Indian Institute of Science Bangalore
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Studies on Rotary Atomization
No full textIn rotary fuel injection systems, the fuel is atomized with the help of centrifugal force provided by the rotation of the injector. The rotational power can be directly provided by the shaft of a gas turbine, which can go up to the rotational speed of 100000 RPM. Therefore, there is no need for extra systems such as a fuel pump or a pressure chamber. This makes the rotary injection system very compact and suitable for small gas turbines. Also, the injection orifice diameter in these systems is large enough to be used for high viscosity and slurry fuels, as small diameters can cause clogging. There is a strong need to study and understand the spray characteristics of these types of injectors to adapt combustors for challenging fuels such as high energetic fuels containing boron nanoparticles. Also, there is a need to develop correlations for improved prediction of spray characteristics.
The present work is motivated by the demand for high-energy-density fuels, such as fuel slurries. In the first part of the study, the spray characteristics of rotary atomization were investigated experimentally. A high-speed rotating facility has been developed in which a 1.5-kW electric motor was used with a maximum operating speed of 40000-RPM. The spray characteristics of pure water, water-ethanol blend, and water-glycerol blend were investigated to understand the variation of liquid physical properties such as viscosity and surface tension on spray characteristics. It is shown that at lower rotational speeds, the effect of viscosity and surface tension are evident, but at higher speeds, the inertial effects dominate. To understand the spray characteristics from slurry fuels, aqueous colloidal silica nanoparticle suspension with different nanoparticle loading is used. Results showed that even with the particle loading, the variation in SMD is the function of liquid physical properties and does not have a unique dependency on particle loading. The jet visualization and spray characteristics such as breakup length and droplet size were measured using the shadowgraphy technique. For droplet size measurements, a long-distance microscope (LDM) attached to a CCD camera with laser-induced fluorescent backlight is used. The Spray patternation has been studied with water by passing a laser sheet to the spray and capturing scattering images. The images of the liquid flow inside the orifice were captured using a transparent atomizer made of acrylic. The second part of the study comprises of analyzing the liquid flow physics. A novel non-dimensional number has been derived to account for the Coriolis force induced inside the orifice. From the images of liquid flow inside the orifice, it has been shown that the flow physics can be explained using this non-dimensional number. There are eight different parameters affecting the spray characteristics, viz. viscosity, density, surface tension, liquid feed rate, atomizer rotational speed, atomizer diameter, orifice diameter, and orifice length. A new empirical correlation has been established to account for the variation in all these parameters, and this correlation is shown to be very effective over a wide range of parametric space. The last part of the study involves the design proposal of a novel orifice structure, which comprises of two stages; also, the feasibility of the design was investigated
Aqueous Binary Mixtures: A Study Using NMR Measurements and ab initio Molecular Dynamics Simulations
No full textBinary mixtures having a combination of hydrophobic and hydrophilic groups, such as waterdimethyl
sulfoxide (DMSO), water-methanol, water-ethanol, water-tertiary butyl alcohol
(TBA), water dioxane, and water-glycerol, have been widely studied. These mixtures
often show non-ideal mixing behaviour with properties such as electrical conductivity,
permittivity, speci fic volume, enthalpy of mixing and light-scattering behaviour exhibiting
non-monotonic behaviour as a function of the composition of the mixture. The waterethanol
system that is miscible over the entire composition range is perhaps one of the most
elementary but nontrivial model system that exhibits non-ideal behaviour. The mixtures
exhibit negative excess entropy and a strong increase in heat capacity as compared to
an ideal solution, with properties such as, molar volume, excess entropy, compressibility,
viscosity, diffusion coeffcient, and sound attenuation coeffcient exhibiting composition
dependent anomalies. The occurrence of different solvation regimes in water-alcohol
binary mixtures has been widely reported, although the values of the transition points,
the mole fraction of alcohol in the mixture, show minor differences depending on the
experimental technique. It is generally accepted that the anomalous properties of aqueous
binary mixtures is a consequence of the perturbation of the local and global hydrogen
bond network due to the presence of the second component. Despite a large number of
investigations encompassing both theories and experiments, a molecular understanding of
the anomalous properties of the water-ethanol system has remained elusive. The focus of
this thesis is on the role of hydrogen bonding and the modi fication of the hydrogen bonding network due to the presence of the second component in water-ethanol and water-ethylene
glycol systems using solution Nuclear Magnetic Resonance (NMR) spectroscopy aided
by ab initio Molecular Dynamics (AIMD) simulations for interpreting the experimental
observations.
In summary, this thesis has attempted to show how 1H NMR measurements in combination
with ab initio MD simulations can be used to probe H-bonding and the nature of
association in aqueous-binary mixtures of ethanol and ethylene glycol. The strategy was to
first establish a geometrical de nition of H-bonds in pure ethanol and ethylene glycol before
attempting the same for aqueous-binary mixtures. It is shown here that the secondary
isotope effect can be used to distinguish intra and intermolecular interactions in the 1H
NMR of mixtures of ethanol and ethylene glycol along with deuterted counter parts. It is
shown how 1D transient NOE measurements of these mixtures, in conjuction with AIMD
simulations can establish the geometry of H-bonds in pure ethanol and ethylene glycol. A
simple procedure to determine the geometry of H-bonds between different donor-acceptor
pairs ethanol-ethanol, water-ethanol and water-water in water-ethanol mixture from fragment
obtained from AIMD is outlined. Having established the geometry of H-bonds it is
possible to determine the thermodynamics of hydrogen bond formation in water-ethanol
system at different compositions. The nature of association in the aqueous-binary mixtures
could be established by 1H NMR measurements. The fact that there are water-rich and
ethanol-rich clusters in dynamic equilibrium could be established from these measurements.
The results presented in this thesis could pave the way for a better understanding, at the
molecular level, intriguing properties of binary mixtures using a combination of NMR
measurements and molecular dynamics simulations
Designing Authentication and Privacy Schemes for Ubiquitous Services
No full textUbiquitous services in ubiquitous environments provide personalized and adaptive services to a user anytime anywhere, by considering the context information such as profile information and history of the users. Ubiquitous services, like ubiquitous health care service, ubiquitous shopping complex, ubiquitous tourist guide systems, ubiquitous learning systems, provide adaptive, user-centric, context-aware services intelligently in healthcare, product purchase, tourism and education fields respectively.
The objective of the work is to design secured ubiquitous services for ubiquitous users. Proposes user authentication, mutual authentication between user and service provider, service access control and user privacy schemes for ubiquitous services. Ubiquitous services require context aware information of a user such as interests, activities or preferences. Proposed work uses hybrid Genetic Algorithm - Agent technology. Agents are intelligent and have the capability to take decisions by themselves, and dynamically resolve the local problems. Two variants of agents, Static Agent (SA) and Mobile Agents (MAs) are employed, where the SA resides at the ubiquitous service provider and deploys MA on requirement. Correlations among contexts are established and relevant contexts are obtained through genetic algorithm(GA).
To have a secured ubiquitous service for ubiquitous users, we have proposed the following works:
• Ubiquitous User Authentication:
To avail ubiquitous services, users need to be authenticated. Using GA-Agent technology, authentication certificates are generated based on user context information considering user personal and professional information, activity, relation, location, status, etc. The proposed authentication scheme is analysed and validated using BAN Logic. The designed scheme is implemented over the ubiquitous health care service and the results demonstrate that a consistent authentication was rendered to the users and health-workers. The certificate generated is simple, accurate, and smaller.
• Mutual Authentication:
To enable users to roam seamlessly to avail ubiquitous services, one should be authenticated by the new subnetworks which he visits, also users need to authenticate the entity who provides the services, and with whom the user shares all his data. We propose mutual authentication of entities involved in ubiquitous services and GA are assisted with SA-MA to enable mutual authentication.
Authentication based access control: On mutual authentication, dynamic access control policies are assigned to the users, and ubiquitous services are allocated fairly, by the SA and MA’s . It calculates the context priorities of the users - by considering the importance of the services executed, service utilization history and cost effectiveness of the service. The scheme is validated using BAN Logic and we have tested the proposed scheme over ubiquitous health care services, and the results shown are satisfactory.
• Preservation of user privacy:
The main goal of ubiquitous service is to create the information space, where users can share information like events, knowledge, activities, etc., securely without giving up their privacy. We propose a privacy preserving technique by considering the user’s interests,preferences, history, etc. In this method, intelligent agents are deployed, where the SA is responsible for determination of privacy policies of users. On the other hand, MAs are responsible for ensuring enforcement of privacy policies over the users. The proposed scheme is tested over an ubiquitous tourist guide service, where we simulated different sets of services related to the tour-guide. Subsequently, privacy threats and their prevention are also discussed
Spectrotemporal Processing of Speech Signals Using the Riesz Transform
No full textSpeech signals possess a rich time-varying spectral content, which makes their analysis a challenging signal processing problem. Developing methods for accurate speech analysis has a direct impact on applications such as speech synthesis, speaker recognition, speech recognition, voice morphing, etc. A widely used tool to visualize the time-varying spectral content is the spectrogram, which represents the spectral content of the signal in the joint time-frequency plane. A spectrogram can be viewed as a collection of several localized spectrotemporal patches. By analyzing the structure of two-dimensional (2-D) patterns in the spectrogram, we propose modeling it using 2-D amplitude-modulated and frequency-modulated (AM-FM) sinusoids. The justification for the 2-D AM-FM model for speech can be provided based on the physical process behind its generation. From a speech production perspective, the AM and FM components correspond to the vocal-tract smooth envelope and excitation signal, respectively. We demonstrate that analyzing speech jointly in time and frequency reveals several important characteristics, which are otherwise not evident either in purely time-domain or frequency-domain analysis.
The central problem in this dissertation is 2-D demodulation of a speech spectrogram, which yields 2-D AM and FM components. We advocate the use of the Riesz transform, which is a 2-D extension of the Hilbert transform, to demodulate narrowband and pitch adaptive spectrograms. Interestingly, the 2-D AM and FM components obtained as a result of demodulation have potential benefits for speech analysis. We demonstrate the impact of the proposed modeling technique for vocal tract filter estimation, voiced/unvoiced component separation, pitch tracking, speech synthesis, and periodic/aperiodic decomposition of speech signals. The accuracy of the estimated speech parameters is validated considering the task of speech reconstruction.
The first part of the thesis is focused on theoretical developments related to 2-D modeling. We consider prototypical 2-D cosine signals, analyze their Fourier transform properties, solve the problem of demodulation of a 2-D AM-FM cosine signal and extend the model to spectrotemporal patches. Following this, we examine the taxonomy of time-frequency patterns in the FM component, highlighting the salient attributes of different types of phonation in speech. We show that 2-D patterns specific to different speech sounds (voiced/unvoiced) can be captured by computing two novel time-frequency maps from the 2-D FM component: the coherencegram and orientationgram. The usefulness of the maps is demonstrated for the problem of periodic and aperiodic decomposition of speech signals.
In the second part, we use the FM component for estimating the source parameters. We show that the FM component is a rich representation of the source signal in 2-D and use it to estimate the speaker’s fundamental frequency (or pitch), speech aperiodicity, and voiced/unvoiced segmentation of the speech signal. We propose novel spectrotemporal features for voiced and unvoiced segmentation of speech. In contrast to time-domain features such as short-time energy, zero crossings, and autocorrelation coefficients, the proposed features are relatively insensitive to local variations of the speech waveform. The FM component is obtained by demodulating the narrowband speech spectrogram, which exhibits high frequency resolution. Consequently, the FM component encodes the speaker’s pitch. Hence, we propose methods for estimating the pitch from the FM component. Another critical component of a speech signal is its aperiodicity. Voiced sounds are quasi-periodic and have a noise component of strength relatively weaker than unvoiced sounds. Utilizing the time-frequency properties of the FM component, we propose methods for the estimation of speech aperiodicity.
While the FM component is used to estimate the source parameters, the 2-D AM component models the slowly varying vocal-tract filter. However, estimation of the vocal-tract filter is challenging due to its interaction with the quasi-periodic excitation. Two issues arise in this context: the first one is related to the length of the analysis window used for computing the spectrogram. We argue that a fixed-length analysis window is not ideal for vocal tract estimation. We show that the best results can be obtained by adapting the window length to the speaker’s pitch while computing the spectrogram. Such a spectrogram is referred to as the pitch-adaptive spectrogram. The second issue is related to the processing involved in demodulation, which has the undesirable effect of broadening the formant bandwidths. Hence, we propose a method to compensate for the formant broadening. It is crucial to estimate the optimum formant bandwidths as they determine the shape of the vocal tract filter and govern speech intelligibility during synthesis.
The effectiveness of the estimated source and filter parameters is shown by incorporating them in a spectral synthesis model and a neural vocoder for speech reconstruction. For neural vocoder, we use WaveNet, which is a deep generative model for audio generation. By conditioning the model on acoustic features, one can guide WaveNet to produce realistic speech waveforms. We use the Riesz transform-based acoustic features as conditional features in WaveNet vocoder. The quality of generated speech waveforms is evaluated by using objective and subjective measures
Compiler controlled Task Management in Runtime Systems for Dynamic Data ow Model of Execution
No full textFor the past 40 years, relentless focus on Moore's Law transistor scaling has provided ever-increasing transistor performance and density. An ever increasing demand for large scale parallelism has driven hardware designers to fit in more cores per die, reaching physical limits of power dissipation. Attention has now turned to low power light-weight cores such as ARM with thousands of wimpy and brawny cores per die. The responsibility of running applications on such cores, however, still remains with the operating system, adding to the overheads of an otherwise light-weight shared-memory based application. A massively parallel low power chip with high scalability as a building block for a larger compute infrastructure is the preferred design. Such chips are expected to sport features such as inexpensive computation and communication along with a low-latency runtime interface. State of the art runtime systems incur significant performance penalties as they are tied to traditional parallel computing models. Performance concerns have led researchers to consider alternative models of computing such as Dynamic Dataflow. Such models have proven to be more scalable and power budget friendly, making parallelism exploitation more amenable even with irregular applications that usually are tricky to parallelize and scale. An ideal runtime implementation exposes runtime management primitives to the software abstraction layer to use. We introduce one such distributed hardware runtime for a massively parallel manycore processor (called REDEFINE), that exposes parallelism handles as instructions that are part of the its ISA.We present a compilation strategy that utilizes the primitives to effectively manage tasks on the hardware. REDEFINE's compiler controls task creation and deletion and manages communication between them, and balances task loads on REDEFINE's distributed execution fabric
Studies on the roles of rffG and rfbB encoding dTDP-glucose 4,6- dehydratase in Salmonella Typhimurium
No full textSalmonellosis is a major health concern which causes significant morbidity and mortality worldwide. Infection with Salmonella Typhimurium leads to self-limiting diarrhea in healthy individuals and invasive disease in immune-compromised hosts. Emerging drug resistance in Salmonella is making treatment options increasingly challenging. A key barrier for the development of new classes of antibiotics for Gram negative pathogens is presence of the outer membrane. The presence of the outer membrane prevents the entry of many antibiotics rendering them ineffective in their clinical utility. Consequently, research on finding new potential drug targets in Salmonella will have a great impact on food industry and public health worldwide.
The bacterial surface is the first point of contact with the host and hence it’s a target of a variety of antimicrobial defense mechanisms. Lipopolysaccharide (LPS) and enterobacterial common antigen (ECA) are two important virulence determinants which are present on the outer membrane of S. Typhimurium. LPS consists of three components; Lipid A, core oligosaccharide and O-antigen which comprises structural repeating units of four sugar residues. ECA is made up of repeating trisaccharide units composed of 4-acetamide-4,6-dideoxy-D-galactose, N-acetyl-D-mannosaminuronic acid and N-acetyl-D-glucosamine. The gene cluster rfb encodes genes involved in O-antigen biosynthesis whereas rff codes for genes responsible for ECA biosynthesis. The genes rfbB and rffG encode the protein dTDP-glucose 4,6 -dehydratase, an intermediate in the synthesis of both O-antigen and ECA. Since, the enzyme dTDP-glucose- 4,6-dehydratase is involved in the intermittent steps of the synthesis of both O-antigen and ECA, the functional loss of the enzyme renders the cell incapable of synthesizing both O-antigen and the ECA. The function of dTDP-glucose 4,6-dehydratase in organisms such as Candida and Mycobacterium have been well characterized. However, to the best of our knowledge, a detailed study into the major physiological changes associated with the functional loss of both rffG and rfbB in S. Typhimurium is lacking.
With this objective in mind, we generated single deletion strains of rffG, rfbB and a double deletion strain of both the genes. First, we studied the growth characteristic of the strains. We did not observe any difference with respect to growth in nutrient rich as well as deficient media. However, we observed that the rffGrfbB strain showed a distinct colony morphology on agar. The colonies formed by the rffGrfbB strain were significantly smaller in size when compared with the wildtype (WT) and the single deletion strains. Also, the colonies formed by the rffGrfbB strain appeared distinct in texture when observed under the microscope. Further, we investigated the individual cell morphology with the help of atomic force microscopy (AFM). Our investigations with AFM revealed that the rffGrfbB strain showed round cell morphology whereas the WT and the single deletion strains displayed rod shaped morphology. Next, we subjected our strains to stresses commonly encountered by S. Typhimurium in the environment or during host colonization. We did not observe any difference in growth in osmolarity, pH and high temperature induced stresses. However, we found that the rffGrfbB strain was highly susceptible to bile as well as antibiotics such as polymyxin B, meropenem and amoxicillin-clavulanic acid but not to ciprofloxacin or cefotaxime. To address the question whether the increased susceptibility to different substances seen in the rffGrfbB strain was due to the loss in the outer membrane integrity, we performed 1-N-phenylnaphthylamine (NPN) dye accumulation assay. We found the outer membrane permeability was markedly increased in the rffGrfbB strain when compared with the WT and the single deletion strains. However, analysis of the outer membrane protein profile of the WT and the rffGrfbB strains did not reveal any noticeable differences. Next, we analyzed the LPS profile of our strains. Our finding revealed that the WT and the single deletion strains displayed a complete LPS profile whereas the rffGrfbB strain displayed a truncated version of the LPS molecule with the absence of the O-antigen repeating units.
To obtain mechanistic insights into the observable phenotypic characteristics of the rffGrfbB strain, we performed RNA-seq analysis of the WT and the rffGrfbB strain. Among the key pathways which were downregulated in the rffGrfbB strain, as compared to the WT strain, were genes related to flagellar assembly, chemotaxis, quorum sensing and Salmonella pathogenicity island 1 (SPI-1) pathways. We focused our study on the motility and SPI-1 pathway by performing qRT-PCRs to validate the differentially expressed genes (DEGs) as well as functional responses. Our qRT-PCRs results indicated that the flagellar assembly pathway genes flhD, fliC and fljB were downregulated in the single deletion strains as well as the double deletion strain when compared to the WT, although the extent of downregulation of the genes was more pronounced in the rffGrfbB strain. We also performed swimming and swarming motility assays to assess motility defect in our strains. No difference was observed between the WT and the single deletion strains during swimming motility. On the other hand, in swarming motility, the single deletion strains were less motile when compared to the WT, whereas the rffGrfbB strain was completely non-motile. Next, we performed qRT-PCRs of representative genes of the SPI-1 pathway. We studied the expression of hilD, hilA and sipC and found them to be downregulated in the single deletion strains as well as the double deletion strain compared to the WT strain. However, the extent of down regulation of the genes were more pronounced in the rffGrfbB strain than the single deletion mutants. We also performed in-vitro infection in RAW264.7 and HeLa cells. In HeLa, the single deletion mutants displayed reduced adhesion and invasion ability as compared to the WT, whereas the rffGrfbB strain was highly compromised in both adhesion and invasion. In RAW264.7 cells, we did not detect any difference between the WT and the single deletion strains in terms of adhesion and invasion. However, the rffGrfbB strain displayed reduced adhesion as well as invasion in the RAW264.7 cells as compared to the
WT and the single deletion strains. Furthermore, we performed in vivo infection in C57BL/6 with S. Typhimurium WT and rffGrfbB strain through both oral as well as intraperitoneal routes. In both the models, we found that the rffGrfbB strain was less proficient in colonizing different organs than the WT strain. We also observed that the rffGrfbB strain did not generate pro-inflammatory cytokine responses in mice and was highly attenuated during both oral and intraperitoneal infection model systems.
Overall, this detailed study highlights the importance of rffG and rfbB in maintaining cell wall integrity, colony and cell morphology, motility and virulence in S. Typhimuriu
Role of Hydrogen Sulfide Gas in Modulating HIV-1 Latency and Reactivation
No full textHuman Immunodeficiency Virus 1 (HIV-1) remains a global public health threat, claiming 690 thousand people’s lives in 2020 and causing 1.5 million new infections. The advent of combinatorial antiretroviral therapy (ART) have curbed the spread of the HIV-1 epidemic by limiting new infections rate. However, ART is not a curative therapy, and HIV-1 persists in latent reservoirs mainly comprising long-lived memory CD4+ T cells. Notably, low ART treatment coverage and cases of poor therapy adherence lead to replenishment of latent reservoirs and the emergence of drug-resistant variants. Thus, to eradicate HIV-1, it is important to understand how the virus establishes latency, maintains stable cellular reservoirs, and promotes rebound upon interruption of antiretroviral therapy (ART). Cellular redox status has been observed as a key determinant modulating HIV-1 latency and reactivation. HIV-1 patients display the hallmark of oxidative stress with reduced levels of major cellular antioxidants, glutathione (GSH), and thioredoxin (Trx) systems. The current approach to target latent HIV-1 includes a ‘shock and kill’ approach, which utilizes latency reversing agents (LRAs) to reactivate HIV-1 and kill infected cells by immune-based mechanisms [Chapter 1]. The LRAs belonging to histone deacetylase inhibitors class, when used in combination with GSH biosynthesis inhibitor, BSO, induce robust oxidative stress and heightened HIV-1 reactivation. In this direction, the use of antioxidant molecules, e.g., N-acetyl cysteine (NAC), has been shown to limit HIV-1 reactivation, but the molecular mechanism involved in NAC action remains understudied. Recently, NAC has been shown to exert its effect by inducing the biogenesis of a novel antioxidant gasotransmitter molecule, hydrogen sulfide (H2S). Previously considered as a toxic gas, but literature in the past two decades suggests the cytoprotective and antioxidant role of H2S in several patho-physiological conditions. In this study, we were interested in comprehensively characterizing the role of H2S gas in modulating HIV-1 latency and reactivation program. Here we observed that HIV-1 reactivation is associated with the downregulation of the H2S biogenesis enzyme, CTH, resulting in the depletion of endogenous H2S levels. Moreover, depleting endogenous H2S levels via knockdown of CTH expression in the cells with latent HIV-1 results in an imbalance of redox homeostasis, increased GSSG levels, enhances mitochondrial ROS (mito-ROS), dysfunctional mitochondria, and thereby resulting in robust HIV-1 reactivation [Chapter 2]. Moreover, chemical complementation of H2S deficiency using GYY4137, a slow- releasing H2S donor, inhibited distinct LRAs induced HIV-1 reactivation in monocytic, T- lymphocytic cell line model and primary CD4+ T cells derived from ART-treated HIV-patients [Chapter 3]. Next, using the NanoString based targeted gene expression analysis, we found that HIV-1 reactivation with phorbol ester, PMA, is associated with the upregulation of genes known to induce ROS production, inflammation, and transcription factors involved in HIV-1 reactivation, e.g., NF-kB and FOS with concomitant downregulation of antioxidant machinery and viral suppressive factor, YY1. GYY4137 treatment reversed the PMA mediated effects and led to downregulation of genes involved in ROS production, inflammation and, concomitantly, enhanced the expression of NRF2 dependent antioxidant and antiviral genes [Chapter 4]. Furthermore, GYY4137 inhibited the activity of proviral factor NF-kB and enhanced the occupancy of an epigenetic repressor, YY1, on HIV-1 LTR. These mechanistic insights suggest that H2S inhibits HIV-1 reactivation by maintaining cellular redox status, limiting the activation of transcription factors required for active viral transcription, and inducing epigenetic suppression of viral LTR
Mode Coupling and Nonlinearities in Micro/Nano Electromechanical Systems
No full textMicro and nanoelectromechanical systems have shown tremendous potential in applications ranging from sensing to obtaining ultrastable oscillators for timing. They have also opened avenues for fundamental quantum studies and exploring nonlinear dynamics. The advent of CNTs and two-dimensional materials has enabled extreme miniaturization of resonators, allowing mass sensitivities down to a proton limit. This is possible since the mass resolution is proportional to the mass of the resonator itself. The limit of detection is also proportional to the frequency stability of the resonator. This is a measure of the uncertainty associated with the frequency measurement. Frequency stability can be effected either by the measurement noise or noise intrinsic to the device's mechanical response.
In this thesis, we have explored the room temperature frequency stability of MoS2 resonators in the linear regime. The work involves the fabrication of local gated MoS2 resonators. The devices are characterized using capacitive actuation and homodyne detection techniques. Allan deviation is used as a tool to measure the frequency stability of MoS2 resonators. We study the effect of actuation drive (both AC and DC) on the resonator's frequency stability and correlate it with the signal to noise ratio of the device. The frequency stability measured in MoS2 resonators corresponds to a mass resolution of few attograms. We further identify the various noise sources present in the system through the slope of Allan deviation plots.
Recently, Antonio et al. have demonstrated improved frequency stability due to nonlinear intermodal coupling. Coupled resonators have also been shown to enhance the sensitivity of mass sensors and hold promise for future nanomechanical technologies. The linear and nonlinear coupling between modes and/or resonators has enabled the observation of dynamics similar to optomechanics, such as phonon lasing and state squeezing. Nonlinear coupling enables the transfer of energy between vibrational modes having resonant frequencies far apart. Internal resonance is the most common form of nonlinear coupling mechanism. The necessary condition for mechanical modes to be coupled through internal resonance is that the ratio of resonant frequencies of coupled modes should be close to an integer (n=1,2,3). Previous studies on internal resonance have been restricted to clamped-clamped beams. However, our expriemental understanding of modal coupling through internal resonance is limited as it requires the meticulous design of device parameters to obtain resonant modes that are commensurate. Two-dimensional materials such as graphene and metal dichalcogenides have highly tunable resonant frequencies, enabling internal resonance conditions to be easily satisfied. Moreover, vibrational modes of a two-dimensional resonator are coupled through the intrinsic strain in the membrane. Thus, two-dimensional materials serve as a great platform to understand the dynamics of coupled systems. In this work, we demonstrate strong tunable intermodal coupling due to 2:1 internal resonance in MoS2 drum resonators. The modal peak splitting, a signature of coupling, is observed in the linear regime itself in addition to the nonlinear regime. We show the tunability of this coupling with applied gate bias. The simulations enabled us to qualitatively understand the effect of excitation force, frequency detuning and modal coupling strength on the resonator dynamics. Understanding internal resonance in two-dimensional membranes would enable new possibilities in signal transduction and frequency conversion. It could also help in improving the frequency stability of MoS2 resonators through the intermodal coupling.
Coupling between different modes of a resonator is not just limited to two-dimensional materials but has also been reported in MEMS structures like clamped-clamped beams and curved arches. Advanced fabrication techniques have paved the way for a new class of MEMS structures, the piezo-micromachined ultrasonic transducer (pMUT). The majority of pMUTs/diaphragms are designed to operate in a linear dynamic range. But, at larger vibrational amplitudes, the nonlinear effect strongly affects the device dynamics. Careful control of these nonlinearities could pave the way to improved stability in microsensors, such as phase fluctuation reduction, frequency control and in-situ amplification schemes. Thus, it is imperative to understand and tune device nonlinearities. Previously tuning of nonlinearities has been achieved in mechanical resonators using capacitive techniques. But the same has not been demonstrated for piezoelectrically actuated ZnO diaphragms. In this thesis, we present the tuning of nonlinearity through diaphragm curvature in these devices. We calculate the effective nonlinearity through the device's backbone curve response and relate it with the diaphragm curvature. Nonlinearity in these resonators also leads to intermodal coupling and energy exchange between the commensurate vibrational modes. We further demonstrate the transfer of energy from the coupled higher vibrational mode to the fundamental mode of the pMUT. This coupling in the future would enable ultrastable piezo-based oscillators
Allylic Halogenation Route to Latent-Active Trans-Glycosylation of Allyl Glycoside Donors
No full textAllylic halogenation of allyl glycosides as a new route to allyl glycoside donors in glycosylations is investigated in this thesis. Allyl functionality is one of the commonly adopted protecting groups to hydroxyl groups in sugar chemistry. In addition, allyl glycosides act as glycosyl donors, through isomerization to the corresponding vinyl glycosides. Facile conversion of allyl moiety to other functionalities, as well as, stabilities under acidic and basic conditions offer rich possibilities of this moiety in sugar chemistry. Chapter 1 provides a succinct overview of glycosylation reactions and mechanisms.
An area of intense interest is to transform a latent allyl moiety to an active glycosyl donor. In this effort, allylic halogenation reaction is considered appealing, due to the expected reactivity of the mixed halo-acetal of allyl glycoside towards an electrophile and the subsequent transformation to a glycosylation-active intermediate, suitable as an active glycosyl donor. Early experiments show that allylic bromination of allyl glycosides, using N-bromosuccinimide (NBS)/azo-bis-isobutyronitrile (AIBN) in CCl4 generates mixed halo-allyl glycoside intermediate, the reaction of which with an acceptor in the presence of Ag(I) or triflic acid (TfOH) affords the corresponding trans-glycoside in a good yield. The reaction is verified with a number of glycoside acceptors, including allyl glycoside acceptors. In the case of allyl glycoside acceptors, the resulting trans-glycoside possesses allyl moiety at the reducing end, which, in turn, is subjected allylic activation and subsequent glycosylation. Di-, tri- and tetrasaccharide syntheses are accomplished in good yields by this new route. Chapter 2 describes the development of this new method.
Radical halogenations in CCl4 warranted a replacement to the solvent, as well as, further optimizations of the reaction. In these efforts, diethylcarbonate (Et2O)2CO) is identified as a suitable solvent to conduct (i) radical halogenation and (ii) the subsequent glycosylation. The glycosylation is promoted either by TfOH or trimethylsilyl triflate (TMSOTf). A one-pot methodology is developed and method is verified with the synthesis of xyloyranoside, mono-, di- and trisaccharides. Chapter 3 provides the details of these developments.
Halo-allyl mixed acetal of allyl glycoside is found to undergo S¬N2 and S¬¬¬¬N2’ reactions with thiolate nucleophiles. The SN2’ reaction leads to 3-thiocresylpropenyl (TCP) glycoside, as a stable vinyl glycoside, which can be stored for longer duration, unlike, vinyl glycosides that are quite unstable due to faster hydrolysis. TCP glycoside is subjected to remote activation using iodonium reagent and activation leads to the formation of glycosylation active intermediate. Glycosylations with aglycosyl and glycosyl acceptors are conducted facile and the corresponding trans-glycosides are obtained in excellent yields. Chapter 4 describes the development of this new, stable TCP-based vinyl glycoside methodology in glycosylations.
Overall, the thesis illustrates establishing allyl glycosides as glycosyl donors as allylic halogenations and subsequent glycosylations. The new method merits in the repertoire of contemporary glycosylation techniques of remote activation-based glycosylations.UG
A study on Expatriate's adjustment stressors and coping strategies
No full textAdjustment to the cross-cultural environment is inherently taxing for expatriates.
Researchers have investigated the implications of cultural stress on occupational stress.
However, gaps remain, as follows: (i) There are not many studies available in the literature
which focuses on the theoretical model of stress and coping to understand expatriate's
adjustment, (ii) Previous studies focus on adjustment on only three dimensions, namely,
general adjustment, social interaction and workplace adjustment, there are limited studies on
stressors of adjustment, (iii) Majority of the studies captures the positive and negative form of
stress in work-stress literature; however, limited studies on the distinct effect of positive and
negative forms of stress on expatriate's adjustment, (iv) There are minimal studies which have
examined the coping strategies specifically linking with stressful situations, (v) There are no
adequate studies which have explored expatriate's adjustment pattern in Indian and Japanese
context.
The thesis aims to develop a model to guide successful expatriate adjustment through
the lens of the popular model of stress and coping, including the challenge –hindrance
framework and the transactional model of stress. The following research objectives are
explored, namely, 1. Explore the factors related to challenge and hindrance stressors, expat's
overall adjustment, and coping strategies, 2. Explore the demographic variations in challenge
and hindrance stressors, expat's overall adjustment, and coping strategies used by expatriates,
3. Determine the influence of challenge and hindrance stressors on expat's overall adjustment,
4. Study the relationship between challenge and hindrance stressors with coping strategies, 5.
Explore the expatriate adjustment pattern in the Japanese and Indian context.
Measures were captured using a questionnaire, on a Likert scale, point allocation
method, multiple-choice and open-ended questions. We have used various statistical methods
to analyze the data, such as; 1. Exploratory factor analysis to identify the underlying structure
of challenges & hindrance stressors, 2. Kruskal-Wallis test to analyze demographic variations
and regression analysis to understand influences on expat's overall adjustment. The study
identifies a new set of variables related to expatriates' everyday adjustment, family’s
adjustment, language comfort, problems-faced, and discrimination-faced by expatriates in a
foreign environment. The study highlights the type of stressors faced and coping strategies used
and the relationship between them. We have proposed a new framework for understanding the
adjustment stressors, emphasizing non-work day-to-day adjustment in the host societ