Wyatt C. Nelson

Droplet Actuation by Electrowetting-on-Dielectric (EWOD): A Review

Abstract This paper reviews publications that have fortified our understanding of the electrowetting-on-dielectric (EWOD) actuation mechanism. Over the last decade, growing interest in EWOD has led to a wide range of scientific and technological investigations motivated by its applicability in microfluidics, especially for droplet-based optical and lab-on-a-chip systems. At this point in time, we believe that it is helpful to summarize the observations, insights, and modeling techniques that have led to the current picture showing how forces act on liquid droplets and how droplets respond in EWOD microfluidic devices. We discuss the basic physics of EWOD and explain the mechanical response of a droplet using free-body diagrams. It is our hope that this review will inspire new research approaches and help design useful devices.

Electrowetting on dielectric-based microfluidics for integrated lipid bilayer formation and measurement

We present a microfluidic platform for the formation and electrical measurement of lipid bilayer membranes. Using electrowetting on dielectric (EWOD), two or more aqueous droplets surrounded by a lipid-containing organic phase were manipulated into contact to form a lipid bilayer at their interface. Thin-film Ag/AgCl electrodes integrated into the device enabled electrical measurement of membrane formation and the incorporation of gramicidin channels of two bilayers in parallel.

Incubated protein reduction and digestion on an electrowetting-on-dielectric digital microfluidic chip for MALDI-MS.

Localized heating of droplets on an electrowetting-on-dielectric (EWOD) chip has been implemented and shown to accelerate trypsin digestion reaction rates, sample drying, and matrix crystallization for matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS). Achieving this involved extending the functionality of previous EWOD droplet-based techniques by developing a multifunctional electrode with closed-loop temperature control, while minimizing overall system complexity and addressing challenges associated with rapid evaporation. For the EWOD chip design, we discuss the performance of multifunctional surface electrodes for actuation, localized Joule heating, and thermistic temperature sensing. Furthermore, a hydrophilic pattern is formed in the multifunctional electrode to control the location of an evaporating droplet on the electrode. To demonstrate the capabilities and limitations of this technique, we performed three experiments and measured the results using MALDI-MS: (i) insulin disulfide reductions in dithiothreitol (DTT) over a range of heater temperatures (22-70 °C) to show how reaction rates can be affected by thermal control, (ii) insulin disulfide reductions at 130 °C in dimethyl sulfoxide (DMSO) to demonstrate a reaction in a high boiling point solvent, and (iii) tryptic digestions of cytochrome c at 22 and 40 °C to show that heated droplets can yield reasonably higher peptide sequence coverage than unheated droplets. Although they do not decouple the effects of changing temperatures and concentrations, these experiments verified that thermal cycling by EWOD electrodes accelerates reaction rates in liquid droplets in air.

Dynamic Contact Angles and Hysteresis under Electrowetting-on-Dielectric

By designing and implementing a new experimental method, we have measured the dynamic advancing and receding contact angles and the resulting hysteresis of droplets under electrowetting-on-dielectric (EWOD). Measurements were obtained over wide ranges of applied EWOD voltages, or electrowetting numbers (0 ≤ Ew ≤ 0.9), and droplet sliding speeds, or capillary numbers (1.4 × 10(-5) ≤ Ca ≤ 6.9 × 10(-3)). If Ew or Ca is low, dynamic contact angle hysteresis is not affected much by the EWOD voltage or the sliding speed; that is, the hysteresis increases by less than 50% with a 2 order-of-magnitude increase in sliding speed when Ca < 10(-3). If both Ew and Ca are high, however, the hysteresis increases with either the EWOD voltage or the sliding speed. Stick-slip oscillations were observed at Ew > 0.4. Data are interpreted with simplified hydrodynamic (Cox-Voinov) and molecular-kinetic theory (MKT) models; the Cox-Voinov model captures the trend of the data, but it yields unreasonable fitting parameters. MKT fitting parameters associated with the advancing contact line are reasonable, but a lack of symmetry indicates that a more intricate model is required.

A miniature capillary breakup extensional rheometer by electrostatically assisted generation of liquid filaments

A micromachined chip capable of generating liquid microfilaments has been developed for a miniature version of the Capillary Breakup Extensional Rheometer (CaBER®). The proposed system is exceptionally simple and compact because liquid samples are actuated by voltages administered on-chip, which therefore requires only electrical connections (rather than a linear motor, an integral part of the CaBER®). Since chip features are photolithographically defined, the miniature rheometer can handle sub-microlitre samples. Following the CaBER®, we show that a commercial LED micrometer effectively measures diameters of filaments generated by the electrowetting-on-dielectric (EWOD) forces. Since negligible electric fields are sustained within the liquid far away from the measurement region, the applied EWOD voltage does not influence tested material properties. Through breakup experiments using a wide range of Newtonian and complex fluids (e.g., glycerol, xanthan gum, dilute polystyrene, and dilute solutions of various molecular weight polyethylene oxide) we demonstrate a versatile testing platform for scarce and precious samples such as biochemical fluids and novel materials. Measured Newtonian and complex dynamics agree well with published theories and experiments.

Monolithic Fabrication of EWOD Chips for Picoliter Droplets

We report monolithic fabrication of parallel-plate electrowetting-on-dielectric (EWOD) chips for digital micro-fluidics of picoliter droplets. Instead of assembling a second substrate to form a top plate-the common practice with all previous parallel-plate EWOD chips-the top plate is surface micromachined as a transparent thin-film membrane that forms a monolithic cavity having a gap height on the order of micrometers with excellent accuracy and uniformity. The membrane is embedded with EWOD driving electrodes and confines droplets against the device substrate to perform digital microfluidic operations. Two main attributes of the monolithic architecture that distinguish it from tradition methods are: (i) it enables excellent control of droplet dimensions down to the micrometer scale, and (ii) it does not require the typical alignment and assembly steps. Basic device functions such as creation and splitting are verified by EWOD actuation of ~100 picoliter droplets surrounded by air or oil inside a 10 μm-high cavity. Additionally, flow focusing of droplets containing 5.3 μm beads demonstrates one example of the utilities afforded by monolithic fabrication.

A micro extensional filament rheometer enabled by EWOD

We present a miniature system for generating and measuring liquid microfilaments for capillary breakup rheometry. The key component is a chip that splits samples in open air, creating shear-free liquid threads that can be measured by optical micrometry. For testing polar samples, electrowetting-on-dielectric (EWOD) is used to induce spreading, which causes necking and capillary instability-driven breakup. Low-surface-tension samples spread spontaneously, and thus reach instability without EWOD. We use LED optical micrometry to measure inelastic and elastic microfilaments, and the results are consistent with capillary breakup theory and comparable to those obtained by established experimental methods.

An EWOD Droplet Microfluidic Chip with Integrated Local Temperature Control for Multiplex Proteomics

We present a microfluidic chip for multiplex thermal processing of discrete micro- and nano-liter sample droplets driven by electrowetting-on-dielectric (EWOD). The system features local temperature control integrated on chip in addition to the basic fluidic functionalities previously reported for EWOD devices [1]. Multifunctioning indium-tin-oxide (ITO) electrodes serve as optically transparent resistive heaters, temperature sensors, and EWOD actuation pads. With a 1 ¿L sample, heaters reach programmed set points in a few seconds and stabilize to ±1 °C. We demonstrate automated on-chip proteomics sample processing and direct characterization by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). The results, showing increased insulin disulfide reduction efficiency with heater temperatures from 22 to 130 °C, highlight the efficacy of the chip.

High pressure EWOD digital microfluidics

We are developing new electrowetting-on-dielectric (EWOD) digital microfluidic systems for operating at non-atmospheric conditions. The first generation is a compact pressure chamber with an electric feed-through, enabling EWOD operation within a gaseous medium of well-controlled pressure and composition. EWOD performance is insensitive to chamber pressure because the chip is of open-channel architecture. We demonstrate two different types of previously unachievable processes - (i) controlling evaporation rates of common solvents (water, methanol, acetonitrile) by adjusting the pressure of an inert gaseous medium (N2), and (ii) controlling the reaction rate of a solid-liquid-gas-phase reaction by adjusting the pressure of a gas-phase reagent (H2).

Resistance of droplets sliding by EWOD actuation

We report an experimental investigation of the resistive forces imparted on droplets actuated by electrowetting-on-dielectric (EWOD). While the advancing contact angle is always larger than the receding of a droplet moving on a solid surface under conventional actuation means, EWOD actuation is unique because the advancing angle is smaller than the receding. We recorded high-speed videos of sliding droplets under electrowetting to elucidate this seemingly paradoxical contact angle hysteresis. The results indicate a transition point at Ca≈ 10−3, after which the dominant resistance force becomes strongly speed-and voltage-dependent.