Electrowetting Research Articles

An electrowetting on dielectrics based lab-on-a-chip utilizing an integrated high fundamental frequency quartz crystal resonator as a biosensor

We demonstrate the operation of an Electro Wetting on Dielectrics (EWOD) hybrid lab-on-a-chip system by utilizing a High Fundamental Frequency (HFF; 50MHz) Quartz Crystal Microbalance (QCM) resonator as masssensitive sensor. In a first experiment we have tested the reversible formation of a phosphor-lipid monolayer out of an aqueous buffer suspension of phospholipid vesicles onto the integrated sensor, coated by a Self-Assembled Monolayer (SAM) of 1-octadecanethiol on a gold-covered surface. The altered mass load resulted in a shift of the resonance frequencies. In the second experiment, the formation of a protein multilayer composed of streptavidin and biotinylated immunoglobulin G was monitored. The original resonance frequency was recovered by the complete lipid removal by the detergent octyl-D-glucopyranoside. Using these sample applications, we were able to demonstrate and verify the feasibility of our prototype combining a mass-sensitive QCM sensor with digital microfluidics based on EWOD.

Reversible Electrowetting on Superhydrophobic Double-Nanotextured Surfaces

The paper reports on wetting, electrowetting (EW), and systematic contact angle hysteresis measurements after electrowetting of superhydrophobic silicon nanowire surfaces (NWs). The surfaces consist of C4F8-coated silicon nanowires grown on Si/SiO2 substrate. Different surfaces modulating (i) the dielectric layer thickness and (ii) the nanotexturation were investigated in this study. It was found that the superhydrophobic NWs display different EW behaviors according to their double nanotexturation with varying droplet impalement levels. Some surfaces exhibited a total reversibility to EW with no impalement (contact angle variation of 35+/-2 degrees at 190 VTRMS with deionized water), whereas other surfaces showed nonreversible behavior to EW with partial droplet impalement. A scenario is proposed to explain the unique properties of these surfaces.

Preventing the Cassie−Wenzel Transition Using Surfaces with Noncommunicating Roughness Elements

Control and switching of liquid droplet states on artificially structured surfaces have significant applications in the field of microfluidics. The present work introduces the concept of using structured surfaces consisting of noncommunicating roughness elements to prevent the transition of a droplet from the Cassie to the Wenzel state. The use of noncommunicating roughness elements leads to a confinement of the medium under the droplet in its Cassie state. Transition to the Wenzel state on such surfaces requires expulsion of this confined medium, which offers significantly increased resistance to the Wenzel transition unlike surfaces consisting of communicating roughness elements. This enhances the robustness of the Cassie state and significantly minimizes the possibility of the Cassie-Wenzel transition. In the present work, the resistance of a surface to the Wenzel transition is measured in terms of the electrowetting (EW) voltage required to trigger this transition. It is seen that surfaces with noncommunicating roughness elements (cratered surfaces) require significantly higher voltages to trigger the Wenzel transition than corresponding surfaces with communicating roughness elements. The findings from the present work also indicate that EW-induced droplet morphology control characteristics show a strong dependence on the nature of the roughness elements (communicating versus noncommunicating). Different aspects of droplet morphology and EW-induced state transition control on surfaces with noncommunicating roughness elements are analyzed; it is seen that such surfaces offer significant possibilities for the development of robust superhydrophobic surfaces.

Electromechanical model for actuating liquids in a two-plate droplet microfluidic device

Both conducting and insulating liquids can be actuated in two-plate droplet ("digital") microfluidic devices. Droplet movement is accomplished by applying a voltage across electrodes patterned beneath the dielectric-coated top and bottom plates. This report presents a general electromechanical model for calculating the forces on insulating and conducting liquids in two-plate devices. The devices are modeled as an equivalent circuit in which the dielectric layers and ambient medium (air or oil) are described as capacitors, while the liquid being actuated is described as a resistor and capacitor in parallel. The experimental variables are the thickness and dielectric constant of each layer in the device, the gap between plates, the applied voltage and frequency, and the conductivity of the liquid. The model has been used to calculate the total force acting on droplets of liquids that have been studied experimentally, and to explain the relative ease with which liquids of different conductivities can be actuated. The contributions of the electrowetting (EW) and dielectrophoretic (DEP) forces to droplet actuation have also been calculated. While for conductive liquids the EW force dominates, for dielectric liquids, both DEP and EW contribute, and the DEP force may dominate. The general utility of the model is that it can be used to predict the operating conditions needed to actuate particular liquids in devices of known geometry, and to optimize the design and operating conditions to enable movement of virtually any liquid.

T1101-1-1 表面張力を利用した液滴の三次元輸送に関する研究(マイクロナノ理工学:nmからmmまでの表面制御とその応用(1))

This paper describes a new method for three-dimensional transportation of liquid droplets using EWOD (Electro Wetting on Dielectric). This method makes use of surface tension and electrostatic forces, which become dominant in microscale devices. The droplet transported on a vertical or upside down surface is affected by gravity force. In order to reduce the gravity effect, volume of water was reduced and shape and size of electrodes were changed. A water droplet was transported on a inclined plate, and the angle of inclination was changed from 10° to 360° by every 10°. The results show that water droplet can be transported on all inclined surfaces. Horizontal transportation of a droplet on a vertical surface was also made successfully.

Electrowetting of Complex Fluids: Perspectives for Rheometry on Chip

We explore the possibilities of electrowetting (EW) as a tool to assess the elastic properties of aqueous jellifying materials present in the form of a small droplet on a hydrophobic substrate. We monitored the EW response of aqueous solutions of gelatin (2-10 wt %) in ambient oil for various temperatures (8-40 degrees C) below and above the gel point. Whereas the drops remained approximately spherical cap-shaped under all conditions, the voltage-induced reduction of the contact angle became progressively less pronounced upon entering the gel state at lower temperatures. We modeled the decrease in contact angle by minimizing the total energy of the drops consisting of interfacial energies, electrostatic energy, and the elastic energy due to the deformation of the drop, which was taken into account in a modified Hertz model. This allowed fitting the data and extracting the elastic modulus G, which were found to agree well with macroscopic storage moduli G' obtained with oscillatory shear rheometry. These results show that EW can be used as a tool for characterizing soft materials with the elastic moduli ranging (at least) from 10 to 1000 Pa. Our observations also create interesting perspectives for performing in situ rheological measurement inside microfluidic chips.

Extreme Resistance of Superhydrophobic Surfaces to Impalement: Reversible Electrowetting Related to the Impacting/Bouncing Drop Test

The paper reports on the comparison of the wetting properties of superhydrophobic silicon nanowires (NWs), using drop impact impalement and electrowetting (EW) experiments. A correlation between the resistance to impalement on both EW and drop impact is shown. From the results, it is evident that when increasing the length and density of NWs (i) the thresholds for drop impact and EW irreversibility increase and (ii) the contact-angle hysteresis after impalement decreases. This suggests that the structure of the NW network could allow for partial impalement, hence preserving the reversibility, and that EW acts the same way as an external pressure. The most robust of our surfaces shows a threshold to impalement higher than 35 kPa, while most of the superhydrophobic surfaces tested so far have impalement thresholds smaller than 10 kPa.

Electrowetting-Based Microdrop Tensiometer

We performed electrowetting (EW) contact angle measurements to determine the interfacial tension between aqueous drops laden with various inorganic and organic solutes and various ambient oils. Using low frequency AC voltage, we obtained interfacial tensions from 5 to 72 mJ/m 2, in close agreement with macroscopic tensiometry for drop volumes between 20 and 2000 nL. In addition to the conventional EW geometry, we demonstrate the possibility of performing "contact-less" measurements without any loss of accuracy using interdigitated coplanar electrodes.

Electrowetting-Based Control of Droplet Transition and Morphology on Artificially Microstructured Surfaces

Electrowetting (EW) has recently been demonstrated as a powerful tool for controlling droplet morphology on smooth and artificially structured surfaces. The present work involves a systematic experimental investigation of the influence of electrowetting in determining and altering the state of a static droplet resting on an artificially microstructured surface. Extensive experimentation is carried out to benchmark a previously developed energy-minimization-based model that analyzed the influence of interfacial energies, surface roughness parameters, and electric fields in determining the apparent contact angle of a droplet in the Cassie and Wenzel states under the influence of an EW voltage. The EW voltage required to trigger a transition from the Cassie state to the Wenzel state is experimentally determined for surfaces having a wide range of surface parameters (surface roughness and fraction of surface area covered with pillars). The reversibility of the Cassie-Wenzel transition upon the removal of the EW voltage is also quantified and analyzed. The experimental results from the present work form the basis for the design of surfaces that enable dynamic control of droplet morphology. A significant finding from the present work is that nonconservative dissipative forces have a significant influence in opposing fluid flow inside the microstructured surface that inhibits reversibility of the Cassie-Wenzel transition. The artificially structured surfaces considered in this work have microscale roughness feature sizes that permits direct visual observation of EW-induced Cassie-Wenzel droplet transition; this is the first reported visual confirmation of EW-induced droplet state transition.

Energy minimization-based analysis of electrowetting for microelectronics cooling applications

Electrowetting (EW)-induced droplet motion has been studied over the last decade in view of its promising applications in the field of microfluidics. The objective of the present work is to analyze the physics underlying two specific EW-based applications for microelectronics thermal management. The first of these involves heat absorption by liquid droplets moving on the surface of a chip under EW actuation. Droplet motion between two flat plates under the influence of an electrowetting voltage is analyzed. An energy minimization framework is employed to predict the actuation force on a droplet. This framework, in combination with semi-analytical models for the forces opposing droplet motion, is used to develop a model that predicts transient EW-induced droplet motion. The second application is targeted at hot-spot thermal management and relies on the control of droplet states on artificially structured surfaces through an applied EW voltage. The influence of an electrowetting voltage in determining and altering the state of a static droplet resting on a rough surface is analyzed. An energy minimization-based modeling approach reveals the influence of interfacial energies, surface roughness parameters and electric fields in determining the apparent contact angle of a droplet in the Cassie and Wenzel states under the influence of an EW voltage. The model is used to establish preliminary criteria to design rough surfaces for use in the hot-spot mitigation application. The concept of an electrically tunable thermal resistance switch for hot-spot cooling applications is introduced and analyzed.

Electrowetting --A versatile tool for controlling microdrop generation

Integrating insulator-covered electrodes into a microfluidic flow focusing device (FFD) we demonstrate enhanced flexibility and control of the flow of two non-miscible liquids based on electrowetting (EW). In the parameters space, determined by liquid inlet pressures, we identify a specific region where drops can only be generated and addressed via EW. In this regime we show that the size distribution and the frequency of drop generation can be controlled by the applied voltage and the width of voltage pulses. Moreover it turns out that with EW the drop size and the frequency can be tuned independently. Finally we show that the same drop generation phenomena can also be observed in the presence of surfactants.

2112 Integration of closed microfluidics and open droplet actuation on a single lab-on-a-chip device

This paper reports on the extraction of solution from a PDMS microchannel using two forms of liquid actuation by electrical methods. Electro Wetting on Dielectric (EWOD) and Liquid Dielectrophoresis (LDEP). On the Liquid Dielectrophoresis (LDEP) device, upon application of an AC voltage on a pair of parallel electrodes beneath the PDMS microchannel, a liquid finger protrudes from the microchannel exit and advances along the gap between the electrodes. After the voltage is removed, the liquid finger breaks due to capillarity-driven instability and picolitre-sized droplets are formed at specific periodic bumps.

Electrowetting-Based Control of Static Droplet States on Rough Surfaces

Electrowetting (EW) is a powerful tool to control fluid motion at the microscale and has promising applications in the field of microfluidics. The present work analyzes the influence of an electrowetting voltage in determining and altering the state of a static droplet resting on a rough surface. An energy-minimization-based modeling approach is used to analyze the influence of interfacial energies, surface roughness parameters, and electric fields in determining the apparent contact angle of a droplet in the Cassie and Wenzel states under the influence of an EW voltage. The energy-minimization-based approach is also used to analyze the Cassie-Wenzel transition under the influence of an EW voltage and estimate the energy barrier to transition. The results obtained show that EW is a powerful tool to alter the relative stabilities of the Cassie and Wenzel states and enable dynamic control of droplet morphology on rough surfaces. The versatility and generalized nature of the present modeling approach is highlighted by application to the prediction of the contact angle of a droplet on an electrowetted rough surface consisting of a dielectric layer of nonuniform thickness.

An energy-based model for electrowetting-induced droplet actuation

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.

The Movement of Micro Droplet with the Effects of Dielectric Layer and Hydrophobic Surface Treatment with R.F. Atmospheric Plasma in EWOD Structure

This study is about how to lower the driving voltage that enables to move the micro droplet by the EWOD (Electro Wetting On Dielectric) mechanism. EWOD is well known that it is used µ -TAS digital micro fluidics system. As voltages are applied to the device which is fabricated with dielectric layer between electrode and micro droplet, the hydrophobic surface is changed into the hydrophilic surface by its electrical property. EWOD induces the movement of micro droplet with reducing contact angle of micro droplet. Because the driving voltage depends on the dielectric constant of dielectric layer, it can be reduced by increase of dielectric constant. Typically, driving voltage of these devices was 30V∼100V and Teflon film was coated to provide hydrophobic surface. This high voltage induces many problems. In previous study, we used Ta2O5 as the dielectric layer and driving voltage was 23V that reduced 24 percent compared with SiO2. In this study, we used BZN (Bi2O3-ZnO-Nb2O5) layer which had high dielectric constant. It operated the just on 12V. And micro droplet was moved within 1s on 14V. It reduced the voltage until 35 percents compared with Ta2O5 and 50 percents compared with SiO2. The movement of micro droplet within 1s was achieved with BZN (ferroelectrics) just on 14V. Teflon film which is coated to provide hydrophobic surface is a very difficult to control accurate concentration, and unnecessary processes which uses metal as a protect layer in patterning should be added. So instead of Teflon coating, surface was treated to hydrophobic with R.F. atmospheric plasma.

On the influence of surfactants in electrowetting

On the influence of surfactants in electrowetting

High-transmission electrowetting light valves

High-efficiency spatial light modulation has been demonstrated for transmissive electrowetting (EW) light valves (ELVs). The ELV structure consists of a competitive oil/water-on-dielectric EW cell fabricated on an optically transparent substrate. ELVs are configured as display devices by attaching a diffuse backlight powered by white converted InGaN light emitting diodes. The oil film contains ∼1wt.% nonpolar organic chromophores which absorb with near-neutral optical density across the visible light spectrum. Using the EW effect, spatial light modulation is achieved as the water layer locally displaces the oil film. The transmissivity of the cell can be modulated from ∼5% (zero bias) to >80% (∼30V). ELV switching speed depends on cell size, typically ∼10–100ms for 1 and 3mm2 cells. Additional optical enhancement can decrease the off-state ELV transmissivity to <1%.

Intense switchable fluorescence in light wave coupled electrowetting devices

Switchable fluorescence has been obtained from light wave coupled (LWC) electrowetting (EW) devices fabricated on an optical waveguide substrate. The LWC device structure contains a polar water component and a nonpolar oil component that compete for placement on a hydrophobic surface under the influence of an applied electric field. The oil film contains organic lumophores that fluoresce intense red (608nm), green (503nm), and blue (433nm) light with ∼90% quantum efficiency when excited by violet light. Violet InGaN light-emitting diodes (LEDs) couple ∼405nm excitation light into the waveguide substrate. EW of the water layer displaces the fluorescent oil film such that it is either optically coupled to, or decoupled from, the underlying waveguide. The output luminance can be modulated from >100cd∕m2to<5cd∕m2 as a dc voltage ranging from 0to−24V is applied to the water layer. Maximum luminance of 15×30 arrays of the devices may exceed ∼500cd∕m2 by simply increasing the output of the InGaN LEDs.

Electrowetting (EW)-Based Valve Combined with Hydrophilic Teflon Microfluidic Guidance in Controlling Continuous Fluid Flow

Electrowetting (EW)-based techniques have been widely used in manipulating discrete liquid. However, few articles discussed the controlling of continuous fluid flow by using EW-based techniques. In this paper, an EW-based valve combined with plasma-modified Teflon surface, which serves as a microfluidic guidance, in controlling continuous fluid flow has been demonstrated. The plasma-modified Teflon surface is firstly demonstrated for confining continuous fluid flow. The EW-based microfluidic device possesses the functions of a valve and a microchannel without complex moving parts and grooved microchannels. The quantitative characteristics of the EW-based valve are also studied. Propylene carbonate (PC) is firstly demonstrated as the working liquid in the EW-based device because of its applications in parallel oligonucleotide synthesis. It is found that lower valve actuation voltage reduces the deterioration of the valve and improves the valve stability.

Electrowetting and electrowetting-on-dielectric for microscale liquid handling

This paper deals with electrowetting (EW) and electrowetting-on-dielectric (EWOD) principles applied to microfluidic devices. EW and EWOD are principles that can control wettability of liquids on solid surfaces using electric potential. While EW is controlling wettability of a certain electrolyte on a metal electrode by varying electric energy across the electrical double layer (EDL), EWOD applies to virtually any aqueous liquid by varying electric energy across the thin dielectric film between the liquid and conducting substrate. These driving mechanisms have many advantages. By electrically changing the wettability of each of the electrode patterns on a surface, a liquid on these electrodes can be shaped and driven along the active electrodes, making microfluidics extremely simple both for device fabrication and operation. It is also worth noting that, driven by surface tension, the mechanism becomes more effective as the size of the device becomes smaller. This paper describes fundamental concepts and the proof-of-concept experiments, modeling and design, microfabrication processes, and initial testing results for the microfluidic devices based on the EW and EWOD principles.