Myeongsub Kim

Measurement of CO2 diffusivity for carbon sequestration: a microfluidic approach for reservoir-specific analysis.

Predicting carbon dioxide (CO(2)) security and capacity in sequestration requires knowledge of CO(2) diffusion into reservoir fluids. In this paper we demonstrate a microfluidic based approach to measuring the mutual diffusion coefficient of carbon dioxide in water and brine. The approach enables formation of fresh CO(2)-liquid interfaces; the resulting diffusion is quantified by imaging fluorescence quenching of a pH-dependent dye, and subsequent analyses. This method was applied to study the effects of site-specific variables--CO(2) pressure and salinity levels--on the diffusion coefficient. In contrast to established, macro-scale pressure-volume-temperature cell methods that require large sample volumes and testing periods of hours/days, this approach requires only microliters of sample, provides results within minutes, and isolates diffusive mass transport from convective effects. The measured diffusion coefficient of CO(2) in water was constant (1.86 [± 0.26] × 10(-9) m(2)/s) over the range of pressures (5-50 bar) tested at 26 °C, in agreement with existing models. The effects of salinity were measured with solutions of 0-5 M NaCl, where the diffusion coefficient varied up to 3 times. These experimental data support existing theory and demonstrate the applicability of this method for reservoir-specific testing.

Enhancement of the thermo-mechanical properties of PDMS molds for the hot embossing of PMMA microfluidic devices

We present a cost-efficient and rapid prototyping technique for polymethylmethacrylate (PMMA) microfluidic devices using a polydimethylsiloxane (PDMS)-based hot embossing process. Compared to conventional hot embossing methods, this technique uses PDMS molds with enhanced thermo-mechanical properties. To improve the replication performance, increases in both PDMS stiffness and hardness were achieved through several processing and curing means. First, the amount of curing agent was increased from 1/10 to 1/5 with respect to the amount of prepolymer. Second, the cured PDMS was thermally aged either over three days at 85 ◦ C or for 30 min at 250 ◦ C. Those combined steps led to increases in stiffness and hardness of up to 150% and 32%, respectively, as compared to standard PDMS molds. Using these enhanced molds, structures with features of the order of 100 μm in PMMA are successfully embossed using a standard laboratory press at 150 ◦ C. The PDMS molds and process produce identical structures through multiple embossing cycles (10) without any mold damage or deterioration. A Y-shaped microfluidic mixer was fabricated with this technique. The successful demonstration of this enhanced PDMS-based hot embossing technique introduces a new approach for the rapid prototyping of polymer-based microfluidic devices at low-cost. (Some figures may appear in colour only in the online journal)

Extending Fluorescence Thermometry to Measuring Wall Surface Temperatures Using Evanescent-Wave Illumination

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 a spatial resolution of O(10 μm) for water flowing through a heated channel. Fluorescence thermometry is typically used to estimate water temperature fields based on variations in the emission intensity of a fluorophore dissolved in the water. The accuracy of FT can be improved by taking the ratio of the emission signals from two different fluorophores (dual-tracer FT or DFT) to eliminate variations in the signal due to (spatial and temporal) variations in the excitation intensity. In this work, two temperature-sensitive fluorophores, fluorescein and sulforhodamine B, with emission intensities that increase and decrease, respectively, with increasing temperature, are used to further improve the accuracy of the temperature measurements. Water temperature profiles were measured in steady Poiseuille flow at Reynolds numbers of 3.3 and 8.3 through a 1 mm2 heated minichannel. Water temperatures in the bulk flow (i.e., away from the walls) were measured using DFT with an average uncertainty of 0.2 °C at a spatial resolution of 30 μm. Temperatures within the first 0.3 μm next to the wall were measured using evanescent-wave illumination of a single temperature-sensitive fluorophore with an average uncertainty of less than 0.2 °C at a spatial resolution of 10 μm. The results are compared with numerical predictions, which suggest that the water temperatures at an average distance of ∼70 nm from the wall are identical within experimental uncertainty to the wall surface temperature.

Numerical simulation of high inertial liquid-in-gas droplet in a T-junction microchannel

Aqueous microdroplet generation involving high inertial air flow inside a T-junction microchannel was studied numerically. The volume of fluid method was employed to track the interface between two immiscible fluids: water and air. The effects of high inertial air flow on the water droplet generation were investigated. At various Re and Ca numbers, unique flow regime mapping including squeezing, dripping, jetting, unstable dripping, and unstable jetting and their transitions were determined. Unstable dripping and unstable jetting flow regimes are new regimes which have not been previously reported in the liquid–liquid system. The flow structure in these two flow regimes is affected by the high inertial nature of the continuous phase which is negligible in the conventional liquid–liquid system. It was found that the 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 is fragmented into multi-satellite droplets and threads of different sizes as it moves downstream. The behavior of the unstable jetting flow regime cannot be characterized due to the effect of high inertial air flow on the water stream. The results show that droplet size increases as Ca and Re numbers increase and decrease, respectively. As both Ca and Re numbers increase, droplet generation frequency increases, reaching its maximum at 223 Hz. Finally, the effect of different contact angles at 120–180° on droplet size, detachment time, and droplet generation frequency was investigated. The results of this research provide valuable insight into the understanding of high throughput oil-free aqueous droplet generation within a gas flow.

Microbubbles Loaded with Nickel Nanoparticles: A Perspective for Carbon Sequestration

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 a proportional increase in 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 concentrations. The optimal concentration for the Ni NPs suspension was determined to be 30 mg L-1; increasing the concentration up to 30 mg L-1 showed a significant increase in the dissolution of CO2 bubbles, but increasing from 30 to 50 mg L-1 displayed a decrease in catalytic potential, due to the decreased translational diffusion coefficient that occurs at higher concentrations. The optimal additive concentration of Ni NPs was tested with variations of solution at acidic and basic conditions and different levels of salinity to reveal how effectively the Ni NPs behave under real reservoir conditions. At the acidic level, Ni NPs proved to be more effective in catalyzing CO2 dissolution and can sufficiently alleviate the negative impact of salinity in brine.

Thermal Characterization of Microheated Microchannels With Spatially Resolved Two-Color Fluorescence Thermometry

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.

Studying Interfacial Transport With Evanescent Wave-Based Particle Velocimetry and Thermometry

Interfacial phenomena due to surface forces are important in microfluidic devices with their relatively large surface areas and small volumes. This article reviews our recent studies measuring fluid velocities in Poiseuille and electrokinetically driven flows over the first ∼0.4 μm next to the wall from the motion of an ensemble of O(105) fluorescent particles illuminated by evanescent waves. Because the evanescent-wave intensity decays exponentially with wall-normal distance, the particle–wall separation can be determined from the brightness of each particle image, and used to estimate the steady-state distribution of the tracers near the wall. More recently, evanescent-wave illumination has been combined with fluorescence thermometry, to estimate water temperature fields from changes in the fluorescence intensity of aqueous fluorophore solutions. Poiseuille flow of a fluorescein solution at Reynolds numbers of 3.3 and 8.3 through a heated minichannel was illuminated with evanescent waves to measure solution temperatures at an average distance of 74 nm from the wall. The temperature results obtained over three different regions were in good agreement with numerical predictions of the wall surface temperature, even in the presence of temperature variations exceeding 9°C over the 1-mm width of the channel.

Infrared Quantum Dots for Liquid-Phase Thermometry in Silicon Microchannels

The exponential growth in the component density of integrated circuits has created huge thermal management challenges for next-generation microelectronics, with projected local heat fluxes approaching 1 kW/cm2 within five years. There is thus an urgent need to develop compact, high heat flux cooling methodologies. Developing and evaluating effective microelectronic single-phase liquid cooling systems, however, also requires measuring liquid-phase and wall surface temperatures to determine the local convective heat transfer coefficient in complex piping systems with dimensions comparable to the diameter of a human hair. Yet there are few if any practical thermometry techniques that can measure temperatures in such small geometries without disturbing the coolant flow and hence affecting cooling performance. Most optically based non-intrusive liquid-thermometry techniques exploit the temperature-sensitive spectral characteristics such as emission intensity and lifetime of the photoluminescence from various tracers at visible wavelengths. Applying such techniques to measure temperatures in silicon (Si) devices, however, has been hampered by a lack of suitable temperature indicators because Si is opaque at visible wavelengths. Silicon is, however, partially transparent in the near-infrared (IR), with absorption coefficients as high as 15 cm−1 at wavelengths of 1.2–1.6 μm. We have therefore investigated the temperature sensitivity of the photoluminescent emission from oleate-capped lead sulfide (PbS) quantum dots (QD) suspended in toluene that emit at a wavelength of 1.55 μm. These QD are found to have an emission intensity that decreases by as much as 0.5% per K for temperatures ranging from 293 K to 333 K. Results are presented for temperature measurements through Si surfaces using PbS QD. The accuracy and reproducibility of these temperature measurements are discussed.Copyright

Catalytic activity of nickel nanoparticles stabilized by adsorbing polymers for enhanced carbon sequestration

This work shows the potential of nickel (Ni) nanoparticles (NPs) stabilized by polymers for accelerating carbon dioxide (CO2) dissolution into saline aquifers. The catalytic characteristics of Ni NPs were investigated by monitoring changes in diameter of CO2 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 CO2 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 anchoring sites of Ni NPs by the monovalent cation, Na+. Considerable dispersion of Ni NPs by an optimal concentration of the anionic polymers improved their catalytic capabilities even under unfavorable conditions for CO2 dissolution. This study has implications for enhancing geologic sequestration into deep saline aquifers for the purposes of mitigating atmospheric CO2 levels.

Using Quantum Dots for Liquid-Phase Thermometry at Near-Infrared Wavelengths in Silicon Devices

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). Previous work has shown that the intensity of emissions at 1.55 μm from PbS IRQD suspended in toluene are temperature-sensitive, decreasing by as much as 15% as the temperature increased from 20 °C to 60 °C. The accuracy of temperature measurements using PbS IRQD was estimated to be about 5 °C, based on 95% confidence intervals, where the major limit on the accuracy of the technique was the poor photostability of this material [1]. Recently, a new method for creating a cadmium sulfide (CdS) overcoat layer on PbS “cores” has been developed [2]. The experimental results presented here on the temperature sensitivity of these PbS/CdS core-shell infrared quantum dots with an emission peak around 1.35 μm and a diameter of 5.7 nm (with a core diameter of 4 nm) suggest that these new core-shell structures are more temperature-sensitive than the PbS cores. These core-shell quantum dots, when suspended in toluene, were found to have a 0.5% decrease in emission power per °C increase in temperature at suspension temperatures ranging from 20 °C to 60 °C. The uncertainty in the liquid-phase temperatures derived from these emissions was estimated to be less than 0.3 °C based on the standard deviation. Furthermore, the PbS/CdS quantum dots were highly photostable, with a consistent response more than 100 days after suspension. These results imply that that these new IRQD can be used to measure liquid-phase coolant temperatures without disturbing the flow of coolant at an accuracy comparable to commercially available thermocouples in monolithic Si devices.© 2010 ASME