Characterization of Dynamic Response of AFM Cantilevers for Microscale Thermofluidic and Biophysical Sensors

My doctoral research has focused on the characterization of dynamic response of atomic force microscope (AFM) cantilevers for thermofluidic and biophysical sensors, a novel scanning thermal microscopy technique development using a tipless microcantilever to investigate micro/nanoscale transport properties in liquid, and the characterization of the surface nanomechanical properties of biocompatible polyelectrolyte hydrogels with AFM for biomedical applications. The temperature effects on Sader‟s viscous model for multilayered microcantilevers immersed in an aqueous medium were experimentally verified as a preliminary work. Next, temperature dependence of the near-wall oscillation of microcantilevers submerged in an aqueous medium was investigated to explore the possibility of a near-wall thermometry sensor. By correlating the frequency response of a microcantilever immersed in an aqueous medium near a solid surface (within the width of a microcantilever) with the surrounding liquid temperature, the near-wall region microscale temperature distributions at the probing site were successfully determined. For biological applications, this work has been extended to examine the effect of adsorption-induced surface stress change on the stiffness of a microcantilever immersed in saline solution with varying salt concentrations. It was found that adsorption-induced surface stress change increased the stiffness of a microcantilever in saline solution with increasing salt concentration ranging from 0 to 2 molality. The surface nanomechanical properties of 2-hydroxyethyl methacrylate (HEMA) and 2-methacryloxyethyl trimethyl ammonium chloride (MAETAC) copolymer hydrogels were probed using AFM. The HEMA-MAETAC polyelectrolyte hydrogels with increasing positive charge concentrations ranging from 0 to 400 mM in increments of 40 mM, were fabricated using different proportions of HEMA and MAETAC monomers. Increasing proportions of positively charged MAETAC monomers produced hydrogels with increasingly swollen states and correspondingly decreasing measures of surface elasticity, or Young‟s modulus. The attachment of porcine pulmonary artery endothelial cells (PPAECs) increased with increasing prepared hydrogel charge concentration and subsequently decreasing surface elasticity. Keywords: Atomic Force Microscope (AFM), Microcantilever, Thermofluidic sensor, Hydrogel, Surface elasticit

Discriminatory vs Uniform Price Auction: Auction Revenue

We compare auction revenues from discriminatory auctions and uniform price auctions in the case of the Korean treasury bonds auction market. For this purpose, we employ detailed bidder level data for each of 16 discriminatory auctions recently carried out in Korea. We first theoretically recover unobserved individual bidding functions under counter-factual uniform price auctions from the observed bidding functions under the actual discriminatory auctions, and then empirically estimate revenue differences. To test significance of the auction revenue differences, we use Bootstrap re-sampling methods where uncertainty in the cut-off yield spreads and uncertainty in the bidders are addressed individually as well as simultaneously. Our results indicate that uniform price auction increases the auction revenue relative to the discriminatory auction in most of the 16 cases, justifying the Korean government’s decision to switch to the uniform price auction mechanism in August 2000Treasury bonds auction, discriminatory auction, uniform price auction, hazard rate, Bootstrap re-sampling, yield spread, bidding function, bid shading

A 2D Graph-Based Generative Approach For Exploring Transition States Using Diffusion Model

The exploration of transition state (TS) geometries is crucial for elucidating chemical reaction mechanisms and modeling their kinetics. In recent years, machine learning (ML) models have shown remarkable performance in TS geometry prediction. However, they require 3D geometries of reactants and products that can be challenging to determine. To tackle this, we introduce TSDiff, a novel ML model based on the stochastic diffusion method, which generates the 3D geometry of the TS from a 2D graph composed of molecular connectivity. Despite of this simple input, TSDiff generated TS geometries with high accuracy, outperforming existing ML models that utilize geometric information. Moreover, the generative model approach enabled the sampling of various valid TS conformations, even though only a single conformation for each reaction was used in training. Consequently, TSDiff also found more favorable reaction pathways with lower barrier heights than those in the reference database. We anticipate that this approach will be useful for exploring complex reactions that require the consideration of multiple TS conformations

Automated phase unwrapping in digital holography with deep learning

Digital holography can provide quantitative phase images related to the morphology and content of biological samples. After the numerical image reconstruction, the phase values are limited between −π and π; thus, discontinuity may occur due to the modulo 2π operation. We propose a new deep learning model that can automatically reconstruct unwrapped focused-phase images by combining digital holography and a Pix2Pix generative adversarial network (GAN) for image-to-image translation. Compared with numerical phase unwrapping methods, the proposed GAN model overcomes the difficulty of accurate phase unwrapping due to abrupt phase changes and can perform phase unwrapping at a twice faster rate. We show that the proposed model can generalize well to different types of cell images and has high performance compared to recent U-net models. The proposed method can be useful in observing the morphology and movement of biological cells in real-time applications. © 2021 Optical Society of America1

On the Nature of Propagating Intensity Disturbances in Polar Plumes during the 2017 Total Solar Eclipse

The propagating intensity disturbances (PIDs) in plumes are still poorly understood and their identity (magnetoacoustic waves or flows) remains an open question. We investigate PIDs in five plumes located in the northern polar coronal hole observed during the 2017 total solar eclipse. Three plumes are associated with coronal bright points, jets and macrospicules at their base (active plumes) and the other two plumes are not (quiet plumes). The electron temperature at the base of the plumes is obtained from the filter ratio of images taken with the X-ray Telescope on board Hinode and the passband ratio around 400 nm from an eclipse instrument, the Diagnostic Coronagraph Experiment (DICE). The phase speed (v_r), frequency (omega), and wavenumber (k) of the PIDs in the plumes are obtained by applying a Fourier transformation to the space-time (r-t plane) plots in images taken with the Atmospheric Imaging Assembly (AIA) in three different wavelength channels (171 A, 193 A, and 211 A). We found that the PIDs in the higher temperature AIA channels, 193 and 211 A, are faster than that of the cooler AIA 171 A channel. This tendency is more significant for the active plumes than the quiet ones. The observed speed ratio (~1.3) between the AIA 171 and 193 A channels is similar to the theoretical value (1.25) of a slow magnetoacoustic wave. Our results support the idea that PIDs in plumes represent a superposition of slow magnetoacoustic waves and plasma outflows that consist of dense cool flows and hot coronal jets.Comment: 18 pages, 8 figures, accepted for publication in Ap

Intense pulsed light-based synthesis of hybrid TiO2–SnO2/MWCNT doped Cu-BTC for room temperature ammonia sensing

Ammonia gas is an irritating and corrosive gas commonly used as fertilizers, refrigerants and cleaners. In this work, a sensitive and selective nanocomposite film is fabricated on a quartz crystal microbalance (QCM) with hybrid TiO2-SnO2/multi-walled carbon nanotubes (MWCNTs) doped Cu-BTC for trace amount of ammonia detection under ambient conditions. Cu-BTC exhibits excellent porosity for gas sensing, but poor stability in the presence of water and ammonia which limits the long term viability. Hybrid TiO2-SnO2/MWCNTs doping prevents framework decomposition and improves longevity. The nanocomposite film is rapidly synthesized under ambient conditions using a facile intense pulsed light (IPL) technique to stimulate in-situ growth. IPL also assists in the thermal conversion of anatase TiO2 to rutile TiO2. This functionalized nanocomposite film-based QCM sensor demonstrates ammonia gas sensing capability with a limit of detection estimated to be 0.77 ppm and strong stability even under humid conditions. The sensor exhibits reversibility during cyclic testing and minimal drift suggesting that the reaction mechanism is primarily surface adsorption. Changes in the motional resistance and resonance frequency of the sensor can be linearly related to the ammonia gas concentration

Cultural Hybridity of Early Childhood Education in South Korea: Exploring the Biographical Narratives of Mrs. Kim

The purpose of this study is to elucidate what experiences made Mrs. Kim decide to be an educator and how she stepped into the early childhood education field. Also, this study aims to illustrate Mrs. Kim’s experiences and meanings while she has served as a leading educator in elementary schools and her kindergarten class. Lastly, this study seeks to unravel the culturally diverse thoughts and approaches to Mrs. Kim’s pedagogic beliefs. For the purposes, I investigated her stories guided by the following research questions: What socio-political changes and education policy have Mrs. Kim experienced over time? How has her interpretation of her experiences over this long period of time influenced Mrs. Kim’s pedagogic beliefs? What are the implications of Mrs. Kim’s pedagogic beliefs in terms of cultural hybridity? I employed a narrative inquiry research design to capture Mrs. Kim’s experiences and meanings formed from her 60-year pedagogic journey. I interviewed her eight times in total and examined the literature about culturally intermingled features of educational cultures by looking at cultural hybridity in the context of South Korea. Then, I reviewed the changes in early childhood education and care in South Korea embedded within historical contexts. In the findings section, using the first-person perspective, I chronologically arranged Mrs. Kim’s discursive life narratives, which were told in biographical narrative interviews. Also, I organized Mrs. Kim's narratives according to emergent themes and present short life stories. I composed 12 short stories: the stories are told in Mrs. Kim’s voice. In the discussion section, I discussed the meaning of Mrs. Kim's experiences through the theoretical lens of cultural hybridity as framed by themes from cultures of curriculum. As a result, I examined what cultural factors are emergent from Mrs. Kim's stories and how they are interrelated with each other

Plasmonic absorbers with optical cavity for the enhancement of photothermal/opto-calorimetric infrared spectroscopy

Plasmonic absorbers with optical cavity are fabricated using deposition of thin gold films on both sides of nanoporous anodic aluminum oxide (AAO) microcantilevers. Photothermal/optocalorimetric responses of the microcantilevers are investigated with respect to various pore sizes and gold coating thicknesses. Photothermal/opto-calorimetric response of the AAO cantilever is significantly amplified when the thickness of the gold layer is at 40 nm due to scattering and trapping of infrared (IR) radiation in the gold nanomesh/AAO nanochannels/gold layer structure. Unlike previous photothermal IR spectroscopy of adsorbed molecules with a bimetallic AAO cantilever, the reversed IR spectra are obtained due to significantly amplified baseline photothermal signal and IR absorption of free molecules. Molecular detection sensitivity is enhanced by an order of magnitude compared with the previous photothermal IR spectroscopy of adsorbed molecules on a bimetallic AAO cantilever. The acetone vapor molecules are selectively recognized using the reversed IR spectra.Ye