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1. EWOD-aided droplet transport on texture ratchets

   https://doi.org/10.1063/1.5142571  

   Sun, Di; Böhringer, Karl F. (March 2020, Applied Physics Letters)

   We report a digital microfluidic device to transport aqueous droplets on an open surface in air using electrowetting-on-dielectric (EWOD) with anisotropic ratchet conveyors (ARCs). ARCs are micro-sized periodic semicircular hydrophilic regions on a hydrophobic background, providing anisotropic wettability. SiNx and Cytop are used as the dielectric layer between the water droplet and working electrodes. By adopting parylene as a stencil mask, hydrophilic patterning on the hydrophobic Cytop thin film layer is achieved without the loss of Cytop hydrophobicity. While the traditional EWOD platform requires the control of multiple electrodes to transport the droplet, our system utilizes only two controlling electrodes. We demonstrate that 15 μl water droplets are transported at a speed of 13 mm/s under 60 Vpeak sinusoid AC signal at 50 Hz. Droplet transport at 20 Hz is also presented, demonstrating that the system can operate within a range of frequencies. 

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2. Metasurfaces for Sensing Applications: Gas, Bio and Chemical

   https://doi.org/10.3390/s22186896  

   Tabassum, Shawana; Nayemuzzaman, SK; Kala, Manish; Kumar Mishra, Akhilesh; Mishra, Satyendra Kumar (September 2022, Sensors)

   Performance of photonic devices critically depends upon their efficiency on controlling the flow of light therein. In the recent past, the implementation of plasmonics, two-dimensional (2D) materials and metamaterials for enhanced light-matter interaction (through concepts such as sub-wavelength light confinement and dynamic wavefront shape manipulation) led to diverse applications belonging to spectroscopy, imaging and optical sensing etc. While 2D materials such as graphene, MoS2 etc., are still being explored in optical sensing in last few years, the application of plasmonics and metamaterials is limited owing to the involvement of noble metals having a constant electron density. The capability of competently controlling the electron density of noble metals is very limited. Further, due to absorption characteristics of metals, the plasmonic and metamaterial devices suffer from large optical loss. Hence, the photonic devices (sensors, in particular) require that an efficient dynamic control of light at nanoscale through field (electric or optical) variation using substitute low-loss materials. One such option may be plasmonic metasurfaces. Metasurfaces are arrays of optical antenna-like anisotropic structures (sub-wavelength size), which are designated to control the amplitude and phase of reflected, scattered and transmitted components of incident light radiation. The present review put forth recent development on metamaterial and metastructure-based various sensors. 

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3. Optical Dielectrophoretic (DEP) Manipulation of Oil-Immersed Aqueous Droplets on a Plasmonic-Enhanced Photoconductive Surface

   https://doi.org/10.3390/mi13010112  

   Thio, Si; Park, Sung-Yong (January 2022, Micromachines)

   We present a plasmonic-enhanced dielectrophoretic (DEP) phenomenon to improve optical DEP performance of a floating electrode optoelectronic tweezers (FEOET) device, where aqueous droplets can be effectively manipulated on a light-patterned photoconductive surface immersed in an oil medium. To offer device simplicity and cost-effectiveness, recent studies have utilized a polymer-based photoconductive material such as titanium oxide phthalocyanine (TiOPc). However, the TiOPc has much poorer photoconductivity than that of semiconductors like amorphous silicon (a-Si), significantly limiting optical DEP applications. The study herein focuses on the FEOET device for which optical DEP performance can be greatly enhanced by utilizing plasmonic nanoparticles as light scattering elements to improve light absorption of the low-quality TiOPc. Numerical simulation studies of both plasmonic light scattering and electric field enhancement were conducted to verify wide-angle scattering light rays and an approximately twofold increase in electric field gradient with the presence of nanoparticles. Similarly, a spectrophotometric study conducted on the absorption spectrum of the TiOPc has shown light absorption improvement (nearly twofold) of the TiOPc layer. Additionally, droplet dynamics study experimentally demonstrated a light-actuated droplet speed of 1.90 mm/s, a more than 11-fold improvement due to plasmonic light scattering. This plasmonic-enhanced FEOET technology can considerably improve optical DEP capability even with poor-quality photoconductive materials, thus providing low-cost, easy-fabrication solutions for various droplet-based microfluidic applications. 

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4. Plasmonic-enhanced floating electrode optoelectronic tweezers (FEOET) for effective optical droplet manipulation

   S. Thio, S. Bae (January 2021, The 21st International Conference on Solid-State Sensors, Actuators and Microsystems (Transducers 2021))

   A plasmonic-enhanced floating electrode optoelectronic tweezers (FEOET) device is presented for effective optical droplet manipulation. Due to the importance of having a high-quality photoconductive layer, conventional FEOET devices face the issue between ineffective DEP performance and cost-ineffective fabrication. In this study, the use of metallic nanoparticles enables plasmonic light scattering to significantly enhance light absorption onto a photoconductive layer of the device, resulting in a largely improved dielectrophoretic (DEP) performance. Two numerical simulation studies have demonstrated the working principle of plasmonic-enhanced DEP and were further validated experimentally by an improved spectrophotometric light absorbance of the TiOPc layer, as well as demonstrating an 11-fold increase in light-actuated droplet speed. With much-improved DEP performance, this plasmonic-enhanced FEOET technology can provide a low-cost solution for various digital microfluidic (DMF) applications with the benefits of device simplicity. 

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5. Advancing Reverse Electrowetting‐on‐Dielectric from Planar to Rough Surface Electrodes for High Power Density Energy Harvesting

   https://doi.org/10.1002/ente.202100867  

   Adhikari, Pashupati_R; Patwary, Adnan_B; Kakaraparty, Karthik; Gunti, Avinash; Reid, Russell_C; Mahbub, Ifana (January 2022, Energy Technology)

   Reverse electrowetting‐on‐dielectric (REWOD)‐based energy harvesting has been studied over the last decade as a novel technique of harvesting energy by actuating liquid droplet(s) utilizing applied mechanical modulation. Much prior research in REWOD has relied on planar electrodes, which by its geometry possess a limited surface area. In addition, most of the prior REWOD works have applied a high bias voltage to enhance the output power that compromises the concept of self‐powering wearable motion sensors in human health monitoring applications. In order to enhance the REWOD power density resulting from an increased electrode–electrolyte interfacial area, high surface area electrodes are required. Herein, electrical and multiphysics‐based modeling approaches of REWOD energy harvester using structured rough surface electrodes are presented. By enhancing the overall available surface area, an increase in the overall capacitance is achieved. COMSOL and MATLAB‐based models are also developed, and the empirical results are compared with the models to validate the performance. Root mean square (RMS) power density is calculated using the RMS voltage across an optimal load impedance. For the proposed rough electrode REWOD energy harvester, maximum power density of 3.18 μW cm⁻²is achieved at 5 Hz frequency, which is ≈4 times higher than that of the planar electrodes. 

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