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1. Electrokinetic particle trapping in microfluidic wells using conductive nanofiber mats

   https://doi.org/10.1002/elps.202400051  

   West, J_Hunter; Mondal, Tonoy_K; Williams, Stuart_J (September 2024, ELECTROPHORESIS)

   Abstract The frequency dependence of electrokinetic particle trapping using large‐area (>mm²) conductive carbon nanofiber (CNF) mat electrodes is investigated. The fibers provide nanoscale geometric features for the generation of high electric field gradients, which is necessary for particle trapping via dielectrophoresis (DEP). A device was fabricated with an array of microfluidic wells for repeated experiments; each well included a CNF mat electrode opposing an aluminum electrode. Fluorescent microspheres (1 µm) were trapped at various electric field frequencies between 30 kHz and 1 MHz. Digital images of each well were analyzed to quantify particle trapping. DEP trapping by the CNF mats was greater at all tested frequencies than that of the control of no applied field, and the greatest trapping was observed at a frequency of 600 kHz, where electrothermal flow is more significantly weakened than DEP. Theoretical analysis and measured impedance spectra indicate that this result was due to a combination of the frequency dependence of DEP and capacitive behavior of the well‐based device. 

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2. Multi-frequency MEMS acoustic emission sensor

   https://doi.org/10.1016/j.sna.2023.114648  

   Khan, Talha Masood; Taha, Raguez; Zhang, Tonghao; Ozevin, Didem (November 2023, Sensors and Actuators A: Physical)

   In this paper, a multi-frequency MEMS acoustic emission (AE) sensor is designed, characterized, and tested. The sensor includes sixteen individual resonators tuned in the range of 100 kHz to 700 kHz. The resonator frequencies are selected to form constructive interference when they are connected in parallel to increase the signal-to-noise ratio. Each resonator is comprised of a membrane that forms the mass and four beams that provide stiffness. The membrane size is kept the same for each resonator to have approximately the same sensitivity per frequency. The influence of spring elements on the resonant frequency and the sensitivity is numerically demonstrated. The sensor is manufactured using MEMSCAP PiezoMUMPs. The characterization experiments show a slight shift in the resonant frequency of individual resonators compared to the design values. The MEMS sensor is packaged using a custom-designed printed circuit board to improve the signal-to-noise ratio. The sensor performance is compared with a conventional AE sensor. The sensitivity and frequency bandwidth of the MEMS AE device is brought to a comparable level to bulky AE sensors. 

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3. Wehnelt photoemission in an ultrafast electron microscope: Stability and usability

   https://doi.org/10.1063/5.0214246  

   Willis, Simon A; Curtis, Wyatt A; Flannigan, David J (July 2024, Journal of Applied Physics)

   We tested and compared the stability and usability of three different cathode materials and configurations in a thermionic-based ultrafast electron microscope: (1) on-axis thermionic and photoemission from a custom 100 μm diameter LaB6 source with a graphite guard ring, (2) off-axis photoemission from the Ni aperture surface of the Wehnelt electrode, and (3) on-axis thermionic and photoemission from a custom 200 μm diameter polycrystalline Ta source. For each cathode type and configuration, including the Ni Wehnelt aperture, we illustrate how the photoelectron beam-current stability is deleteriously impacted by simultaneous cooling of the source following thermionic heating. Furthermore, we demonstrate usability via collection of parallel- and convergent-beam electron diffraction patterns and by formation of the optimum probe size. We find that usability of the off-axis Ni Wehnelt-aperture photoemission is at least comparable to on-axis LaB6 thermionic emission, as well as to on-axis photoemission [the heretofore conventional approach to ultrafast electron microscopy (UEM) in thermionic-based instruments]. However, the stability and achievable beam currents for off-axis photoemission from the Wehnelt aperture were superior to that of the other cathode types and configurations, regardless of the electron-emission mechanism. Beam-current stability for this configuration was found to be ±1% (one standard deviation from the mean) for 70 min (longest duration tested), and steady-state beam current was reached within the sampling-time resolution used here (∼1 s) for 15 pA beam currents (i.e., 460 electrons per packet for a 200 kHz repetition rate). Repeatability and robustness of the steady-state condition were also found to be within ±1% of the mean. We discuss the implications of these findings for UEM imaging and diffraction experiments, for pulsed-beam damage measurements, and for practical switching between optimum conventional TEM and UEM operation within the same instrument. 

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4. Capacitance Effects of a Hydrophobic-Coated Ion Gel Dielectric on AC Electrowetting

   https://doi.org/10.3390/mi12030320  

   Lee, Taewoo; Park, Sung-Yong (March 2021, Micromachines)

   We present experimental studies of alternating current (AC) electrowetting dominantly influenced by several unique characteristics of an ion gel dielectric in its capacitance. At a high-frequency region above 1 kHz, the droplet undergoes the contact angle modification. Due to its high-capacitance characteristic, the ion gel allows the contact angle change as large as Δθ = 26.4°, more than 2-fold improvement, compared to conventional dielectrics when f = 1 kHz. At the frequency range from 1 to 15 kHz, the capacitive response of the gel layer dominates and results in a nominal variation in the angle change as θ ≈ 90.9°. Above 15 kHz, such a capacitive response of the gel layer sharply decreases and leads to the drastic increase in the contact angle. At a low-frequency region below a few hundred Hz, the droplet’s oscillation relying on the AC frequency applied was mainly observed and oscillation performance was maximized at corresponding resonance frequencies. With the high-capacitance feature, the ion gel significantly enlarges the oscillation performance by 73.8% at the 1st resonance mode. The study herein on the ion gel dielectric will help for various AC electrowetting applications with the benefits of mixing enhancement, large contact angle modification, and frequency-independent control. 

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5. Antifouling Studies of Unsymmetrical Oligo(ethylene glycol) Spiroalkanedithiol Self-Assembled Monolayers

   https://doi.org/10.3390/micro1010012  

   St. Hill, Lydia R.; Tran, Hung-Vu; Chinwangso, Pawilai; Lee, Han Ju; Marquez, Maria D.; Craft, John W.; Lee, T. Randall (September 2021, Micro)

   The antifouling properties of self-assembled monolayers (SAMs) on gold generated from custom-designed bidentate unsymmetrical spiroalkanedithiols containing both oligo(ethylene glycol) and hydrocarbon tailgroups (EG3C7-C7 and EG3C7-C18) were evaluated and compared to SAMs derived from analogous monodentate octadecanethiol (C18SH) and the tri(ethylene glycol)-terminated alkanethiol EG3C7SH. Complementary techniques, including in situ surface plasmon resonance spectroscopy (SPR), ex situ electrochemical quartz crystal microbalance (QCM) measurements, and ex situ ellipsometric thickness measurements, were employed to assess the protein resistance of the SAMs using proteins having a wide range of sizes, structures, and properties: protamine, lysozyme, bovine serum albumin (BSA), and fibrinogen. The studies found that SAMs generated from the bidentate adsorbates EG3C7-C7 and EG3C7-C18, which contain a 1:1 mixture of OEG and hydrocarbon tailgroups, exhibited a diminished capacity to resist protein adsorption compared to the EG3C7SH SAMs, which possess only OEG tailgroups. The data highlight the critical role of hydration of the OEG matrix for generating antifouling OEG-based surface coatings. 

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