Continuous electrowetting via electrochemical diodes

We describe a novel method for droplet transport combining electrowetting on dielectric (EWOD) and the diode-like behavior of valve metals to achieve unique actuation performance. While traditional EWOD droplet transport requires switching of voltage between multiple electrodes, our method, which we term continuous rectified electrowetting, utilizes a simple single electrode and a DC voltage to move a 50 μl droplet 28 mm with velocities up to 32 mm s(-1).     

A control method for steering individual particles inside liquid droplets actuated by electrowetting

An algorithm is developed that allows steering of individual particles inside electrowetting systems by control of actuators already present in these systems. Particles are steered by creating time varying flow fields that carry the particles along their desired trajectories. Results are demonstrated using an experimentally validated model developed in ref. . We show that the current UCLA electro-wetting-on-dielectric (EWOD) system contains enough control authority to steer a single particle along arbitrary trajectories and to steer two particles, at once, along simple paths. Particle steering is limited by contact angle saturation and by the small number of actuators that are available to actuate the flow in practical electrowetting systems.     

Model description of contact angles in electrowetting on dielectric layers

The electrowetting on dielectric (EWOD) technique has considerable potential for microfluidic and biomedical applications. The Lippmann-Young model based on the force balance concept has long been used to predict the contact angles of droplets under electrowetting. However, recent experimental evidence has indicated that this model fails to provide accurate predictions of the lower contact angles associated with saturation conditions at higher electric potentials. Hence, the study simulates the internal flow in an actuated droplet and treats it as stagnation-point flow. This kinetic energy is then taken into consideration while calculating the contact angles using an energy balance model. The energy of an actuated droplet is contributed by the combination of the side surface tension energy, the base tension energy, the dielectric energy, and the kinetic energy when deriving the energy balance model. Consequently, the new energy balance model modifies the Lippmann-Young equation, thereby providing enhanced reasonable predictions of the droplet contact angle across the higher electric potential where the contact angles are close to the saturated condition.     

Superior performance of multilayered fluoropolymer films in low voltage electrowetting

The requirement for low operational voltage in electrowetting devices, met using thin dielectrics, is usually connected with serious material failure issues. Dielectric breakdown (visible as electrolysis) is frequently evident slightly beyond the onset of the contact angle saturation. Here, plasma-enhanced chemical vapor deposition (PECVD) is used to deposit thin fluorocarbon films prior to the spin-coating of Teflon® amorphous fluoropolymer. The resulting multilayered hydrophobic top coating improves the electrowetting performance of the stack, by showing high resistance to dielectric breakdown at high applied voltages and for continuous long term application of DC and AC voltage. Leakage current measurements during electrowetting experiments with the proposed composite coating showed that current remains fairly constant at consecutive electrowetting tests in contrast to plain Teflon® coating in which material degradation is evident by a progressive increase in the leakage current after multiple electrowetting tests. Since the proposed composite coating demonstrates increased resistance to material failure and to dielectric breakdown even at thin configurations, its integration in electrowetting devices may impact their reliability, robustness, and lifetime.     

Wetting and electrowetting properties of carbon nanotube templated parylene films

In this study, we compared the wetting and electrowetting properties of a planar parylene (poly(p-xylylene)) film to those of a nanostructured parylene film. To generate the nanostructured film, we used an aligned array of multiwalled carbon nanotubes as a template; a thin coating of parylene was deposited on the nanotube template to generate a parylene film with a nanoscale roughness structure. Static contact angle measurements indicated a very significant increase in the water contact angle from approximately 73 degrees for planar parylene to approximately 110 degrees for the nanotemplated parylene. In addition, we performed electrowetting experiments to dynamically tune the contact angle by application of electric potential. Interestingly, the flat parylene film showed contact angle saturation at an applied voltage of approximately 40 V, while the nanotemplated parylene film did not experience saturation in the contact angle response even for voltages up to 80 V. These results show that engineering a nanoscale roughness structure to a polymer film results in significant changes to the wetting and electrowetting properties of the polymer.     

Micro air bubble manipulation by electrowetting on dielectric (EWOD): transporting, splitting, merging and eliminating of bubbles

This paper describes various manipulations of micro air bubbles using electrowetting on dielectric (EWOD): transporting, splitting, merging and eliminating. First, in order to understand the response of bubbles to EWOD, the contact angle modulation is measured in a capped air bubble and confirmed to be in good agreement with the Lippmann-Young equation until saturation. Based on the contact angle measurement, testing devices for the bubble manipulations are designed and fabricated. Sequential activations of patterned electrodes generate continuous bubble transportations. Bubble splitting is successfully realized by activating a single electrode positioned in the middle of bubble base. However, it is found that there are criteria that make splitting possible only in certain conditions. For successful splitting, smaller channel gap, larger bubble size, wider splitting electrode and/or larger contact angle changes by EWOD are preferred. These criteria are verified by a series of experiments as well as a static analysis. Bubble merging is achieved by moving bubbles towards each other in two different channel configurations: (1) channel I, where bubbles are in contact with the bottom channel plate only, and (2) channel II, where bubbles in contact with the top as well as bottom channel plates. Furthermore, eliminating a bubble to the ambient air is accomplished. All the bubble manipulation techniques may provide a versatile integrated platform not only to manipulate micro objects by utilizing micro bubbles as micro carriers, but also to enable a discrete bubble-based gas analysis system.     

On the performance of thermostable electrowetting agents

Electrowetting of ionic liquids (ILs) in different alkane ambient and at different temperatures were studied under different AC voltages. The performance of IL‐based electrowetting at elevated temperatures and that at room temperature was compared. It turns out that the electrowetting of ILs at elevated temperatures resulted in faster response and lower operation voltage. Furthermore, the relationship between the structure of the ion and the contact angle change was specifically investigated and theoretically explained. Finally, the properties of IL‐based electrowetting in different alkane ambient were studied. It was demonstrated that the long length of the alkyl chain guaranteed a smaller contact angle under the same applied voltage. All the experimental results showed attractive features involving wide operating temperatures and in particular high stability, fast response, and good reversibility at elevated temperatures. The performance of IL‐based electrowetting could be further improved through an optimal choice of an ambient phase or a rational design and synthesis of ionic liquids. Copyright © 2011 John Wiley & Sons, Ltd.     

Dynamics of droplet motion under electrowetting actuation

The static shape of droplets under electrowetting actuation is well understood. The steady-state shape of the droplet is obtained on the basis of the balance of surface tension and electrowetting forces, and the change in the apparent contact angle is well characterized by the Young-Lippmann equation. However, the transient droplet shape behavior when a voltage is suddenly applied across a droplet has received less attention. Additional dynamic frictional forces are at play during this transient process. We present a model to predict this transient behavior of the droplet shape under electrowetting actuation. The droplet shape is modeled using the volume of fluid method. The electrowetting and dynamic frictional forces are included as an effective dynamic contact angle through a force balance at the contact line. The model is used to predict the transient behavior of water droplets on smooth hydrophobic surfaces under electrowetting actuation. The predictions of the transient behavior of droplet shape and contact radius are in excellent agreement with our experimental measurements. The internal fluid motion is explained, and the droplet motion is shown to initiate from the contact line. An approximate mathematical model is also developed to understand the physics of the droplet motion and to describe the overall droplet motion and the contact line velocities.     

Low voltage electrowetting using thin fluoroploymer films

This paper investigates the nonideal electrowetting behavior of thin fluoroploymer films. Results are presented for a three phase system consisting of: (1) an aqueous water droplet containing sodium dodecyl sulfate (SDS), (2) phosphorous-doped silicon topped with SiO2 and an amorphous fluoroploymer (aFP) insulating top layer on which the droplet is situated, and (3) a dodecane oil that surrounds the droplet. The presented measurements indicate that the electrowetting equation is valid down to a 6 nm thick aFP film on a 11 nm thick SiO2. At this dielectric thickness, a remarkable contact angle change of over 100 degrees can be achieved with an applied voltage less than 3 V across the system. The data also shows that for this water/surfactant/oil system, contact angle saturation is independent of the electric field, and is reached when the surface energy of the solid-water interface approaches zero.     

Frequency-dependent electromechanics of aqueous liquids: electrowetting and dielectrophoresis

Electrowetting on dielectric and dielectrophoretic electromechanical mechanisms dominate microfluidic actuation in the low- and high-frequency limits, respectively. The frequency-dependent relationship between these two mechanisms has been clarified by the Maxwell stress tensor and a simple RC circuit model. In this paper, we report extensive height-of-rise measurements obtained with vertical, parallel, dielectrically coated electrodes to test this relationship using deionized water and solutions containing sugar and salt. For DC and AC (20 Hz to 20 kHz) voltage magnitudes up to approximately 150 V-rms, the data are highly reproducible and, within experimental error, consistent with the square-law predictions of the model. Eventually as voltage is increased, a saturation phenomenon is observed which exhibits a weak dependence on frequency and is probably correlated to contact angle saturation.     

Irreversible electrowetting on thin fluoropolymer films

A study was conducted to investigate electrowetting reversibility associated with repeated voltage actuations for an aqueous droplet situated on a silicon dioxide insulator coated with an amorphous fluoropolymer film ranging in thickness from 20 to 80 nm. The experimental results indicate that irreversible trapped charge may occur at the aqueous-solid interface, giving rise to contact angle relaxation. The accumulation of trapped charge was found to be related to the applied electric field intensity and the breakdown strength of the fluoropolymer. On the basis of the data, an empirical model was developed to estimate the amount of trapped charge in the fluoropolymer as well as the voltage threshold for the onset of irreversible electrowetting.     

Impact of pinning of the triple contact line on electrowetting performance

Pinning of the triple contact line adversely affects electrowetting on dielectric. Electrowetting response of substrates with contact angle hysteresis ranging from 1° to 30° has been characterized, and the results are interpreted within the framework of electromechanics corrected for pinning. The relationship between contact angle hysteresis, threshold potential for liquid actuation, and electrowetting hysteresis is quantified. Our results demonstrate that a modified electrowetting equation, based on balance of forces (including the pinning forces) acting on the triple contact line and on the drop, describes the electrowetting response of substrates with significant contact angle hysteresis. Finally, the surface properties of PDMS Sylgard 184 were found to be influenced by the electric field.     

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.     

Dual-frequency electrowetting: application to drop evaporation gauging within a digital microsystem

This paper addresses a method to estimate the size of a sessile drop and to measure its evaporation kinetics by making use of both Michelson interferometry and coplanar electrowetting. From a high-frequency electrowetting voltage, the contact angle of the sessile droplet is monitored to permanently obtain a half-liquid sphere, thus complying perfectly with the drop evaporation theory based on a constant contact angle (Bexon, R.; Picknett, R. J. Colloid Interface Sci. 1977, 61, 336-350). Low-frequency modulation of the electrowetting actuation is also applied to cause droplet shape oscillations and capillary resonance. Interferometry allows us to measure a time-dependent capillary spectrum and, in particular, the shift in natural frequencies induced by drop evaporation. Consequently, diffusive kinetics of drop evaporation can be properly estimated, as demonstrated. Because of coplanar electrode configuration, our methodology can be integrated in open and covered microsystems, such as digital lab-on-a-chip devices.     

Nanoscale electrowetting effects observed by using friction force microscopy

We report the study of electrowetting (EW) effects under strong electric field on poly(methyl methacrylate) (PMMA) surface by using friction force microscopy (FFM). The friction force dependence on the electric field at nanometer scale can be closely related to electrowetting process based on the fact that at this scale frictional behavior is highly affected by capillary phenomena. By measuring the frictional signal between a conductive atomic force microscopy (AFM) tip and the PMMA surface, the ideal EW region (Young-Lippmann equation) and the EW saturation were identified. The change in the interfacial contact between the tip and the PMMA surface with the electric field strength is closely associated with the transition from the ideal EW region to the EW saturation. In addition, a reduction of the friction coefficient was observed when increasing the applied electric field in the ideal EW region.     

Reversible electrowetting of vertically aligned superhydrophobic carbon nanofibers

Reversible electrostatically induced wetting (electrowetting) of vertically aligned superhydrophobic carbon nanofibers has been investigated. Carbon nanofibers on a 5 x 5 microm pitch were grown on Si substrates, electrically insulated with a conformal dielectric, and hydrophobized with fluoropolymer. This nanostructured scaffold exhibited superhydrophobic behavior for saline (theta approximately 160 degrees). Electrowetting induced a contact angle reduction to theta approximately 100 degrees. Competitive two-liquid (dodecane/saline) electrowetting exhibited reversibility on the same nanostructured scaffold. Without applied bias, ultra-fine-point tip (approximately 25 nm radius) nanofibers result in effectively zero capacitance with the overlying saline layer. Complete electrowetting of the substrate is confirmed as capacitance values increase by several orders of magnitude with increased wetting. These results demonstrate the applicability of reversible electrowetting on nanostructured scaffolds and use of nanofabricated structures that can be integrated with various micro- and nanoelectronic technologies.     

Size-selective sliding of sessile drops on a slightly inclined plane using low-frequency AC electrowetting

When placed on an inclined solid plane, drops often stick to the solid surface due to pinning forces caused by contact angle hysteresis. When the drop size or the plane's incline angle is small, the drop is difficult to slide due to a decrease in gravitational force. Here we demonstrate that small drops (0.4-9 μL) on a slightly inclined plane (~12°, Teflon and parylene-C surface) can be mobilized through patterned electrodes by applying low-frequency ac electrowetting under 400 Hz (110-180 V(rms)), which has a mechanism different from that of the high-frequency ac method that induces sliding by reducing contact angle hysteresis. We attribute the sliding motion of our method to a combination of contact angle hysteresis and interfacial oscillation driven by ac electrowetting instead of the minimization of contact angle hysteresis at a high frequency. We investigated the effects of ac frequency on the sliding motion and terminal sliding of drops; the terminal sliding velocity is greatest at resonance frequency. Varying the electrowetting number (0.21-0.56) at a fixed frequency (40 Hz) for 5 μL drops, we found an empirical relationship between the electrowetting number and the terminal sliding velocity. Using the relationship between the drop size and ac frequency, we can selectively slide drops of a specific size or merge two drops along an inclined plane. This simple method will help with constructing microfluidic platforms with sorting, merging, transporting, and mixing of drops without a programmable control of electrical signals. Also, this method has a potential in heat transfer applications because heat removal capacity can be enhanced significantly through drop oscillation.     

Electrowetting — From statics to dynamics

More than one century ago, Lippmann found that capillary forces can be effectively controlled by external electrostatic forces. As a simple example, by applying a voltage between a conducting liquid droplet and the surface it is sitting on we are able to adjust the wetting angle of the drop. Since Lippmann's findings, electrocapillary phenomena – or electrowetting – have developed into a series of tools for manipulating microdroplets on solid surfaces, or small amounts of liquids in capillaries for microfluidic applications. In this article, we briefly review some recent progress of fundamental understanding of electrowetting and address some still unsolved issues. Specifically, we focus on static and dynamic electrowetting. In static electrowetting, we discuss some basic phenomena found in DC and AC electrowetting, and some theories about the origin of contact angle saturation. In dynamic electrowetting, we introduce some studies about this rather recent area. At last, we address some other capillary phenomena governed by electrostatics and we give an outlook that might stimulate further investigations on electrowetting.     

Shape Oscillation of a drop in ac electrowetting

A sessile drop oscillates when an ac voltage is applied in electrowetting. The oscillation results from the time-varying electrical force concentrated on the three-phase contact line. Little is known about the feature of drop oscillation in electrowetting. In the present work, the drop oscillations are observed systematically, and a theoretical model is developed to analyze the oscillation. It is revealed that resonance occurs at certain frequencies and the oscillation pattern is significantly dependent on the applied ac frequencies. The domain perturbation method is used to derive the shape-mode equations under the assumptions of a weak viscous effect and small drop deformation. The electrical force concentrated on the three-phase contact line is approximated as a delta function, which is decomposed and substituted into each shape-mode equation as a forcing term. The theoretical results for the shape and frequency responses are compared with experimental results, which shows qualitative agreement.