Electrowetting Devices Research Articles

Lifetime experiment and analysis of an electrowetting ionic liquid electrospray thruster

Ionic liquid electrospray thrusters are notable for their small size, low mass, and high specific impulse, making them ideal for use in micro/nano satellites. This study evaluates the lifetime of an electrowetting ionic liquid electrospray thruster. Experimental results indicate that the decrease in emission current is attributed to electrochemical reactions that increase the curvature of the emitter tips and the increased flow resistance. The online replenishment of insufficient propellant is achieved using an electrowetting device, which restores the emission current. Over the entire test period, the thruster operated for a total of 331.81 h, consuming 2.68083 g of propellant. This study offers valuable insights for improving the lifetime and total impulse of ionic liquid electrospray thrusters.

A Wettability-Mediated Microdroplet Under Electrowetting Effect for Hotspot Cooling

This work reports an experimental investigation of electrowetting (EW)-driven microdroplets for on-chip hotspot cooling applications. A discrete microdroplet of deionized (DI) water with 4 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</i> <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mu }\text {l}$ </tex-math></inline-formula> volume is transported over an electrode array in a single-plate EW device. The instantaneous shape of the droplet and its wetting behavior during the two-phase heat dissipation process have been analyzed for different operating conditions. In conventional EW devices, the cooling of a hydrophobic hotspot by a microdroplet exhibits evaporation with low contact line pinning, and the droplet skitters away from the hotspot, resulting in underperformance of the device. In this work, we have introduced a super hydrophilic surface using an anodization technique to improve the heat dissipation capacity of an EW device. The droplet eventually wets the hydrophilic hotspot and maintains a relatively large surface area with low thermal resistance, enhancing heat dissipation capacity. A maximum 210.7% enhancement in heat transfer efficiency of a coolant droplet over a superhydrophilic hotspot is observed compared to that on the hydrophobic surface. Furthermore, continuous droplet feeding has been applied over the hydrophilic surface to tackle the quick evaporation of the droplet. The coalescence of two successive droplets produces extra cooling effects on the hotspot.

Programmable Control of Two‐Phase Fluid Interface Relative Motion in Electrowetting Device (Adv. Mater. Interfaces 21/2021)

Dynamic Control of Two-Phase Fluid Interface In article number 2101086, Yuanyuan Guo, Guofu Zhou, and co-workers demonstrate the inkjet-printed conductive micropillars (CMPs) on the hydrophobic surface can be used to manipulate the dynamic motion of the oil-water interface in an electrowetting device. The oil film rupture position, contract direction and rupture voltage can be precisely controlled by varying the position and height of CMPs in the confined structure.

A Novel Hexagonal Beam Steering Electrowetting Device for Solar Energy Concentration.

Traditional tracking devices for solar energy applications have several disadvantages, such as bulky mechanical structure, large wind loads, and ease of misalignment. This study aims to design a flat, thin, and adaptive beam steering device to eliminate these drawbacks. A proof of concept device was fabricated to demonstrate this design. The novelty of the proof of concept device is the hexagonal structure of the electrowetting cell design. The hexagonal cell was dosed with two immiscible liquids with different refractive indices. The hypothesis of this design is that by deforming the liquid shape with the application of voltage, light can be steered and concentrated for solar energy applications. A maximum contact angle change of 44° was observed with the application of 26 V to one of the electrodes of the hexagonal cell. The device demonstrated a 4.5° change of laser beam path with only a 0.2 refractive index difference of the liquids. The 3D simulation model developed in this study shows that a tilted and flat interface can be achieved using higher dielectric constant dielectric materials. The device can facilitate the planer steering and concentration of sunlight for rooftop applications without moving mechanical parts.

Inkjet-printed patterned polytitanosiloxane-fluoropolymer composite dielectric layer for micro-optical electrowetting valves

Fluid motion control has always been essential in electrowetting devices. Using the inkjet printing technique, an organic-inorganic hybrid material, polytitanosiloxane, was patterned as a secondary dielectric under the hydrophobic fluoropolymer coating in an optical electrowetting device (OEWD). The effects of different structural parameters of the inkjet-printed polytitanosiloxane (IPP) layer on both the fluid motion at the microscopic level and the macroscopic visual performances of the device were studied experimentally. The leakage-current test was also performed to evaluate the insulating performance of the composite dielectric layer. In addition, COMSOL Multiphysics was used to simulate the fluid flow inside the OEWD with/without the polytitanosiloxane layer. The results showed that, by introducing this IPP layer, precise fluid motion control and improved performance could be obtained.

Statistical modeling of electrowetting-induced droplet coalescence for condensation applications

Coalescence of water droplets strongly influences dropwise condensation on hydrophobic surfaces. This work reports a study on experimental data-based statistical modeling to predict the coalescence dynamics of an ensemble of water droplets under the influence of an electrowetting (EW) field. Previous related studies have primarily used high speed visualization to characterize coalescence. However, this study uses statistical modeling to analyze the parameter space associated with EW-induced coalescence, noting that physics-based modeling of EW-induced coalescence is challenging. The objective of this study is to quantify the influence of the applied voltage, frequency of the AC waveform and the geometry of the EW device on two parameters related to droplet coalescence (droplet radius enhancement and reduction in wetted area). Multiple supervised learning techniques are used to identify dominant variables and statistically model the influence of these variables on coalescence. Data for the statistical models is obtained via image analysis from coalescence experiments.The statistical models lead to a reference tool to predict droplet coalescence-related parameters versus the applied voltage and electrode geometry. Importantly, data analysis shows that droplet coalescence is independent of the AC frequency; this conclusion would be challenging to infer from conventional analysis. It is also seen that an EW field significantly narrows the droplet size distribution. Overall, this study leads to a detailed understanding of the factors that impact EW-induced coalescence and provides a tool (which matches experimental data) to predict the change in droplet size distribution. These findings are key to quantifying the influence of EW on condensation rates and heat transfer. This work leverages the large amount of data from experiments to develop statistical analysis-based predictive models. This approach can be utilized for predictive modeling of other data-rich but complex physical phenomena.

Approximately symmetric electrowetting on an oil-lubricated surface.

As the most widely used insulator materials in the electrowetting (EW) systems, amorphous fluoropolymers (AFs) provide excellent hydrophobicity, dielectric properties and chemical inertness; however, they suffer from charge trapping during electrowetting with water and the consequent asymmetric phenomenon. In this study, an ultra-thin oil-lubricated AF surface was proposed to release the charge trapping in the dielectric layer and further suppress the polarity-dependent asymmetry during electrowetting. The negative spontaneously trapped charges gathering on the dielectric/water interface with aging time were characterized by various measurements and calculations, which explained the polarity dependence of the asymmetric electrowetting. Approximately symmetric EW curves withstanding water aging were obtained for the oil-lubricated AF surface, confirming the blocking effect on charge trapping induced by the lubricated surface. The improved reversibility of EW with low contact angle hysteresis brought by the oil-lubricated surface was also demonstrated. This study reveals the mechanism behind the asymmetric EW phenomenon and offers an attractive oil-lubricated EW material system for suppressing the charge trapping on the dielectric/water interface, which can significantly improve the manipulation of the EW devices.

Bistable electrowetting device with non-planar designed controlling electrodes for display applications

Bistable electrowetting display (EWD) is a promising low-power electronic paper technology, where power is consumed only during the switching between two stable states; however, it is not required for state maintenance once switched. In this paper, a bistable electrowetting device with non-planar designed controlling electrodes is fabricated by a fully conventional photolithography process. The device has potential for video display applications with a controllable gray scale. The novel electrode design realizes a lower driving voltage and a higher contrast between two stable states than the EWDs with planar electrodes reported previously.

Enhanced dielectric constant and hydrophobicity of P(VDF–TrFE)-based composites

In this research, we report a facile and low-lost method to prepare single-layer hybrid films with high permittivity and hydrophobicity, which exhibited promising potential application as insulator layer in low-voltage electrowetting devices. α-terpineol treated BaTiO3 nanoparticles with less agglomeration were incorporated into P(VDF–TrFE) matrix. The dielectric behavior and hydrophobic property of these composites were studied. Typically, the composite with 25 vol% surface treated BaTiO3 nanoparticles reveals dielectric constant of 46.5, three times of that of pure P(VDF–TrFE), and a lower dielectric loss of 0.091 at 103 Hz compared to the composite filled with commercial BaTiO3 nanoparticles, although its breakdown strength still remains 100 MV m−1. Besides, electric conductivity of all the composites were lower than 10−7 S cm−1 even at high frequency, indicating their excellent insulativity. Further, the composites film exhibits the high static contact angle with excellent AC electrowetting response. These results demonstrated the great potential of such single-layer composite film for low-voltage electrowetting devices.

Electro-wetting lenticular lens with improved diopter for 2D and 3D conversion using lens-shaped ETPTA chamber.

In this paper, we introduce a method for improving the lens diopter of 2D/3D convertible devices using electro-wetting. For stable operation, an electro-wetting device requires high dioptric performance and this was achieved using bi-convex liquid-liquid-solid phases. 1-Chloronaphthalene with a refractive index of 1.633 was used as an oil phase to achieve high diopters. ETPTA (trimethylolpropane ethoxylate triacrylate), a UV-sensitive material with low chemical reactivity to the 1-Chloronaphthalene, was used as a chamber material. This resulted in a diopter of 3030D for high quality multi-view images without unstable oil movement or trembling. The ETPTA was molded on a 0.3mm thick glass substrate that was coated with UV adhesive (NOA 81). The maximum diopter capable of stable operation was 3425D. 2D and 3D conversion and parallax motion were demonstrated.

Deposition method and performance of SiO2 as a dielectric material for beam steering electrowetting devices

Electrowetting has the potential to be applied to steering and concentrating sunlight using liquid lenses. However, the performance of electrowetting devices depends on the careful selection/fabrication of their constituent dielectric material. The dielectric material should have high optical transparency, chemical stability, high relative permittivity, be relatively thin and have a high breakdown voltage. Recent studies noted that inorganic dielectric materials such as SiO2 are more suitable for beam steering compared to organic dielectric materials such as fluoropolymers or parylene C for example. When optical transmission is not required, electrowetting devices are fabricated by thermally growing SiO2 on top of silicon wafers. However, this deposition method is not suitable for beam steering electrowetting devices as the silicon substrate is not optically transparent. In this paper, we investigated the performance of SiO2 as a dielectric material for beam steering electrowetting devices. Specifically, we measured the contact angle modulation capability of SiO2 produced by electron-beam and sputtering deposition methods and correlated the results with morphological defects of the SiO2 layer introduced by the deposition methods. We investigated the effect of annealing and the performance with ionic and DI water. Finally, we compared the performance of SiO2 with other dielectric materials such as PDMS and Ta2O5 in air/water and silicone oil/water medium.

Scaling Electrowetting with Printed Circuit Boards for Large Area Droplet Manipulation

ABSTRACTDroplet based microfluidics (digital microfluidics) with Electrowetting on dielectric (EWOD) has gained popularity with the promise of being technology for a true lab-on-chip device with applications spanning across assays/library prep, next-gen sequencing and point-of-care diagnostics. Most electrowetting device architecture are linear electrode arrays with a shared path for droplets, imposing serious limitations -- cross contamination and limited number of parallel operations. Our work is in addressing these issues through large 2D grid arrays with direct addressability providing flexible programmability.Scaling electrowetting to larger arrays still remains a challenge due to complex and expensive cleanroom fabrication of microfluidic devices. We take the approach of using inexpensive PCB manufacturing, investigate challenges and solutions for scaling electrowetting to large area droplet manipulation. PCB manufactured electrowetting arrays impose many challenges due to the irregularities from process and materials used. These challenges generally relate to preparing the surface that interfaces with droplets -- a dielectric material on the electrodes and the top most hydrophobic coating that interfaces with the droplets. A requirement for robust droplet manipulation with EWOD is thin (&lt;10um) hydrophobic dielectric material which does not break down at droplet actuation voltages (AC/DC, 60V to 200V) and has a no droplet pinning. For this, we engineered materials specifically for large area PCBs.Traditionally, digital microfluidic devices sandwich droplets between two plates and have focussed on sub-microliter droplet volumes. In our approach, droplets are on an open surface with which we are able to manipulate droplets in microliter and milliliter volumes. With milliliter droplet manipulation ability on our electrowetting device, we demonstrate “digital millifluidics”. Finally, we report the performance of our device and to motivate the need for large open arrays we show an example of running multiple parallel biological experiments.

Development of an electrowetting device for thermal conductance control

In this study, we investigate thermal conductance switching characteristics using liquid metal gallium indium eutectic alloy. Gallium indium eutectic alloy was fabricated by mixing gallium and indium. Liquid metal is poured into the micro channel saturated with electrolyte aqueous solution which is driven by applying electric field and known as electrowetting. The filling rate of liquid metal in the action part can control thermal conductance. Thermal switching using electrowetting process shows that the value of thermal conductance ratio was about 1.2 using small channel and about 1.8 using big channel for our liquid metal. In the future, we will be developing high efficient device using MEMS technique.

Numerical analysis of wavefront aberration correction using multielectrode electrowetting-based devices.

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.

Protein droplet actuation on superhydrophobic surfaces: a new approach toward anti-biofouling electrowetting systems

Anti-biofouling behaviour of an electrowetting device using off-the-shelf superhydrophobic materials is demonstrated through protein adsorption measurement and protein-laden droplet actuation.

A Contact Line Dynamic Model for a Conducting Water Drop on an Electrowetting Device

AbstractThe static shape of drop under electrowetting actuation is well studied and recent electrowetting theory and experiments confirm that the local contact angle (microscopic angle) is unaffected while the apparent contact angle (macroscopic angle) is characterized by the Lippmann-Young equation. On the other hand, the evolution of the drop motion under electrowetting actuation has received less attention. In this paper, we investigate the motion of a conducting water drop on an electrowetting device (EWD) using the level set method. We derive a contact line two-phase flow model under electrowetting actuation using energy dissipation by generalizing an existing contact line model without the electric field. Our model is consistent with the static electrowetting theory as the dynamic contact angle satisfies the static Young's equation under equilibrium conditions. Our steady state results show that the apparent contact angle predicted by our model satisfies the Lippmann-Young's relation. Our numerical results based on the drop maximum deformation agree with experimental observations and static electrowetting theory. Finally, we show that for drop motion our results are not as good due to the difficulty of computing singular electric field accurately. Nonetheless, they provide useful insights and ameaningful first step towards the understanding of the drop dynamics under electrowetting actuation.

Ultra-low voltage electrowetting using graphite surfaces.

The control of wetting behaviour underpins a variety of important applications from lubrication to microdroplet manipulation. Electrowetting is a powerful method to achieve external wetting control, by exploiting the potential-dependence of the liquid contact angle with respect to a solid substrate. Addition of a dielectric film to the surface of the substrate, which insulates the electrode from the liquid thereby suppressing electrolysis, has led to technological advances such as variable focal-length liquid lenses, electronic paper and the actuation of droplets in lab-on-a-chip devices. The presence of the dielectric, however, necessitates the use of large bias voltages (frequently in the 10-100 V range). Here we describe a simple, dielectric-free approach to electrowetting using the basal plane of graphite as the conducting substrate: unprecedented changes in contact angle for ultra-low voltages are seen below the electrolysis threshold (50° with 1 V for a droplet in air, and 100° with 1.5 V for a droplet immersed in hexadecane), which are shown to be reproducible, stable over 100 s of cycles and free of hysteresis. Our results dispel conventional wisdom that reversible, hysteresis-free electrowetting can only be achieved on solid substrates with the use of a dielectric. This work paves the way for the development of a new generation of efficient electrowetting devices using advanced materials such as graphene and monolayer MoS2.

Electro-responsively reversible transition of polythiophene films from superhydrophobicity to superhydrophilicity.

An electro-responsively reversible switching of wettability between superhydrophobicity and superhydrophilicity has been obtained from a highly porous structured polythiophene film. The polythiophene film was prepared by two-step electrochemical deposition on an indium tin oxide (ITO) substrate. The underlying poly(3,4-ethylenedioxythiophene) (PEDOT) provides a highly porous structured conductive support, and poly(3-methylthiophene) (P(3-MTH)) deposited thereon plays the role of a low-surface-energy conductive coating. The wettability switching of this double-layer film between superhydrophobicity and superhydrophilicity has been investigated by doping and dedoping in an electrolyte solution containing ClO4(-). Electrochromism of the film was also seen to accompany the electrochemical process of conversion between the two superwetting states. On the basis of this porous electro-active film, an in situ electro-wetting device was also demonstrated.

Tailoring Pigment Dispersants with Polyisobutylene Twin-Tail Structures for Electrowetting Display Application

We have designed a class of highly hydrophobic dispersants for finely dispersing carbon black and organic pigment nanoparticles in apolar mediums. The synthesis involved the use of polyisobutylene-g-succinic anhydride (PIB-SA) and judiciously selected amines by amidation and imidation. The structures were characterized by infrared spectroscopy for anhydride functionalities in the starting materials and amide/imide linkages in the products. These polymeric forms of dispersants were structurally varied with respects to their PIB molecular weight, twin-tails, and linkages. Their relative performance for dispersing six different pigments in decane was evaluated against solution homogeneity, viscosity, stability, and particle size. The fine dispersion was achieved at particle sizes of ca. 100 nm, with the viscosity as low as 2-3 cP. The measurement of zeta potentials, which varied from -39.8 to -5.1 mV with pigment addition, revealed a strong surface-charge interaction between pigment and PIB dispersant molecules. Examination by TEM (transmission electronic microscope) showed the homogeneous dispersion of the primary structures of pigment particles at ca. 20 nm in diameter. The polymeric dispersants with different PIB tails and imide functionalities could be tailored for pigment stability in the oil phase, which is potentially suitable for the electrowetting devices.

Degradation of electrowetting for upward and downward electrolyte droplets containing microparticles

Electrowetting (EW) of an electrolyte droplet containing zinc or silica particles was investigated for durability and degradation of devices. Two types of experimental geometries exist, an upward droplet standing on a substrate and a downward droplet hanging under a substrate. Different gravitational effects of droplets for the two geometries shift the contact angle of the EW dynamics. Added zinc and silica particles significantly accelerated the degradation in the EW switching with increased driving time, which may result from the surface damage due to the collision between the particles and surface. The degradation was significantly influenced by many factors such as the particle dispersity, droplet direction, particle concentration, and viscosity of dispersions. Interestingly, the device with a downward droplet degraded much less than that with the upward droplet. Our findings will be highly useful in designing durable microfluidic and EW devices.