Microsystem Technologies

, 16:449

Dielectric materials for electrowetting-on-dielectric actuation

Authors

    • Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR)
  • Saman Dharmatilleke
    • Innovative Nano Systems Pte Ltd
  • Devendra K. Maurya
    • Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR)
  • Andrew A. O. Tay
    • Department of Mechanical EngineeringNational University of Singapore
Technical Paper

DOI: 10.1007/s00542-009-0933-z

Cite this article as:
Liu, H., Dharmatilleke, S., Maurya, D.K. et al. Microsyst Technol (2010) 16: 449. doi:10.1007/s00542-009-0933-z

Abstract

The fundamental building blocks of typical electrowetting-on-dielectric (EWOD) actuation and their importance in the EWOD mechanism are introduced and reviewed, respectively. The emphasis in this experimental study of EWOD is on dielectric materials, upon which the performance of EWOD devices is heavily dependent. Dielectric breakdown of several typical polymeric and inorganic insulators employed as dielectrics for EWOD has been analytically investigated, which is forced to occur between the electrodes and conductive liquids under certain threshold potential. The electric breakdown occurring in both dielectric layer and surrounding medium (air or silicon oil) has been studied to build up a mathematical model of breakdown voltage as a function of dielectric thickness. Contact angle measurement of some polymeric materials and self-assembled monolayer using pure water has been carried out to demonstrate the contact angle tunability and reversibility, respectively, upon EWOD actuation.

1 Introduction

Electrowetting-on-dielectric (EWOD) (Berge 1993; Verheijen and Prins 1999), which originates from the phenomenon of electrowetting (Lippmann 1875; Beni and Hackwood 1981), has been extensively investigated since the last decade and internationally recognized as a promising technology for liquid manipulation and actuation. With the well-established micro/nanofluid manipulation techniques using EWOD, the latest research efforts have concentrated on demonstration of optical/display technology (Berge and Peseux 2000; Kuiper and Hendriks 2004; Hayes and Feenstra 2003) and lab-on-chip applications (Pollack et al. 2002; Belaubre et al. 2004; Hoshino et al. 2004; Yi and Kim 2004; Cheng and Hsiung 2004) of EWOD systems and devices. Several papers have reviewed in detail the recent development of electrowetting and EWOD covering topics from theory, material properties, surface morphologies to applications (Quilliet and Berge 2001; Darhuber and Troian 2005; Mugele et al. 2005).

Generally a typical EWOD device must consist of four compulsory components—an ionized liquid working as a medium, a conductive material serving as the electrodes and a dielectric material providing both the capacitance between liquid and electrode, and hydrophobicity of its surface at the solid–liquid interface (some dielectrics require an additional coating of a thin hydrophobic material due to inadequate hydrophobicity of their natural surface). However, these components of different intrinsic material properties influence the performance of an EWOD device in different ways.

The effect of a variety of liquids on EWOD has been investigated in recent years and the results indicate that the requirement of liquid properties such as concentration and charge carrying ability are not that severe for EWOD initiation. Using typical liquid electrolytes like KCl and K2SO4 with concentrations of the order of 0.01–1.0 M/l, Verheijen and Prins (1999) reported the independence of ion type, ion molarity and ion valence of electrolytes on the applied voltages for EWOD. Mugele et al. (2005) presented an EWOD experiment with DI water on indium-tin-oxide (ITO) electrode coated with a thin layer of Teflon® AF and obtained a noticeable contact angle change. The electrowetting phenomenon was also observed for mixtures of salt solutions (NaCl or Na2SO4) with other chemicals such as glycerol (Mugele and Herminghaus 2002; Klingner et al. 2004), ethanol (Vallet et al. 1999) and methanol (Krupenkin et al. 2004), without any degradation of EWOD performance.

Since more and more research interests on EWOD have been driven by its biomedical applications, researchers have exploited the applicability of EWOD devices in the presence of biomolecules and physiological fluids. Yoon and Garrell (2003) showed that EWOD occurred with biofluids containing protein, DNA and adult bovine whole serum of a biomolecule concentration of 4 μg/ml, although the performance was degraded by the adsorption of biomolecules into the insulation layer giving rise to the reduced contact angle change and hysteresis. The compatibility of EWOD with physiological fluids such as insulin, cytochrome, myoglobin etc. has been demonstrated by Wheeler et al. (2004) using an EWOD-based chip. Recently, EWOD actuation of human physiological fluids containing human whole blood, serum, plasma, urine, saliva sweat and tears was demonstrated at 20 Hz and less than 65 V by Fair et al. (Srinivasan et al. 2004) using an integrated digital microfluidic chip. Hitherto, all the corresponding experimental results identically show that the performance of an EWOD device has very limited dependence on liquids properties.

Another component, the metal electrode, which is dominant in the semiconductor and printed circuit board (PCB) industry, has also been commonly employed in EWOD and patterned by photolithography and lift off/wet etching processes. These materials include gold (Au) (Yi and Kim 2004), silver (Ag) (Kuo et al. 2003), aluminum (Al) (Verheijen and Prins 1999), chromium (Cr) (Cho et al. 2003), platinum (Pt) (Cho et al. 2003; Moon et al. 2002), titanium (Ti) (Cho et al. 2003), and Si (Wheeler et al. 2004). The typical thickness of the electrode is in the range of several nanometers to 200 nanometers. Stainless steel sheets have also been reportedly used as electrodes for EWOD (Vallet et al. 1999). Because of the increasing demand to visualize the device operation or induce light transmission in practical experiments, the top-substrate conductive material should be transparent or semi-transparent to light. ITO films of 50–200 nm thickness intrinsically have a transmittance of above 80% with sheet resistances of less than 10 Ω/sq. Therefore, ITO has been popularly used as the material for the top-substrate electrode in the EWOD device (Pollack et al. 2002; Wheeler et al. 2004; Srinivasan et al. 2004; Cho et al. 2003). Comparatively, the influence of electrode property on EWOD is insignificant.

In contrast with these components mentioned above, the properties of dielectric materials have been experimentally verified to have a remarkable impact on EWOD performance and have hence been investigated intensively since the discovery of EWOD. A significant amount of research focusing on optimizing the extrinsic properties (e.g., thickness, applied voltages) and intrinsic properties (e.g., surface property, dielectric strength or constant) of EWOD have provide valuable information for practical EWOD design and application.

Referring to the Young-Lippmann equation
$$ \cos \theta = \cos \theta_{ 0} + {\frac{{\varepsilon_{0} \varepsilon V^{2} }}{{2\gamma_{lv} d}}} $$
(1)
where, θ is the contact angle under applied voltage of V and θ0 is the contact angle without externally applied potentials, ε is the dielectric constant of a dielectric layer of thickness d and ε0 is the vacuum permittivity, there are two essential criteria for EWOD design. Firstly, the inherent contact angle (without external applied voltage) should be large enough to obtain a maximum contact angle variation; and secondly, the thickness of dielectric layer should be as small as possible to reduce applied potential. To obtain large contact angle (normally >90°) at zero potential, one can either use highly hydrophobic insulators or hydrophilic insulators deposited with a very thin hydrophobic layer without blocking electrical connection. Minimizing the dielectric thickness may give rise to poor dielectric strength, and may cause dielectric breakdown at a much lower applied potential. Nevertheless, the dielectrics ideally should be of high dielectric constant for achievement of large contact angle tunability and high dielectric strength to facilitate application of a high potential without dielectric breakdown. Beside these considerations, applied potential or power consumption should be minimized when operating the devices/systems. Materials employed should be chemically inert/resistant to achieve reliability. Mechanical, thermal and charge stability are important for manufacturing reproducibility. Biocompatibility and minimal bimolecular adsorption of materials must be taken into consideration when handling bio- and physiological fluids.

In this study, we present a sufficient comparison of the properties of polymeric and inorganic dielectrics widely reported and employed in EWOD, together with a critical analysis of their merits/demerits for EWOD applications. The role of dielectrics on the evolution of EWOD technology is summarized. We also demonstrate dielectric breakdown analysis to obtain the electrowetting voltage and breakdown voltage as functions of dielectric thickness for several typical dielectric materials. Contact angle measurements were conducted to compare the contact angle tunability and reversibility of those dielectrics upon EWOD actuation. The results show that Cytop is the most promising and competitive dielectric material for EWOD applications.

2 Polymeric and inorganic dielectrics

Polymeric materials which have been used as candidates for EWOD includes parylene-C (Pollack et al. 2002; Mugele et al. 2005; Srinivasan et al. 2004; Moon et al. 2002; Welters and Fokkink 1998) and parylene-N (Verheijen and Prins 1999) serving as the prime insulator coated with a thin hydrophobic layer of Teflon® AF (Verheijen and Prins 1999; Welters and Fokkink 1998), conventional Teflon® films (Vallet et al. 1999; Bienia et al. 2003), and Cytop™ amorphous fluoropolymer (Yi and Kim 2004; Mach et al. 2002; Acharya et al. 2003), polydimethylsiloxane (PDMS) (Kuo et al. 2003), and polyimide (Welters and Fokkink 1998; Mach et al. 2002), as well as other commercially available polymers like PET (Vallet et al. 1999) and polyethylene (PE) (Klauk et al. 2000) etc. Parylene has been reported exclusively to work together with a top-coated thin hydrophobic layer like Teflon® AF 1600. Teflon® AF has the lowest dielectric constant of 1.93 among available polymers, which makes itself a candidate for next-generation inter-level dielectrics1 but a poor dielectric for capacitor. Therefore, it has frequently served only as a hydrophobic layer instead of a prime insulator in electrowetting. It has a typical thickness on the order of 100 nm in between liquid and insulation layer without electrical isolation in order to provide a low contact angle hysteresis (8°) (Welters and Fokkink 1998). Cytop™ has been actively employed as a new member of dielectrics for EWOD since the early 2000s. Compared to Teflon, Cytop™ yields lower contact angle hysteresis (4°) and impermeability, and better light transmission for optical applications. Poly(dimethylsiloxane) (PDMS) Sylgard® 184, Dow Corning, becomes a potentially attractive material for integrated electrowetting devices because of its very hydrophobic surface property and low material cost compared to Teflon. However, a potential of 500 V, a rather high voltage for EWOD, was required to obtain a contact angle change of 35° on a 38 μm-thick PDMS film (Kuo et al. 2003). Compared to its high popularity as a molding material for microfluidic devices, currently there are almost no interests in using PDMS as an insulating material. Polyimide has a dielectric strength of 22 kV/mm and its dielectric constant of 3.4 is higher than that of parylene-C. However, it requires much higher applied voltages to achieve the same magnitude of contact angle change as that of parylene-C (Welters and Fokkink 1998). Thus, its use for EWOD devices is not popular nowadays. Table 1 summarizes the corresponding material properties and specifications of the above-mentioned polymers employed in EWOD.
Table 1

Comparisons of material properties of polymeric dielectrics for EWOD

Dielectrics

Parylene -N and -C

Teflon® AF 1600

Teflon (PTFE)

Cytop™

PDMS

Polyimide

Dielectric strength (kV/mm)

276 (-N)

21

60

110

21.2

22

268 (-C)

Dielectric constant

2.65 (-N)

1.93

2.1

2.1

2.3–2.8

3.4

3.15 (-C)

Applied voltage (V)

±240 (DC)

<1 k (AC 50–20 kHz)

<300 (DC)

<600 (AC 1 kHz)

120 (DC)

<800 (AC 2 kHz)

±500 (DC)

<400 (DC)

Typical thickness (μm)

3.5–30

0.01–0.1

25–50

0.1–1

38

6–35

Contact angle of water (°)

126

120

114

110

120

50–80

Fabrication method

Chemical vapor deposition

Spin or dip coating

Commercial

Spin coating

Spin coating

Spin coating

Another interesting group of materials employed as inorganic insulators in EWOD includes silicon dioxide (SiO2) (Moon et al. 2002), silicon nitride (SiNx) (Acharya et al. 2003) and the Sol–Gel derived barium strontium titanate [(Ba,Sr)TiO3] (Cho et al. 2003; Moon et al. 2002) or BST. As their original surface properties exhibit hydrophilicity (water contact angle is 46.7° (Thomas 1996), 30° (Sung et al. 1999), and 40.8° (Tan et al. 2005) for SiO2, Si3N4 and BST, respectively), normally they have to work with a hydrophobic top-coating layer such as Teflon® AF 1600 for electrowetting actuation. The prime advantages of SiO2 as a capacitive insulator are its high dielectric constant, high dielectric strength and availability as one of the predominant materials in the MEMS and microelectronics industries. Compared to Teflon® AF as an insulator, SiO2 is able to offer a larger contact angle variation at the same applied voltage due to its higher dielectric constant (Moon et al. 2002). Silicon nitride was incorporated into a tunable liquid lens in combination with a 1-μm thick Cytop™ layer (hydrophobic coating) (Acharya et al. 2003). No changes were observed under the applied voltage of 40 V for a droplet of 6 μl. Barium Strontium Titanate (BST) has a significantly higher dielectric constant (200–300) than that of SiO2 and SiNx. Therefore, it can provide an even lower operation voltage (15 Vdc) to reduce the contact angle from 120° to 80° (Cho et al. 2003; Moon et al. 2002). Metal organic chemical vapor deposition (MOCVD) can be used to deposit this thin film with a typical thickness of the order of less than 100 nm. Table 2 summarizes the corresponding material properties of SiO2, SiN4 and BST.
Table 2

Comparisons of material properties of inorganic dielectrics for EWOD

Dielectrics

SiO2 (Chang et al. 1996)

Si3N4 (Thomas 1996)

BST (Imanaka et al. 2002)

Dielectric strength (kV/mm)

400–600

500

18–54

Dielectric constant

3.9

7.5

225–265

Applied voltage (V)

Vdc ≥ 25

>40

Vdc ≥ 15

Typical thickness

100 nm–1 μm

150 nm

70 nm

Contact angle of water (°)

46.7

30

40.8

Fabrication method

Thermal oxidation or PECVD

Chemical vapor deposition

MOCVD

Self-assembled monolayers (SAMs) has been studied and incorporated with electrowetting since the last decade. SAMs normally can be prepared by immersing the substrates coated with gold films (~100 nm thick) into ethanoic solution for several hours and its typical thickness is around 20–30 Å (Bain et al. 1989). The value of the dielectric constant for SAMs was obtained analytically (Chang and Kwok 2004) at nearly 2, which is in agreement with the results through experimental results (Sondag-Huethorst and Fokkink 1994a, b). In short, limited stability (Gorman et al. 1995) and contact angle irreversibility (Saeki et al. 2001) of SAMs have noticeably weakened its application in EWOD devices. Progress of EWOD development is summarized in Table 3, listing the corresponding applied voltages, dielectric materials and features.
Table 3

Progress of EWOD development

Ref (year)

Applied voltage

Dielectrics

Features

Minnema et al. (1980)

AC 0–2 kV (50 or 1 k Hz)

PE (200–900 μm)

θ changes from 80° to ~20°

Irreversibility

Berge (1993)

AC (50–3 k Hz)

PTFE (53 μm)

Reversibility

Saturation

Sondag-Huethorst and Fokkink (1994a, b)

DC (0–0.9 V)

SAM

Limited reversibility

Hysteresis

Gorman et al. (1995)

1.0 mW/cm2 (light intensity)

Photoisomerizable monolayer

Reversibility

θ changes from 24° to 11°

Vallet et al. (1996)

AC (0–3 kV at 50 Hz–a few kHz)

PET (12 μm)

Reversibility at moderate voltage

Contour line instability at high voltages

Welters and Fokkink (1998)

DC (0–400 V)

Parylene C (6–10 μm) or polyimide (18–35 μm) + Teflon® AF 1600 (0.1 μm)

Hysteresis (8°)

Reversibility

θ changes from 110° to ~60°

Verheijen and Prins (1999)

DC (±240 V)

Parylene N (10  μm) + Teflon® AF 1600 (30 nm) + silicon oil

Hysteresis (2°)

Reversibility

Polarity, ion type, molarity and ion valence independent

Prins et al. (2001)

DC (±227 V)

Parylene C (11.5 μm) + Teflon® AF1600 (10 nm)

Weak hysteresis

High transportation speed

Mach et al. (2002)

DC (0-100 V)

Polyimide (2 μm) + Cytop™ (1 μm)

Reversibility

Pinning-free voltage control

Cho et al. (2003)

DC (15-25 V)

SiO2 (0.1 μm)/BST (70 nm) + Teflon® (20 nm)

Low voltage operation

Reproducibility

Versatile droplet handling

Srinivasan et al. (2004)

AC (65 V at 20 Hz)

Parylene C (800 nm) + Teflon® AF1600 (50 nm) + silicon oil

Biocompatibility

High transportation rate

High reproducibility

3 Dielectric breakdown analysis

3.1 Dielectric breakdown in dielectrics

Dielectric breakdown is a phenomenon where insulators are forced to conduct electricity. In EWOD, it specifically refers to the failure of insulating materials which results in a short circuit between the electrodes and conductive liquids. Dielectric breakdown damages dielectric layers, and charges do not accumulate at the interface of the dielectric and the conducting liquid to give rise to a destructive malfunction of the EWOD device. Hence, caution must be taken in determining the minimum thickness of an insulating layer, the dielectric strength of which is one of the most important factors in materials selection.

Rearranging Eq. 1, the electrical potential required to induce a desirable variation of contact angle change (Δθ) is given by:
$$ V_{\Updelta \theta } = \sqrt {{\frac{{2d\gamma_{lv} (\cos \theta - \cos \theta_{0} )}}{{\varepsilon_{0} \varepsilon }}}} $$
(2)
Also, the dielectric breakdown voltage Vbd, i.e., the minimum voltage to make an insulator act as a conductor, is given by
$$ V_{\text{bd}} = E_{\text{ds}} \times d $$
(3)
where Eds is the dielectric strength and d is the thickness of the insulator. Figure 1a shows the variation of the required electrowetting actuation voltage and the breakdown voltage with the thickness of Cytop™ insulation material. The electrowetting actuation voltage to acquire a specific Δθ is proportional to the square root function of the thickness of the dielectric layer of a fixed dielectric constant. In contrast, the breakdown voltage is a linear function of the thickness of the dielectric layer. As illustrated in Fig. 1a, the minimum insulator thickness (correspondingly a minimum voltage) necessary for induction of a specific Δθ with a given insulator of fixed dielectric strength and dielectric constant is determined by the intersection of these two functions. Hence the thickness of the dielectric layer should always be greater than this minimum value, otherwise dielectric breakdown always occurs before obtaining the desired contact angle change. In the same manner, electrowetting and dielectric breakdown voltage as functions of the dielectric thickness for several typical dielectric materials are graphically shown in Fig. 1b–f. The data on the contact angle variations of the dielectric materials mentioned in Fig. 1 are based on the measurement results listed in Table 4, which will be discussed in Sect. 4.
https://static-content.springer.com/image/art%3A10.1007%2Fs00542-009-0933-z/MediaObjects/542_2009_933_Fig1_HTML.gif
Fig. 1

Electrowetting actuation voltage to obtain a specific contact angle change (Δθ) and dielectric breakdown voltage as a function of dielectric layer thickness: a Cytop™ with ε = 2.1, Eds = 110 MV/m, and Δθ is 30° (from 110°to 80°), b PDMS, c Teflon® AF 1600, d Parylene C, e BST, f SiO2

Table 4

The dielectric breakdown voltage for corresponding minimum thickness of typical dielectrics (under 100 V)

Dielectrics

dmin (μm)

Vbd (V)

100 V

Δθ (°)

dmin (μm)

dmax (μm)

Cytop™

0.33

36.74

0.91

2.47

30 (110–80)

PDMS

4.27

90.59

4.72

5.21

20 (120–100)

Teflon® AF 1600

13.04

273.84

1.74

4.76

40 (120–80)

Parylene C

74.16

511.704

2.83

14.49

41 (126–85)

BST

2.87 × 10−3

0.14

2

1,392

10.8 (40.8–30)

SiO2

3.05 × 10−3

1.5

0.2

13.14

16.7 (46.7–30)

Table 4 lists the minimum dielectric thickness (dmin) and dielectric breakdown voltage (Vbd) corresponding to different typical dielectric materials. The dielectric breakdown voltage Vbd is the minimum theoretical actuation voltage required to initiate the EWOD effect in air without any failure of the insulator. A practically acceptable maximum driving voltage is 100 V and for this voltage, the minimum (limited by dielectric strength Eds) and maximum (limited by driving voltage Vew) value of thickness for all the dielectric materials studied are listed in Table 4. Other material properties are given in Tables 1 and 2.

To summarize, Parylene C coated with a thin layer of Teflon® AF 1600 exhibits the highest variation of contact angle (~41°) which can sustain up to 500 V before dielectric breakdown. In contrast, the small initial contact angles (<50°) of BST and SiO2 due to their natural hydrophilic surfaces are not able to generate adequate electrowetting force for EWOD actuation. Hence, BST and SiO2 are not competitive enough as dielectric materials in EWOD. A moderate contact angle change (20°) is available with PDMS at a relatively high applied voltage of about 100 V. For Cytop™, the contact angle variation is as large as 30° under the same voltage and EWOD can always be operated at as low as 40 V before dielectric breakdown.

3.2 Dielectric breakdown in surrounding medium

Besides the dielectric layer, dielectric breakdown may also occur in the medium (air or silicon oil) surrounding the liquid. As reported, silicon oil immiscible with water has normally been employed to reduce contact angle hysteresis and prevent evaporation of liquid. A safety gap or channel height between the top and bottom substrate is essential to avoid the short circuiting of electrodes through the surrounding medium. For simplicity, we only consider a perfect model of parallel-plate capacitor of interfaces of three dielectric regions (liquid, medium and dielectric) parallel to the substrates with a negligible morphological influence of liquid and electrode on the electrical field.

Assuming an applied potential of V0 across the substrates, the corresponding electrical field E in the dielectrics is given by
$$ E = {\frac{{V_{0} }}{{d_{1} + {\frac{{\varepsilon_{1} }}{{\varepsilon_{2} }}}d_{2} + {\frac{{\varepsilon_{1} }}{{\varepsilon_{3} }}}d_{3} }}} = {\frac{{V_{0} }}{{\sum\limits_{i = 1}^{3} {{\frac{{\varepsilon_{1} }}{{\varepsilon_{i} }}}d_{i} } }}} $$
(4)
where εi and di are the dielectric constant and thickness respectively for each dielectric region. Assuming that the top and bottom solid insulation are identical, we can simplify the electrical field in the surrounding medium Esm as
$$ E_{sm} = {\frac{{V_{0} }}{{d_{1} + {\frac{{\varepsilon_{1} }}{{\varepsilon_{2} }}}d_{2} + {\frac{{\varepsilon_{1} }}{{\varepsilon_{3} }}}d_{3} }}} = {\frac{{V_{0} }}{{d + \sum\limits_{i = 1}^{2} {{\frac{{\varepsilon_{sm} }}{{\varepsilon_{i} }}}d_{i} } }}} $$
(5)
where d and di are the channel height (or liquid thickness) and thickness of solid insulating layer, respectively, and εsm and εi are the dielectric constant of surrounding medium and solid insulation, respectively. Consequently, the breakdown voltage of the surrounding medium as a function of the ratio of dii is depicted in Fig. 2. Some typical values of di in both air (ε = 1.0, Eds = 3.0 × 106 V/m) and silicone oil (ε = 2.5, Eds = 15 × 106 V/m) were used in the case studies. Applying a maximum operating voltage of 100 V, the minimum gap depth for silicon oil is less than 10 μm and it is about 35 μm for air.
https://static-content.springer.com/image/art%3A10.1007%2Fs00542-009-0933-z/MediaObjects/542_2009_933_Fig2_HTML.gif
Fig. 2

Breakdown voltage of surrounding medium (air or silicone oil) as a function of ratio d/ε of insulator for different channel heights d

To sum up, precautions to avoid dielectric breakdown effectively such as using thicker dielectrics and utilizing high-dielectric-constant materials, etc., should be taken into consideration in EWOD design. However, increasing the thickness of the dielectric layer requires a higher voltage to achieve the same effect, thus, consuming more power. A hydrophobic dielectric of high dielectric constant and dielectric strength is ideal for EWOD. Unfortunately, such materials are very limited. Certain trade-offs are needed to balance the requirement of the material properties and the performance of EWOD devices.

4 Contact angle measurement

This section describes the measurement of contact angles for several dielectric materials. The accuracy of the measurement depends on factors such as composition of liquid droplet, water evaporation, droplet size/volume, humidity and temperature. Other factors such as air pressure, relative velocity between air and liquid droplet are negligible.

Liquid evaporation is almost inevitable for droplets and it is undesirable for contact angle measurement. In microfluidics, liquid evaporation becomes significant for micro-droplets. Under typical experimental conditions (T = 20°C and relative humidity 40%), the evaporation rate of water is about 3 μg/s (King et al. 1997; Bonn et al. 2006). In addition, water evaporation decreases with increasing humidity increases rapidly with air temperature. Therefore, in this case, each contact angle measurement of a typical size of liquid droplet (3–5 μl) was completed within 2 min before water evaporation becomes significant at room temperature. The humidity in the lab is controlled at 40% and monitored during the experiment.

A non-transparent plastic cover (like a box) was made to cover the goniometer and to allow it to be operated only from the front. All illumination sources in the laboratory were turned off during the experiments in order to minimize the influence of illumination. Hence, the effect of illumination on contact angle measurement has been significantly reduced and thus considered to be negligible in this experiment.

4.1 Sample preparations

Only pure water and saturated sodium chloride (NaCl) were used as electrolytes for this study. Four-inch silicon wafers were firstly diced into 25 × 25 mm chips and then sputter-deposited with Au thin films (2500, with 100 thick Cr underlayer). Parylene C, PDMS, Cytop™ and self assembled monolayers (SAMs), which were able to serve as prime insulators without additional hydrophobic coatings, were prepared and the contact angles of electrolyte droplets on them were measured.

Parylene C thin films (2.25 μm thick) were deposited onto the Au-coated silicon chips by a customized parylene coating equipment.

PDMS (Sylgard® 184, Dow Corning) thin films were firstly prepared by mixing the base and curing agent at a ratio of 10:1 either by weight or volume (10 parts of base and 1 part of curing agent). Then, they were placed into a VWR vacuum oven to degas air bubbles at 101.6 kPa for 30 min. After that, PDMS was dispensed onto Au-coated silicon chips and spin-coated at 2,000 rpm, and finally cured at around 150°C for 1 h. Its thickness was measured by a surface profilometer to be about 38.2 μm.

Cytop™ (TL809M, Asahi Glass Co. Ltd) was deposited onto Au-coated silicon chips by spin-coating at 2,000 rpm with a ramp rate of 500 rpm. Then, they were cured at 60°C for 30 min. The thickness was about 950 nm.

A hexadecanethiol (C16H34S, from Fluka) SAM was formed by immersing the Au-coated silicon chips in 1 mM hexadecanethiol (15.28 μl) in ethanolic solution (50 ml) sealed in a glass beaker placed in a fume hood for 12 h. Then the monolayer was thoroughly rinsed with ethanol and its thickness was measured at about 24 by a variable angle spectroscopic ellipsometer (WVASE32, J. A. Woollam Co. Inc.).

4.2 Experimental setup

Figure 3 schematically shows the contact angle measurement of EWOD in open air using a contact angle goniometer (model no. 100-00-220-S, Rame-hart Instrument Co.). 3-μl droplets were dispensed by a micropipette for each measurement to minimize the gravitational effects.
https://static-content.springer.com/image/art%3A10.1007%2Fs00542-009-0933-z/MediaObjects/542_2009_933_Fig3_HTML.gif
Fig. 3

Schematic diagram of the experimental setup for contact angle measurement of EWOD

A DC potential was applied between the Au electrode underneath the dielectric layer and a thin 25 μm diameter Au wire dipping into the droplet. Each measurement was repeated three times using fresh droplets to eliminate the evaporation effect and trapped charges. The droplet images were recorded by a camera to calculate contact angle and to analyze the shape of the droplet using the software of DROPimage. An amp meter was used to monitor the dielectric breakdown. Figure 4 shows the images of water droplets on these dielectrics in the presence and absence of different applied potentials, respectively.
https://static-content.springer.com/image/art%3A10.1007%2Fs00542-009-0933-z/MediaObjects/542_2009_933_Fig4_HTML.gif
Fig. 4

Contact angle (water droplet) of dielectrics with and without applied potentials

5 Results and discussions

5.1 Contact angle dependence on applied potentials

Contact angle dependence upon the applied potential with water has been studied experimentally and Fig. 5 plots the contact angle variation of water on those sample dielectrics as a function of applied voltage. The theoretical predictions of this variation are obtained from Eq. 6 which can be directly derived from Eq. 1, and are plotted as solid lines in Fig. 5 for comparison.
https://static-content.springer.com/image/art%3A10.1007%2Fs00542-009-0933-z/MediaObjects/542_2009_933_Fig5_HTML.gif
Fig. 5

Contact angle as a function of applied potential for water on dielectrics Cytop™ (950 nm), parylene C (2.25 μm), SAMs (2.4 nm) and PDMS (38.2 μm)

$$ \theta = a\cos \left( {{\frac{{\varepsilon_{0} \varepsilon V^{2} }}{{2\gamma_{lv} d}}} + \cos \theta_{0} } \right) $$
(6)

In the derivation of this theoretical model, it is assumed that the substrate is homogenous and that liquid evaporation and gravitational effects are negligible. A desirable contact angle modulation of 40° (from 128° to 88°) on Parylene C required about 120 V potential to be applied. The contact angle stabilized at ~88° on the 2.25 μm-thick Parylene C sample. A small contact angle modulation of 16° (117° to 101°) on a 38.2-μm thick PDMS sample was obtained under a potential of about 140 V without observation of contact angle saturation. A moderate potential of 65 V was applied to achieve the contact angle variation of 35° (from 110° to 75°) with Cytop. Contact angle saturation was observed on this 0.95 μm-thick Cytop sample when the potential was beyond 65 V.

A contact angle variation of 40° (from 105° to 65°) with SAMs could be obtained when applying a potential as small as 7.5 V, beyond which the contact angle saturated. However, it was observed that its hydrophobicity degraded significantly and the surface became hydrophilic in 1–2 days. The instability and contact angle irreversibility make SAMs impossible to be used in the device. Hence, SAMs as a prime insulator was not considered for the rest of the contact angle measurements. In addition, it should be noted that the theoretical predication given by Eq. 6 is not applicable for nanoscale effects of SAMs, as reported by Moon et al. (2002).

The dependence of EWOD on polarity was measured using both saturated sodium chloride solution and water. The results for Cytop™ and Parylene C are plotted and compared with the theoretical predictions based on water from Eq. 6 (which are shown as solid lines) in Fig. 6a and b, respectively.
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Fig. 6

EWOD dependence on polarity with a Cytop™ and b Parylene C using water and saturated NaCl

It can be seen that the experimental results follow very well the trend predicated by Eq. 6. Each data point in Fig. 6 is the average of three separate measurements and standard deviations of 1° for Cytop (with water) and 1.6° for Parylene C (with water) were obtained respectively. Contact angle hysteresis of 1° and 3.6° were observed for Cytop and Parylene C, respectively. It was found that saturation of the contact angle occurred with both of these materials, which was independent of the potential polarity. The change in contact angle for Cytop™ and Parylene C on both water and NaCl solution was found to be independent of the polarity of the applied voltage, which is in agreement with literature2 and theoretical prediction.

5.2 The effect of dielectric thickness on contact angle variation

The effect of dielectric thickness on contact angle variation for a specified applied electrical potential was studied in a series of experiments using gold electrodes coated with dielectrics of varying thickness. These different thicknesses of Cytop and PDMS layers having different thickness were achieved by spin-coating silicon substrates from 2,000 to 5,000 rpm with a ramp rate of 500 rpm per second. Figure 7 shows the experimentally measured voltage required for modulation of the contact angle of Cytop and PDMS on water, as well as theoretically predicted values from Eq. 2.
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Fig. 7

The voltage required to generate a specific change in contact angle as a function of the dielectric thickness. The contact angle modulation is 35° for (a) Cytop and 16° for (b) PDMS

The contact angle variation for Cytop is from 110° to 75° (35° modulation) and it is from 117° to 101° (16° modulation) for PDMS. The standard deviation in the experimental data is about 1.33 V for Cytop and 1.18 V for PDMS, respectively. The results show good agreement with the theoretical predication up to the point where the contact angle saturates. Using the spin coating method, it is difficult to make PDMS thickness thinner than 10 μm because the maximum spinning speed (about 5,000–6,000 rpm) has been reached. The thickness of Cytop achieved is above 500 nm, which is more than the minimum thickness of 330 nm required to avoid dielectric breakdown.

5.3 Contact angle reversibility

Figure 8 shows the reversibility of the contact angle of a water droplet on a 700 nm thick Cytop™ film upon voltage actuation. Contact angle hysteresis varies from 3.2° to 4°, which is acceptable for EWOD devices. Other dielectrics tested (parylene C and PDMS) show uncompetitive and even poor reversibility of contact angle changes for both water and the NaCl solution. Figure 9 shows the corresponding experimental images of the reversible contact angle of water droplet in the experiment.
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Fig. 8

The reversibility of contact angle of water droplet on Cytop™ film (700 nm thick) with a contact angle hysteresis of 3.2°–4°

https://static-content.springer.com/image/art%3A10.1007%2Fs00542-009-0933-z/MediaObjects/542_2009_933_Fig9_HTML.jpg
Fig. 9

Experimental images showing the reversibility of contact angle of water droplet on Cytop™

Comparing the tested dielectrics, Parylene C requires a high driving voltage to obtain a desirable contact angle modulation of 40° and patterning it using common microfabrication processes is difficult. PDMS presents a small contact angle variation of only 16° under an extremely high potential of 140 V. Microstructures of PDMS are quite difficult to be photo-defined by dry/wet processes such as RIE etching. In contrast, a desirable contact angle modulation of Cytop (35°) can be obtained under a relatively low applied potential (65 V). Cytop is also relatively easy o pattern using dry etching processes.

6 Conclusions

It has been shown by this study that the dielectric layer has the most dominant influence on EWOD performance compared to the other materials in the system. Based on the Young–Lippmann equation, the dielectric breakdown voltage and electrowetting actuation voltage as functions of dielectric thickness have been obtained for several typical dielectric materials. The minimum dielectric thickness was obtained from the intersection of these two functions. The contact angle measurements were made for droplets of water and NaCl solution on several dielectric materials. Comparisons with respect to contact angle tunability and reversibility upon electrowetting actuation were made. The effect of the thickness of the dielectric layer on contact angle variations was also studied. The results show that among the several dielectric materials studied, Cytop™ is the best dielectric material for EWOD with a good combination of mechanical, electrical, chemical and thermal properties. Cytop also exhibits good reversibility of contact angle and patternability with dry etching process. Its favorable surface properties make it suitable for commercial EWOD systems and devices. It has been successfully incorporated into a system for pumping liquids based on the EWOD effect, which has been internationally patented (Dharmatilleke and Liu 2006). The exploiting of powerful dielectric materials for EWOD will always be challenging for researchers to make breakthroughs in real applications.

Acknowledgments

This work was supported by Institute of Materials Research and Engineering (IMRE) and Agency for Science, Technology and Research (A*STAR).

Copyright information

© Springer-Verlag 2009