Indian Institute of Science Bangalore
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Studies on exosomal microRNAs and proteins associated with Hepatitis C and Dengue virus induced pathogenesis
No full textStudies on the exosomal microRNAs and proteins associated with Hepatitis C and Dengue virus-induced pathogenesis.
Hepatitis C virus and Dengue virus are the group of positive-strand RNA viruses belonging to the family Flaviviridae which causes hepatitis and dengue fever, respectively. Various host factors including small non-coding RNAs, micro RNAs (miRNAs) and proteins are the major regulators of the cellular pathways that are involved in virus induced pathogenesis.
Exosomes are the small extracellular vesicles, secreted out by the cells and have significant role in cell to cell signaling. It comprises of various host factors like miRNA, lncRNAs, mRNAs and cellular proteins. As the disease progresses from an early stage to severe form of infection, the composition of the exosome changes which might directly correlate to the pathogenesis. Study of the exosomal miRNAs and proteins can provide an insight into its role in disease progression, which has been highly under explored. The current study focuses on the differential expression of the exosomal miRNAs and proteins upon two distinctly different viruses with diverse phenotype viz. Hepatitis C virus (HCV) causes chronic infection and Dengue virus causes acute infection.
Part I: Elucidation of the role of differentially expressed exosomal miRNAs in HCV infection and pathogenesis.
Hepatitis C virus (HCV) is one of the most common causes of liver disease. In some infected patients, the virus gets cleared by the host immune system, but others develop chronic disease, cirrhosis and often lead to hepatocellular carcinoma (HCC). In this part, the role of exosome-associated microRNAs (miRNAs) in HCV-induced disease pathology has been investigated. RNA-sequencing was performed to identify miRNAs that are differentially regulated in the exosomes isolated from patient sera at different stages of HCV infection viz. cirrhosis and hepatocellular carcinoma. Among the differentially expressed miRNAs, miR-375 was shortlisted for further studies. miR-375 was found to be significantly upregulated in the exosomes isolated from patients with both cirrhosis and HCC. A similar upregulation pattern was observed in the intracellular and extracellular/exosomal levels of miR-375 in HCV-JFH1 RNA transfected Huh7.5 cells. The depletion of miR-375 in infected cells inhibited the HCV-induced cell migration and proliferation without affecting viral replication or release, suggesting a supportive role for miR-375 in HCV induced pathogenesis. Naïve Huh 7.5 cells would uptake miR-375 that is secreted through exosomes derived from HCV-infected cells that will lead to an increase in cell proliferation and migration in the recipient cells. Further, IGFBP4 was identified as a potential target of miR-375, which was validated using luciferase reporter constructs. Since IGFBP4 is involved in cell growth and malignancy, it is possible that miR-375 exerts its effect on cell proliferation through IGFBP4. Taken together, the results demonstrate the critical involvement of exosome-associated miR-375 in HCV-induced pathogenesis.
Part II: To study the exosomal miRNAs signatures during disease progression in dengue virus infection.
Dengue virus is a causative agent of the dengue fever, a most common mosquito transmitted disease. As an epidemic disease, dengue has put almost half of the world’s population at risk. With the lack of specific treatment and effective vaccine, it is most important to diagnose and prognose the disease as early as possible. Based on its severity, dengue infection has been categorized into 3 group plans viz. WHO Plan A (dengue without warning), WHO Plan B (dengue with warning) and WHO Plan C (Dengue haemorrhagic fever). In this part, we examined the exosome associated micro-RNA in human patient’s serum sample at different stages of dengue infection using RNA-sequencing. We could identify 50 differentially expressed microRNAs that were either significantly up regulated or down regulated in dengue infection. After extensive validation, we observed that miR-96-5p was significantly upregulated in all stages of dengue infection whereas miR-146a-5p was found to be significantly downregulated in all stages of dengue infection. Further study on miR-146a-5p showed that it regulates the expression of most of the proteins involved in immune response viz. IL-8, NF-kappa, HuR, indicating its role in regulating the immune response during dengue infection. Further, we could identify a few potential miRNAs that are upregulated in the patients that progress to severe form of dengue infection.
Part III: Studying the exosomal proteins differentially expressed in HCV and Dengue infection: Possible implications
Protein composition of the exosomes are also known to change during the viral infection. In the final part of the study, the exosomal proteins expressed during different stages of HCV and Dengue virus infection were evaluated using LC-MS and 2D gel analysis respectively. Interestingly, in case of HCV infection, certain proteins, such as Apo B100, Tetranectin and Gelsolin were found to be gradually upregulated with the disease progression from chronic to cirrhosis to HCC. Whereas Transthyretin and Hsp90 co-chaperon showed differential expression at different stages (Plan A, Plan B and Plan C) of dengue virus infection. Interestingly, Haptoglobin was found to be upregulated in both the viral infections, indicating some common role. Results suggest possible involvement of some common and different exosomal proteins in mediating cellular pathogenesis in the context of two different viruses with diverse phenotype.
Overall, the study reveals the potential role of exosomal miRNA and proteins, as part of host response and viral strategies, which could collectively contribute to disease progression, underpinning novel targets for possible therapeutic intervention
Lignin and Extractive Tracing - an Alternative approach to Assess and Predict Biogas Yields and Production Rates among Tropical Biomass Feedstocks
No full textPredicting biogas yields from the composition of biomass feedstocks has been difficult primarily due to the variations found in their chemical composition and structure of lignocellulosics. This study aims at understanding, assessing and predicting biogas yields from the chemical composition cutting across different types of biomass feedstocks: fruit peels and rinds, cereal straws, crop stalks, husks, leaves and conifers. A wide range of substrates were chosen to account for the variation in composition by choosing these feedstocks with large variations in the lignin (range, type and steric arrangement). This research answers a few fundamental questions pertaining to predicting biogas yields, the role played by extractives and lignin and also the arrangement of the constituents.
A classical Biochemical Methane Potential (BMP) approach was taken up to understand the fermentation behavior of various feedstocks and understand the issues/ problems in a systematic manner. The first part of this study deals with predicting the pattern of biogas production. The results show that the biogas yield is a strong function of extractive , i.e., pectins, soluble carbohydrates that leach out into the water during the hot water extraction, content in the biomass species. The feedstocks with high concentration of extractives produced higher amount of biogas and at higher rates. As the lignin concentration rises the extent and rates of gas production falls down. Based on the preliminary results and prior understanding, a new methodology was developed where the gas production can be ascribed to sequential/ semi-sequential degradation of biomass components. The sequential decomposition leading to biogas production can be explained by a two-component fit, wherein the first stage of gas production is ascribed predominantly to the breakdown of the easily accessible extractives and hemicellulose (unbound). In the second stage the gas production is slower and can be ascribed to the conversion of difficult to degrade hemicellulose (bound) and cellulose. The point where the first stage and second stage join has been termed as the “inflection point”. The inflection point was found to be ≈20 days in case of easy to digest feedstocks, that are usually rich in extractives. As the lignin concentration increases the time taken to achieve inflection increases, in this case ≥30 days. The concept of two-component fit and inflection point also shows that ≈4/5th of the total gas production is evolved till it reaches the inflection point.
In a first, a classification-based approach was taken up to arrive at regression equations correlating the biogas yields and composition of the biomass. The 26 feedstocks studied were grouped into three classes based on: a) the degradability in nature and cross verified with b) statistical K-means clustering. The classes obtained were
1. Extractive rich peels: that degrade faster.
2. Lignin rich leaves: that have slow degradability rates.
3. Holo-cellulose rich straws, stalks, husks and grasses: degradation rates at levels in between the above two classes mentioned.
Based on this classification two multiple regression equations were developed. The equations showed strong correlation between the biogas yields and chemical composition with a correlation coefficient of ≥0.95. The equation shows the significance of extractives and hemicellulose in the gas production. In contrast, lignin and ash hinder the gas production. A new indicator L/E (L= lignin and E= extractives) has shown that these two components play a significant role in understanding the AD process. The correlation between gas yields and L/E follows power law with a R2 of 0.70 and needs more refinement to incorporate additional recalcitrants such as waxes, silica, etc.
Lignin has been extensively used as a marker as it has been considered not to degrade under the typical anaerobic environment of an AD system. The correlation between gas yields and lignin has always been considered to follow a negative linear relationship. This study showed that the gas yields follow a power law with lignin as an independent variable. The relationship shows that at some point (in this case 15-20%) the extent of access limitation becomes significant, finally leading to decreased rate and extent of gas production. In order to study the effect of extractive concentration on the AD process, substrate with high extractive was chosen and the inoculum concentration was varied. At low levels of inoculum, the gas production was inhibited due to the inability to take up and rapidly convert the VFA flux (pH was found to be low). On the other hand, when the lignin concentration is high (30%) the degradation is low and methanogens are starved, hence low methane production takes place. Such feedstocks do not show sensitivity to S/I ratios. In this study it was termed as the access limited fermentation. It shows that when lignin concentration is higher the propensity to eclipse underlying hemicellulose and cellulose becomes higher thereby reducing access to enzyme hydrolysis and finally biogas production.
Here lignin is considered as an ‘obstructor’ to access and degradation, an effort has been made to understand the biogas production after simple pretreatment like autoclaving with and without alkali. The classification developed earlier was used to understand the effect of alkali treatment which selectively delignifies the biomass. It was found that a simple autoclaving is sufficient for high gas yields compared to the alkali pretreatment, although alkali pretreatment gives high gas yields in some cases, the loss of volatile solids is about 40-50% during the alkali pre-treatment. Therefore, the delignification step to make cellulosics more accessible, needs to sacrifice a large fraction of extractables that would have contributed to biogas production
Inverse Problems in 3D Full-wave Electromagnetics
No full textAn inverse problem in Electromagnetics (EM) refers to the process of reconstructing the physical system by processing the measured data of its electromagnetic properties. Inverse problems are typically ill-posed, and this makes them far more challenging than the typically well-posed forward problem. The solution of such inverse problems finds applications in nondestructive testing and evaluation, biomedical imaging, geophysical exploration etc. This thesis addresses some inverse problems specific to the area of electromagnetics, arising in three different scenarios.
The first problem is 3-D quantitative imaging primarily targeted towards bio-medical applications. The task is to retrieve the dielectric properties, location and the shape of an unknown object from the measured scattered field. The unknown object is modeled by discretization into several voxels, with each voxel having its own dielectric property. As the inverse problem is non-linear, typically an iterative optimization process is adopted, and a forward problem needs to be solved at every iteration. The total time for reconstruction depends on the forward solver time and the number of iterations. In many cases, the number of unknowns to be reconstructed is prohibitively large. Further, the non-convergence or false-convergence of the optimization process presents its own challenge. This thesis proposes two methodologies to solve these challenges. In the first approach a multilevel methodology is proposed where voxels are hierarchically decomposed into smaller voxels based on an appropriate indicator, leading to a non-uniform multilevel voxel structure aimed at reducing the eventual number of unknowns to be solved for, also enabling faster convergence. In the second approach, a two-stage framework is proposed comprising of Machine Learning classification followed by optimization (ML-OPT). The first stage generates an appropriate adaptive grid for the optimization process and provides a suitable initial guess aiding convergence to the global minima. This approach is aimed at detecting breast tumors where the optimization algorithm can aim for higher resolution in the suspected tumor region, while using lower resolution elsewhere.
The second problem is in the domain of high-speed circuits and is focused on synthesis of transmission line physical parameters given the desired electrical parameters like characteristic impedance and propagation constant. A forward solver is used to train Neural network for several different configurations for analysis and an optimization algorithm is used for synthesis.
The third problem is focused on finding the source of radiation in an electronic system e.g. an automotive ECU, given the measured field at the antenna in the radiated emissions setup. The source of radiation can be from common mode current on the cable harness or from the Design Under Test (DUT). A method based on Huygens box is proposed to quantify the radiation from cable and DUT at each frequency. On each cell of the Huygens box the value of electric field computed at the observation point taking the Electric Current (J) and Magnetic Current (M) on that cell as sources and this information on the Huygens box is used to quantify the radiation.
Some part of the presented work is used via technology-transfer at Simyog Technology Pvt. Ltd., an IISc incubated startup, to develop a simulation software called Compliance-scope which allows the hardware designer to predict the EMI/EMC performance of electronics modules from an early design stage
Excitons in monolayer transition metal dichalcogenides
No full textExcitons are quasiparticles formed due to electrostatic attraction between the electrons
and the holes in a semiconductor. This Coulomb attraction is very strong in the mono-
layers of Transition Metal Dichalcogenides (TMDs) mainly because of strong quantum
confinement, reduced dielectric screening, and high effective mass of electrons and holes
in these material systems. A 2D hydrogen atom is a simple model to describe confined
excitons in these monolayer films. A more formal way to describe excitons in thin semi-
conductors is through the Bethe-Salpeter formalism which describes these excitons as a
superposition of the electronic states in momentum space. In order to understand exci-
tons further, we explore the following excitonic features in this thesis:
Probing intrinsic exciton linewidth: Monolayer TMDs are highly luminescent
materials despite being sub-nanometer thick. This is due to the ultrashort radiative life-
time of the strongly bound bright excitons hosted by these materials. The intrinsically
short radiative lifetime results in a large broadening in the exciton band with a magnitude
that is about two orders greater than the spread of the light cone itself. The situation
calls for a need to revisit the conventional light cone picture. We present a modified light
cone concept which places the light line as the generalized lower bound for allowed radia-
tive recombination. A self-consistent methodology, which becomes crucial upon inclusion
of large radiative broadening in the exciton band, is proposed to segregate the radiative
and the nonradiative components of the homogeneous exciton linewidth. We estimate
a fundamental radiative linewidth of 1:54 0:17 meV, owing purely to finite radiative
lifetime in the absence of nonradiative dephasing processes. As a direct consequence of the large radiative limit, we nd a surprisingly large ( 0:27 meV) linewidth broadening
due to zero-point energy of acoustic phonons. This obscures the precise experimental
determination of the intrinsic radiative linewidth and sets a fundamental limit on the
nonradiative linewidth broadening at T=0 K.
Modulating exciton binding energy: Screening due to the surrounding dielectric
medium reshapes the electron-hole interaction potential and plays a pivotal role in decid-
ing the binding energies of strongly bound exciton complexes in quantum confined TMD
monolayers. However, owing to strong quasiparticle band-gap renormalization in such
systems, a direct quantification of estimated shifts in binding energy in different dielectric
media remains elusive using optical studies. By changing the dielectric environment, we
show a conspicuous photoluminescence peak shift at low temperature for higher energy
excitons (2s,3s,4s,5s) in monolayer MoSe2, while the 1s exciton peak position remains
unaltered a direct evidence of varying compensation between screening induced exciton
binding energy modulation and quasiparticle band-gap renormalization. The estimated
modulation of binding energy for the 1s exciton is found to be 58.6% (72.8% for 2s,
75.85% for 3s, and 85.6% for 4s) by coating an Al2O3 layer on top, while the correspond-
ing reduction in quasiparticle band-gap is estimated to be 246 meV. Such direct evidence
of large tunability of the binding energy of exciton complexes as well as the band-gap in
monolayer TMDs holds promise of novel device applications.
Enhancing exciton valley coherence time: In monolayer TMDs, valley coher-
ence degrades rapidly due to a combination of fast scattering and inter-valley exchange
interaction. This leads to a sub-picosecond valley coherence time, making coherent manip-
ulation of exciton a highly formidable task. Using monolayer MoS2 sandwiched between
top and bottom graphene, we demonstrate perfect valley coherence by observing 100%
degree of linear polarization (DOLP) of excitons in steady state photoluminescence. This
is achieved in this unique design through a combined effect of (a) suppression in exchange
interaction due to enhanced dielectric screening, (b) reduction in exciton lifetime due to a fast inter-layer transfer to graphene, and (c) operating in the motional narrowing regime.
We disentangle the role of the key parameters affecting valley coherence by using a com-
bination of calculation (solutions of Bethe-Salpeter and steady-state Maialle-Silva-Sham
equations) and choice of systematic design of experiments using four different stacks with
varying screening and exciton lifetime. To the best of our knowledge, this is the first
time where the valley coherence timescale has been significantly enhanced in monolayer
semiconductors.
Probing the role of motional narrowing in exciton valley coherence: We
observe a strong effect of motional narrowing (regime of random phase cancellation) by
observing a high DOLP from a defected MoS2 sample, as compared to a clean MoS2 sam-
ple which shows relatively lower exciton DOLP. Similar observations hold good for both
monolayer and bilayer MoS2 samples, which results from random phase cancellation in
the exciton pseudospin in the motional narrowing regime. This highlights the counter-
intuitive role of sample quality in the exciton DOLP measurements: a clean sample does
not necessarily guarantee large exciton DOLP and vice versa.
Generating highly luminescent, highly-polarized, ultra-narrow exciton peak :
On generation, the excitons relax to the lowest energy 1s state by scattering with phonons
through multiple possible pathways. We use a simple technique in which, by tuning the
excitation laser wavelength, the excitons resonantly come down to the 1s state in a single-
shot manner through scattering with a specific phonon mode. Using this technique in a
monolayer WS2 sample sandwiched between few-layer graphene flakes, we obtain exciton
peaks that are: (1) highly luminescent, (2) highly linearly polarized - demonstrating near-
perfect valley coherence, and (3) ultra-narrow - due to a reduction in the inhomogeneous
broadening. The lowest exciton linewidth obtained using this technique is 1:5 meV,
which after deconvolution with the excitation laser gives an upper bound of 0:23 meV
on the homogeneous linewidth of the exciton peak. We demonstrate the above features
all the way from cryogenic temperature to room-temperature
Investigation into the Synthesis, Structure and Dielectric Property of Lead-Free Double Perovskite Structured Materials
No full textLead based materials have shown to be important materials showing high
piezoelectricity, colossal dielectric constant, ferroelectricity along with their good mechanical
behaviour and resistance to corrosion. As a result, they have wide range of applications such
as sensors, actuators, detectors, batteries, lead-painting pigments etc. However, due to high
toxicity of lead and its nature to accumulate in the environment with gradually increasing
concentrations, increasingly more health hazards are encountered. In this regard, replacing
these lead based materials by lead free material with similar/higher level of dielectric
properties is the driving force for research on lead free dielectric materials. In this work, we
have studied few double perovskites structured dielectric materials and their solid solutions.
The present work attempts to understand synthesis, structure and dielectric property of
lead free double perovskite structure BaFe0.5Nb0.5O3, BaFe0.5Ta0.5O3 and solid solution of (x)
BaFe0.5Nb0.5O3–(1-x)KNbO3 (x= 0, 0.2, 0.4, 0.6, 0.8 and 1). This thesis work organized in
seven chapters as given below. First two chapters describe the motivation behind the work
and experimental technique used for synthesis and characterizes materials, chapter 3, 4, 5 and
6 described investigated material and chapter 7 summarizes the thesis. Details of chapters as
follows
Hypothesis Testing under Communication Constraints - Theory and an Application in IoT
No full textApplications in the Internet of Things (IoT) often demand enabling low-compute devices to perform distributed inference and testing by communicating over a low bandwidth link. This gives rise to a plethora of new problems which may broadly be termed resource-constrained statistical inference problems. In this thesis, we consider two such problems.
In the first part of the thesis, we study the following distributed hypothesis testing problem. Two parties observing sequences of uniformly distributed bits want to determine if their bits were generated independently or not. To that end, the first party communicates to the second. A simple communication scheme involves taking as few sample bits as determined by the sample complexity of independence testing and sending it to the second party. But is there a scheme that uses fewer bits of communication than the sample complexity, perhaps by observing more sample bits? We show that the answer to this question is in the affirmative. More generally, for any given joint distribution, we present a distributed independence test that uses linear correlation between functions of the observed random variables. Furthermore, we provide lower bounds for the general setting that use hypercontractivity and reverse hypercontractivity to obtain a measure change bound between the joint and the independent distributions. The resulting bounds are tight for both a binary symmetric source and a Gaussian symmetric source. The proposed scheme is then extended to handle high dimensional correlation testing with interactive communication, wherein one party observes a Gaussian vector X and the other party observes a jointly Gaussian scalar Y, and we seek to test if the norm of the vector of correlation between X and Y exceeds a given value or is it 0. We provide corresponding lower bounds to establish the optimality of the proposed scheme. Furthermore, we derive a lower bound which implies that distributed correlation testing requires less communication than distributed estimation of the correlation vector.
In the second part of the thesis, we study streaming compression of electrical signals sampled at a very high frequency by Intelligent Electronic Devices (IEDs), enabled to capture anomalous signal behavior. Under normal operation, this oversampling is redundant and leads to excessive data being stored or transmitted. This gives rise to a new compression problem where the collected samples should be further subsampled and quantized based on the presence of an anomaly in the underlying signal. We propose an Anomaly-aware Compressive Sampler (ACS) which tests the signal for the presence of an anomaly in a block of samples, and subsamples in a hierarchical manner to retain the desired sampling rate. ACS has been designed keeping hardware constraints in mind, using integer operations, an appropriate bit-packing, a simple iterated delta filter, and a streaming data pipeline. ACS competes with the state-of-the-art algorithm for the better-behaved transmission system data from DOE/EPRI, and outperforms it significantly on real-time distribution system data recorded in our laboratory. ACS is lightweight and was implemented on an ARM processor. Further, we present a mathematical analysis of the anomaly detection module of ACS. Finally, the performance of the proposed scheme in compressing a nonstationary signal with frequency band uncertainty is studied, with a focus on the dependence of the compression ratio and reconstruction error on the oversampling rate. We modify a zero-crossings-based compression scheme proposed in literature for bandlimited signals to incorporate resolution of frequency band uncertainty using our anomaly detection procedure. While this new scheme is theoretically appealing, we point out some of its limitations when it comes to implementation
Exploring carrier dynamics in van-der-Waals heterostructures with shot noise spectroscopy, thermoelectricity and opto-electronic study
No full textIn the last two decades, there has been tremendous progress in two-dimensional (2D) - material research. It is mainly
due to the availability of a large selection of 2D-materials and their van-der-Waals heterostructures. The 2D-material
based heterostructures, in particular, have significant fundamental and technological importance due to their tunable
electrical and optical properties. In this thesis, we have studied three types of 2D-heterostructures: graphene pn
junction, magic-angle twisted bilayer graphene, and MoTe2-MoS2 based hetero-pn junction. We investigate the carrier
transport and its dynamics using cutting-edge probes like shot noise spectroscopy, thermoelectricity, and optoelectronic
studies.
In the first part of the thesis, we will present our research on equilibration dynamics of quantum Hall (QH)
edge states in graphene pn junction (PNJ). Graphene pn junction with co-propagating spin-valley polarized QH edges
is a promising platform for studying electron interferometry. Though several conductance measurements have been
attempted for such PNJs, the edge dynamics of the spin-valley symmetry broken edge states remain unexplored. In this
study, we present the measurements of conductance together with shot noise, an ideal tool to unravel the dynamics,
in dual graphite gated hexagonal boron nitride encapsulated high mobility graphene devices. The conductance data
show that the symmetry broken QH edges at the PNJ follow spin selective equilibration. The shot noise results as a
function of both p and n side filling factors reveal the unique dependence of the scattering mechanism. Remarkably, the
scattering is found to be fully tunable from incoherent to a coherent regime with the increasing number of QH edges
at the PNJ, shedding crucial insights into the velocity-dependent phase coherence of the QH edges at a pn junction.
Furthermore, we study the bias-dependent tunneling between the ν = ±1 QH edges of the pn junction. At zero bias,
we observe finite tunneling (t ∼ 0.5), which remains almost constant up to a bias energy of few hundreds of µV . The
tunneling sharply falls with further increasing bias voltage and exhibits repeated smaller peaks at discrete energies
before completely vanishing. The tunneling at zero bias is anomalous as it is expected to be zero between ν = ±1
edges due to their orthogonal spin polarization. Thus, from tunneling to fully blockade regime suggests bias-controlled
switching of spin polarization at graphene pn junction.
In the second part we will discuss the interaction driven resetting of the band-structure of a Magic-angle twisted
bilayer graphene (MtBLG) using thermoelectricity as a probe. MtBLG has proven to be an extremely promising
new platform to realize and study a host of emergent quantum phases arising from the strong correlations in its
narrow bandwidth flat band. In this regard, thermal transport phenomena like thermopower, in addition to being
coveted technologically, is also sensitive to the particle-hole (PH) asymmetry, making it a crucial tool to probe the
underlying electronic structure of this material. In this study, we have carried out thermopower measurements of
MtBLG as a function of carrier density, temperature and magnetic field, and report the observation of an unusually
large thermopower reaching up to a value as high as ∼ 100µV/K at a low temperature of 1K. Surprisingly, our observed thermopower exhibits peak-like features in close correspondence to the resistance peaks around the integer
Moire fillings, including the Dirac Point, violates the Mott formula. We show that the large thermopower peaks and
their associated behaviour arise from the emergent highly PH asymmetric electronic structure due to the cascade of
Dirac revivals.
In the last part, we will present the optoelectronic properties of MoTe2-MoS2 hetero pn junction. MoS2 and
MoTe2 are, respectively, n-type and p-type transition metal di-chalcogenides with band-gaps of ∼ 1.8 eV and 0.8 eV.
Over and above the antiambipolar transfer characteristics observed similar to other hetero pn junction, our experiments
reveal a unique feature as a dip in the transconductance near the maximum. We further observe that the modulation of
the dip in the transconductance depends on the doping concentration of the 2D flakes and also on the power density
of the incident light. We also demonstrate high photo-responsivity of ∼ 105A/W at room temperature for a forward
bias of 1.5V. We explain these new findings based on interlayer recombination rate-dependent semi-classical transport
model
Liquefaction Resistance and Cyclic Response of Air - Injected Desaturated Clean Sandy Soil: Experimental and Numerical Investigations
No full textLiquefaction has been one of the major concerns in the field of geotechnical earthquake engineering. Various mitigation techniques such as vibroflotation, deep dynamic compaction, explosive compaction, grouting, deep soil mixing etc. have been employed over the last fifty years. These techniques either densify the in-situ soil or fill the voids with some external agent. Some of these techniques are costly, while others pose a threat to the adjacent structures and environment. Over the last couple of years, induced desaturation is emerging as a possible cost-effective and environment-friendly liquefaction mitigation technique. In this technique, the degree of saturation of the in situ saturated soil is reduced either by injecting air or by generating some kind of gas within the soil matrix.
The present study investigates the liquefaction resistance and cyclic response of the air-injected desaturated clean sandy soil. The large number of stress controlled undrained cyclic triaxial tests were conducted on samples with the degree of saturation in the range of 70 % to 99 %. Three relative densities of 30 %, 40 % and 60 % and initial effective confining pressure of 25 kPa, 50 kPa and 100 kPa were considered for the investigation. Cyclic shear stress ratio (CSR) was 0.175, 0.250, 0.300 and 0.400. It was found that the number of cycles required for initial liquefaction increased exponentially with the reduction in the degree of saturation. Cyclic strength of desaturated sand with relative density of 40 % with the degree of saturation of 80 % was found to be almost twice that of fully saturated sand. Depending upon the degree of saturation, relative density, confining pressure and CSR, five distinct failure modes were observed: 1) Hybrid cyclic liquefaction 2) Cyclic mobility-gradual/catastrophic 3) Cyclic softening-CE 4) Cyclic softening-E and 5) FAPSC-SS: Failure due to Accumulation of Plastic Strain on the Compression side as a result of gradual Strain Softening. Samples with a high degree of saturation underwent cyclic
mobility or hybrid cyclic liquefaction failure while those at a low degree of saturation underwent cyclic softening-CE or cyclic softening-E. Insight from the critical state soil mechanics framework revealed that nearly saturated samples with the initial state closer to critical state undergoes either catastrophic cyclic mobility or hybrid cyclic liquefaction whereas that farther from it undergoes either gradual cyclic mobility or FAPSC-SS. The phase transformation line was found to play an important role in understanding the cause of pore pressure reduction during the extension stage of loading in samples with a low degree of saturation. Longevity test revealed that at the end of seven days, there was negligible change in the degree of saturation of the triaxial specimen. It was found that Konstadinoun and Georgiannou (2014) pore pressure model, when modified for CSR and desaturation effect, could be used to predict pore pressure evolution in desaturated sand. Further, assessment of hypothetical desaturated soil domain by approximate method, revealed that desaturated soil with degree of saturation of around 70 % is adequate to prevent liquefaction under strong to very strong earthquakes having peak acceleration as high as 0.36 g.
The numerical investigation was performed employing open-source finite element software “OpenSees” developed by Pacific Earthquake Engineering Research (PEER) Center, University of California, Berkeley. Constitutive behavior of the soil was modelled using in-built pressure dependent multi yield material model “PressureDependMultiYield”. The triaxial specimen was modeled by eight-node linear isoparametric hexahedral element “BrickUP”. This element simulates the undrained response of coupled solid-fluid material based on Biot’s theory of poroelasticity. Further, an evaluation study was conducted to determine the constitutive parameters for the saturated and desaturated condition. Finite element model of 30 m thick soil domain was prepared and subjected to four earthquake motions from India, having peak acceleration between 0.10 g and 0.36 g. This soil domain was discretized into four node plane strain bilinear
isoparametric elements entitled as “quadUP”. This element captures coupled solid-fluid response when subjected to dynamic loading. The parametric study was conducted to understand the effect of permeability, degree of saturation, the thickness of the desaturated zone, on liquefaction resistance of the soil, measured in terms of pore pressure ratio. With an increase in the permeability for fully saturated condition, excess pore pressure was reduced, and dissipation of pore pressure was accelerated. Further, the higher the permeability, the higher was the acceleration amplification for fully saturated condition. With the reduction in the degree of saturation, reduction in the thickness of liquefied zone was observed. Moreover, for the degree of saturation of 81.4 %, the thickness of the non-liquefied zone was found to be equal to the thickness of the desaturated zone, irrespective of the input motion.
Keeping in view, experimental and numerical findings, the following points can be recommended: 1) Desaturation up to the degree of saturation of 80 % is adequate to double the cyclic strength of clean sand. 2) From approximate method it was found that desaturation up to degree of saturation of 70 % was adequate to prevent liquefaction against strong to very strong earthquakes having peak acceleration in the range of 0.10 g to 0.36 g, on the other hand numerical simulation revealed that degree of saturation of 81.4 % was sufficient to avoid liquefaction under same earthquakes. As numerical analysis is more rigorous than approximate method, it can be concluded that desaturation up to the degree of saturation of 80 % is adequate to prevent liquefaction. 3) Injected air gets entrapped into the voids and remains there for a long period of time. 4) The thickness of the desaturation zone, with the degree of saturation of 81.4 %, can be kept between 5 m to 15 m to prevent liquefaction under strong to very strong earthquakes having peak acceleration between 0.10 to 0.36 g. 5) Amplification factor at the ground surface for the degree of saturation of 81.4 % was found to be in the range of 0.32 to 1.76, under strong to very
strong earthquakes having peak acceleration between 0.10 to 0.36 g. This implies that desaturation of clean sand up to degree of saturation of 80 % is enough to achieve the two-fold goal: 1) to prevent liquefaction 2) to keep the acceleration amplification low. Further reduction in the degree of saturation may amplify motion significantly owing to presence of high matric suction, though this issue needs further investigatio
Steady Sedimentation of Particles in Long Vertical Tube and Effect of End Boundary Conditions on Convective Motion
No full textSedimentation – settling of particles in a fluid- is observed in nature like rain droplets and dust particles in the atmosphere, and in a variety of industrial processes, like to clarify liquid as well as separate particles of different size and density. The simplest system is the sedimentation of mono-disperse particles in a vast stationary fluid. The main parameters are Particle Reynolds Number (〖Re〗_p based on terminal velocity), ratio of particle density to fluid density (ρ_p/ρ_f ), particle volume fraction (φ), and container dimensions for experimental and numerical methods. Two main questions arise: what is the mean settling velocity (V_g), and nature and values of fluctuation in particle velocity (V^/), and how do they compare with the terminal velocity (V_t) of an isolated particle in an infinite fluid. At low particle Reynolds number, V_t is given by the Stokes law. Experiments have been typically performed in a tank containing the fluid with particles initially well mixed and tracking the motion of the particles or performing PIV to obtain mean settling velocity (V_g), fluctuating particle velocities (V^/) etc. The main focus of these studies has been to correlate different parameters like mean settling velocity, velocity fluctuation, correlation length with volume fraction, and dimension of the container. Though this apparently simple problem has been studied theoretically, experimentally, and numerically over many decades, there are several unanswered questions. For example, the experimental results for velocity fluctuations do not agree with the theoretical predictions. The origin of scalings for velocity fluctuations are unclear. In our study, we try to address some of these issues using a new type of experiment.
In our experiment, particles are fed at a constant rate at the top and allowed to settle in a long vertical tube containing quiescent fluid, closed at the bottom. The constant particle feed rate ensures mean steady particle settling in contrast to the standard experiments done previously where the settling process is transient. Also, the long vertical extent of the tube ensures Axial Homogeneity. We have done two types of experiments: water droplets (10 μm, 〖Re〗_p~〖10〗^(-3)) falling in the air, and spherical glass beads (110 μm, 〖Re〗_p~1 ) settling in water. The estimated volume fractions for the former is 〖10〗^(-7) and for the latter, it is〖 10〗^(-3). For the droplet-air system, the tube dimension is 5×5 〖cm〗^2 and for the particle-water system, three tube dimensions (4×4 〖cm〗^2 , 5×5 〖cm〗^2, 7×7 〖cm〗^2 ) have been used. Experiments have been done with different mass flux values. We have used high-speed imaging illuminated by a sheet of laser light to visualize the particle motion fields and Particle Image Velocimetry (PIV) to get the mean and fluctuating particle velocities and the spatial and temporal correlations. We have observed a variety of sedimentation-induced convective motions, including regions of particle patches moving upwards. The conditions at the tube end significantly alter the convective patterns and the fluctuating velocities. Convective motions, though hypothesized to exist, have not been observed in earlier experiments. We present results for the mean and fluctuating velocities and spatial and temporal correlations of the velocity fields for the range of mass fluxes and different tube dimensions. Besides the existence of convective motion, the main findings are: the mean settling velocity varies between 0.80-1.1 V_t. The fluctuating velocities are in the range 0.30-0.80 V_t and strongly depend on mass flux. Correlation lengths scale with tube width. We present these results in a non-dimensional form which suggest different scaling laws
A Study of R&D Team Leadership Roles, Gaps in Role Execution and Team Performance Outcomes
No full textR&D is the mother of many technological innovations, provides an economic advantage and is imperative in the growth story of any nation. Particularly for a developing nation like India, R&D growth and sustenance helps to keep pace with the developed countries, who are the competitive forerunners and exporters of advanced technologies. Literature suggests several internal and external factors that lead to success and failures of R&D. This research focuses its scope on R&D team leadership, which literature reports as one of the most highly regarded internal/humanistic factors, that significantly contributes to R&D team performance. Much of the leadership research since 1980s is on leadership traits, styles, theories, etc. We narrow down the wide taxonomies of leadership to a few specific roles. We build a role-inventory and propose the ‘critical five’ roles which are indispensable for R&D team performance. We propose a ‘role-based model of R&D team leadership’ as the conceptual framework. This research stresses on dual focus of R&D team leaders towards their team, considering roles both internal and external to teams. We also acknowledge the idea of various contingency factors influencing the ‘role-performance’ relationship. A gap in execution of critical roles may undermine the performance and productivity of R&D teams. Thus, further to the exploration of ‘role-performance’ relationship and effect of moderators on this association, we conduct a gap analysis of critical R&D team leadership roles.
This research focusses on the public sector R&D labs, and data primarily collected from the CSIR labs of India, is subjected to validation. At the outset, we assess the factor structure and measurement scales for the five roles. This is an additional step to ensure the suitability of the theory-derived scales, for the purpose of this study. We further use PLS-SEM, to assess the measurement and structural aspects of the ‘role-based’ model and provide its empirical validation. After having ascertained the ‘role-performance’ association, we perform interaction moderation analysis to study the moderating effect of situational variables on the ‘role-performance’ relationship. Further, we analyse the presence and severity of a role-gaps. Findings suggest a significantly positive association between the five proposed roles and team performance outcomes. Moderating effects suggest the extent of emphasis that a team leader needs to put on each role, depending upon situation confronted. Findings also suggest presence of low-moderate gap in the execution of the five roles critical for team performance. Suggestions to address the gaps are also proposed. Finally, based on the research findings, we make recommendations to the R&D labs, which would be useful in the staffing, appraisal, training and development exercises of R&D team leadership