Article pubs.acs.org/Langmuir
Insulating Material Requirements for Low-Power-Consumption Electrowetting-Based Liquid Lenses Stéphanie Chevalliot,*,† Géraldine Malet,† Herbert Keppner,‡ and Bruno Berge† †
Varioptic, 24B rue Jean Baldassini, 69007 Lyon, France Haute Ecole Arc Ingénierie, NeodeEplaturesGrise 17, CH-2300 La Chaux-de-Fonds, Switzerland
‡
ABSTRACT: Insulating materials from the parylene family were investigated for use in low-power-consumption electrowetting-based liquid lenses. It was shown that for DC-driven operations, parylene C leads to hysteresis, regardless of the presence of a hydrophobic top coat. This hysteresis was attributed to the non-negligible time needed to reach a stable contact angle, due to charge injection and finite conductivity of the material. It was further demonstrated that by using materials with better insulating properties, such as parylene HT and VT4, satisfactory results can be obtained under DC voltages, reaching a low contact angle hysteresis of below 0.2°. We propose a simplified model that takes into account the injection of charges from both sides of the insulating material (the liquid side and the electrode side), showing that electrowetting response can be both increased and decreased.
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INTRODUCTION Electrowetting on dielectrics (EWOD) started as a research area in the 1990s. A lot of aspects in this field have been discussed in scientific papers since that time.1−3 More recently, EWOD has appeared in several practical applications that have been commercialized, namely, microfluidic actuators4 and liquid optical lenses for high-end industrial cameras.5 In these recent applications, the EWOD device works closely to a source of electric power so that the tens of milliwatts needed to produce liquid actuation is easily available. There are a number of situations where it would be interesting to benefit from such microsystems based on EWOD and no or little electric power is available, such as medical implant systems, dormant systems that have to be awakened in rare events, and so forth. It is thus necessary to achieve liquid actuation using extremely low electric power. In EWOD, lowering the electric power needed to produce actuation depends essentially on the frequency of the carrier wave of the driving voltage. Usually AC voltage is used for actuation because it prevents charges from being injected from the liquid into the dielectric. AC EWOD then uses approximately tens of milliwatts, including the power required by the voltage converter IC. The natural way of decreasing this amount is to go to DC EWOD, which then allows power consumption in the tens of microwatt range. Charge injection in EWOD has been the subject of several studies.6−8 Verheijen and Prins6 showed that ions injected from the liquid into the top layer of the insulator stack decreased the EWOD response. Other groups7,8 studied the chemistry of the conducting liquid and of the ions present in the electrolyte, sometimes leading to a finite charge transport through the whole dielectric thickness. In addition, charge injection has © XXXX American Chemical Society
been proposed as a possible mechanism responsible for EWOD saturation.6,9,10 Several scientific studies about EWOD were performed under DC voltages, but none of them reached the requirements of a high-accuracy optical application. This paper investigates new aspects of working with EWOD under DC voltages. It first presents existing material solutions and their problems related to charge injection when working in DC. This paper introduces a model that stresses that EWOD response can also be increased by charge injection, when the injected charges come from the underneath electrode. This paper addresses how materials can be chosen to achieve a high EWOD accuracy (high reproducibility of the contact angle vs voltage curves, no hysteresis) and fast response (no long time lags, etc.). It is shown how charge injection can be entirely suppressed to achieve an “engineering-ready” EWOD system.
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EXPERIMENTAL DETAILS
Boron-doped Si ⟨1-0-0⟩ wafers (sheet resistance of 1−25 Ω·cm, purchased from Nova Electronic Materials, Flower Mound, TX, USA) and commercial parts from Varioptic (conical cavities made of metal, which are used to manufacture Varioptic liquid lenses A316) were used as substrates for the experiments. Polymers from the parylene family were investigated as the main dielectric: either parylene C or parylene HT or parylene VT4. Because these materials present different dielectric constants, layers of different thicknesses were deposited to obtain a similar contact angle change at a given voltage and for a given liquid couple. A layer of 5.6 μm parylene C was vapordeposited onto the substrates using a Kisco DACS-0600V S diX Received: September 8, 2016 Revised: November 11, 2016 Published: November 22, 2016 A
DOI: 10.1021/acs.langmuir.6b03237 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
θ(V)] as a function of the square of the applied voltage (V2). Linear curves are expected, according to the Young’s equation1
automatic coating system. The samples were sent to Specialty Coating Systems (Woking, Surrey, UK) to be coated with 4.4 μm parylene HT. Parylene VT4 deposition (4.8 μm layer) was carried out by Comelec (La Chaux-de-Fonds, Switzerland). Several fluoropolymer top coats were studied. Cytop CTL-809M (Asahi glass, Tokyo, Japan) was diluted in Cytop CT-Solv180 to form a 3 wt % solution. Fluoropel 1601V (Cytonix, Beltsville, MD, USA) was used as purchased (1 wt % solution). The fluoropolymers were deposited by spin coating on top of parylene to yield ∼50 nm layers after annealing (at 180 °C for 30 min for flat substrates and at 90 °C for 2 h for liquid lens parts). The insulating fluid was composed of silicone and germane oils (proprietary formulation). The conducting liquid consisted of a saline solution of CaCl2 in a mixture of 20 wt % deionized water and various polar nonaqueous solvents (proprietary formulation). A salt concentration of 1 wt % was used for the flat substrate experiments to ensure that sufficient charge carriers were present for the EWOD effect to take place under an AC bias. To avoid any disturbance coming either from dielectric loss inside of the insulating material or from the finite conductivity of the electrolyte, the quality factor Q (Q = 1/D, D being the loss-dissipation factor) was closely monitored. It was kept above 10 for each experiment (exact values not reported in the Experimental Results section). For liquid lens experiments, the salt concentration of the conducting liquid was reduced to 0.01 wt % because the lenses were driven only by DC voltages. For all experimental results, the contact angle is the one formed by the insulating fluid on the substrate in the presence of a conducting liquid, as shown in Figure 1a.
(1 − cos θ(V )) = (1 − cos θY ) +
CV 2 2γic
(1)
where θY is the contact angle without the applied voltage, C represents the surface capacitance of the system, and γic is the interfacial tension between the insulating and conducting fluids. These plots enable us to directly and easily compare the EWOD response under positive and negative voltages because they take into consideration the square of the voltage applied instead of the voltage. Moreover, they present the advantage to better underline hysteresis. The optical performances of the liquid lenses were measured using a standard cycle test. The conducting liquid was electrically biased with the lens cap with which it is in direct electrical contact. Thus, the voltage sign convention is the same as the one defined for the flat sample experiments. DC positive and DC negative voltages were applied in 2 V/s step increments from 0 to ±60 V and back to 0 V. The optical power (1/f) as a function of the applied voltage was recorded using an optical bench based on a Shack−Hartmann wave front analyzer (Imagine Optic, Orsay, France).
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EXPERIMENTAL RESULTS Flat Substrate Results. The EWOD contact angle stability was first investigated on bare parylene C. The experiment was repeated on different samples. As explained in the experimental details section, the quality factor Q was above 10 for all samples. Figure 2 presents the results of two representative samples. A large contact angle modulation is obtained as soon as a voltage of 60 V AC 1 kHz is applied. The subsequent contact angle measurements do not show any evolution. The different values of the equilibrium contact angle between the two samples (90° vs 85°) can be explained by a slight difference in the sample thicknesses and by the measurement uncertainty. For a DC positive voltage, the response is more complex. A fast transition is first observed (from 45° to 105° in Figure 2a and from 45° to 90° in Figure 2b). It is followed by a “slow lag” reaching 115° and 97° with a typical decay time of several seconds as shown in Figure 2a and Figure 2b, respectively. In this article, slow lag refers to the long response time of EWOD, typically longer than a few seconds, which makes any practical use of this system difficult. The strong difference in contact angles reached depending on the sample cannot simply be explained by the measurement conditions. Rather, it reveals a real reproducibility issue. At equilibrium, the EWOD response attained under a DC positive bias is higher than the one under an AC voltage for both samples. Under a DC negative voltage, in addition to an initial slow lag from approximately 110 to 120°, a strong decrease in the contact angle over time is observed for both samples (down to approximately 105° after 25 s). The angle still keeps on decreasing after 1 min of voltage application (down to ∼95°) and does not seem to reach an equilibrium value (data not displayed for clarity). Figure 3 presents the results obtained with a ∼50 nm Cytop layer deposited on top of parylene C. Fluoropel as a top coat displayed a similar behavior (results not shown). With fluorinated top coats, reproducible results between samples were obtained. Therefore, only one typical plot is reported. As was observed with bare parylene C, a lag in the EWOD response is present under DC voltages, whereas an immediate stable contact angle is reached under AC bias. However, with fluorinated top coats, no contact angle decrease is observed under a DC negative voltage and an equilibrium state is reached. The EWOD response attained under DC voltages is
Figure 1. (a) Schematic of the experimental setup. The contact angle (θ) formed by a drop of oil on the substrate in the presence of a conducting liquid was measured. (b) Representation of the configuration adopted for the theoretical model derivation. The contact angle formed by a drop of the conducting fluid on the top coat in a surrounding oil medium was considered.
EWOD contact angle stability measurements were performed on flat substrates under a constant voltage. The samples were immersed in the conducting fluid. A drop of oil (∼1 μL) was deposited using a syringe. The metal tip of the syringe was used as an electrode and was connected to an Agilent 4284A precision LCR meter coupled with a Tegam 2340 high-voltage amplifier. The wafer was held at electrical ground. A 20 V AC (1 kHz, sine wave, rms values reported for all experiments) voltage was applied to the conducting liquid. After ∼5 s, the voltage was switched to 60 V and held for at least 25 s. After the AC data were taken, the same voltage ramp was applied to the same sample with DC positive and DC negative biases. The evolution of the oil contact angle as a function of time was recorded using a Krüss DSA10 drop-shape analysis system. For low contact angles (typically below 5°), the software had trouble detecting drop shapes automatically. By starting the experiments at 20 V (contact angles higher than 15°) rather than at 0 V, this issue was avoided. EWOD cycle experiments were further carried out on fresh flat samples. An AC voltage was applied in 10 V step increments from 20 to 60 V and back to 20 V. The same voltage ramp was subsequently performed on the same sample with DC positive and DC negative biases. The oil contact angle as a function of the applied voltage [θ(V)] was recorded. The results are displayed as a plot of [1 − cos B
DOI: 10.1021/acs.langmuir.6b03237 Langmuir XXXX, XXX, XXX−XXX
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Langmuir
higher than the one under an AC bias (equilibrium contact angle of ∼75° for AC, as opposed to ∼90° for both DC polarities). Comparing Figures 2 and 3, it can be seen that for AC bias, the contact angles displayed on Cytop are lower than those on bare parylene C. This results from a better natural wetting of the liquids used on Cytop. The EWOD contact angle stability measurements of Cytopcoated parylene HT and parylene VT4 samples are presented in Figure 4. Regardless of the type of voltage, stable contact angles
Figure 2. Contact angle as a function of time for two different samples of bare parylene C.
Figure 4. Contact angle as a function of time for Cytop-coated (a) parylene HT and (b) parylene VT4.
are reached immediately after the application of voltage. Moreover, a similar EWOD response is achieved for all types of biases. EWOD cycle experiments were next performed using a Cytop-coated parylene HT sample. As can be seen in Figure 5, a very low hysteresis (less than 2°) is observed for all types of biases. Moreover, a similar EWOD response is obtained. For instance, contact angles of 68°, 71°, and 67° are reached after the application of ±60 V AC, DC positive, and DC negative voltages, respectively. Liquid Lens Results. Liquid lenses with bare parylene C do not display satisfactory optical performances under DC biases
Figure 3. Contact angle as a function of time for Cytop-coated parylene C.
C
DOI: 10.1021/acs.langmuir.6b03237 Langmuir XXXX, XXX, XXX−XXX
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Figure 5. EWOD curves under different types of voltages for Cytopcoated parylene HT.
(high hysteresis), regardless of the voltage polarity. The addition of a top coat (fluorinated polymer) leads to lower but still non-negligible hysteresis (not shown). Figure 6 presents the typical optical performance of Cytopcoated parylene HT liquid lenses. DC voltage was applied from 0 to 60 V and back to 0 V. The same cycle was then performed under a DC negative voltage. A very low optical power hysteresis (difference of 1/f up and 1/f down at a given voltage) is obtained for both DC positive and DC negative voltages (
