J112021 表面張力を利用した液滴輸送の研究([J112-02]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(2))

Application of voltage across a liquid-dielectric interface resulting in the change of fluid contact angle is known as electrowetting on dielectric (EWOD) effect [1]. EWOD actuation is one of the most preferred techniques to move liquid at microscale due to relatively low voltages, low currents and power consumption requirements, and the absence of electrolysis. Our recent work has shown that nanofluids containing semiconductor nanoparticles exhibit a very strong EWOD effect [2, 3]. In particular, in the tested voltage range of 0 to 60 V, nanofluids loaded with molecularly-capped bismuth telluride nanoparticles show enhanced stability and increased actuation range of contact angle change. Developing a fundamental understanding of the role of nanostructure inclusions in controlling and potentially enhancing the EWOD effect would pave the way for the efficient use of EWOD with nanofluids in a wide range or microfluidic applications. Here we extend the studies on the EWOD behavior of nanofluids, to a system containing noble metal nano particles. In this abstract we report the preliminary results of the electro-wetting studies on nanofluids containing silver nano particles.

Liquid-liquid extraction techniques are one of the major tools in chemical engineering, analytical chemistry, and biology, especially in a system where two immiscible liquids have an interface solutes exchange between the two liquid phases along the interface up to a point where the concentration ratios in the two liquids reach their equilibrium values [1]. Solutes including nucleic acids and proteins of interests can be extracted from one liquid phase to the other immiscible liquid phase as a preparation step for many analytical processes. There are several advantages in miniaturizing the liquid-liquid extraction methods to on-chip level extraction. Usual advantages of miniaturization are the reduction in the sample size and portability. In addition, transport phenomena is faster in Micro-systems than in ordinary size systems, and therefore, one may expect that liquid-liquid extraction takes less time to achieve in miniaturized devices. It is due to shorter diffusion time in micro scale as well as high surface to volume ratio of Microsystems. Electrowetting on dielectric (EWOD) digital microfluidics is an efficient platform to process droplet based analytical processes [2]. Nanoliter (nL) or smaller volume of aqueous liquid droplets can be generated and transported on a chip by EWOD process. In addition to the high surface to volume ratio, high chemical potential can be expected in droplet based extraction when the droplets are in motion. In this paper, we propose to use room temperature ionic liquid (RTIL) as a second liquid phase for extraction, which forms immiscible interface with aqueous solutions. Properties of RTIL can be tailored by choice of cation, anion and substituents. RTIL has been investigated as replacements for the organic solvents and various “task-specific” ionic liquid are being developed which exhibit many attractive properties such as very low vapor pressure, high thermal stability [3]. We recently published EWOD properties of various RTILs toward microfluidic applications [4]. To demonstrate liquid-liquid micro extraction on chip, we fabricated and tested EWOD digital microfluidic devices. Fig. 1 shows (a) top and (b) cross sectional views of EWOD device. Two model extraction systems were tested. One is organic dye extracted from RTIL (1-butyl-3-methylimidazolium bis(trifluoromethanesulfonylimide or BMIMNTf2) to water and the other is iodine (I2) extracted from water to BMIMNTf2. The later model experiment is demonstrated in Fig. 2. Droplets of aqueous solution and BMIMNTf2 solution were generated on chip reservoir then transported for extraction and separated by EWOD actuation. When an aqueous solution and BMIMNTf2 solution join together, they created an interface, since water and BMIMNTf2 are immiscible. Extraction of I2 was done along the interface. After successful extraction, two immiscible liquid phases were separated by EWOD actuation and formed two separate droplets. From the result shown in Fig 2 (g), it is expected that extraction performance at the interface of moving droplet would be enhanced compared to the stationary droplet, because a moving interface prevent the chemical equilibrium, thus more chemical extraction potential can be provided with a moving interface than at a stationary interface. This demonstration is the first step toward total analysis system. The presented result opens the way to on-chip micro extraction, which will be readily integrated with other sample preparation microfluidic components and detection components. Currently, micro extraction systems for larger molecules such as nucleic acids, proteins and biological cells are being developed for further analytical applications.

Electrowetting-on-dielectric (EWOD) technology has been considered as a promising candidate for digital microfluidic (DMF) applications due to its outstanding flexibility and integrability. The dielectric layer with a hydrophobic surface is the key element of an EWOD device, determining its driving voltage, reliability, and lifetime. Hereby, inspired by the ionic-liquid-filled structuring polymer with high capacitance independent on thickness, namely ion gel (IG), we develop a polymer (P)-ion gel-amorphous fluoropolymer, namely, PIGAF, composite film as a replaceable hydrophobic dielectric layer for fabrication of a high-efficiency and stable EWOD-DMF device at relatively low voltage. The results show that the proposed EWOD devices using the PIGAF-based dielectric layer can achieve a large contact angle (θ) change of ∼50° and excellent reversibility with a contact angle hysteresis of ≤5° at a relatively low voltage of 30 Vrms. More importantly, the EWOD actuation voltage did not change obviously with the PIGAF film thickness in the range of several to tens of microns, enabling the thickness of the film to be adjusted according to the demand within a certain range while keeping the actuation voltage low. An EWOD-DMF device can be prepared by simply stacking a PIGAF film onto a PCB board, demonstrating stable droplet actuation (motion) at 30 Vrms and 1 kHz as well as a maximum moving velocity of 69 mm/s at 140 Vrms and 1 kHz. The PIGAF film was highly stable and reliable, maintaining excellent EWOD performance after multiple droplet manipulations (≥50 cycles) or long-term storage of 1 year. The proposed EWOD-DMF device has been demonstrated for digital chemical reactions and biomedical sensing applications.

Electrowetting (EW) effect is defined as changing in the interfacial energy of solid–liquid interface which causes to change in solid–liquid contact angle. In EW on dielectric (EWOD) actuations, the microliter drops are usually placed between two substrates which have some advantages. By modifying the voltages applied to the electrodes, it is possible to manipulate the droplets by setting up surface tension gradients along the plate. In this paper, the free energy based lattice Boltzmann method which has been recently extended by the authors for modeling and simulation of EW phenomenon has been applied to simulate EWOD. Also here we report the results of simulation of EW on superhydrophobic surfaces and pinning problem. The results have been compared with experimental data and show good accuracy of the numerical simulation.

The optical application of electrowetting-on-dielectric (EWOD) using thin dielectric layers is the focus of this paper. An optical switching configuration is designed with transparent indium-tin-oxide (ITO) electrodes and glass substrates as well as a transparent dielectric film between the liquid and the electrodes. A polyimide layer with a thickness of 0.5 μm to 1.5 μm and a top coating of 0.5 μm PTFE as hydrophobic surface has been used as dielectric film. Experimentally we present a significant change in the contact angle up to 56° applying a voltage of 155 V. The wettability of the surface can be controlled and a liquid flow is achieved by applying a voltage below 100 V. The saturation of contact angle is described by a model including a contact angle dependent resistance of the dielectric layer as fitting parameter and a constant resistance of the water droplet. The resistances have been confirmed by independent measurements. Based on these results optical switching has been performed by the principle of total reflection. Thereby the refractive index of the optical beam path is changed between the total reflection condition at an interface and transmission. This operation is realised by moving a water droplet between two glass plates. Within this concept addressable operations of a liquid on a fluidic chip and the integration of optical guiding and switching of light is possible. The application of EWOD for optics can be fundamental for the integration of micro-optics and fluidics in one device and the development of new micro-opto-electro-mechanical systems (MOEMS).

We report on experiments of electrowetting on dielectric (EWOD) on functional fibers in air. Firstly, we investigate the evolution of drop contact angle θ with the applied voltage Vrms. θ(Vrms) relationship is compared to the theoretical Young–Dupre model and shows a good agreement at low voltage. However it appears that the saturation effect is different when compared to the EWOD response on a planar surface. We then study reversibility of the phenomenon to show that the drop recovers its initial shape when the voltage is switched off. Secondly, EWOD is used to modulate the maximal volume ΩM of a hanging drop. Results are compared to numerical simulations performed with Surface Evolver software. A good agreement is observed and it appears that ΩM increases linearly with Vrms. Finally, we present a proof of concept of a drop manipulation tool on fiber networks. This consists of an EWOD driven valve that is capable of regulating the drop traffic at a cross-junction between two fibers.

Active surface tension driven micropump using droplet/meniscus pressure gradient

Study of cyanoethyl pullulan as insulator for electrowetting

We report electrowetting on dielectric (EWOD) experiments using graphene; a transparent, flexible and stretchable nanomaterial. Graphene sheets were synthesized by chemical vapor deposition, and transferred to various substrates (including glass slides and PET films). Reversible contact angle changes were observed on the Teflon-coated graphene electrode with both AC and DC voltages. Nyquist plots of the EWOD reveal that the graphene electrode has higher capacitive impedance than gold electrodes under otherwise identical conditions, suggesting a lower density of pin-holes and defects in the Teflon/graphene electrode than in the Teflon/gold electrode. Furthermore, we have observed reduced electrolysis of the electrolyte and smaller leakage current in the dielectric layer (Teflon) on graphene electrodes than on Au electrodes at the same Teflon thickness and applied voltage. We expect that the improved EWOD properties using graphene as an electrode material will open the door to various applications, including flexible displays and droplet manipulation in three-dimensional microfluidics.

This paper presents a low temperature technological process ahle to integrate an all-glass microfluidic network with an ElectroWetting On Dielectric (EWOD) structure for the digital handling of liquids. The fluidic channels result from the wet-etching of the glass, while the electrodes necessary for the droplet movement are deposited on the bottom and top surfaces of the microfluidic structure. The bottom electrodes are produced by a selective and sequential photolithographic pattern of a stack of metals, insulation layer and hydrophobic film. The top common electrode is made by a continuous transparent conductive oxide, covered by a hydrophobic layer. Compatibility of the technological steps and mechanical robustness of the proposed device have been tested designing and fabricating a microfluidic network integrating a central chamber, with a volume of about 9 μΐ, two reservoirs, two microfluidic channels and 26 EWOD electrodes. The maximum temperature reached during the device fabrication was 330°C, which is two times lower than the one used for the anodic bonding of glass-based microfluidic network.

Dielectric breakdown is a common problem in a digital microfluidic system, which limits its application in chemical or biomedical applications. We propose a new fabrication of an electrowetting-on-dielectric (EWOD) device using Si3N4 deposited by low-pressure chemical vapor deposition (LPCVD) as a dielectric layer. This material exhibits a greater relative permittivity, purity, uniformity, and biocompatibility than polymeric films. These properties also increase the breakdown voltage of a dielectric layer and increase the stability of an EWOD system when applied in biomedical research. Medium droplets with mouse embryos were manipulated in this manner. The electrical properties of the Si3N4 dielectric layer-breakdown voltage, refractive index, relative permittivity, and variation of contact angle with input voltage-were investigated and compared with a traditional Si3N4 dielectric layer deposited as a plasma-enhanced chemical vapor deposition to confirm the potential of LPCVD Si3N4 applied as the dielectric layer of an EWOD digital microfluidic system.

Wearable sweat sensors can analyze the abundant composition of solutes and metabolites in sweat to reflect the health state of the wearers in real time. The realization of active motion control for sweat droplets is significant for a multifunctional sweat monitoring device with several analysis chambers. Here, a wearable droplet‐based human sweat monitoring platform (WSMP), by combining an electrowetting on dielectrics (EWOD) device and a triboelectric nanogenerator (TENG), is demonstrated. It allows to collect and transport sweat droplets in different chambers by dielectric wetting effect and eventually merge and react with a pH indicator. The mechanical and electrical model of WSMP is introduced to describe the relationship between the open‐circuit voltage of the TENG and the voltage applied on the EWOD device. The high‐voltage electrical field generated by the TENG can change the wettability of solid–liquid interfaces and realize the controlling of droplet motion. The contact angle of electrolyte droplets changes over 30% with the triboelectric voltage of 5 kV. The driving, merging, and color reaction can be realized by actively controlling the motion of droplets. Finally, a wearable WSMP worn on the shank successfully demonstrates the preliminary detection of the pH level of human sweat.

This paper reviews the principles, operations, and applications of bubble-based electrowetting-on-dielectric (EWOD). EWOD has proved to be an efficient tool in digital microfluidics that employs discrete droplets, and various applications that use the principles of EWOD have been developed from lab-on-a-chip to optical systems. Similar to its use with droplets, EWOD can also be applied to gaseous bubbles. This review begins with a discussion of the principles of EWOD for a bubble on an electrode covered with a hydrophobic dielectric layer. It then addresses EWOD actuation and the transportation of a bubble in an aqueous medium, along with a physical explanation of bubble motion. The operation of EWOD is then extended to the on-chip creation/elimination and splitting of bubbles. In particular, micro-mixers and pumps are discussed as potential applications of these operations. Unlike droplets, bubbles can be easily oscillated by external excitation, which provides additional functionalities. By integrating EWOD with external excitation, a number of new advanced applications are introduced, including the capture/separation of particles and the propulsion of objects. In these advanced operations, cavitational microstreaming flows and acoustic radiation forces are mainly responsible for the physical mechanisms. This paper also discusses these advanced operations along with their underlying physics. It is expected that in addition to bubble oscillation, other bubble actuation modes will create new functionalities and new potential applications.

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

Electrowetting is a technique which allows to modify the wettability of a surface through the action of an electric field. At the time of its discovery the phenomenon was believed to have no practical use due to the electrochemical reactions occurring on the surface, which in fact limit the overall effect obtainable. In the last few decades the interest toward this technique arose and the first applications appeared. Fundamental to allow such renewed interest was the use of a dielectric hydrophobic layer interposed between the metallic surface and the electrolyte: it is known in this case as Electrowetting on Dielectric (EWOD). Possible applications of this technique include variable focal lenses, microfluidic systems and displays. The latter in particular can find possible wide industrial diffusion due to the development of the Liquavista technology by Samsung. The challenge of the next future is the transfer of the electrowetting technology from rigid and planar displays to flexible and curved ones. Flexible electronics are expected to be the technology of the future and applications include wearable devices, flexible screens and smart labels for packaging. The present work investigates the possibility of creating compact layers of copper oxide via copper anodization for the EWOD technology. Copper was chosen instead of metals that can be more efficiently anodized due to its ease of deposition by mean of wet techniques. In fact copper can be easily electroless deposited on flexible substrates, while metals like aluminium or titanium need PVD deposition or thermal coupling techniques. Other advantages of copper include high conductivity and good mechanical properties. The first part of the study involves the fundamental study of electrowetting on copper oxide coated with a fluoropolymer. Hysteresis and saturation of contact angle are analysed, as well as the adherence of the behaviour to the Young-Lippman equation. In the second part a flexible substrate is tested for electrowetting on a curved surface. Such specimen is manufactured by electroless plating of copper on PET, anodizing the resulting layer and coating the oxide layer thus obtained with a fluoropolymer. Finally the result is compared with an equivalent sample obtained coupling an aluminium layer with a polymeric substrate and thermally bonding them. The result is then anodized to obtain a compact oxide layer and coated with the fluoropolymer.