Namwon Kim

A three-compartment graphical illustration showing the sources of PFOA, properties of biochar, and adsorption mechanisms. Per- and polyfluoroalkyl substances (PFAS) are synthetic chemicals widely used in consumer products, firefighting foams, and various industrial applications. These chemicals are pervasive, persistent, and degradation-resistant, posing a critical threat to human and environmental health. Currently, adsorption remains the most widely used technology for the remediation of PFAS. However, large-scale PFAS removal would require extensive usage of adsorbents such as activated carbon, which tends to be relatively expensive. Therefore, we investigated the upcycling of water hyacinth, an invasive aquatic plant, as a sustainable source for biochar production to remove perfluorooctanoic acid (PFOA), a type of PFAS. Different kinetic and isotherm models assessed PFOA adsorption to water hyacinth biochar. Pseudo-second-order kinetics and Langmuir isotherm models proved to be better fits in most experiments. Intra-particle diffusion and surface adsorption were determined to contribute to the rate-limiting step, and a monolayer distribution of PFOA through chemisorption was found to be the primary mechanism of adsorption. Water hyacinth biochar exhibited an adsorption capacity of 154.39 μg/g for PFOA removal at a solid-to-liquid ratio of 1 (in mg/mL). Experiments at different pH levels revealed that electrostatic interactions influenced PFOA adsorption, and, in acidic conditions, the removal capacity increased to 164.91 μg/g.

Graphene produced by chemical vapor deposition finds applications in a wide range of technologies. However, the transfer of monolayer graphene from the native substrate (commonly Cu foils) to a desired substrate remains challenging. In this study, we report a solvent-free transfer technique for monolayer graphene using a sublimable supporting coating, cyclododecane (CDD). After spin-coating CDD on graphene grown on Cu foil, we rapidly heat the stack at 80 °C to melt and recrystallize the CDD coating. The recrystallized coating top surface becomes composed of larger and interconnected CDD crystals, which form a mechanically strong coating that takes almost 2 h to sublimate completely. Having a bottom surface that closely conforms to graphene's topography, the CDD coating can effectively support graphene during the whole transfer process without compromising its structural integrity, maintaining its superior electrical and optical properties. In this way, CDD becomes capable of transferring monolayer graphene over square centimeter areas. Unlike traditional polymer-based techniques, after transfer, the CDD coating sublimates completely, without the need of any post-transfer cleaning treatment. Our work shows that the CDD coating can be an alternative transfer medium for the efficient and clean transfer of monolayer graphene. This technique paves the way for the widespread adoption of graphene in various applications, including those not compatible with solvents.

Continuous delivery of segmented reagents using pressure-driven multiphase flow in microchannels is a promising technology for high throughput microfluidic bioassays. Separation and encapsulation of the target reagents with another inert fluid provide many advantages over single phase flow in microfluidic applications of biotechnology. In order to achieve these advantages and control these multiphase flows, it is necessary to understand their generation and transport characteristics as influenced by geometrical miniaturization, channel wall properties, the effects of surfactants and operating conditions. For gas-liquid two-phase flow, dry air and deionized water were driven into hot embossed PMMA microchannels with 200 μm square test microchannels. Flow regimes, flow maps and the lengths of the gas bubbles and liquid plugs in terms of the liquid volumetric flow ratio (βL) were determined. Continuous generation of regular segmented flow was also discussed. Three sub-regimes of the Segmented flow were identified based on the statistical phase length scales observed over a substantial test channel length. For the liquid-liquid segmented flow, deionized water and perfluorocarbon with a surfactant were used as test fluids in the hot embossed polycarbonate microchannels. The effects of three expansion ratios from the injection to the test channels of 2, 4, and 16 were investigated comparing the flow regimes, transitions and maps in terms of a fixed carrier fluid volumetric flow ratio. The length of the dispersed fluids and the distance between consecutive droplets or plugs in terms of the carrier fluid volumetric flow ratio (βC) were determined. Velocities of the dispersed droplets and plugs were measured using double-pulsed laser illumination and were found to be 1.46 ± 0.08 and 1.25 ± 0.05 times faster than the superficial velocity of the segmented flow, respectively. The multiphase flow pressure drops were measured for all of the flow regimes in gas-liquid two-phase and liquid-liquid segmented flows. Each flow regime identified on the basis of topological observations, including the length scale of each fluid phase and the number of the gas bubbles or dispersed droplets in unit length with respect to the volumetric flow ratio, was associated with different trends in the pressure drop variation.

Covalent functionalization of the surface is crucial in 2D materials because of their atomic thinness, large surface-to-volume ratio, and uniform surface chemical potential.

The improvement in the quantitative and qualitative heat transfer performances of working fluids is trending research in the present time for heat transfer applications. In the present work, the first and second law analyses of a microplate heat exchanger with single-particle and hybrid nanofluids are conducted. The microplate heat exchanger with single-particle and hybrid nanofluids is analyzed using the computational fluid dynamics approach with symmetrical heat transfer and fluid flow analyses. The single-particle Al₂O₃ nanofluid and the hybrid Al₂O₃/Cu nanofluid are investigated for different nanoparticles shapes of sphere (Sp), oblate spheroid (OS), prolate spheroid (PS), blade (BL), platelet (PL), cylinder (CY) and brick (BR). The first law characteristics of NTU, effectiveness and performance index and the second characteristics of thermal, friction and total entropy generation rates and Bejan number are compared for Al₂O₃ and Al₂O₃/Cu nanofluids with considered different-shaped nanoparticles. The OS- and PL-shaped nanoparticles show superior and worse first and second law characteristics, respectively, for Al₂O₃ and Al₂O₃/Cu nanofluids. The hybrid nanofluid presents better first and second law characteristics compared to single-particle nanofluid for all nanoparticle shapes. The Al₂O₃/Cu nanofluid with OS-shaped nanoparticles depicts maximum values of performance index and Bejan number as 4.07 and 0.913, respectively. The first and second law characteristics of the best combination of the Al₂O₃/Cu nanofluid with OS-shaped nanoparticles are investigated for various volume fractions, different temperature and mass flow rate conditions of hot and cold fluids. The first and second law characteristics are optimum at higher hot fluid temperature, lower cold fluid temperature, lower hot and cold fluid mass flow rates. In addition, the first and second law characteristics have improved with increase in volume fraction.

Mass production of affordable microfluidic devices is a subject of importance to the biological and medical communities. If devices are cheap enough to be disposable, tests can be run at a higher rate and lower cost without concern for contamination. Achieving this cost-effectiveness requires devices to be of polymer material, made by injection molding or hot embossing. Though

Internal short circuit in lithium-ion battery by penetrating element leads to exothermic behavior due to accumulated heat. In the present study, investigations are conducted on the thermal behavior of the LIR2450 micro coin cell haivng capacity of 120 mAh, with internal short circuit by penetrating element. The experimental coin cell discharge study was conducted and validated with numerical study within ±5.0%. The effect of penetrating element size, location of penetrating element, state of charge, discharge rate, short-circuit resistance, and heat transfer co-efficient on maximum coin cell temperature and heat generation rate are analyzed. The penetrating element diameters of 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, and 3.5 mm are considered. The effect of initial state of charge (SOC) is considered with 100%, 80%, 60%, and 40%. Three locations for penetrating element are considered with the center, the middle of the radius, and on the edge of the coin cell radius. The different discharge rates of 1C, 2C, 3C, and 4C are considered. The higher-penetrating element size of 3.5 mm with location at the center of the coin cell with 100% SOC showed maximum heat generation rate and maximum temperature of the coin cell. In addition, the optimum value of the dimensionless heat generation rate is obtained at dimensionless short-circuit resistance. The study provides comprehensive insights on the thermal behavior of the lithium-ion cell during thermal abuse condition with internal short circuit by penetrating element.