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Journal ArticleDOI

Creating, transporting, cutting, and merging liquid droplets by electrowetting-based actuation for digital microfluidic circuits

TL;DR: In this paper, the authors report the completion of four fundamental fluidic operations considered essential to build digital microfluidic circuits, which can be used for lab-on-a-chip or micro total analysis system (/spl mu/TAS): 1) creating, 2) transporting, 3) cutting, and 4) merging liquid droplets, all by electrowetting.
Abstract: Reports the completion of four fundamental fluidic operations considered essential to build digital microfluidic circuits, which can be used for lab-on-a-chip or micro total analysis system (/spl mu/TAS): 1) creating, 2) transporting, 3) cutting, and 4) merging liquid droplets, all by electrowetting, i.e., controlling the wetting property of the surface through electric potential. The surface used in this report is, more specifically, an electrode covered with dielectrics, hence, called electrowetting-on-dielectric (EWOD). All the fluidic movement is confined between two plates, which we call parallel-plate channel, rather than through closed channels or on open surfaces. While transporting and merging droplets are easily verified, we discover that there exists a design criterion for a given set of materials beyond which the droplet simply cannot be cut by EWOD mechanism. The condition for successful cutting is theoretically analyzed by examining the channel gap, the droplet size and the degree of contact angle change by electrowetting on dielectric (EWOD). A series of experiments is run and verifies the criterion.
Citations
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Journal ArticleDOI
TL;DR: A review of the physics of small volumes (nanoliters) of fluids is presented, as parametrized by a series of dimensionless numbers expressing the relative importance of various physical phenomena as mentioned in this paper.
Abstract: Microfabricated integrated circuits revolutionized computation by vastly reducing the space, labor, and time required for calculations. Microfluidic systems hold similar promise for the large-scale automation of chemistry and biology, suggesting the possibility of numerous experiments performed rapidly and in parallel, while consuming little reagent. While it is too early to tell whether such a vision will be realized, significant progress has been achieved, and various applications of significant scientific and practical interest have been developed. Here a review of the physics of small volumes (nanoliters) of fluids is presented, as parametrized by a series of dimensionless numbers expressing the relative importance of various physical phenomena. Specifically, this review explores the Reynolds number Re, addressing inertial effects; the Peclet number Pe, which concerns convective and diffusive transport; the capillary number Ca expressing the importance of interfacial tension; the Deborah, Weissenberg, and elasticity numbers De, Wi, and El, describing elastic effects due to deformable microstructural elements like polymers; the Grashof and Rayleigh numbers Gr and Ra, describing density-driven flows; and the Knudsen number, describing the importance of noncontinuum molecular effects. Furthermore, the long-range nature of viscous flows and the small device dimensions inherent in microfluidics mean that the influence of boundaries is typically significant. A variety of strategies have been developed to manipulate fluids by exploiting boundary effects; among these are electrokinetic effects, acoustic streaming, and fluid-structure interactions. The goal is to describe the physics behind the rich variety of fluid phenomena occurring on the nanoliter scale using simple scaling arguments, with the hopes of developing an intuitive sense for this occasionally counterintuitive world.

4,044 citations


Cites background from "Creating, transporting, cutting, an..."

  • ...…1998; Yun et al., 2002 , creation, manipulation, and metering of fluid droplets Lee and Kim, 2000; Pollack et al., 2000; Lee, Moon, et al., 2002; Cho, Moon, and Kim, 2003; Kuo et al., 2003; Ren et al., 2004 , and chemical reactors that merge and mix droplets presumably by electrowetting…...

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  • ...Droplet manipulation is likewise fast—velocities on solid substrates of order 1 mm/s–10 cm/s have been demonstrated Washizu, 1998; Pollack et al., 2000, 2002; Cho, Moon, and Kim, 2003 and drops have been split in less than a second Cho, Moon, and Kim, 2003 Fig....

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Journal ArticleDOI
TL;DR: In this paper, the authors compare the various approaches used to derive the basic electrowetting equation, which has been shown to be very reliable as long as the applied voltage is not too high.
Abstract: Electrowetting has become one of the most widely used tools for manipulating tiny amounts of liquids on surfaces. Applications range from 'lab-on-a-chip' devices to adjustable lenses and new kinds of electronic displays. In the present article, we review the recent progress in this rapidly growing field including both fundamental and applied aspects. We compare the various approaches used to derive the basic electrowetting equation, which has been shown to be very reliable as long as the applied voltage is not too high. We discuss in detail the origin of the electrostatic forces that induce both contact angle reduction and the motion of entire droplets. We examine the limitations of the electrowetting equation and present a variety of recent extensions to the theory that account for distortions of the liquid surface due to local electric fields, for the finite penetration depth of electric fields into the liquid, as well as for finite conductivity effects in the presence of AC voltage. The most prominent failure of the electrowetting equation, namely the saturation of the contact angle at high voltage, is discussed in a separate section. Recent work in this direction indicates that a variety of distinct physical effects?rather than a unique one?are responsible for the saturation phenomenon, depending on experimental details. In the presence of suitable electrode patterns or topographic structures on the substrate surface, variations of the contact angle can give rise not only to continuous changes of the droplet shape, but also to discontinuous morphological transitions between distinct liquid morphologies. The dynamics of electrowetting are discussed briefly. Finally, we give an overview of recent work aimed at commercial applications, in particular in the fields of adjustable lenses, display technology, fibre optics, and biotechnology-related microfluidic devices.

1,962 citations


Cites background or methods from "Creating, transporting, cutting, an..."

  • ...23 Popular inorganic insulator materials include SiO2 [39, 63, 68-71] and SiN [68, 72, 73]....

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  • ...Popular inorganic insulator materials include SiO2 [34, 56, 61–64] and SiN [61, 65, 66]....

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  • ...Both current approaches, composite substrates of solid state insulators (e.g. SiO2) with hydrophobic top coatings as well as homogeneous polymeric insulators,have reached the range of submicrometre thickness....

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  • ...Routine tasks such as moving, merging, mixing, and splitting of droplets have been demonstrated, first independently and later integrated into more complex devices (see [5], [71], and references therein)....

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  • ...Lab-on-a-chip applications of electrowetting were mainly promoted by two laboratories, the one of Richard Fair at Duke University (for a recent review focused on the activities of this laboratory, see [5]) and the one of CJ Kim at UCLA (see [71] and references there) with several other groups joining in more recently [105-107]....

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Journal ArticleDOI
TL;DR: Fundamental and applied research in chemistry and biology benefits from opportunities provided by droplet-based microfluidic systems, which enable the miniaturization of reactions by compartmentalizing reactions in droplets of femoliter to microliter volumes.
Abstract: Fundamental and applied research in chemistry and biology benefits from opportunities provided by droplet-based microfluidic systems. These systems enable the miniaturization of reactions by compartmentalizing reactions in droplets of femoliter to microliter volumes. Compartmentalization in droplets provides rapid mixing of reagents, control of the timing of reactions on timescales from milliseconds to months, control of interfacial properties, and the ability to synthesize and transport solid reagents and products. Droplet-based microfluidics can help to enhance and accelerate chemical and biochemical screening, protein crystallization, enzymatic kinetics, and assays. Moreover, the control provided by droplets in microfluidic devices can lead to new scientific methods and insights.

1,702 citations

Journal ArticleDOI
TL;DR: This critical review summarizes developments in microfluidic platforms that enable the miniaturization, integration, automation and parallelization of (bio-)chemical assays and attempts to provide a selection scheme based on key requirements of different applications and market segments.
Abstract: This critical review summarizes developments in microfluidic platforms that enable the miniaturization, integration, automation and parallelization of (bio-)chemical assays (see S. Haeberle and R. Zengerle, Lab Chip, 2007, 7, 1094–1110, for an earlier review). In contrast to isolated application-specific solutions, a microfluidic platform provides a set of fluidic unit operations, which are designed for easy combination within a well-defined fabrication technology. This allows the easy, fast, and cost-efficient implementation of different application-specific (bio-)chemical processes. In our review we focus on recent developments from the last decade (2000s). We start with a brief introduction into technical advances, major market segments and promising applications. We continue with a detailed characterization of different microfluidic platforms, comprising a short definition, the functional principle, microfluidic unit operations, application examples as well as strengths and limitations of every platform. The microfluidic platforms in focus are lateral flow tests, linear actuated devices, pressure driven laminar flow, microfluidic large scale integration, segmented flow microfluidics, centrifugal microfluidics, electrokinetics, electrowetting, surface acoustic waves, and dedicated systems for massively parallel analysis. This review concludes with the attempt to provide a selection scheme for microfluidic platforms which is based on their characteristics according to key requirements of different applications and market segments. Applied selection criteria comprise portability, costs of instrument and disposability, sample throughput, number of parameters per sample, reagent consumption, precision, diversity of microfluidic unit operations and the flexibility in programming different liquid handling protocols (295 references).

1,536 citations


Cites background or methods from "Creating, transporting, cutting, an..."

  • ...A similar technology can be used for the splitting of a droplet into several smaller droplets [32]....

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  • ...Control and manipulation of such droplets can be achieved by another favorable aspect of the high surface-to-volume ratio in microfluidics: the possibility to control the liquid flow by electrically induced forces or electrowetting [32]....

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Journal ArticleDOI
TL;DR: The dynamics and stability of thin liquid films have fascinated scientists over many decades: the observations of regular wave patterns in film flows along a windowpane or along guttering, the patterning of dewetting droplets, and the fingering of viscous flows down a slope are all examples that are familiar in daily life.
Abstract: The dynamics and stability of thin liquid films have fascinated scientists over many decades: the observations of regular wave patterns in film flows down a windowpane or along guttering, the patterning of dewetting droplets, and the fingering of viscous flows down a slope are all examples that are familiar in daily life. Thin film flows occur over a wide range of length scales and are central to numerous areas of engineering, geophysics, and biophysics; these include nanofluidics and microfluidics, coating flows, intensive processing, lava flows, dynamics of continental ice sheets, tear-film rupture, and surfactant replacement therapy. These flows have attracted considerable attention in the literature, which have resulted in many significant developments in experimental, analytical, and numerical research in this area. These include advances in understanding dewetting, thermocapillary- and surfactant-driven films, falling films and films flowing over structured, compliant, and rapidly rotating substrates, and evaporating films as well as those manipulated via use of electric fields to produce nanoscale patterns. These developments are reviewed in this paper and open problems and exciting research avenues in this thriving area of fluid mechanics are also highlighted.

1,226 citations

References
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Book
01 Jan 1997
TL;DR: The second edition of the Fundamentals of Microfabrication as discussed by the authors provides an in-depth coverage of the science of miniaturization, its methods, and materials, from the fundamentals of lithography through bonding and packaging to quantum structures and molecular engineering.
Abstract: MEMS technology and applications have grown at a tremendous pace, while structural dimensions have grown smaller and smaller, reaching down even to the molecular level. With this movement have come new types of applications and rapid advances in the technologies and techniques needed to fabricate the increasingly miniature devices that are literally changing our world.A bestseller in its first edition, Fundamentals of Microfabrication, Second Edition reflects the many developments in methods, materials, and applications that have emerged recently. Renowned author Marc Madou has added exercise sets to each chapter, thus answering the need for a textbook in this field.Fundamentals of Microfabrication, Second Edition offers unique, in-depth coverage of the science of miniaturization, its methods, and materials. From the fundamentals of lithography through bonding and packaging to quantum structures and molecular engineering, it provides the background, tools, and directions you need to confidently choose fabrication methods and materials for a particular miniaturization problem.New in the Second EditionRevised chapters that reflect the many recent advances in the fieldUpdated and enhanced discussions of topics including DNA arrays, microfluidics, micromolding techniques, and nanotechnology In-depth coverage of bio-MEMs, RF-MEMs, high-temperature, and optical MEMs.Many more links to the WebProblem sets in each chapter

2,334 citations

Journal ArticleDOI
TL;DR: In this article, a microactuator for rapid manipulation of discrete microdroplets is presented, which is accomplished by direct electrical control of the surface tension through two sets of opposing planar electrodes fabricated on glass.
Abstract: A microactuator for rapid manipulation of discrete microdroplets is presented. Microactuation is accomplished by direct electrical control of the surface tension through two sets of opposing planar electrodes fabricated on glass. A prototype device consisting of a linear array of seven electrodes at 1.5 mm pitch was fabricated and tested. Droplets (0.7–1.0 μl) of 100 mM KCl solution were successfully transferred between adjacent electrodes at voltages of 40–80 V. Repeatable transport of droplets at electrode switching rates of up to 20 Hz and average velocities of 30 mm/s have been demonstrated. This speed represents a nearly 100-fold increase over previously demonstrated electrical methods for the transport of droplets on solid surfaces.

1,471 citations


"Creating, transporting, cutting, an..." refers background in this paper

  • ...Devices made with known-good-dice give us good repeatable EWOD actuations at as low as 25 V, which is much lower than the previously reported EWOD actuation voltages in air (over 50 V) [10], [12]–[14], [16], [17]....

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  • ...Among the known configurations of electrowetting, electrowetting on dielectric (EWOD), which allows for control of the wettability of liquids on a dielectric solid surface using electric potential [10], [12]–[19], is considered most promising, thanks to the electrochemical inertness of the surface....

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  • ...Furthermore, we found that when high-voltage ac potential (150 V) is applied, the transporting speed increases up to 250 mm/s [18], one order of magnitude faster than previously reported [10], depending on the frequency of the ac potential and the channel gap....

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  • ...[10] showed that an aqueous liquid droplet could be transported with EWOD....

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  • ...All four fundamental operations were successfully performed in an air-filled environment with 25 V , much lower than the EWOD actuation voltages in air previously reported [10], [12]–[14], [16], [17]....

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Book
01 Feb 1998
TL;DR: In this paper, the authors present an overview of Micromachining Techniques, Mechanical Transducers, Optical Transducers and Ionizing Radiation Transducers for Microfluidic Devices.
Abstract: 1 Introduction and Overview2 Micromachining Techniques3 Mechanical Transducers4 Optical Transducers5 Ionizing Radiation Transducers6 Thermal Transducers7 Magnetic & Electromagnetic Transducers8 Chemical & Biological Transducers9 Microfluidic Devices

1,212 citations

Journal ArticleDOI
TL;DR: In this paper, an alternative approach to microfluidics based upon the micromanipulation of discrete droplets of aqueous electrolyte by electrowetting is reported.
Abstract: The serviceability of microfluidics-based instrumentation including ‘lab-on-a-chip’ systems critically depends on control of fluid motion. We are reporting here an alternative approach to microfluidics based upon the micromanipulation of discrete droplets of aqueous electrolyte by electrowetting. Using a simple open structure, consisting of two sets of opposing planar electrodes fabricated on glass substrates, positional and formational control of microdroplets ranging in size from several nanoliters to several microliters has been demonstrated at voltages between 15–100 V. Since there are no permanent channels or structures between the plates, the system is highly flexible and reconfigurable. Droplet transport is rapid and efficient with average velocities exceeding 10 cm s−1 having been observed. The dependence of the velocity on voltage is roughly independent of the droplet size within certain limits, thus the smallest droplets studied (∼3 nl) could be transported over 1000 times their length per second. Formation, mixing, and splitting of microdroplets was also demonstrated using the same microactuator structures. Thus, electrowetting provides a means to achieve high levels of functional integration and flexibility for microfluidic systems.

1,078 citations