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An Overview of Restricted Boltzmann Machines
No full textThe restricted Boltzmann machine (RBM) is a two-layered network of stochastic units with undirected connections between pairs of units in the two layers. The two layers of nodes are called visible and hidden nodes. In an RBM, there are no connections from visible to visible or hidden to hidden nodes. RBMs are used mainly as a generative model. They can be suitably modified to perform classification tasks also. They are among the basic building blocks of other deep learning models such as deep Boltzmann machine and deep belief networks. The aim of this article is to give a tutorial introduction to the restricted Boltzmann machines and to review the evolution of this model
Bubble and conical forms of vortex breakdown in swirling jets
No full textExperimental investigations of laminar swirling jets had revealed a new form of vortex breakdown, named conical vortex breakdown, in addition to the commonly observed bubble form. The present study explores these breakdown states that develop for the Maxworthy profile (a model of swirling jets) at inflow, from streamwise-invariant initial conditions, with direct numerical simulations. For a constant Reynolds number based on jet radius and a centreline velocity of 200, various flow states were observed as the inflow profile's swirl parameter (scaled centreline radial derivative of azimuthal velocity) was varied up to 2. At low swirl ( ) a helical mode of azimuthal wavenumber (co-winding, counter-rotating mode) was observed. A `swelling' appeared at , and a steady bubble breakdown at . On further increase to , a helical, self-excited global mode ( , counter-winding and co-rotating) was observed, originating in the bubble's wake but with little effect on the bubble itself - a bubble vortex breakdown with a spiral tail. Local and global stability analyses revealed this to arise from a linear instability mechanism, distinct from that for the spiral breakdown which has been studied using Grabowski profile (a model of wing-tip vortices). At still higher swirl ( ), a pulsating type of bubble breakdown occurred, followed by conical breakdown at 1.6. The latter consists of a large toroidal vortex confined by a radially expanding conical sheet, and a weaker vortex core downstream. For the highest swirls, the sheet was no longer conical, but curved away from the axis as a wide-open breakdown. The applicability of two classical inviscid theories for vortex breakdown - transition to a conjugate state, and the dominance of negative azimuthal vorticity - was assessed for the conical form. As required by the former, the flow transitioned from a supercritical to subcritical state in the vicinity of the stagnation point. The deviations from the predictions of the latter model were considerable
Influence of alkyl chains on fluoranthene ensembles towards fluorescence-based detection of 2,4,6-trinitrophenol
No full textA series of novel p-electron rich fluoranthene derivatives (P1-P3) adorned with symmetrical and unsymmetrical alkoxy chains were developed using Diels-Alder reaction. The use of peripheral triethyleneglycol (TEG) chains on fluoranthene enhances its electron donating ability and furnishes decrease in their energy levels and optical band gap. The intense sky-blue fluorescence exhibiting fluoranthene ensembles have been investigated as potential fluorescent chemosensors for the detection of explosive nitroaromatics (NACs). Fluorescence studies demonstrate that emission intensity of P1-P3 was efficiently decreased upon interaction with picric acid (PA) in a dominant static quenching phenomena which is attributed to photoinduced electron transfer process. The limit of detection (LOD) was found to be in the range of 2-20 ppb. The vapour phase sensing study involving thin films of P1-P3 showed efficient quenching response and sensing process is found to be highly reversible. Surface morphology of thin films significantly varied with the type of alkyl chain present on fluoranthene. The thin films are highly selective towards PA in aqueous medium containing commonly interfering nitro-explosives. Contact mode approach using silica gel substrates allows femto-gram detection of trinitrotoluene (TNT) and PA makes these materials as potential chemosensors for real time applications
Microstructure and corrosion behaviour of NiCo-Carbon nanotube composite coatings
No full textMicrostructure-corrosion property correlation in electrodeposited NiCo-carbon nanotube (NiCo-CNT) composite coatings has been explored. NiCo-CNT composite coatings were electrodeposited over mild steel substrate by dispersing different amounts of CNTs into the electrolyte bath. CNTs were synthesized by the chemical vapor deposition (CVD) method. Morphological characterization revealed crack free morphology for all the coatings. The coating morphology progressively became finer with increase in the addition of CNTs up to a certain amount of CNT after which the morphology again became rougher. Finer morphology was due to a uniform distribution of CNTs in the coating matrix. Lower amount of CNT and non-uniform distribution of agglomerated CNTs in case of higher concentrations produced the rougher morphology. All the NiCo-CNT composite coating were relatively hydrophobic when compared to the only NiCo coating. Structural characterization revealed that an intimate interaction between the growing metal matrix and CNT promoted coating growth along the low energy (111) direction. Corrosion behaviour of the coatings was examined through potentiodynamic polarization and electrochemical impedance spectroscopy methods. It was observed that there exists an optimum with respect to the amount of CNT in the NiCo-CNT composite coating for achieving highest corrosion resistance performance. This optimum CNT amount was the same that produced finer morphology and preferred growth along low energy planes. Further characterization of the composite coating microstructure revealed that the optimum CNT concentration also produced a microstructure with largest fraction of low energy low angle grain boundaries which also resulted in higher corrosion resistance performance
Magnetism in two-dimensional materials beyond graphene
No full textMagnetic materials enjoy an envious position in the area of data storage, electronics, and even in biomedical field. This review provides an overview of low-dimensional magnetism in graphene, h-BN, and carbon nitrides, which originates from defects like vacancy, adatom, doping, and dangling bonds. In transition metal dichalcogenides, a tunable magnetism comes from doping, strain, and vacancy/ defects, and these materials offer spintronics, as well as photoelectronic potentials, since they have an additional degree of freedom called valley state (e.g. MoS2). Strain- and layer-dependent magnetic ordering has been observed in layered compounds like CrXTe3, CrI3, and trisulfides. The magnetism in 2D oxides like MoO3, Ni(OH)(2), and perovskites are also interesting as they are potential candidates for next-generation devices having faster processing and large data storage capacity. Quasi 2D magnetism in MXene and in atomically thin materials supported on 3D materials will also be discussed. Finally, some of the challenges related to the control of defects and imperfections in 2D lattice, promising approaches to overcome them will be covered
Flaring stellar disk in the low surface brightness galaxy UGC 7321
No full textWe theoretically study the vertical structure of the edge-on low surface brightness (LSB) galaxy UGC 7321. This is one of the few well-observed LSBs. We modeled it as a gravitationally coupled disk system of stars and atomic hydrogen gas in the potential of the dark matter halo and treated the realistic case where the rotation velocity varies with radius. We used a dense and compact halo as implied by the observed rotation curve in this model. We calculated the thickness of stellar and HI disks in terms of the half-width at half-maximum of the vertical density distribution in a region of R = 0-12 kpc using input parameters constrained by observations. We obtain a mildly increasing disk thickness up to R = 6 kpc, in a good agreement with the observed trend, and predict a strong flaring beyond this. To obtain this trend, the stellar velocity dispersion has to fall exponentially at a rate of 3.2R(D), while the standard value of 2R(D) gives a decreasing thickness with radius. Interestingly, both stellar and HI disks show flaring in the outer disk region although they are dynamically dominated by the dark matter halo from the very inner radii. The resulting vertical stellar density distribution cannot be fit by a single sech(2/n) function, in agreement with observations, which show wings at larger distances above the mid-plane. Invoking a double-disk model to explain the vertical structure of LSBs as done in the literature may therefore not be necessary
Fractional Regularization to Improve Photoacoustic Tomographic Image Reconstruction
No full textPhotoacoustic tomography involves reconstructing the initial pressure rise distribution from the measured acoustic boundary data. The recovery of the initial pressure rise distribution tends to be an ill-posed problem in the presence of noise and when limited independent data is available, necessitating regularization. The standard regularization schemes include Tikhonov, l(1)-norm, and total-variation. These regularization schemes weigh the singular values equally irrespective of the noise level present in the data. This paper introduces a fractional framework to weigh the singular values with respect to a fractional power. This fractional framework was implemented for Tikhonov, l(1)-norm, and total-variation regularization schemes. Moreover, an automated method for choosing the fractional power was also proposed. It was shown theoretically and with numerical experiments that the fractional power is inversely related to the data noise level for fractional Tikhonov scheme. The fractional framework outperforms the standard regularization schemes, Tikhonov, l(1)-norm, and total-variation by 54% in numerical simulations, experimental phantoms, and in vivo rat data in terms of observed contrast/signal-to-noise-ratio of the reconstructed images
Isolation and molecular characterization of dengue virus clinical isolates from pediatric patients in New Delhi
No full textObjective: To characterize the in vitro replication fitness, viral diversity, and phylogeny of dengue viruses (DENV) isolated from Indian patients. Methods: DENV was isolated from whole blood collected from patients by passaging in cell culture. Passage 3 viruses were used for growth kinetics in C6/36 mosquito cells. Parallel efforts also focused on the isolation of DENV RNA from plasma samples of the same patients, which were processed for next-generation sequencing. Results: It was possible to isolate 64 clinical isolates of DENV, mostly DENV-2. Twenty-five of these were further used for growth curve analysis in vitro, which showed a wide range of replication kinetics. The highest viral titers were associated with isolates from patients with dengue with warning signs and severe dengue cases. Full genome sequences of 21 DENV isolates were obtained. Genome analysis mapped the circulating DENV-2 strains to the Cosmopolitan genotype. Conclusions: The replication kinetics of isolates from patients with mild or severe infection did not differ significantly, but the viral titers varied by two orders of magnitude between the isolates, suggesting differences in replication fitness among the circulating DENV-2
Dislocation mechanism based model for Portevin-Le Chatelier like instability in microindentation of dilute alloys
No full textAlthough the topic of intermittent plastic flow manifesting as load fluctuations or displacement jumps in nanoindentation (depths less than 100 nm) has attracted considerable attention, the existence of steps on load-indentation (F-z) curves reported in microindentation (depths of several microns) of samples of dilute alloys of varying concentrations and load dates, has received little attention from a modeling point of view. Following our earlier approaches to nanoindentation instabilities and indentation size effect, we develop a minimal dislocation mechanism based model that predicts all the generic experimental features by setting up time-evolution equations for the mobile, the forest, dislocations with solute atmosphere, and the geometrically necessary dislocation densities. The model includes basic dislocation mechanisms common to most plastic deformations, such as dislocation multiplication, storage, and recovery mechanisms. We model the indentation instability as a variant of the standard Portevin-Le Chatelier (PLC) effect seen in the constant strain rate condition by including collective pinning and unpinning of dislocations from solute atmosphere. The instability mechanism is further generalized to include concentration-dependent dislocation-solute interaction to capture both concentration dependence of the indentation instability and strengthening of alloy samples. Based on recent experimental observations that show small misorientation at small depths suggesting limited geometrically necessary dislocation density, we model the growth of the geometrically necessary dislocation density by the number of loops that can be activated under the contact area and the mean strain gradient. The equations are then coupled to the load rate equation. The model predicts all the generic experimental features, such as (a) the stepped nature of the F-z curves, (b) the existence of a critical load and critical indentation depth for the onset of the instability, (c) the decreasing dependence of the maximum indentation depth with concentration of the alloying element, (d) the mean critical indentation depth z* for the onset of the instability increasing with decreasing concentration with a concomitant increase in levels of fluctuations, (e) the decreasing power-law dependence of the critical indentation depth with concentration, (f) the manifestation of intermittent stepped response in a window of load rates, and (g) the magnitude of the load steps scaling linearly with the load. In essence, the basic physical mechanisms responsible for predicting all the experimental results (a)-(g) are the generalization of pinning and unpinning mechanism (of dislocations from solute atmosphere) to include concentration-dependent dislocation-solute interaction and solution hardening of alloy samples with concentration together with the inherent rate-dependent nature of the PLC instability
Toward Closing the Gap between Hexoses and N-Acetlyhexosamines: Experimental and Computational Studies on the Collision-Induced Dissociation of Hexosamines
No full textMotivated by the fundamental difference in the reactivity of hexoses and N-acetylhexosamines under collision-induced dissociation (CID) mass spectrometry conditions, we have investigated the CID of two hexosamines, glucosamine (GlcN) and galactosamine (GalN), experimentally and computationally. Both hexosamines undergo ring-opening and then dissociate via the (0,2)A and the (0,3)A (X-0,X-3) cross-ring cleavage channels. The preference for the ring-opening is similar to the behavior of N-acetylhexosamines and explains why the two anomers of the same sugar give the same mass spectrum. While the spectrum for GlcN is dominated by the (0,2)A signal, the signal intensities for both (0,2)A and the (0,3)A (X-0,X-3) dissociation channels are comparable for GalN, which allows GlcN and GalN to be distinguished easily. Calculations at MP2 level of theory indicate that this is related to the differences in the relative barrier heights for the (0,2)A and the (0,3)A (X-0,X-3) cross-ring cleavage channels. This, in return, reflects the circumstance that the (0,2)A cross-ring cleavage barriers are different for the two sugars, while the barriers of all other dissociation channels are comparable. While the mechanisms of the cross-ring dissociation channels of hexoses are well described using the retro-aldol mechanism in the literature, this study proposes a new mechanism for the (0,3)A (X-0,X-3) cross-ring cleavage of hexosamines that involves the formation of an epoxy intermediate or a zwitterionic intermediate