz on a droplet of 100 mM KCl. (b) Correlation coefficient β for droplets of 100 mM KCl and 6.25 mM KCl at frequencies where β < 1(n=3). In contrast, the dataset of the highest frequency (7 = 1 MHz) reveals no significant linear correlation between − and the electrowetting number based on a Student’s twotailed t-test with 95 % confidence limit. This suggests that EWOD effect is not the main cause of contact angle change at this high frequency. This frequency-based difference shown in the correlation behavior reveals that, electrowetting occurs below a frequency threshold. The frequency-based relationship between electrowetting


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and dielectrophoresis was studied by Jones et al.39-40: electrowetting dominates at lower frequencies while dielectrophoresis dominates at higher frequencies. Due to the asymmetric electrode configuration (Figure 2) in the measurement setup here, the aqueous droplet experiences an inhomogeneous electric field during electrowetting. For a comparable system as reported here, Shapiro et al.36 have demonstrated the non-uniform electric potential scaling inside a slightly resistive droplet and in its underneath EWOD electrode. Since the voltage mainly drops across the droplet rather than across the MDS at high frequencies, the gradient of the electric field may be sufficiently high to cause a significant volume polarization force

41

on the

droplet and thus induce dielectrophoresis here. In addition, at a low salt concentration of 6.25 mM KCl, the correlation in the YoungLippmann equation behaves similarly: Below a frequency threshold of 10 kHz, significant linear correlations deliver comparable slope values below 1 (h = 0.63 ± 0.05) at various frequencies (Figure 9b, left bars). This confirms that electrowetting dominates in the low frequency range. Moreover, this indicates that the frequency and the salt concentration have no significant influence on the correlation in the Young-Lippmann equation. However, the frequency region for electrowetting is limited by a lower threshold at the low salt concentration (7n = 10 kHz) than at the high salt concentration (7n = 100 kHz). This implies that the salt concentration in the droplet regulates the frequency threshold, below which the Young-Lippmann equation can well describe the electrowetting behavior at alternating voltages.

V.

CONCLUSIONS

The low-voltage electrowetting (≤ 5 VAC) was realized in this study for an on-line electrical investigation of the frequency dependence of AC electrowetting.


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To satisfy the conditions for the low-voltage electrowetting, an EWOD electrode was developed as a multilayer dielectric stack (MDS) with a thin Ta2O5 layer (d = 59.35 nm) processing high relative permittivity ( = 26.34) and a silane thin layer for a hydrophobic surface. This low-voltage EWOD electrode enables the simultaneous electrowetting and investigation with EIS. Moreover, an equivalent circuit model was developed to electrically describe the system under investigation: a µL-droplet of a salt solution in oil on the EWOD electrode. The validity of the circuit model was confirmed by the impedance measurements. The frequency dependence of all three variables in the Young-Lippmann equation was investigated: the voltage drop across MDS, the MDS-capacitance per unit area and the contact angle under voltage. Firstly, the frequency dependence of the voltage drop across MDS indicates a “low-pass filter” behavior; the low-pass frequency region can be regulated by the salt concentration. Secondly, the MDS-capacitance per unit area shows a voltage-dependence at low frequencies due to the deformation of an entrapped oil layer on the droplet-electrode interface during electrowetting. Meanwhile, the decrease of the MDS-capacitance per unit area at high frequencies reveals that the time limitation may result in the incomplete energy storage. Thirdly, the frequency dependence of the contact angle under voltage is visible above a voltage threshold and below a frequency threshold. Finally, the correlation between both sides of the Young-Lippmann equation was studied with regard to frequency dependence. Below a frequency threshold, electrowetting dominates and can be well described by the Young-Lippmann equation independently of frequency and salt concentration. In this frequency region, the correlation-coefficient β is less than one, thus the electrostatic potential energy stored in MDS is indicated to only partially contribute to electrowetting.


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SUPPORTING INFORMATION Comparison between the Double Layer Capacitance and the MDS Capacitance; Regression Analysis AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Tel. +49 03606 671 600. Fax: +49 03606 671 200.

Author Contribution The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGEMENT The authors gratefully thank Prof. A. Janshoff and Prof. P. Vana at the University of Göttingen for their valuable discussions and Dr. B. Schröder for the editorial revisions. ABBREVIATIONS AC, alternating current; DC, direct current; EWOD, electrowetting-on-dielectric; OTS, octadecyltrichlorosilane; pp/2, half of the peak-to-peak voltage; EIS, electrochemical impedance spectroscopy; AWG, arbitrary wave generator; MDS, multilayer dielectric stack; RMS, rootmean-square value.

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23.

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Abstract Graphic


33

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OTS Ta2O5 Ta

silicon wafer


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Measurement cell

Digital oscilloscope

Amp Platinum wire

Amp

Oil Droplet High-speed EWOD-electrode camera


Light source

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

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(a)

(b)

Rdrop

Cdrop

Rdrop

Coil RMDS

CMDS

COTS CTa2O5

electrode substrate

MDS: Multilayer dielectric stack


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(a)

1.0 cm

(b)

1.5 cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

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Ra = 0.23 nm


Langmuir

(a)

|Z| / 

1E7

1E6

1E5

1E4

f / Hz (b) 1E6

|Z| / 

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

1E5

1E4

1E3

f / Hz ACS Paragon Plus Environment

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Langmuir

U RMS

U(RMS)MDS

f / Hz

c

U RMS

/%

(b)

U(RMS)MDS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

/%

(a)


A

CMDS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

/ nF/mm2

Langmuir

U

f / Hz


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(a)

qu (b)

ft

qu / °

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Langmuir


Langmuir

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CMDS 2s A

2

U(RMS)MDS

(b)

Slope b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

cosqu - cosq0

(a)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

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oil

coun elect ter rode

Page 43 of 43

Rdrop

aqueous µL-droplet

Coil COTS CTa2O5

EWOD electrode

electrode substrate


Frequency Dependence of Low-Voltage Electrowetting Investigated by

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