Mechanism for the Compressive Strain Induced Oscillations in the Conductance of Carbon Nanotubes

We report on the monotonic increase and the oscillation of electrical conductance in multiwalled carbon nanotubes with compressive strain. Combined experimental and theoretical analyses confirm that the conductance variation with strain is because of the transition from sp(2) to configurations that are promoted by the interaction of walls in the nanotubes. The intrawall interaction is the reason for the monotonic increase in the conduction, while the oscillations are attributable to interwall interactions. This explains the observed electromechanical oscillation in multiwalled nanotubes and its absence in single-walled nanotubes, thereby resolving a long-standing debate on the interpretation of these results. Moreover, the current carrying capability of nanotubes can be enhanced significantly by controlling applied strains. DOI: 10.1103/PhysRevLett.110.095504

A multifold reduction in the transition Reynolds number, and ultra-fast mixing, in a micro-channel due to a dynamical instability induced by a soft wall

A dynamical instability is observed in experimental studies on micro-channels of rectangular cross-section with smallest dimension 100 and 160 mu m in which one of the walls is made of soft gel. There is a spontaneous transition from an ordered, laminar flow to a chaotic and highly mixed flow state when the Reynolds number increases beyond a critical value. The critical Reynolds number, which decreases as the elasticity modulus of the soft wall is reduced, is as low as 200 for the softest wall used here (in contrast to 1200 for a rigid-walled channel) The instability onset is observed by the breakup of a dye-stream introduced in the centre of the micro-channel, as well as the onset of wall oscillations due to laser scattering from fluorescent beads embedded in the wall of the channel. The mixing time across a channel of width 1.5 mm, measured by dye-stream and outlet conductance experiments, is smaller by a factor of 10(5) than that for a laminar flow. The increased mixing rate comes at very little cost, because the pressure drop (energy requirement to drive the flow) increases continuously and modestly at transition. The deformed shape is reconstructed numerically, and computational fluid dynamics (CFD) simulations are carried out to obtain the pressure gradient and the velocity fields for different flow rates. The pressure difference across the channel predicted by simulations is in agreement with the experiments (within experimental errors) for flow rates where the dye stream is laminar, but the experimental pressure difference is higher than the simulation prediction after dye-stream breakup. A linear stability analysis is carried out using the parallel-flow approximation, in which the wall is modelled as a neo-Hookean elastic solid, and the simulation results for the mean velocity and pressure gradient from the CFD simulations are used as inputs. The stability analysis accurately predicts the Reynolds number (based on flow rate) at which an instability is observed in the dye stream, and it also predicts that the instability first takes place at the downstream converging section of the channel, and not at the upstream diverging section. The stability analysis also indicates that the destabilization is due to the modification of the flow and the local pressure gradient due to the wall deformation; if we assume a parabolic velocity profile with the pressure gradient given by the plane Poiseuille law, the flow is always found to be stable.

On the mass flow rate from silos with lateral exit holes

The mass flow rate, (m) over dot, associated with the lateral outflow of dry, cohesionless granular material through circular orifices of diameter D made in vertical walls of silos was measured experimentally in order to determine also the influence of the wall thickness of the silo, w. Geometrical arguments, based on the outflow happening, are given in order to have a general correlation for (m) over dot embracing both quantities, D and w. The angle of repose appears to be an important characterization factor in these kinds of flows.

Indium Assisted Growth of Silicon Nanowires by Electron beam evaporation

Silicon nanowires were grown on Si substrates by electron beam evaporation (EBE) was demonstrated using Indium as an alternate catalyst to gold. We have studied the effect of substrate (growth) temperature, deposition time on the growth of nanowires. It was observed that a narrow temperature window from 300 degrees C to 400 degrees C for the nanowires growth. At growth temperature >= 400 degrees C suppression of nanowires growth was observed due to evaporation of catalyst particle. It is also observed that higher deposition times also leading to the absence of nanowire growth as well as uncatalyzed deposition on the nanowires side walls due to limited surface diffusion of ad atoms and catalyst evaporation.

Rheometry of granular materials in cylindrical Couette cells: Anomalous stress caused by gravity and shear

The cylindrical Couette device is commonly employed to study the rheology of fluids, but seldom used for dense granular materials. Plasticity theories used for granular flows predict a stress field that is independent of the shear rate, but otherwise similar to that in fluids. In this paper we report detailed measurements of the stress as a function of depth, and show that the stress profile differs fundamentally from that of fluids, from the predictions of plasticity theories, and from intuitive expectation. In the static state, a part of the weight of the material is transferred to the walls by a downward vertical shear stress, bringing about the well-known Janssen saturation of the stress in vertical columns. When the material is sheared, the vertical shear stress changes sign, and the magnitudes of all components of the stress rise rapidly with depth. These qualitative features are preserved over a range of the Couette gap and shear rate, for smooth and rough walls and two model granular materials. To explain the anomalous rheological response, we consider some hypotheses that seem plausibleapriori, but showthat none survive after careful analysis of the experimental observations. We argue that the anomalous stress is due to an anisotropic fabric caused by the combined actions of gravity, shear, and frictional walls, for which we present indirect evidence from our experiments. A general theoretical framework for anisotropic plasticity is then presented. The detailed mechanics of how an anisotropic fabric is brought about by the above-mentioned factors is not clear, and promises to be a challenging problem for future investigations. (C) 2013 AIP Publishing LLC.

Classification, representation, and automatic extraction of deformation features in sheet metal parts

This paper presents classification, representation and extraction of deformation features in sheet-metal parts. The thickness is constant for these shape features and hence these are also referred to as constant thickness features. The deformation feature is represented as a set of faces with a characteristic arrangement among the faces. Deformation of the base-sheet or forming of material creates Bends and Walls with respect to a base-sheet or a reference plane. These are referred to as Basic Deformation Features (BDFs). Compound deformation features having two or more BDFs are defined as characteristic combinations of Bends and Walls and represented as a graph called Basic Deformation Features Graph (BDFG). The graph, therefore, represents a compound deformation feature uniquely. The characteristic arrangement of the faces and type of bends belonging to the feature decide the type and nature of the deformation feature. Algorithms have been developed to extract and identify deformation features from a CAD model of sheet-metal parts. The proposed algorithm does not require folding and unfolding of the part as intermediate steps to recognize deformation features. Representations of typical features are illustrated and results of extracting these deformation features from typical sheet metal parts are presented and discussed. (C) 2013 Elsevier Ltd. All rights reserved.

Role of outer membrane exopolymers of Acidithiobacillus ferrooxidans in adsorption of cells onto pyrite and chalcopyrite

Bacterial surface polymers play a major role in the adhesion of bacterial cells to solid surfaces. Lipopolysaccharides (LPS) are essential constituents of the cell walls of almost all Gram-negative bacteria. This paper reports the results of the investigations on the role of outer membrane exopolymers (LPS) of the chemolithotroph, Acidithiobacillus ferrooxidans, in adsorption of the cells onto pyrite and chalcopyrite. Optimization of EDTA treatment for removal of LPS from cell surface and the surface characterization of EDTA-treated cells are outlined. There was no change in cell morphology or loss in cell motility upon treatment with upto 0.04 mM EDTA for 1 h. Partial removal of LPS by EDTA treatment resulted in reduced adsorption of the cells on both pyrite and chalcopyrite. The protein profile of the EDTA-extractable fraction showed presence of certain outer membrane proteins indicating that EDTA treatment results in temporary gaps in the outer membrane. Also, specificity towards pyrite compared to chalcopyrite that was exhibited by untreated cells was lost when their exopolymer layers were stripped off, which could be attributed to the role of outer membrane proteins in the mineral-specificity exhibited by the bacteria. (C) 2013 Elsevier B.V. All rights reserved.

Effect of hinged leaflets on vortex pair generation

We experimentally study the effect of having hinged leaflets at the jet exit on the formation of a two-dimensional counter-rotating vortex pair. A piston-cylinder mechanism is used to generate a starting jet from a high-aspect-ratio channel into a quiescent medium. For a rigid exit, with no leaflets at the channel exit, the measurements at a central plane show that the trailing jet in the present case is never detached from the vortex pair, and keeps feeding into the latter, unlike in the axisymmetric case. Passive flexibility is introduced in the form of rigid leaflets or flaps that are hinged at the exit of the channel, with the flaps initially parallel to the channel walls. The experimental arrangement closely approximates the limiting case of a free-to-rotate rigid flap with negligible structural stiffness, damping and flap inertia, as these limiting structural properties permit the largest flap openings. Using this arrangement, we start the flow and measure the flap kinematics and the vorticity fields for different flap lengths and piston velocity programs. The typical motion of the flaps involves a rapid opening and a subsequent more gradual return to its initial position, both of which occur when the piston is still moving. The initial opening of the flaps can be attributed to an excess pressure that develops in the channel when the flow starts, due to the acceleration that has to be imparted to the fluid slug between the flaps. In the case with flaps, two additional pairs of vortices are formed because of the motion of the flaps, leading to the ejection of a total of up to three vortex pairs from the hinged exit. The flaps' length (L-f) is found to significantly affect flap motions when plotted using the conventional time scale L/d, where L is the piston stroke and d is the channel width. However, with a newly defined time scale based on the flap length (L/L-f), we find a good collapse of all the measured flap motions irrespective of flap length and piston velocity for an impulsively started piston motion. The maximum opening angle in all these impulsive velocity program cases, irrespective of the flap length, is found to be close to 15 degrees. Even though the flap kinematics collapses well with L/L-f, there are differences in the distribution of the ejected vorticity even for the same L/L-f. Such a redistribution of vorticity can lead to important changes in the overall properties of the flow, and it gives us a better understanding of the importance of exit flexibility in such flows.

Numerical analysis of geocell reinforced ballast overlying soft clay subgrade

Geotextiles and geogrids have been in use for several decades in variety of geo-structure applications including foundation of embankments, retaining walls, pavements. Geocells is one such variant in geosynthetic reinforcement of recent years, which provides a three dimensional confinement to the infill material. Although extensive research has been carried on geocell reinforced sand, clay and layered soil subgrades, limited research has been reported on the aggregates/ballast reinforced with geocells. This paper presents the behavior of a railway sleeper subjected to monotonic loading on geocell reinforced aggregates, of size ranging from 20 to 75 mm, overlying soft clay subgrades. Series of tests were conducted in a steel test tank of dimensions 700 mm x 300 mm x 700 mm. In addition to the laboratory model tests, numerical simulations were performed using a finite difference code to predict the behavior of geocell reinforced ballast. The results from numerical simulations were compared with the experimental data. The numerical and experimental results manifested the importance that the geocell reinforcement has a significant effect on the ballast behaviour. The results depicted that the stiffness of underlying soft clay subgrade has a significant influence on the behavior of the geocell-aggregate composite material in redistributing the loading system.

Regional variations in the nonlinearity and anisotropy of bovine aortic elastin

Arterial walls have a regular and lamellar organization of elastin present as concentric fenestrated networks in the media. In contrast, elastin networks are longitudinally oriented in layers adjacent to the media. In a previous model exploring the biomechanics of arterial elastin, we had proposed a microstructurally motivated strain energy function modeled using orthotropic material symmetry. Using mechanical experiments, we showed that the neo-Hookean term had a dominant contribution to the overall form of the strain energy function. In contrast, invariants corresponding to the two fiber families had smaller contributions. To extend these investigations, we use biaxial force-controlled experiments to quantify regional variations in the anisotropy and nonlinearity of elastin isolated from bovine aortic tissues proximal and distal to the heart. Results from this study show that tissue nonlinearity significantly increases distal to the heart as compared to proximally located regions (). Distally located samples also have a trend for increased anisotropy (), with the circumferential direction stiffer than the longitudinal, as compared to an isotropic and relatively linear response for proximally located elastin samples. These results are consistent with the underlying tissue histology from proximally located samples that had higher optical density (), fiber thickness (), and trend for lower tortuosity () in elastin fibers as compared to the thinner and highly undulating elastin fibers isolated from distally located samples. Our studies suggest that it is important to consider elastin fiber orientations in investigations that use microstructure-based models to describe the contributions of elastin and collagen to arterial mechanics.

Compressibility behaviour of bentonite and organically modified bentonite slurry

Bentonite in slurry walls needs to be amended with organo-clay to control the migration of organic contaminants. Consolidation behaviour of the slurry is important because it will reduce the total effective stress owing to mobilisation of frictional force between the side wall of the trench and the slurry. Compressibility of the slurry of bentonite is expected to undergo significant changes owing to amendment with organo-clay and according to the nature of the fluid. Standard one-dimensional consolidation tests were carried out on slurries of bentonite, organo-clay and their mixtures by remoulding them to their respective liquid limit consistency with water as well as fluids of low polarity, such as carbon tetrachloride, and inundating with different fluids. Organo-clay and its mixture with bentonite when moulded with water exhibit lower compressibility than bentonite in any pore fluid, but their compressibility increases when moulded with carbon tetrachloride and inundated with the same fluid. These changes in the compressibility of bentonite amended with organo-clay are explained by particle rearrangements, changes in the development of the diffuse double layer and contribution from the water adsorbed in the inter-lamellar space of the clay.

Effects of elastase and collagenase on the nonlinearity and anisotropy of porcine aorta

We use enzymatic manipulation methods to investigate the individual and combined roles of elastin and collagen on arterial mechanics. Porcine aortic tissues were treated for differing amounts of time using enzymes elastase and collagenase to cause degradation in substrate proteins elastin and collagen and obtain variable tissue architecture. We use equibiaxial mechanical tests to quantify the material properties of control and enzyme treated tissues and histological methods to visualize the underlying tissue microstructure in arterial tissues. Our results show that collagenase treated tissues were more compliant in the longitudinal direction as compared to control tissues. Collagenase treatment also caused a decrease in the tissue nonlinearity as compared to the control samples in the study. A one hour collagenase treatment was sufficient to cause fragmentation and degradation of the adventitial collagen. In contrast, elastase treatment leads to significantly stiffer tissue response associated with fragmented and incomplete elastin networks in the tissue. Thus, elastin in arterial walls distributes tensile stresses whereas collagen serves to reinforce the vessel wall in the circumferential direction and also contributes to tissue anisotropy. A microstructurally motivated strain energy function based on circumferentially oriented medial fibers and helically oriented collagen fibers in the adventitia is useful in describing these experimental results.

Low embodied energy cement stabilised rammed earth building-A case study

Rammed earth is a monolithic construction and the construction process involves compaction of processed soil in progressive layers in a rigid formwork. Durable and thinner load bearing walls can be built using stabilised rammed earth. Use of inorganic additives such as cement for rammed earth walls has been in practice since the last 5-6 decades and cement stabilised rammed earth (CSRE) buildings can be seen across the world. The paper deals with the construction aspects, structural design and embodied energy analysis of a three storey load bearing school building complex. The CSRE school complex consists of 15 classrooms, an open air theatre and a service block. The complex has a built-up area of 1691.3 m(2) and was constructed employing manual construction techniques. This case study shows low embodied energy of 1.15 GJ/m(2) for the CSRE building as against 3-4 GJ/m(2) for conventional burnt clay brick load bearing masonry buildings. (C) 2013 Elsevier B.V. All rights reserved.

Asymptotics of Harish-Chandra expansions, bounded hypergeometric functions associated with root systems, and applications

A series expansion for Heckman-Opdam hypergeometric functions phi(lambda) is obtained for all lambda is an element of alpha(C)*. As a consequence, estimates for phi(lambda) away from the walls of a Weyl chamber are established. We also characterize the bounded hypergeometric functions and thus prove an analogue of the celebrated theorem of Helgason and Johnson on the bounded spherical functions on a Riemannian symmetric space of the noncompact type. The L-P-theory for the hypergeometric Fourier transform is developed for 0 < p < 2. In particular, an inversion formula is proved when 1 <= p < 2. (C) 2013 Elsevier Inc. All rights reserved.

Mesoporous bioactive glass and glass-ceramics: Influence of the local structure on in vitro bioactivity

Mesoporous quaternary bioactive glasses and glass-ceramic with alkali-alkaline-earth oxide were successfully synthesized by using non-ionic block copolymer P123 and evaporation induced self assembly (EISA) process followed by acid treatment assisted sal-gel method. As prepared samples has been characterized for the structural, morphological and textural properties with the various analytical techniques. Glass dissolution/ion release rate in simulated body fluid (SBF) was monitored by inductively coupled plasma (ICP) emission spectroscopy, whereas the formation of apatite phase and its crystallization at the glass and glass-ceramic surface was examined by structural, textural and microscopic probes. The influence of alkaline-earth oxide content on the glass structure followed by textural property has become more evident. The pristine glass samples exhibit a wormhole-like mesoporous structure, whereas the glass-ceramic composition is found to be in three different phases, namely crystalline hydroxyapatite, wollastonite and a residual glassy phase as observed in Cerabone (R) A/W. The existence of calcium orthophosphate phase is closely associated with the pore walls comprising nanometric-sized ``inclusions''. The observed high surface area in conjunction with the structural features provides the possible explanation for experimentally observed enhanced bioactivity through the easy access of ions to the fluid. On the other hand, presence of multiple phases in glass-ceramic sample inhibits or delays the kinetics of apatite formation. (C) 2013 Elsevier Inc. All rights reserved.