Search

In electrowetting-on-dielectric (EWOD) microfluidics, a motion of a fluid is created by a voltage applied to the fluid/surface interface. Water and aqueous solutions are the most frequently used fluids in EWOD devices. In order for EWOD microfluidics to be a versatile platform for various applications, however, movability of different types of fluids other than aqueous solutions should be understood. An electromechanical model using a simple RC circuit has been used to predict the mechanical force exerted on a liquid droplet upon voltage application. In this present study, two important features missed in previous works are addressed. Energy dissipation by contact line friction is considered in the new model as the form of resistor. The phase angle is taken into account in the analysis of the AC circuit. The new electromechanical model and computation results are validated with experimental measurements of forces on two different liquids. The model is then used to explain influences of contact angle hysteresis, surface tension, conductivity, and dielectric constant of fluids to the mechanical force on a liquid droplet. [ABSTRACT FROM AUTHOR]

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 ([DELTA][theta]) 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. 10.1021/ac9021852

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.

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.

A new technique for preparing samples for matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is reported. The technique relies on electrowetting-on-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.

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 ([micro]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.sub.dc] applied voltage. [862] Index Terms--Contact angle, electrowetting, electrowetting on dielectric (EWOD), lab-on-a-chip, microfluidics, micro total analysis system ([micro]TAS), surface tension.

Precipitation related to atmospheric rivers (ARs) is critical in shaping climate and extreme hydrologic events in the mid‐latitude coastal regions, making it a recent topic of intense research. This study has analysed the AR climatology, AR‐precipitation characteristics and their regional variations in East Asia (EA) for 1979–2015 using an AR inventory based on vertically‐integrated water‐vapour transport and AR geometry. The AR frequency over EA varies seasonally with highest (lowest) frequency in summer (winter). The AR frequency peaks in the coastal regions of the mid‐latitude western North Pacific (WNP), then decreases rapidly away from the region towards higher/lower latitudes and inlands. ARs are related to 30–60% of the annual precipitation in EA with maximum in summer; secondary peaks occur in spring in eastern China, Korea and southern Japan, and in winter for the southern China coast. Calculating the AR precipitation on days of partial AR occurrences from daily precipitation data is a key source of uncertainties in the AR‐precipitation fraction in this study. The AR days are substantially more related to heavy precipitation than non‐AR days for all seasons and regions. Seasonal cycle of AR frequency over EA and the WNP follows the northward march of the main rainband from spring to mid‐summer, but is decoupled from it from late summer to winter as the North Pacific storm track migrates into the Eastern North Pacific. The annual cycle of AR frequency and AR precipitation varies regionally; Japan and southern China adjacent to WNP show high (low) frequency of weak‐ (heavy‐) precipitating ARs in early spring (late fall) with highly frequent heavy‐precipitating ARs in summer while the mid‐latitude coastal regions, eastern China and Korea, show low frequency of heavy‐precipitating ARs in early fall with highly frequent heavy‐precipitating ARs in mid‐summer. [ABSTRACT FROM AUTHOR]