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This paper studies and characterizes electrowetting-on-dielectric (EWOD) actuations on coplanar electrodes with an electrode-free cover plate or no cover plate. By arranging driving and reference electrodes on one plate, such an EWOD configuration can accommodate more sensing mechanisms from above and thus allows increased flexibility for system development. Various coplanar electrodes are tested for contact angle changes by EWOD with a focus on the effect of the percentage gap between electrodes and are found to be in good agreement with a simple analytical model. The droplet-moving devices demonstrate the successful moving, cutting and merging of droplets (~0.1 µl) in a parallel-plate configuration, i.e., between the driving plate with coplanar electrodes and the passive plate with no electrode. EWOD actuation in single plate configuration, i.e. no cover plate, is also demonstrated, adding additional flexibility for the system design.

Electrowetting (EW) induced droplet motion has been explored in the past decade in view of its promising applications in the field of microfluidics. This paper demonstrates the potential of energy-based analyses for modeling the performance of EW-based fluid actuation systems. Analyses based on system energy minimization offer simplified modeling tools to predict the overall performance of EW systems while circumventing the need to model the numerous complexities in the system. An analytical model is developed to estimate the actuation force on a droplet moving between two electrodes. The origins and contributions of various components of the actuation force are analyzed. The effects of dielectric parameters, electrode layout, droplet geometry and shape are discussed with the objective of maximizing the actuation force. The actuation force model is combined with semi-analytical models for predicting the forces opposing droplet motion to develop a model that predicts transient EW-induced droplet motion. Parametric results are obtained to evaluate the importance of operating voltage, fluid properties and droplet geometry on droplet motion.

We propose a novel method for fabricating high-aspect-ratio micro-/nano-structures by dielectrophoresis-electrocapillary force (DEP-ECF)-driven UV-imprinting. The force of DEP-ECF, acting on an air–liquid interface and an air–liquid–solid three-phase contact line, is generated by applying voltage between an electrically conductive mold and a substrate, and tends to pull the dielectric liquid (a UV-curable pre-polymer) into the mold micro-cavities. The existence of DEP-ECF is explained theoretically and demonstrated experimentally by the electrically induced reduction of the contact angle. Furthermore, DEP-ECF is proven to play a critical role in forcing the polymer to fill into the mold cavities by the real-time observation of the dynamic filling process. Using the DEP-ECF-driven UV-imprinting process, high-aspect-ratio polymer micro-/nano-structures (more than 10:1) are fabricated with high consistency. This patterning method can overcome the drawbacks of the mechanically induced mold deformation and position shift in conventional imprinting lithography and maximize the pattern uniformity which is usually poor in capillary force lithography.

Electrowetting-on-dielectric (EWOD) lab-on-a-chip systems have already proven their potential within a broad range of bio-assays. Nevertheless, research on the analytical performance of those systems is limited, yet crucial for a further breakthrough in the diagnostic field. Therefore, this paper presents the intrinsic possibilities of an EWOD lab-on-a-chip as a versatile platform for homogeneous and heterogeneous bio-assays with high analytical performance. Both droplet dispensing and splitting cause variations in droplet size, thereby directly influencing the assay's performance. The extent to which they influence the performance is assessed by a theoretical sensitivity analysis, which allows the definition of a basic framework for the reduction of droplet size variability. Taking advantage of the optimized droplet manipulations, both homogeneous and heterogeneous bio-assays are implemented in the EWOD lab-on-a-chip to demonstrate the analytical capabilities and versatility of the device. A fully on-chip enzymatic assay is realized with high analytical performance. It demonstrates the promising capabilities of an EWOD lab-on-a-chip in food-related and medical applications, such as nutritional and blood analyses. Further, a magnetic bio-assay for IgE detection using superparamagnetic nanoparticles is presented whereby the nanoparticles are used as solid carriers during the bio-assay. Crucial elements are the precise manipulation of the superparamagnetic nanoparticles with respect to dispensing and separation. Although the principle of using nano-carriers is demonstrated for protein detection, it can be easily extended to a broader range of bio-related applications like DNA sensing. In heterogeneous bio-assays the chip surface is actively involved during the execution of the bio-assay. Through immobilization of specific biological compounds like DNA, proteins and cells a reactive chip surface is realized, which enhances the bio-assay performance. To demonstrate this potential, on-chip adhesion islands are fabricated to immobilize MCF-7 human breast cancer cells. Viability studies are performed to assess the functionalization efficiency.

Most electrowetting and liquid crystal optical devices are created by standard planar microfabrication. Arrayed electrowetting microprisms are a newer approach that offers unique performance, but which requires a challenging non-planar microfabrication process. This paper reviews a full description of a scalable fabrication process for an ~1500 element array of ~150 µm size electrowetting microprisms. The description includes creation of high aspect-ratio sidewalls, using a conventional i-line mask aligner to vertically pattern electrodes, conformal hydrophobic dielectric deposition, self-assembled and volume-controlled liquid dosing and module sealing. Also presented is a theoretical model which explores the resolution limits for vertically patterned electrodes. In addition to creating a first-generation fabrication process for arrayed electrowetting microprisms, this work may be further useful to investigators seeking methods of forming 3D arrayed electro-optic, electro-chemical or electro-mechanical devices.

We present a low-voltage microgripper utilizing a liquid droplet to pick up and release micro-objects. Lifting force is generated by a liquid bridge formed between the gripper surface and an object. Electrowetting was used to dynamically change the capillary lifting forces and enable easy object release. The driving voltage was applied to a pair of coplanar interdigitated electrodes, eliminating the need for an electrode on top of the droplet and thus significantly facilitating object manipulation. A barium strontium titanate insulation layer was used to lower the driving voltage. Experiments indicated that the lifting forces can be as high as 213 µN at a driving voltage of 28 V. Experiments also demonstrated a low-voltage, low power consumption soft microgripper by picking up and releasing micro glass beads.

This paper describes on-chip microbubble creation and elimination using an electrolytic process integrated with electrowetting on dielectrics (EWOD) actuations. These bubble operation units are developed for the microbubble-based micro-object manipulator in which EWOD-actuated microbubbles can be used not only as micro-object carriers but also as flow amplifiers using ultrasonic excitations. By applying voltages between the anode and cathode, microbubbles of oxygen, hydrogen and their mixture ranging from several microns to several hundred microns in diameter are on-chip created electrolytically. EWOD actuation is used to detach and transport the bubbles from the creation site. On-chip bubble elimination is accomplished by using the reverse electrolytic reaction. A platinum electrode, which acts as a catalyst in the reverse electrolytic reaction, substantially increases the elimination rate of microbubbles, by an order of magnitude, as compared to a Teflon-covered Si surface. The catalytic reaction is effective in eliminating hydrogen, oxygen and mixture bubbles. Interestingly, the Teflon-coated Pt electrodes show a higher elimination rate than the bare Pt electrode. This is attributed to a larger contact area with the Teflon-covered electrode (hydrophobic surface) than with the bare Pt electrode (hydrophilic surface). For the Teflon-covered electrodes, the bubble elimination rate increases with thinner Teflon coating. Integrated bubble operations—creation, transportation and elimination—are realized on a single chip in which coating of the Pt electrode with a Teflon layer allows transporting of bubbles into the elimination site. Finally, as a proof of concept for the micro-object manipulator, it is demonstrated that EWOD-actuated bubbles can push micron- and millimeter-sized objects and release them at a different place. (Some figures in this article are in colour only in the electronic version)

An automatic ultramicro-volume DNA ligation process using a coplanar electrode type of electrowetting-on-dielectric (EWOD) microfluidic system was designed for economy of reagent. The droplets, containing DNA, a ligation enzyme and a multi-salt reaction buffer, served to complete the ligation using a developed EWOD system. Droplets of ultramicro volume (0.3 µL) were successfully generated from reservoirs between one plate with coplanar electrodes and another plate with a hydrophobic surface free of electrodes. In one successful cloning, the total usage of reagents in an ultramicro-volume EWOD chip was 2.1 µL; no volume was wasted, in comparison with 85% waste with the standard protocol and 80% waste with a free-cover coplanar EWOD chip. The results also show that the entire process was accomplished without damage to the chip surface and without biomaterial annulment. With a design consisting of electrode pathways in a flexible pattern and multiplex reservoirs, an EWOD digital microfluidic system with coplanar electrodes would be improved as an efficient parallel DNA-cloning system in the construction of an artificial library or expression library.

This paper reports the development and characterization of a planar liquid lens based on electrowetting. The working concept of electrowetting two immiscible liquids is demonstrated with measurement and characterization of contact angles with regard to externally applied electric voltages. Consequently, a planar liquid lens is designed and implemented based on this competitive electrowetting. A droplet of silicone oil confined in an aqueous solution (1% KCl) works as a liquid lens. Electrowetting then controls the shape of the confined silicone oil and the focal length of the liquid lens varies depending upon an applied dc voltage. A unique feature of this lens design is the double-ring planar electrodes beneath the hydrophobic substrate. While an outer ring electrode provides an initial boundary for the silicone oil droplet, an inner ring works as the actuation electrode for the lens. Further, the planar electrodes, instead of vertical or out-of-plane wall electrodes, facilitate the integration of liquid lenses into microfluidic systems. With the voltage applied in the range of 50–250 V, the confined silicone oil droplet changed its shape and the optical magnification of a 3 mm-diameter liquid lens was clearly demonstrated. Moreover, focal lengths of liquid lenses with diameters of 2 mm, 3 mm and 4 mm were characterized, respectively. The obtained results suggest that a larger lens diameter yields a longer focal length and a wider range of focal length change in response to voltage. The demonstrated liquid lens has a simple structure and is easy to fabricate.

This paper describes droplet manipulation and microparticle sampling where droplets driven by the electrowetting-on-dielectric (EWOD) principle are transported on perforated microfilter membranes and pick up microparticles in their path. Three designs of microfilter membranes that have different hole shapes (rounded rectangle versus circle holes), sizes (≥6 µm) and opening area ratios (5, 9 and 20%) are microfabricated and tested along with a commercial membrane (rounded rectangle holes of 60 µm × 128 µm, 17% opening area ratio). All the tested perforated membranes are embedded with a linear array of electrodes for EWOD actuation. Reversible EWOD actuations and droplet transportations are successfully achieved on the perforated filter membranes. Particle sampling is examined against glass particles (8 µm diameter and ~14° contact angle) and polystyrene particles (8 µm diameter and ~66° contact angle). It is demonstrated that as droplets are moved on the microfilter surfaces by EWOD actuation, they efficiently pick up the microparticles in their path, showing high sampling efficiencies: over 95% for the glass particles and over 85% for the polystyrene particles. This particle sampling method uses a small liquid volume (microliters or smaller) and has the capability of fully automatic handling. Thus, it is expected to be highly compatible and easily integrated with lab-on-a-chip systems for follow-up biological and chemical analyses.

Growth of surface instabilities leading to periodic oil patterns in electrowetting display pixels has been experimentally observed. In the electrowetting display pixels, an electrical potential ranging between 0 V and 80 V is applied among a water electrode, dielectric layers consisting of an oil film, fluoropolymer, Parylene C and a lower electrode. As predicted by spinodal dewetting theory, application of electromechanical pressure to the oil film causes the film to dewet into a periodic arrangement of oil droplets. The separation distance, or wavelength, for the dewetted oil droplets decreases with increasing voltage or with decreasing oil film thickness. Various test voltages of 20–80 V and oil thicknesses values of 3.3–8.9 µm were shown to experimentally generate dewetting wavelengths ranging from 10's to 100's of µm. An increased number of oil droplets inside the display pixel decreases the maximum display transmission or reflection proportional to the cube root of the number of droplets. This work therefore provides a means to further understand oil film dewetting in electrowetting display pixels and explains the implications of the dewetting pattern on display optical performance.