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Coalescence of Multielectron Bubbles in Liquid Helium
No full textIn 1977, Volodin et al. observed that the electrohydrodynamic instability of charged helium surfaces can lead to the loss of the electrons from the surface in the form of bubbles. These are Multielectron Bubbles (MEBs) which contain electrons pinned on their inner surfaces. The MEBs form a model system for studying electrons on curved surfaces, which are predicted to have many interesting properties. Recent experiments showed that above the Lambda point, MEBs could be trapped using a Paul-trap and their sizes are mainly determined by the amount of vapour present inside. Here, we report the experimental observation of the coalescence of two MEBs which were moving upward in bulk liquid helium-4. The charge and radii of the MEBs were determined before and after the coalescence. The merging of two similar charged bubbles was possible because of the presence of vapour inside the merging MEBs
A Model Exact Study of the Properties of Low-Lying Electronic States of Perylene and Substituted Perylenes
No full textThere is a resurgence of interest in the electronic structure of perylene for its applications in molecular devices such as organic photovoltaics and organic light-emitting diodes. In this study, we have obtained the low-lying singlet states of perylene by exactly solving the Parisar-Parr-Pople model Hamiltonian of this system with 20 sites and 20 electrons in the VB basis where dimensionality is similar to 5.92 billion. The triplet states of perylene are obtained using a DMRG scheme with symmetry adaptation. The one- and two-photon states are very close in energy similar to 3.2 eV while the lowest triplet state is slightly below 1.6 eV indicating that perylene is a good candidate for singlet fission. To explore the tunability of the electronic states, we have studied donor-acceptor substituted perylenes. The two donors and two acceptors are substituted symmetrically at either the four bay sites or four peri sites. In all the bay substitution and one peri substitution at moderate D/A strength, the optical gap is lowered to about 2.8 eV. These molecules can be used as blue emitters. We have also reported bond orders in all the cases, and perylene as well as substituted perylenes can be viewed as two weakly coupled naphthalenes in the singlet states, but in triplets these bonds tend to be comparable to other bonds in strength. The charge densities in substituted perylenes are mostly localized around the substitution sites in the ground state. The positive spin densities in triplets are concentrated around the peri and bay sites with the remaining sites having small spin densities of either sign
Stabilization of Brownmillerite-Type SrCoO2.5 by a Cost-Effective Quenching Method for Oxygen-Scavenging Applications
No full textBrownmillerite (BM)-type oxide sorbents have gained attention recently for producing oxygen-enriched streams. Herein, a cost-effective method of quenching with the use of an Al foil pad is adapted for the synthesis of brownmillerite SrCoO2.5. The oxygen storage capacity of this oxide has been investigated using a simple home-built volumetric setup. The oxygen-rich phase was formed by a pressurized heat-treatment of a BM sample. The oxygen storage capacity of the sample has been calculated from the pressure change during desorption. The effect of oxygen pressure on the amount of oxygen stored inside the sample has also been evaluated. Furthermore, selective absorption of oxygen is confirmed by performing the absorption in compressed air. The results indicate that 15.28 cm(3) O-2 g(-1) can be stored in the sample at STP. The change in oxygen content in SrCoO2.5+delta varied reversibly up to a delta value of 0.26, which is confirmed by iodometric titration. It is shown that the new method of quenching proposed does not deteriorate the oxygen storage property of the material
Recent advances in the field of transition metal dichalcogenides for biomedical applications
No full textNanosheets of transition metal dichalcogenide (TMDs), the graphene-like two-dimensional (2D) materials, exhibit a unique combination of properties and have attracted enormous research interest for a wide range of applications including catalysis, functional electronics, solid lubrication, photovoltaics, energy materials and most recently in biomedical applications. Their potential for use in biosensors, drug delivery, multimodal imaging, antimicrobial agents and tissue engineering is being actively studied. However, the commercial translation of exfoliated TMDs has been limited due to the low aqueous solubility, non-uniformity, lack of control over the layer thickness, and the long-term colloidal stability of the exfoliated material. There is wide interest in the synthesis and exfoliation of TMDs resulting in the reporting of increasing numbers of new methods and their biomedical applications. The unique physicochemical characteristics of the TMD nanosheets have been exploited to tether them with biological payload to achieve selective localized delivery in vivo. The large surface-to-volume ratio, good cytocompatibility, ease of surface modification, tunable bandgap, strong spin-orbit coupling, and high optical and thermal conversion efficiency of TMD nanosheets make them favorable over traditional nanomaterials for biomedical research. Moreover, the presence of abundant active edge sites on the 2D TMDs makes them suitable for catalytic activities, while the large surface area and the interspace between layers are particularly conducive to ion or small molecule intercalation, making them useful for energy storage applications with rapid redox reaction capabilities. One of the major limitations of the exfoliated TMDs has been their limited colloidal stability in aqueous media. In this review, we summarize the recent advances in the exfoliation and synthesis of single-layered TMDs, their biomedical efficacy in terms of cytotoxicity, combinatorial therapy and diagnostic imaging, as well as antimicrobial activity. We highlight the current challenges in the field and propose strategies for the future
A common mechanism for evolution of single shear bands in large-strain deformation of metals
No full textShear banding, a type of inhomogeneous plastic flow involving very large local strains, occurs in a variety of material systems. We study dynamics of evolution of single shear bands at strain rates of up to 10(6) per second in three different polycrystalline metal systems, using a special shear deformation framework and a micro-marker technique calibrated to track localised deformation fields at micrometer resolution. Once a band is nucleated as a weak interface, localised plastic flow occurs via Bingham-type viscous sliding between material segments on either side of the interface. As a result, the evolution and magnitude of strains and material displacements in the band vicinity are well-described by a model based on momentum diffusion. The viscosity at the band interface is very small, only a few mPa<bold>sec</bold>, and is comparable to those of liquid metals at their melting point. Based on analysis of various contributions to band viscosity at the microscopic level, a plausible explanation based on phonon drag on dislocation motion is presented for the small viscosity. The accuracy of predictions made by the momentum diffusion model for different materials and deformation rates suggests that once nucleated, a shear band evolves by a common mechanism that is relatively insensitive to microstructure details
Effect of Humidity on Wear of TiN Coatings: Role of Capillary Condensation
No full textCoated ball-on-disk wear configuration was used to study the effect of relative humidity, water vapor pressure, and water on wear of TiN coatings in the temperature range of room temperature to 100 degrees C. Two kinds of experiments were designed: one at constant temperature with varying humidity and the second at constant water vapor pressure with varying temperature. Temperature variation experiments were also conducted in water. The trends in wear volume after a fixed sliding distance were analyzed. At constant temperature, the wear volumes increased with humidity/water vapor pressure. However, at a constant vapor pressure, wear volumes remained roughly invariant with temperature until a critical temperature below which they rose sharply. In contrast, the wear rate increased with temperature for tests in liquid water. Considering calculations based on Kelvin's equations and further characterization of the wear surfaces using profilometry, XPS and FIB techniques, it has been strongly suggested that the anomalous temperature dependence of wear in humid air might be due to capillary condensation occurring at the contacting asperities. The wear of TiN showed two regimes in the influence of humidity. Where the humidity was below a threshold value of similar to 50pct relative humidity (RH), there was negligible condensation and wear was low. Above 50pct RH, the wear rate increased due to the availability of liquid water at asperities. The reason for the difference in the wear rate in vapor and liquid was hypothesized, based on XPS data, to depend on the formation of soft hydroxides in the presence of water, in contrast to the harder barrier oxide formed in the presence of oxygen, the dominant gas species in the vapor phase
Precipitation Behavior of IN718 After Surface Mechanical Attrition Treatment (SMAT) and Its Effect on Wear Properties
No full textInconel 718 is a precipitation-strengthened Ni-based superalloy which finds applications across a wide temperature range (up to 650 degrees C). It shows excellent yield strength, ductility, creep resistance and fatigue strength. Surface treatment is known to improve the fatigue life of materials via nano-crystallization of the surface layer. However, the precipitation behavior of the surface-treated layer is largely unexplored. In this regard, the present study aims to investigate and compare the precipitation in the surface-treated layer and the bulk solution-treated IN718. The material is subjected to surface mechanical attrition treatment (SMAT) after solution treatment and is followed by a two-step aging treatment which promotes the precipitation of / in the alloy. The precipitation behavior shown by the bulk and SMAT layer was different. After aging, the bulk shows a microcrystalline matrix containing precipitates, whereas the microstructure of the surface layer consists of nanograins with nanotwins. No evidence of / precipitates were observed in the SMAT layer. The formation of nanograins in the surface layer after aging is attributed to recrystallization with controlled grain growth. The differential microstructures due to the chosen processing route have resulted in increased hardness of the surface layer and increased wear life of the material
The fully-extended conformation in peptides and proteins
No full textThe intramolecularly H-bonded, fully-extended conformation (C-5) of an alpha-amino acid residue (and the resulting 2.0(5)-helix obtained via its propagation) is one of the least extensively investigated types of peptide and protein backbone secondary structure. This situation does still currently occur despite its unique ability to enjoy by far the largest separation per residue among peptide conformations. In this article, we offer a detailed update of our present knowledge on this intriguing 3D-structure of peptides in the crystal state as obtained from recently published investigations, complemented by a statistical analysis for its occurrence in the crystal structures of alpha-amino acid derivatives and peptides available in the Cambridge Structural Database. We have expanded this useful information to the results of a bioinformatics analysis performed on this (so far largely unappreciated) conformation authenticated in all proteins solved by X-ray diffraction to a resolution of <= 1.5 angstrom. In the last section, we describe the results of our DFT calculations on the conformational preferences of a set of homopeptides (from monomer to octamer) based on as many as six proteins and two noncoded, carefully selected, alpha-amino acids. From this literature survey integrated by new energy calculations, we have definitely provided strong support to the thesis that this polypeptide 3D-structure does indeed exist, it should be not neglected in future studies by structural biochemists, and it represents a very attractive, novel backbone for applications for organic, medicinal, and biomaterials chemists
v-Gap Metric-Based Simultaneous Frequency-Shaping Stabilization for Unstable Multi-Input Multi-Output Plants
No full textEmploying Water as the Hydride Source in Synthesis: A Case Study of Diboron Mediated Alkyne Hydroarylation
No full textWe present an approach to utilize water as the hydride source via Pd(II)/Pd(0) catalysis. As a case study, we have achieved a diboron mediated Pd(II)-catalyzed hydroarylation of alkynes using arylboronic acids. This approach not only complements conventional reactivity of Pd via Pd(0)/Pd(II) cycle for the hydroarylation but also utilizes water as the hydride source. We believe this would particularly be beneficial in utilizing water as a reagent