Hyejin Moon - Academia.edu (original) (raw)

Papers by Hyejin Moon

This paper reports a breakthrough in our quest for digital microfluidic circuits - full completio... more This paper reports a breakthrough in our quest for digital microfluidic circuits - full completion of all four fundamental microfluidic operations: (1) creating, (2) transporting, (3) cutting, and (4) merging of liquid droplets, based on electrowetting-on-dielectric (EWOD) actuation. All the operations were achieved with 25 V/sub DC/ lower than EWOD actuation voltages previously reported. We also report conditions to reduce the driving voltage even further and conditions to drive a droplet as fast as 250 mm/s.

IEEE/ASME Journal of Microelectromechanical Systems, 2003

This paper reports the completion of four fundamental fluidic operations considered essential to ... more This paper 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 ( 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. A smaller channel gap, a larger droplet size and a larger change in the contact angle enhance the necking of the droplet, helping the completion of the cutting process. Creating droplets from a pool of liquid is highly related to cutting, but much more challenging. Although droplets may be created by simply pulling liquid out of a reservoir, the location of cutting is sensitive to initial conditions and turns out unpredictable. This problem of an inconsistent cutting location is overcome by introducing side electrodes, which pull the liquid perpendicularly to the main fluid path before activating the cutting. All four operations are carried out in air environment at 25 V dc applied voltage.

Sensors and Actuators A-physical, 2002

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

Journal of Applied Physics, 2003

This article describes the equilibrium shape of a liquid drop under applied fields such as gravit... more This article describes the equilibrium shape of a liquid drop under applied fields such as gravity and electrical fields, taking into account material properties such as dielectric constants, resistivities, and surface tension coefficients. The analysis is based on an energy minimization framework. A rigorous and exact link is provided between the energy function corresponding to any given physical phenomena, and the resulting shape and size dependent force term in Young's equation.

We present a novel sample preparation technique using electrowetting-based digital microfluidics ... more We present a novel sample preparation technique using electrowetting-based digital microfluidics for matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS). We demonstrate that critical functions, such as generation of droplets from reservoirs and sample purification can be achieved for standard MALDI-MS reagents and analytes by electrowetting-on-dielectric (EWOD) actuation. The results, collected using devices with droplets suspended in air (rather than oil (Pollack et al., 2000)), suggest that the tedious process of sample preparation can be automated on-chip for high-throughput proteomics and other applications of MALDI-MS.

Analytical Chemistry, 2004

A new technique for preparing samples for matrix-assisted laser desorption/ionization mass spectr... more A new technique for preparing samples for matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is reported. The technique relies on electrowettingon-dielectric (EWOD) to move droplets containing proteins or peptides and matrix to specific locations on an array of electrodes for analysis. Standard MALDI-MS reagents, analytes, concentrations, and recipes are demonstrated to be compatible with the technique. Mass spectra are comparable to those collected by conventional methods. Nonspecific adsorption of analytes to device surfaces is demonstrated to be negligible. The results suggest that EWOD may be a useful tool for automating sample preparation for high-throughput proteomics and other applications of MALDI-MS.

Analytical Chemistry, 2005

An in-line sample purification method for MALDI-MS, which relies on the electrowetting-on-dielect... more An in-line sample purification method for MALDI-MS, which relies on the electrowetting-on-dielectric (EWOD)based technique for digital microfluidics, is reported. In this method, a droplet containing peptides and impurities is moved by EWOD and deposited onto a Teflon-AF surface. A droplet of water is subsequently moved over the spot, where it dissolves and removes the impurities. A droplet containing MALDI matrix is then moved to the spot, which is analyzed by MALDI-MS. This purification method reduces the number of salt adduct peaks caused by low concentrations of impurities (e.g., 20 mM sodium phosphate), and reduces or eliminates the catastrophic effects of high concentrations of impurities (e.g., 8 M urea). The method was used to purify spots made by depositing multiple droplets of contaminated peptides. Spectra from the purified spots showed an increase in the S/N ratio as a function of the number of droplets deposited; when not purified, the S/N ratio remained constant regardless of the number of droplets. Finally, the method was used to purify protein digests for peptide mass fragment (PMF) searches, and was shown to be more efficient than the conventional method of purification with reversed-phase-packed pipet tips. We anticipate this new, in-line sample purification technique for EWOD-MALDI-MS will enable development of integrated high-throughput proteomics analysis methodologies.

Sensors and Actuators A-physical, 2003

Optical actuation of liquid droplets has been experimentally demonstrated for the first time usin... more Optical actuation of liquid droplets has been experimentally demonstrated for the first time using a novel optoelectrowetting (OEW) principle. The optoelectrowetting surface is realized by integrating a photoconductive material underneath a two-dimensional array of electrowetting electrodes. Contact angle change as large as 308 has been achieved when illuminated by a light beam with an intensity of 65 mW/cm 2 . A micro-liter droplet of deionized water has been successfully transported by a 4 mW laser beam across a 1 cm Â 1 cm OEW surface. The droplet speed is measured to be 7 mm/s. Light actuation enables complex microfluidic functions to be performed on a single chip without encountering the wiring bottleneck of two-dimensional array of electrowetting electrodes. Published by Elsevier Science B.V.

This paper describes the equilibrium shape of a liquid drop under applied fields such as gravity ... more This paper describes the equilibrium shape of a liquid drop under applied fields such as gravity and electrical fields, taking into account material properties such as dielectric constants, resistivities, and surface tension coefficients. The analysis is based on an energy minimization framework, scaling arguments, and on solutions of Maxwell's electrostatic equations. A rigorous and exact link is provided between the energy function corresponding to any given physical phenomena, and the resulting shape and size dependent force term in the (modified) Young's equation. It is shown that a dielectric solid and a perfectly conducting liquid is all that is needed to exactly recover the Young-Lippmann equation. A dielectric liquid on a conducting solid gives rise to line tension terms. Finally, a slightly resistive liquid on top of a dielectric, highly resistive solid gives rise to contact angle saturation and accurately predicts the experimental data that we observe in our electrowetting devices.

Lab on A Chip, 2006

To realize multiplexed sample preparation on a digital microfluidic chip for high-throughput Matr... more To realize multiplexed sample preparation on a digital microfluidic chip for high-throughput Matrix Assisted Laser Desorption/Ionization Mass Spectrometry (MALDI-MS), several fluidic functions need to be integrated. These include the generation of multiple droplets from a reservoir and parallel in-line sample purification. In this paper, we develop two critical new functions in handling protein solutions and standard proteomic reagents with electrowetting-on-dielectric (EWOD) actuation, leading to an integrated chip for multiplexed sample preparation for MALDI-MS. The first is a voltage sequence designed to generate a series of droplets from each of the three reservoirs--proteomic sample, rinsing fluid, and MALDI reagents. It is the first time that proteomic reagents have been dispensed using EWOD in an air (as opposed to oil) environment. The second is a box-in-box electrode pattern developed to allow droplet passing over dried sample spots, making the process of in-line sample purification robust for parallel processing. As a result, parallel processing of multiple sample droplets is demonstrated on the integrated EWOD-MALDI-MS chip, an important step towards high-throughput MALDI-MS. The MS results, collected directly from the integrated devices, are of good quality, suggesting that the tedious process of sample preparation can be automated on-chip for MALDI-MS applications as well as other high-throughput proteomics applications.

Journal of Applied Physics, 2002

This article discusses and experimentally verifies how to lower the operating voltage that drives... more This article discusses and experimentally verifies how to lower the operating voltage that drives liquid droplets by the principle of electrowetting on dielectric (EWOD). A significant contact angle change (120°-->80°) is desired to reliably pump the droplet in microchannels for applications such as lab-on-a-chip or micrototal analysis systems. Typically, much higher voltages (>100 V) are used to change the wettability of an electrolyte droplet on a dielectric layer compared with a conductive layer. The required voltage can be reduced by increasing the dielectric constant and decreasing the thickness of the dielectric layer, thus increasing the capacitance of the insulating layer. This dependence of applied voltage on dielectric thickness is confirmed through EWOD experiments for three different dielectric materials of varying thickness: Amorphous fluoropolymer (Teflon® AF, Dupont), silicon dioxide (SiO2) and parylene. The dependence on the dielectric constant is confirmed with two different dielectric materials of similar thickness: SiO2 and barium strontium titanate. In all cases, the surface is coated with a very thin (200 Å) layer of amorphous fluoropolymer to provide initial hydrophobicity. Limiting factors such as the dielectric breakdown and electrolysis are also discussed. By using very thin (700 Å) and high dielectric constant (~180) materials, a significant contact angle change (120°-->80°) has been achieved with voltages as low as 15 V. Based on these results, a microfluidic device has been fabricated and tested, demonstrating successful transporting (pumping) of a 460 nL water droplet with only 15 V.

Acs Applied Materials & Interfaces, 2010

RC filters are used to discriminate unwanted frequency elements of a specific signal. Here we rep... more RC filters are used to discriminate unwanted frequency elements of a specific signal. Here we report a new concept for a tunable RC filter. The concept was demonstrated by developing a tunable RC filter "consisting of an ionic liquid drop placed on a dielectric layer." Cut-off frequency of the filter can be altered and controlled by changing the drop shape via electrowetting. The dielectric layer and the solid-liquid interface behave as serially connected capacitors, where the total capacitance is a function of drop shape (or contact angle). The drop shape and hence the total capacitance can be instantly controlled by electrowetting. The change in the capacitance will change the cutoff frequency of the filter. For a 5 µL ionic liquid drop, the achieved "tunability range" was 4.5-9.8 kHz. This demonstrates that the new concept is attainable. This RC filter system could potentially be used as a detecting technique.

Analytical Chemistry, 2010

This paper presents a study of electrowetting of ionic liquids (ILs) under AC voltages, where nin... more This paper presents a study of electrowetting of ionic liquids (ILs) under AC voltages, where nine different ILs (including mono-, di-, and tricationic varieties) with three different AC frequencies (60 Hz, 1 kHz, 10 kHz) were experimentally investigated. The main foci of this study are (i) an investigation of AC frequency dependence on the electrowetting of ILs; (ii) obtaining theoretical relationships between the relevant factors that explain the experimentally achieved frequency dependence; and (iii) a systematic comparison of electrowetting of ILs using AC vs DC voltage fields. The frequency of the AC voltage was found to be directly related to the apparent contact angle change (∆θ) of the ILs. This relationship was further analyzed and explained theoretically. The electrowetting properties of ILs under AC voltages were compared to that under DC voltages. All tested ILs showed greater apparent contact angle changes with AC voltage conditions than with DC voltage conditions. The effect of structure and charge density also was examined. Electrowetting reversibility under AC voltage conditions was studied for few ILs. Finally, the physical properties and AC electrowetting properties of ILs were measured and tabulated.

Analytical Chemistry, 2008

Water or aqueous electrolytes are the dominant components in electrowetting on dielectric (EWOD)-... more Water or aqueous electrolytes are the dominant components in electrowetting on dielectric (EWOD)-based microfluidic devices. Low thermal stability, evaporation, and a propensity to facilitate corrosion of the metal parts of integrated circuits or electronics are drawbacks of aqueous solutions. The alternative use of ionic liquids (ILs) as electrowetting agents in EWOD-based applications or devices could overcome these limitations. Efficient EWOD devices could be developed using task-specific ILs. In this regard, a fundamental study on the electrowetting properties of ILs is essential. Therefore electrowetting properties of 19 different ionic liquids, including mono-, di-, and tricationic, plus mono-and dianionic ILs were examined. All tested ILs showed electrowetting of various magnitudes on an amorphous flouropolymer layer. The effects of IL structure, functionality, and charge density on the electrowetting properties were studied. The enhanced stability of ILs in electrowetting on dielectric at higher voltages was studied in comparison with water. Deviations from classical electrowetting theory were confirmed. The physical properties of ILs and their electrowetting properties were tabulated. These data can be used as references to engineer task-specific electrowetting agents (ILs) for future electrowetting-based applications. . Electrowetting curves of IL4 and water saturated IL4.

Acs Applied Materials & Interfaces, 2009

Efficient and facile synthesis of novel linear tricationic room-temperature ionic liquids was per... more Efficient and facile synthesis of novel linear tricationic room-temperature ionic liquids was performed, and their physiochemical properties were determined. Different physiochemical properties were observed according to the structural variations such as the cationic moiety and the counteranion of the ionic liquid. The electrowetting properties of these ionic liquids were also investigated, and linear tricationic ionic liquids were shown to be advantageous as effective electrowetting materials due to their high structural flexibility.

Surface tension is a line force, directly proportional to the length. However, surface forces (e.... more Surface tension is a line force, directly proportional to the length. However, surface forces (e.g., pressure) are proportional to the square of the length, and body forces (e.g., inertia or gravity) are proportional to the cubic of the length. Therefore, surface tension becomes dominant over pressure or gravity as the dimension of interest gets smaller – in practice, below millimeter range. Many examples are found in our daily lives. An ant can carry a load that is several times heavier than its own weight, demonstrating its relative strength, but it also can be trapped in a water droplet showing its relative weakness. Small creatures climb up walls. Water striders hop on water surfaces. Note these examples are in millimeter scale where inertia forces are still comparable to surface tension. In micrometer scale, surface tension starts to dominate over not only the inertia forces but most other physical forces.

This paper demonstrates a novel drop-to-drop liquid-liquid microextraction (DTD-LLME) device, whi... more This paper demonstrates a novel drop-to-drop liquid-liquid microextraction (DTD-LLME) device, which is based on an electrowetting on dielectric (EWOD) digital microfluidic chip. Droplets of two immiscible liquids, one of which is an ionic liquid, are formed in nanoliter volumes, driven along electrodes, merged and mixed for extraction, and finally separated upon the completion of the extraction process. All the steps are carried out on a microfluidic chip using combined electrowetting and dielectrophoretic forces, which act on the droplet upon the application of electric potential. Specially, the phase separation of two immiscible nanoliter-scale liquid drops was achieved for the first time on an EWOD digital microfluidic chip. To study the on-chip extraction kinetics, an image-based concentration measurement technique with suitable color parameters was studied and compared with the typical UV absorption based technique. Finally, the effect of applied ac voltage frequency on the extraction kinetics was studied. The observations on DTD-LLME, particularly phase separation, are discussed. The image-based method was found to be applicable for precise concentration measurements with the right choice of the color parameter. Results from experiments on finding the frequency dependence on extraction kinetics demonstrate that the application of higher frequencies can be a factor in accelerating the extraction on the proposed microextraction device.