Showing papers on "Electrowetting published in 2017"
TL;DR: It is demonstrated how LM electrochemistry can be exploited to coalesce and separate droplets under moderate voltages of ~1–10 V, and this novel approach to droplet interaction can be explained with a theory that accounts for oxidation and reduction as well as fluidic instabilities.
Abstract: When immersed in an electrolyte, droplets of Ga-based liquid metal (LM) alloy can be manipulated in ways not possible with conventional electrocapillarity or electrowetting. This study demonstrates how LM electrochemistry can be exploited to coalesce and separate droplets under moderate voltages of ~1–10 V. This novel approach to droplet interaction can be explained with a theory that accounts for oxidation and reduction as well as fluidic instabilities. Based on simulations and experimental analysis, this study finds that droplet separation is governed by a unique limit-point instability that arises from gradients in bipolar electrochemical reactions that lead to gradients in interfacial tension. The LM coalescence and separation are used to create a field-programmable electrical switch. As with conventional relays or flip-flop latch circuits, the system can transition between bistable (separated or coalesced) states, making it useful for memory storage, logic, and shape-programmable circuitry using entirely liquids instead of solid-state materials.
100 citations
TL;DR: In this paper, a combined numerical and experimental approach was employed to study the electrowetting response of a droplet subject to both direct current (DC) and alternating current (AC) actuating signals.
57 citations
TL;DR: It is shown that a hydrophobic gate in a model β-barrel nanopore can be functionally opened by electrowetting at voltages that do not electroporate lipid bilayers, providing a framework for the design of functional nanopores with tailored gating functionality.
Abstract: It is desirable that nanopores that are components of biosensors are gated, i.e., capable of controllable switching between closed (impermeable) and open (permeable) states. A central hydrophobic barrier within a nanopore may act as a voltage-dependent gate via electrowetting, i.e., changes in nanopore surface wettability by application of an electric field. We use “computational electrophysiology” simulations to demonstrate and characterize electrowetting of a biomimetic nanopore containing a hydrophobic gate. We show that a hydrophobic gate in a model β-barrel nanopore can be functionally opened by electrowetting at voltages that do not electroporate lipid bilayers. During the process of electrowetting, voltage-induced alignment of water dipoles occurs within the hydrophobic gate region of the nanopore, with water entry preceding permeation of ions through the opened nanopore. When the ionic imbalance that generates a transbilayer potential is dissipated, water is expelled from the hydrophobic gate and ...
51 citations
TL;DR: In this paper, the authors used liquid metals in electrowetting on dielectric (EWOD) devices that resolve all of these challenges by judicious choice of novel electrode materials, dielectrics fluid, and device architecture.
Abstract: Electrowetting is well-established as a fluid manipulation technique in such areas as lab-on-a-chip, visible light optics, and displays, yet has seen far less implementation in the field of radio-frequency (RF) electronics and electromagnetics. This is primarily due to a lack of appropriate materials selection and control in these devices. Low loss RF conductive fluids such as room temperature liquid metals (i.e. Hg, EGaIn, Galinstan) are by far the leading choice of active material due to their superior electrical properties but require high actuating voltages due to their inherently high surface tensions (>400 mN m−1) which often lead to dielectric breakdown. While the toxicity of Hg encourages the pursuit of non-toxic alternatives such as gallium alloys, the native surface oxide formation often prohibits reliable device functionality. Additionally, traditional electrowetting architectures rely on lossy electrode materials which degrade RF transmission efficiencies and result in non-reversible material diffusion at the electrode/liquid metal contact. In this work, we report on approaches to utilize liquid metals in electrowetting on dielectric (EWOD) devices that resolve all of these challenges by judicious choice of novel electrode materials, dielectric fluid, and device architecture. A functional RF device, namely an electromagnetic polarizer, is demonstrated that can be activated on demand through EWOD and provides an average signal attenuation of 12.91 dB in the on state and 1.46 dB in the off state over the range of 8–9.2 GHz, with a switching speed of about 12 ms. These results can be further extended to other RF applications such as tunable antennas, transmission lines, and switchable metasurfaces.
47 citations
TL;DR: In this article, the authors report the forced spreading and subsequent retraction of droplets into liquid films in air on lubricant impregnated surfaces (also known as slippery liquid infused porous surfaces) where the contact line is completely mobile.
Abstract: Electric fields can be used to force a droplet to wet a solid surface using an applied voltage. However, significant hysteresis usually occurs associated with pinning forces at the contact line. Here, we report the forced spreading and subsequent retraction of droplets into liquid films in air on lubricant impregnated surfaces (also known as slippery liquid infused porous surfaces) where the contact line is completely mobile. We first confirm that we achieve a complete removal of hysteresis for the electrowetting of droplets above the saturation voltage. We then show that contact angle hysteresis can be reduced to less than 4° whilst retaining the ability to fully spread a droplet into a liquid film using an interface localized from liquid dielectrophoresis (dielectrowetting). In both cases, we find that the cosine of the contact angle has a quadratic dependence on applied voltage, consistent with previous theoretical expectations. Thus, our work demonstrates that fully reversible spreading encompassing a...
45 citations
TL;DR: Motivated by electrowetting-based flow control in nano-systems, water transport in graphene nano-channels is investigated as a function of the applied electric field, finding force-driven water flows under electric fields exhibit asymmetric velocity profiles and unequal slip lengths.
Abstract: Motivated by electrowetting-based flow control in nano-systems, water transport in graphene nano-channels is investigated as a function of the applied electric field. Molecular dynamics simulations are performed for deionized water confined in graphene nano-channels subjected to opposing surface charges, creating an electric field across the channel. Water molecules respond to the electric field by reorientation of their dipoles. Oxygen and hydrogen atoms in water face the anode and cathode, respectively, and hydrogen atoms get closer to the cathode compared to the oxygen atoms near the anode. These effects create asymmetric density distributions that increase with the applied electric field. Force-driven water flows under electric fields exhibit asymmetric velocity profiles and unequal slip lengths. Apparent viscosity of water increases and the slip length decreases with increased electric field, reducing the flow rate. Increasing the electric field above a threshold value freezes water at room temperature.
39 citations
TL;DR: In this paper, the authors developed an innovative low voltage reversible electrowetting of water droplet based on perfluorinated silane-modified slippery lubricant-infused porous surface (SLIPS) that is impregnated by silicone oil.
Abstract: In this study, we develop an innovative low voltage reversible electrowetting of water droplet based on perfluorinated silane-modified slippery lubricant-infused porous surface (SLIPS) that is impregnated by silicone oil. The electrowetting properties of SLIPS modified with and without 1H,1H,2H,2H-perfluorooctyltrichlorosilane (PFOTS) are investigated initially. After modifying the lubricated PTFE membrane with PFOTS, the driving voltage of electrowetting decreases sharply; the threshold voltage is only 5 V, which exhibits an immediate 15-fold decrease compared to that of the unmodified SLIPS. Subsequently, the electrowetting reversibility based on SLIPS modified with different concentrations of PFOTS solution is studied. Our results show that the initial contact angle increases with an increase in the concentration of PFOTS solution, but the electrowetting reversibility has an obvious depravation. The optimal concentration is about 0.03 wt%, in which the electrowetting hysteresis is less than 5°, and a large modulation can be achieved in the contact angle ranging from 107° to 60° with 60 V driving voltage in ambient air. On the basis of surface topography characterization, this low voltage electrowetting is attributed to the contact interface transformation from liquid–oil interface to the combination of liquid–oil and liquid–PFOTS interface. Therefore, the surfaces maintain low surface energy property of PFOTS and retain SLIPS property. In addition, a marked asymmetric contact angle variation with respect to voltage polarity on SLIPS modified with PFOTS is observed for the first time; the low voltage electrowetting occurs at positive voltage but does not occur at negative voltage. Our study provides a new pathway to achieve low voltage electrowetting without decreasing the thickness of the dielectric layer or increasing the dielectric constant.
37 citations
TL;DR: It is demonstrated that a small fraction of these ions permanently sticks to surfaces of Teflon AF and Cytop, two of the most common fluoropolymer materials, upon prolonged exposure to water.
Abstract: Fluoropolymers are widely used as coatings for their robustness, water-repellence, and chemical inertness. In contact with water, they are known to assume a negative surface charge, which is commonly attributed to adsorbed hydroxyl ions. Here, we demonstrate that a small fraction of these ions permanently sticks to surfaces of Teflon AF and Cytop, two of the most common fluoropolymer materials, upon prolonged exposure to water. Electrowetting measurements carried out after aging in water are used to quantify the density of ‘trapped’ charge. Values up to −0.07 and −0.2 mC m−2 are found for Teflon AF and for Cytop, respectively, at elevated pH. A similar charge trapping process is also observed upon aging in various non-aqueous polar liquids and in humid air. A careful analysis highlights the complementary nature of electrowetting and streaming potential measurements in quantifying interfacial energy and charge density. We discuss the possible mechanism of charge trapping and highlight the relevance of molecular scale processes for the long term stability and performance of fluoropolymer materials for applications in electrowetting and elsewhere.
37 citations
TL;DR: With the increase of the rise time of the input exponential driving voltage, the originally underdamped system response can be damped, enabling a smooth response from the lens, and the technique shows great promise for applications that require fast response times.
Abstract: Adaptive optical lenses based on the electrowetting principle are being rapidly implemented in many applications, such as microscopy, remote sensing, displays, and optical communication. To characterize the response of these electrowetting lenses, the dependence upon direct current (DC) driving voltage functions was investigated in a low-viscosity liquid system. Cylindrical lenses with inner diameters of 2.45 and 3.95 mm were used to characterize the dynamic behavior of the liquids under DC voltage electrowetting actuation. With the increase of the rise time of the input exponential driving voltage, the originally underdamped system response can be damped, enabling a smooth response from the lens. We experimentally determined the optimal rise times for the fastest response from the lenses. We have also performed numerical simulations of the lens actuation with input exponential driving voltage to understand the variation in the dynamics of the liquid–liquid interface with various input rise times. We furt...
35 citations
TL;DR: Wetting and electrowetting on corrugated substrates are studied experimentally and theoretically in this paper. But the experimental results indicated that liquids along the grooves propagate much faster than those in the direction vertical to the groove, whereas spreading in both directions obeys the same scaling law of l∼t4∕5.
Abstract: Wetting and electrowetting (EW) on corrugated substrates are studied experimentally and theoretically in this paper. On corrugated substrates, because of the anisotropy of surface morphology, the droplet shows an elliptical shape and the spreading velocities in different directions are different. Spreading of a droplet is usually controlled not only by the surface tensions but also by hemi-wicking. Our experimental results indicated that liquids along the grooves propagate much faster than those in the direction vertical to the grooves. However, spreading in both directions obeys the same scaling law of l∼t4∕5. EW on corrugated substrates reveals some differences with that on smooth surfaces. The change of contact angles with an applied voltage follows a linear relationship in two stages instead of the smooth curve on flat surfaces. There exists a critical voltage which divides the two stages. The transition of a droplet from the Cassie state to the Wenzel state on corrugated substrates was also discussed...
35 citations
TL;DR: In this paper, the dependence of droplet velocities on different electrode configuration in open wetting-on-dielectric (EWOD) system has been investigated and the electrowetting force is computed by analytical methods as well as by numerical methods and its effect on droplet velocity is studied in detail.
TL;DR: In this paper, two novel electrode design considerations to satisfy two very important aspects of EWOD operation are reported; highly consistent volume of generated droplets and highly improved accuracy in the generated droplet volume.
Abstract: This letter reports two novel electrode design considerations to satisfy two very important aspects of EWOD operation—(1) Highly consistent volume of generated droplets and (2) Highly improved accuracy in the generated droplet volume. Considering the design principles investigated two novel designs were proposed; L-junction electrode design to offer high throughput droplet generation and Y-junction electrode design to split a droplet very fast while maintaining equal volume of each part. Devices of novel designs were fabricated and tested, and the results are compared with those of conventional approach. It is demonstrated that inaccuracy and inconsistency of droplet volume dispensed in the device with novel electrode designs are as low as 0.17 and 0.10%, respectively, while those of conventional approach are 25 and 0.76%, respectively. The dispensing frequency is enhanced from 4 to 9 Hz by using the novel design.
TL;DR: By taking synthetic DNA targets and their corresponding molecular beacon probes as a model system, multiple potential pathogens that cause sepsis are detected rapidly on the 3D-blade-equipped DMF chip, rendering it as a promising tool for parallel diagnosis of diseases.
Abstract: Existing digital microfluidic (DMF) chips exploit the electrowetting on dielectric (EWOD) force to perform droplet splitting. However, the current splitting methods are not flexible and the volume of the droplets suffers from a large variation. Herein, we propose a DMF chip featuring a 3D microblade structure to enhance the droplet-splitting performance. By exploiting the EWOD force for shaping and manipulating the mother droplet, we obtain an average dividing error of <2% in the volume of the daughter droplets for a number of fluids such as deionized water, DNA solutions and DNA-protein mixtures. Customized droplet splitting ratios of up to 20 : 80 are achieved by positioning the blade at the appropriate position. Additionally, by fabricating multiple 3D microblades on one electrode, two to five uniform daughter droplets can be generated simultaneously. Finally, by taking synthetic DNA targets and their corresponding molecular beacon probes as a model system, multiple potential pathogens that cause sepsis are detected rapidly on the 3D-blade-equipped DMF chip, rendering it as a promising tool for parallel diagnosis of diseases.
TL;DR: In this paper, a unique morphological evolution of metal-organic inverse opals (Pb(NO3)2-poly(St-MMA-AA)) subjected to an electrowetting process is presented.
Abstract: This paper presents a unique morphological evolution of metal-organic inverse opals (Pb(NO3)2-poly(St-MMA-AA)) subjected to an electrowetting process. The morphology of the building blocks changes from interconnected pores to separated hollow spheres during the electrowetting process, accompanied by an unusual blue-shift of the stopband position and the decreased wettability of the film. This morphology evolution is attributed to the simultaneous collapse/reconstruction of the metal-organic frame owing to the partial dissolution of the metal salt and the interfacial assembly of the metal-organic coordination around the skeleton. The adjustable morphology can be developed as a novel and simple water-lithography approach for the creation of the photonic crystal pattern.
TL;DR: This work demonstrates axisymmetric and nonaxisymetric oscillations of compound droplets with the biological core of red blood cells, providing crucial first steps for promoting applications such as rapid efficient assays, mixing of biological fluids, and fluidic photonics on hysteresis-free surfaces.
Abstract: Manipulating droplets of biological fluids in an electrowetting on dielectric (EWOD)-based digital microfluidic platform is a significant challenge because of biofouling and surface contamination. This problem is often addressed by operating in an oil environment. We study an alternate configuration of sessile compound droplets having an aqueous core surrounded by a smaller oil shell. In contrast to the conventional EWOD platform, an open digital microfluidic platform enabled by the core–shell configuration will allow electrical, mechanical, or optical probes to get unrestricted access to the droplet, thus enabling highly flexible and dynamically reconfigurable lab-on-chip systems. Understanding droplet oscillations is essential as they are known to enhance mixing. To our knowledge, this is the first study of axisymmetric and nonaxisymmetric oscillations of compound droplets actuated using EWOD platforms. Mode shapes for both axisymmetric and nonaxisymmetric oscillations were studied and explained. Enhanc...
TL;DR: The utility of electrowetting on dielectric (EWOD) prisms as a lateral laser-scanning element integrated in a conventional 2PE microscope is demonstrated, which is the first such demonstration for EWOD prisms.
Abstract: Laser scanners are an integral part of high resolution biomedical imaging systems such as confocal or 2-photon excitation (2PE) microscopes. In this work, we demonstrate the utility of electrowetting on dielectric (EWOD) prisms as a lateral laser-scanning element integrated in a conventional 2PE microscope. To the best of our knowledge, this is the first such demonstration for EWOD prisms. EWOD devices provide a transmissive, low power consuming, and compact alternative to conventional adaptive optics, and hence this technology has tremendous potential. We demonstrate 2PE microscope imaging of cultured mouse hippocampal neurons with a FOV of 130 × 130 μm2 using EWOD prism scanning. In addition, we show simulations of the optical system with the EWOD prism, to evaluate the effect of propagating a Gaussian beam through the EWOD prism on the imaging quality. Based on the simulation results a beam size of 0.91 mm full width half max was chosen to conduct the imaging experiments, resulting in a numerical aperture of 0.17 of the imaging system.
TL;DR: In this paper, the authors investigated both theoretically and numerically a mechanism of bubble detachment by an electrowetting-driven interfacial wave, inspired by droplet control and manipulation via Electrowetting, and showed that an air bubble on a hydrophobic surface can be detached by the trough of such a wave.
Abstract: This article investigates both theoretically and numerically a novel mechanism of bubble detachment by an electrowetting-driven interfacial wave, inspired by droplet control and manipulation via electrowetting. Electrowetting-on-dielectric can be used to modulate the contact point movement at the water-air interface in a thin liquid film. Rapid oscillation of the contact line is achieved by a swift change of voltage under an AC signal. When disturbed with such contact angle changes, the interfacial wave between two immiscible fluids disrupts bubble dynamics. Numerical modeling reveals that an air bubble on a hydrophobic surface can be detached by the trough of such a wave. The frequency of the interfacial wave is twice the voltage frequency. A higher voltage frequency leads to a smaller amplitude and higher celerity of the wave, while a lower voltage frequency leads to a larger wave amplitude and lower celerity. The bubble can easily detach when the voltage frequency is 10 Hz. However, the bubble fails to...
TL;DR: In situ atomic force microscopy shows that the IL adopts a multilayered structure at the solid/IL interface, and a cation-rich layer is present in the innermost layer during cathodic polarization, which provides valuable insights for the interfacial nanostructure and the electrowetting of ILs.
Abstract: In this work, the interfacial nanostructure and electrowetting of ionic liquids having the same 1-ethyl-3-methylimidazolium cation ([EMIm]+) but different anions such as bis(trifluoromethylsulfonyl)imide (TFSI–), trifluoromethylsulfonate (TfO–), methylsulfonate (OMs–), acetate (OAc–), bis(fluorosulfonyl)imide (FSI–), dicyanamide (DCA–), and tris(pentafluorethyl)trifluorphosphat (FAP–) on bare metallic electrodes were investigated. In the investigated voltammetric potential regime, the contact angle versus voltage curve is asymmetric with respect to surface polarity. The electrowetting of the ILs occurs at negative potentials but does not occur at positive potentials. In situ atomic force microscopy (AFM) shows that the IL adopts a multilayered structure at the solid/IL interface, and a cation-rich layer is present in the innermost layer during cathodic polarization. The cations can change their orientation and propagate ahead of the three-phase contact line by diffusion, leading to further spreading on th...
TL;DR: The electrowetting-on-dielectric behavior of Cytop/Tantalum oxide (TaOx) bilayers is studied by measuring their response vs applied voltage and under prolonged periodic cycling, below and above the threshold voltage VT corresponding to the breakdown field for the oxide.
Abstract: The electrowetting-on-dielectric behavior of Cytop/Tantalum oxide (TaOx) bilayers is studied by measuring their response vs applied voltage and under prolonged periodic cycling, below and above the threshold voltage VT corresponding to the breakdown field for the oxide. TaOx exhibits symmetric solid state I–V characteristics, with electronic conduction dominated by Schottky, Poole–Frenkel emission; conduction is attributed to oxygen vacancies (6 × 1016 cm–3), resulting in large currents at low bias. Electrolyte/Metal Oxide/Metal I–V characteristics show oxide degradation at (<5 V) cathodic bias; anodic bias in contrast results in stable characteristics until reaching the anodization voltage, where the oxide thickens, leading eventually to breakdown and oxygen production at the electrode. Electrowetting angle vs applied voltage undergoes three different stages: a parabolic variation of contact angle (CA) with applied voltage, CA saturation, and rebound of the CA to higher values due to degradation of the p...
TL;DR: In this paper, a low-cost open EWOD-based dynamic sensing system is presented, which is capable of tracking multiple droplets on the single substrate in real time. But, it requires a feedback control system whose sensor can continuously monitor the droplet parameters like contact angle, position, size, and velocity.
Abstract: Electrowetting on dielectric (EWOD) based digital microfluidics (DMF) emerges as a modern paradigm for lab on a chip (LOC) application, as it offers advantages of portability, automation, higher sensitivity and high throughput in diagnosis applications. One of the challenges in designing of EWOD devices for such applications is managing many droplets on the single substrate. The controlling of the droplet traffic requires a feedback control system whose sensor can continuously monitor the droplet parameters like contact angle, position, size, and velocity. In this work, we have demonstrated a low-cost open EWOD based dynamic sensing system, which is capable of tracking multiple droplets on the single substrate in real time. Droplet parameters are measured by sensing the capacitance value between electrode pad and ground wire. This makes the systems scalable, portable, robust, highly sensitive and inexpensive. We have used printed circuit board (PCB) based open EWOD device, where polydimethylsiloxane (PDMS) is used as a dielectric as well as a hydrophobic layer. The analytical model for droplet velocity has been developed, which matches quite well with experimental results. The droplet sensing approach presented in this work, adds to the functionality of DMF by allowing measurement with top ground configuration.
TL;DR: Tunable compound eyes made of ionic liquid lenses, of which both curvatures can be individually changed using electrowetting on dielectric (EWOD) and applied pressure are presented.
Abstract: We present tunable compound eyes made of ionic liquid lenses, of which both curvatures (R 1 and R 2 in the lensmaker's equation) can be individually changed using electrowetting on dielectric (EWOD) and applied pressure. Flexible graphene is used as a transparent electrode and is integrated on a flexible polydimethylsiloxane (PDMS)/parylene hybrid substrate. Graphene electrodes allow a large lens aperture diameter of between 2.4 mm and 2.74 mm. Spherical aberration analysis is performed using COMSOL to investigate the optical property of the lens under applied voltage and pressure. The final lens system shows a resolution of 645.1 line pair per millimeter. A prototype of a tunable lens array is proposed for the application of a compound eye.
TL;DR: In this article, a microfluidic mixer utilizing the amplified Marangoni chaotic advection induced by alternating current (AC) continuous electrowetting of a metal droplet situated in electrolyte solution, due to the linear and quadratic voltage-dependence of flow velocity at small or large voltages, respectively.
Abstract: In this work, we proposed a novel design of a microfluidic mixer utilizing the amplified Marangoni chaotic advection induced by alternating current (AC) continuous electrowetting of a metal droplet situated in electrolyte solution, due to the linear and quadratic voltage-dependence of flow velocity at small or large voltages, respectively. Unlike previous researchers exploiting the unidirectional surface stress with direct current (DC) bias at droplet/medium interface for pumping of electrolytes where the resulting flow rate is linearly proportional to the field intensity, dominance of another kind of dipolar flow pattern caused by local Marangoni stress at the drop surface in a sufficiently intense AC electric field is demonstrated by both theoretical analysis and experimental observation, which exhibits a quadratic growth trend as a function of the applied voltage. The dipolar shear stress merely appears at larger voltages and greatly enhances the mixing performance by inducing chaotic advection between the neighboring laminar flow. The mixer design developed herein, on the basis of amplified Marangoni chaotic advection around a liquid metal droplet at larger AC voltages, has great potential for chemical reaction and microelectromechanical systems (MEMS) actuator applications because of generating high-throughput and excellent mixing performance at the same time.
TL;DR: The electrowetting behavior induced by AC voltages is shown to be well described by the Young-Lippmann equation for AC applications below a frequency threshold.
Abstract: An electrowetting-on-dielectric (EWOD) electrode was developed that facilitates the use of low alternating voltages (≤5 VAC). This allows online investigation of the frequency dependence of electrowetting by means of impedance spectroscopy. The EWOD electrode is based on a dielectric bilayer consisting of an anodic tantalum pentoxide (Ta2O5) thin film (d = 59.35 nm) with a high relative permittivity (ed = 26.3) and a self-assembled hydrophobic silane monolayer. The frequency dependence of electrowetting was studied using an aqueous μL-sized sessile droplet on the planar EWOD electrode in oil. Experiments using electrochemical impedance spectroscopy and optical imaging indicate the frequency dependence of all three variables in the Young-Lippmann equation: the voltage drop across the dielectric layers, capacitance per unit area, and contact angle under voltage. The electrowetting behavior induced by AC voltages is shown to be well described by the Young-Lippmann equation for AC applications below a frequen...
TL;DR: The results demonstrate that self-oscillation frequencies of several kHz are possible, which can lead to very high power generation densities in excess of 104 W m-2 and indicate the possibility of high-power energy harvesting from mechanical energy sources with very low frequencies, well below 1 Hz.
Abstract: Mechanical energy harvesting can provide a promising alternative to electrochemical batteries, which are currently widely utilized to power mobile electronics. In this work we present a theoretical analysis of a recently proposed method of mechanical energy harvesting, which combines a reverse electrowetting phenomenon with the fast self-oscillating process of bubble growth and collapse. We investigate the details of the bubble dynamics and analyze the dependence of the energy generation process on the system parameters. The results demonstrate that self-oscillation frequencies of several kHz are possible, which can lead to very high power generation densities in excess of 104 W m-2. The obtained results indicate the possibility of high-power energy harvesting from mechanical energy sources with very low frequencies, well below 1 Hz.
TL;DR: Numerical simulations of multielectrode electrowetting devices used in a novel optical design to correct wavefront aberration are presented, able to accurately simulate the liquid-liquid interface shape using computational fluid dynamics.
Abstract: We present numerical simulations of multielectrode electrowetting devices used in a novel optical design to correct wavefront aberration. Our optical system consists of two multielectrode devices, preceded by a single fixed lens. The multielectrode elements function as adaptive optical devices that can be used to correct aberrations inherent in many imaging setups, biological samples, and the atmosphere. We are able to accurately simulate the liquid-liquid interface shape using computational fluid dynamics. Ray tracing analysis of these surfaces shows clear evidence of aberration correction. To demonstrate the strength of our design, we studied three different input aberrations mixtures that include astigmatism, coma, trefoil, and additional higher order aberration terms, with amplitudes as large as one wave at 633 nm.
TL;DR: In this paper, the presence of synthesized silver nanoparticles not only enhances the wetting characteristics of the resulting nanofluid droplets, but also leads to the augmentation of their heat extraction capability from a hot spot.
Abstract: AC electrowetting is used to induce oscillation in a nanofluid droplet with significant changes in the shape dependent parameters at an optimized frequency. The presence of the synthesized silver nanoparticles not only enhances the wetting characteristics of the resulting nanofluid droplets, but leads to the augmentation of their heat extraction capability from a hot spot. The low frequency AC electrowetting induced droplet shape deformation generates surface waves and associated internal flow inside the droplet. This augments the convective heat transfer process resulting in additional evaporative cooling. The imposed internal flow and mixing also contribute to the reuse of the residual (after evaporation) nanoparticles to recreate the nanofluid droplet on the substrate. The generated surface waves are characterized using image analysis of the oscillating droplets in terms of their amplitude, frequency and damping. A model based on Stoke's drift phenomenon is used to analyze the results indicating augmented heat transfer in the low frequency regime.
TL;DR: In this paper, the use of super-hydrophobic materials as the actuating surface in digital microfluidics (DMF) devices is examined, and the change in contact angle by electrowetting of deionised water and ovalbumin protein samples is characterised on different surfaces.
Abstract: Among Lab-on-a-chip techniques, digital microfluidics (DMF), allowing the precise actuation of discrete droplets, is a highly promising, flexible, biochemical assay platform for biomedical and bio-detection applications. However the durability of DMF systems remains a challenge due to biofouling of the droplet-actuating surface when high concentrations of biomolecules are employed. To address this issue, the use of superhydrophobic materials as the actuating surface in DMF devices is examined. The change in contact angle by electrowetting of deionised water and ovalbumin protein samples is characterised on different surfaces (hydrophobic and superhydrophobic). Ovalbumin droplets at 1 mg ml−1 concentration display better electrowetting reversibility on Neverwet®, a commercial superhydrophobic material, than on Cytop®, a typical DMF hydrophobic material. Biofouling rate, characterised by roll-off angle measurement of ovalbumin loaded droplets and further confirmed by measurements of the mean fluorescence intensity of labelled fibrinogen, appears greatly reduced on Neverwet®. Transportation of protein laden droplets (fibrinogen at concentration 0.1 mg ml−1 and ovalbumin at concentration 1 mg ml−1 and 10 mg ml−1) is successfully demonstrated using electrowetting actuation on both single-plate and parallel-plate configurations with performance comparable to that of DI water actuation. In addition, although droplet splitting requires further attention, merging and efficient mixing are demonstrated.
TL;DR: In this article, a model that describes the surface modes appearing on a liquid lens and predicts the resonance frequencies was proposed, and the effects of those surface modes on a laser beam were simulated using Fresnel propagation.
Abstract: The increased usage of liquid lenses motivates us to investigate surface waves on the liquid's surface. During fast focal switching, the surface waves decrease the imaging quality. We propose a model that describes the surface modes appearing on a liquid lens and predicts the resonance frequencies. The effects of those surface modes on a laser beam are simulated using Fresnel propagation, and the model is verified experimentally.
TL;DR: A sacrificial layer photolithography strategy on highly hydrophobic surface is developed, which is applicable to both rigid and flexible substrate and manufacturing compatible and is important for research and development of devices based on electrowetting effect.
Abstract: Patterning micro-structures on highly hydrophobic surface by photolithography is usually inevitable for fabricating devices based on electrowetting effects. The key challenges for such photolithography processes are how to coat photoresist uniformly and maintain the hydrophobicity of the highly hydrophobic surface, which are usually two contradict aspects. Moreover, the patterned microstructure must adhere to the highly hydrophobic surface excellently, which is critical for device application. However, a simple and robust fabrication process that fulfills all the above requirements was seldom reported. In this paper, we developed a sacrificial layer photolithography strategy on highly hydrophobic surface. Photoresist is easily coated uniformly all over the substrate by introducing a sacrificial layer between the photoresist and the highly hydrophobic surface. The hydrophobicity of the exposed hydrophobic surface was maintained and the adhesion of the microstructures to the substrate is excellent. An electrowetting display sample was demonstrated by this fabrication strategy, which showed dynamic image displaying with response time less than 40 ms. The strategy is applicable to both rigid and flexible substrate and manufacturing compatible. We believe our developed photolithography process is important for research and development of devices based on electrowetting effect.
TL;DR: In this article, a thin layer of dielectric lubricating fluid on top of the solid surface was used to reduce the contact angle hysteresis on smooth and rough surfaces.
Abstract: Low-voltage electrowetting on dielectrics on substrates with a thin layer of lubricating fluid to reduce contact angle hysteresis is reported here. On smooth and homogeneous solid surfaces, it is extremely difficult to reduce contact angle hysteresis (contact angle difference between advancing and receding drop volume cycle) and the electrowetting hysteresis (contact angle difference between increasing and decreasing voltage cycle) below 10°. On the other hand, electrowetting hysteresis on rough surfaces can be relatively large (~30°); therefore, they are not useful for most of the fluidic devices. In the present report, we demonstrate that using a thin layer of dielectric lubricating fluid on top of the solid dielectric surface reduces the contact angle hysteresis as well as electrowetting hysteresis below 2° on smooth as well as rough surfaces. Electrowetting on lubricating fluid-coated surfaces also show a threshold behavior and the threshold voltage depends on the viscosity of the lubricating ...