Myeongsub Kim

ABSTRACT Laser-induced fluorescence (LIF) thermometry measures liquid temperatures based on changes in fluorescence intensity. In dual-tracer or ratiometric LIF thermometry, liquid temperatures are based on the ratio of the fluorescent signals from two different fluorophores, since this ratio is independent of changes in fluorescence intensity due to variations in the excitation. Recently, a dual-tracer LIF technique using two species with opposite temperature sensitivities, fluorescein 27 and Kiton Red (sulforhodamine B), excited at 532 nm has been reported with temperature sensitivities as great as 7% per ^oC [Sutton et al. (2008) Exp. Fluids DOI 10.1007/s00348-008-0506-4]. We describe here a similar technique using fluorescein and sulforhodamine B, which have intensities that increase by 2.2% and decrease by 1.3%, respectively, per ^oC when volumetrically illuminated at 514 nm. The ratio of these two signals gives temperature sensitivities as great as 9% per ^oC. This LIF technique is used to measure temperature distributions in water flowing through a 500 μm x 1000 μm polydimethylsiloxane (PDMS) microchannel covered with a glass lid with localized heating to create temperature gradients up to about 15 ^oC /mm. The results are compared with FLUENT predictions.

Hydrogen is an excellent energy source for long-term storage and free of greenhouse gases. However, its high production cost remains an obstacle to its advancement. The two main parameters contributing to the high cost include the cost of electricity and the cost of initial financial investment. It is possible to reduce the latter by the optimization of system design and operation conditions, allowing the reduction of the cell voltage. Because the CAPEX (initial cost divided by total hydrogen production of the electrolyzer) decreases according to current density but the OPEX (operating cost depending on the cell voltage) increases depending on the current density, there exists an optimal current density. In this paper, a genetic algorithm has been developed to find the optimal evolution parameters and to determine an optimum electrolyzer design. The optimal current density has been increased by 10% and the hydrogen cost has been decreased by 1%.

Hydrogen storage is a promising technology for storage of renewable energy resources. Despite its high energy density potential, the development of hydrogen storage has been impeded, mainly due to its significant cost. Although its cost is governed mainly by electrical energy expense, especially for hydrogen produced with alkaline water electrolysis, it is also driven by the value of the cell tension. The most common means of electrolyzer improvement is the use of an electrocatalyst, which reduces the energy required for electrochemical reaction to take place. Another efficient means of electrolyzer improvement is to use the Computational Fluid Dynamics (CFD)-assisted design that allows the comprehension of the phenomena occurring in the electrolyzer and also the improvement in the electrolyzer’s efficiency. The designed two-phase hydrodynamics model of this study has been compared with the experimental results of velocity profiles measured using Laser Doppler Velocimetry (LDV) meth...

Biosample encapsulation is a critical step in a wide range of biomedical and bioengineering applications. Aqueous two-phase system (ATPS) droplets have been recently introduced and showed a great promise to the biological separation and encapsulation due to their excellent biocompatibility. This study shows for the first time the passive generation of salt-based ATPS microdroplets and their biocompatibility test. We used two ATPS including polymer/polymer (polyethylene glycol (PEG)/dextran (DEX)) and polymer/salt (PEG/Magnesium sulfate) for droplet generation in a flow-focusing geometry. Droplet morphologies and monodispersity in both systems are studied. The PEG/salt system showed an excellent capability of uniform droplet formation with a wide range of sizes (20–60 μm) which makes it a suitable candidate for encapsulation of biological samples. Therefore, we examined the potential application of the PEG/salt system for encapsulating human umbilical vein endothelial cells (HUVECs)....

Aqueous microdroplet generation in gaseous phase is an emerging area of research due to its numerous advantages compared to conventional liquid-liquid system including high system throughput and fast mixing. In this paper, we numerically studied the aqueous droplet generation in an inertial air flow in a T-junction droplet generator to understand the droplet formation mechanisms. The Volume of Fluid method is employed to track the interface between two immiscible fluids. The two-phase flow behavior of water droplet in air in the T-junction microchannel over a wide range of Capillary number (0.0001-0.1), and Reynolds number (0.1-100) was examined. At various Reynolds and Capillary numbers, unique flow regime mapping was determined including squeezing, dripping, jetting, unstable dripping, and unstable jetting. It was found that stable aqueous droplets are generated in the squeezing and dripping flow regimes. On the other hand, the unstable dripping flow regime is unable to sustain spherical droplets as they travel downstream. In the unstable jetting flow regime, a stream of water moves downstream and then it's tip splits into small or large fragments of water. The results show that the droplet size increases as Capillary and Reynolds numbers increases and decreases, respectively. As both Capillary and Reynolds numbers increase, the droplet generation frequency increases, reaching its maximum at 223 Hz.

This work shows the potential of nickel (Ni) nanoparticles (NPs) stabilized by polymers for accelerating carbon dioxide (CO) dissolution into saline aquifers. The catalytic characteristics of Ni NPs were investigated by monitoring changes in diameter of CO microbubbles. An increase in ionic strength considerably reduces an electrostatic repulsive force in pristine Ni NPs, thereby decreasing their catalytic potential. This study shows how cationic dextran (DEX), nonionic poly(vinyl pyrrolidone) (PVP), and anionic carboxy methylcellulose (CMC) polymers, the dispersive behaviors of Ni NPs can be used to overcome the negative impact of salinity on CO dissolution. The cationic polymer, DEX was less adsorbed onto NPs surfaces, thereby limiting the Ni NPs' catalytic activity. This behavior is due to a competition for Ni NPs' surface sites between the cation and DEX under high salinity. On the other hand, the non/anionic polymers, PVP and CMC could be relatively easily adsorbed onto...

The recent advent of aqueous two-phase system (ATPS) has shown great potential to rapidly generate microscale aqueous droplets without tedious post-processing. ATPS provides a more biologically friendly and straightforward method to manufacture aqueous droplets compared with conventional oil-water systems and as such, it has been employed in many biomedical applications. Although the cost-effective manufacturing of aqueous droplets has been feasible by direct generation in ATPS, an understanding of the underlying physics of droplet formation in ATPS is still in its infancy. In this paper, we investigated the hydrodynamic behavior and mechanisms of all-aqueous droplet formation in two flow-focusing droplet generators. This study specifically tests whether ATPS in different geome-tries generates aqueous droplets with different sizes and properties. To achieve these goals, two incompatible polymers, namely polyethylene glycol (PEG) and dextran (DEX), were mixed in water to make ATPS. The influences of inlet pressures and flow-focusing configurations on droplet size, droplet generation frequency, and thread breakup length were quantified. In addition, flow regime mapping for the two different droplet generators at 30° and 90° junction angles was obtained. The results show that droplet size is very susceptible to the junction angle. On the other hand, inlet pressures of the PEG and DEX flows readily control five main flow regimes including PEG Back Flow, DEX Back Flow, Dripping Flow, Jetting Flow and Stratified Flow. It is observed that generated droplets in the Jetting Flow regime are larger in the case of 30°, whereas larger droplets are obtained in the Dripping Flow regime at 90° configuration. The frequency of droplet generation increases and decreases by increasing P PEG and P DEX, respectively. Finally, we characterized flow regimes by the Capillary number (Ca) of the PEG flow.

This work reports a microfluidic study investigating the feasibility of accelerating gaseous carbon dioxide (CO2) dissolution into a continuous aqueous phase with the use of metallic nickel (Ni) nanoparticles (NPs) under conditions specific to carbon sequestration in saline aquifers. The dissolution of CO2 bubbles at different pH levels and salinities was studied to understand the effects that the intrinsic characteristics of brine in real reservoir conditions would have on CO2 solubility. Results showed that an increased shrinkage of CO2 bubbles occurred with higher basicity, while an increased expansion of CO2 bubbles was observed with proportionally increasing salinity. To achieve acceleration of CO2 dissolution in acidic brine containing high salinity content, the catalytic effect of Ni NPs was investigated by monitoring change in CO2 bubble size at various Ni NPs concentration. The optimal concentration for Ni NPs suspension was determined to be 30 mg L(-1); increasing the conc...

Measuring fluid temperature fields at micron-scale spatial resolution is of interest in applications including microelectronic cooling and microfluidics. Fluorescence thermometry (FT), where temperatures are estimated from variations in the emission intensity of various fluorophores, is commonly used to measure liquid temperatures in a variety of flows. Here dual-tracer FT (DFT) where fluorescein (Fl) and sulforhodamine B were volumetrically illuminated was used to measure temperature fields in the Poiseuille flow of water through a heated 1 mm square channel. The average experimental uncertainties in the DFT results are estimated to be

Although a number of optical thermometry techniques estimate fluid temperature fields from changes in the lifetime or intensity of the emissions from fluorescent or phosphorescent species, the majority of these techniques rely on imaging optical signals at visible wavelengths. Silicon (Si), commonly used in microelectronics and microelectromechanical systems (MEMS), is however opaque at these wavelengths, and only becomes partially transparent at near-infrared (IR) wavelengths above ˜1.2 mum. Given the lack of fluorescent species with emissions in the near-IR, colloidal nanocrystals, or "quantum dots" (QD), of lead sulfide overcoated with cadmium sulfide using a new process with a diameter of 5.7 nm were investigated as temperature tracers. The emissions around 1.35 mum from these PbS/CdS QD suspended in toluene at an absorbance of 0.45 were found to decrease by about 0.5% per C increase in the suspension temperature T for T = 20-60 C with a standard deviation that gave an...

Fluorescence thermometry measures liquid temperatures based on changes in fluorescence intensity. Dual-tracer (or ratiometric) fluorescence thermometry (DFT) improves the accuracy of FT by taking the ratio of the emissions from two different fluorescent species excited at the same wavelength by the same illumination, thereby removing changes in fluorescence intensity due to spatial variations in the excitation. Moreover, DFT using two species with opposite temperature sensitivities can significantly increase the sensitivity of the technique. The ratio of the signals from an aqueous solution of fluorescein (Fl) and sulforhodamine B (SrB), which have intensities that increase and decrease, respectively, when volumetrically illuminated at 514 nm, varies by as much as 7% per C for fluid temperatures T = 15-60 C. The method has experimental uncertainties, based on temperature calibrations obtained with volume illumination, of ±1.1 C and ±0.3 C at spatial resolutions of 3.7 mum and 30 mum...

The need for new thermal management technologies to cool electronic components with their ever-increasing density and power requirements has renewed interest in techniques for measuring liquid-phase coolant temperatures, especially nonintrusive techniques with micron-scale spatial resolution. A variety of optical liquid-phase thermometry techniques exploit the changes in the emission characteristics of fluorescent, phosphorescent or luminescent tracers suspended in a liquid-phase coolant. Such techniques are nonintrusive and have micron-scale spatial resolution, but they also require optical access to both excite and image the emissions. Silicon (Si), the leading material for electronic devices, is opaque at visible wavelengths, but is partially transparent in the near-infrared (IR). To date, the only tracers that emit at near-IR wavelengths with reasonable quantum yield are IR quantum dots (IRQD), colloidal nanocrystals of semiconductor materials such as lead sulfide (PbS). Previou...

Laser-induced fluorescence (LIF) thermometry measures liquid temperatures based on changes in fluorescence intensity. In dual-tracer or ratiometric LIF thermometry, liquid temperatures are based on the ratio of the fluorescent signals from two different fluorophores, since this ratio is independent of changes in fluorescence intensity due to variations in the excitation. Recently, a dual-tracer LIF technique using two species with opposite temperature sensitivities, fluorescein 27 and Kiton Red (sulforhodamine B), excited at 532 nm has been reported with temperature sensitivities as great as 7% per ^oC [Sutton et al. (2008) Exp. Fluids DOI 10.1007/s00348-008-0506-4]. We describe here a similar technique using fluorescein and sulforhodamine B, which have intensities that increase by 2.2% and decrease by 1.3%, respectively, per ^oC when volumetrically illuminated at 514 nm. The ratio of these two signals gives temperature sensitivities as great as 9% per ^oC. This LIF technique is use...

ABSTRACT Two-color fluorescence thermometry is a well known, noninvasive, and accurate technique used to measure temperature in liquids. In this paper, we present an improved methodology that enhances the spatial accuracy of the technique by minimizing image-pair distortion errors and its subsequent use in the characterization of heated microchannels. In order to spatially calibrate the image-pair and to quantify the distortion of one image with respect to the other, particle image velocimetry was performed with sandpaper. Results show that the objective lens and the primary dichroic mirror does not significantly affect the beam path and that the main source of distortion is likely to occur between the secondary dichroic mirror and the reflective mirrors within the emission splitting system. This spatial calibration and correlation methodology was used to map the temperature distribution in microheated microchannels. The experimentally calculated advective efficiency results showed good agreement against their numerically computed counterparts. These results suggest that the power supplied to the microheaters should be varied accordingly to maintain fixed heat flux conditions through the microchannel walls as a function of flow rate.

Cooling microelectronics with heat flux values of hundreds of kW/cm2 over hot spots with typical dimensions well below 1 mm will require new single- and two-phase thermal management technologies with micron-scale addressability. However, experimental studies of thermal transport through micro- and mini-channels report a wide range of Nusselt numbers even in laminar single-phase flows, presumably due in part to variations in channel geometry and surface roughness. These variations make constructing accurate numerical models for what would be otherwise straightforward computational simulations challenging. There is, therefore, a need for experimental techniques that can measure both bulk fluid and wall surface temperatures at micron-scale spatial resolution without disturbing the flow in both heat transfer and microfluidics applications. We report here the evaluation of a nonintrusive technique, fluorescence thermometry (FT), to determine wall surface and bulk fluid temperatures with ...