Conductive supports for combined AFM-SECM on biological membranes

Four different conductive supports are analysed regarding their suitability for combined atomic force and scanning electrochemical microscopy (AFM-SECM) on biological membranes. Highly oriented pyrolytic graphite (HOPG), MoS(2), template stripped gold, and template stripped platinum are compared as supports for high resolution imaging of reconstituted membrane proteins or native membranes, and as electrodes for transferring electrons from or to a redox molecule. We demonstrate that high resolution topographs of the bacterial outer membrane protein F can be recorded by contact mode AFM on all four supports. Electrochemical feedback experiments with conductive cantilevers that feature nanometre-scale electrodes showed fast re-oxidation of the redox couple Ru(NH(3))(6)(3+/2+) with the two metal supports after prolonged immersion in electrolyte. In contrast, the re-oxidation rates decayed quickly to unpractical levels with HOPG or MoS(2) under physiological conditions. On HOPG we observed heterogeneity in the re-oxidation rate of the redox molecules with higher feedback currents at step edges. The latter results demonstrate the capability of conductive cantilevers with small electrodes to measure minor variations in an SECM signal and to relate them to nanometre-scale features in a simultaneously recorded AFM topography. Rapid decay of re-oxidation rate and surface heterogeneity make HOPG or MoS(2) less attractive for combined AFM-SECM experiments on biological membranes than template stripped gold or platinum?supports.     

Ergodicity of avalanche transformations

In this paper, we study dynamical systems of product type and some particular inducing scheme motivated by neural dynamics (called avalanche transformation). We derive the distribution of avalanche sizes and give sufficient conditions such that the avalanche transformation is ergodic. Moreover, we deduce a multivariate central limit theorem as a corollary.

In memoriam: Wenbo V. Li (1963–2013)     

Highly wrinkled cross-linked graphene oxide membranes for biological and charge-storage applications

Inspired by the amphiphilicity of graphene oxide (GO), the surface of water is used as a template for the assembly of a GO film. Methacrylate-functionalized GO sheets can be cross-linked instantaneously at the water-air interface to form a highly wrinkled membrane spreading over an extended area. The multiple covalent linkages amongst the GO sheets enhances the in-plane stiffness of the film compared to noncovalently bonded GO films. The highly convoluted GO membrane can be used in two applications: the promoting of spontaneous stem-cell differentiation towards bone cell lineage without any chemical inducers, and for supercapacitor electrodes. Due to reduced van der Waals restacking, capacitance values up to 211 F g(-1) can be obtained. The scalable and inexpensive nature of this assembly route enables the engineering of membranes for applications in regenerative medicine and energy-storage devices where secondary structures like nanotopography and porosity are important performance enhancers.     

Topographic curvature effects in applied avalanche modeling

This paper describes the implementation of topographic curvature effects within the RApid Mass MovementS (RAMMS) snow avalanche simulation toolbox. RAMMS is based on a model similar to shallow water equations with a Coulomb friction relation and the velocity dependent Voellmy drag. It is used for snow avalanche risk assessment in Switzerland. The snow avalanche simulation relies on back calculation of observed avalanches. The calibration of the friction parameters depends on characteristics of the avalanche track. The topographic curvature terms are not yet included in the above mentioned classical model. Here, we fundamentally improve this model by mathematically and physically including the topographic curvature effects. By decomposing the velocity dependent friction into a topography dependent term that accounts for a curvature enhancement in the Coulomb friction, and a topography independent contribution similar to the classical Voellmy drag, we construct a general curvature dependent frictional resistance, and thus propose new extended model equations. With three site-specific examples, we compare the apparent frictional resistance of the new approach, which includes topographic curvature effects, to the classical one. Our simulation results demonstrate substantial effects of the curvature on the flow dynamics e.g., the dynamic pressure distribution along the slope. The comparison of resistance coefficients between the two models demonstrates that the physically based extension presents an improvement to the classical approach. Furthermore a practical example highlights its influence on the pressure outline in the run out zone of the avalanche. Snow avalanche dynamics modeling natural terrain curvature centrifugal force friction coefficients.     

The analysis of ruptures in metropolitan gas pipelines

We model the yearly counts of occurred ruptures in a metropolitan gas distribution network using both a homogeneous Poisson process and a power law process. Covariates such as depth, diameter and site of the pipes are included via standard generalized linear modelling for Poisson data in the first case and via a proportional intensity function in the second case. It is found that the homogeneous Poisson process provides an acceptable interpretation of the data, thus confirming the hypothesis that the cast iron pipes do not suffer from erosion and that only the traffic conditions on the site above the pipes seem to affect the rupture rate. Copyright © 2000 John Wiley & Sons, Ltd.     

Photon avalanche fluorescence and lasers

This paper presents an analysis of the three-level model for avalanche upconversion in the steady state case. The population of the upper level is studied with respect to the ratio between nonresonant and resonant absorptions. We deduce from this variation a limit for avalanche behaviour nearly independent of the actual compound considered. This limit corresponds to a very weak absorption from the ground state which can be estimated for multiphonon assisted absorption. Several examples from the literature are discussed on this basis and we show that a large mismatch between pump photon and electronic transition energies is necessary to observe avalanche upconversion. Application of the photon avalanche to upconversion pumped lasers is also investigated in terms of oscillation threshold and stimulated emission. Avalanche pumping suffers from two drawbacks. First, it results in a lower upconverted emission intensity than that obtained by resonant pumping, especially near avalanche threshold. In the second place, the output power of these lasers is seriously limited because of the weak absorption from the ground state.     

A fiber-optically triggered avalanche transistor

A silicon bipolar transistor operating in the avalanche region was optically triggered into secondary breakdown. This transistor has been given the name fiber-optically triggered avalanche transistor (FOTAT). The FOTAT acts as an optical power discriminator. That is, secondary breakdown occurs when the triggering optical power exceeds the triggering threshold of the FOTAT. This secondary breakdown is seen as a negative resistance between the collector and emitter. High voltage (>100 V) nanosecond transition duration pulses are generated using this negative resistance     

Photon avalanche upconversion in rare earth laser materials

We present an overview of the upconversion mechanisms with special attention to the Photon Avalanche (PA) process and to give the state of the art concerning the upconversion pumped solid state lasers.     

Hysteresis and avalanche phenomena of helium in nuclepore

We report on the hysteresis and avalanche behavior of⁴He capillary condensing into the nearly cylindrical pores of the porous membrane Nuclepore. Following capillary condensation, withdrawal of the⁴ He results in distinct jumps or avalanches where 10⁷ pores can be involved in a single avalanche. By using Nuclepore with 2000Å and 300Å pore diameters, we find that the avalanche distribution depends on details of the pore structure.     

Breakdown of avalanche critical behaviour in polycrystalline plasticity

Acoustic emission experiments on creeping ice as well as numerical simulations argue for a self-organization of collective dislocation dynamics during plastic deformation of single crystals into a scale-free pattern of dislocation avalanches characterized by intermittency, power-law distributions of avalanche sizes, complex space-time correlations and aftershock triggering. Here, we address the question of whether such scale-free, close-to-critical dislocation dynamics will still apply to polycrystals. We show that polycrystalline plasticity is also characterized by intermittency and dislocation avalanches. However, grain boundaries hinder the propagation of avalanches, as revealed by a finite (grain)-size effect on avalanche size distributions. We propose that the restraint of large avalanches builds up internal stresses that push temporally the dynamical system into a supercritical state, off the scale-invariant critical regime, and trigger secondary avalanches in neighbouring grains. This modifies the statistical properties of the avalanche population. The results might also bring into question the classical ways of modelling plasticity in polycrystalline materials, based on homogenization procedures.     

Defect microstructure and microplasmas in silicon avalanche photodiodes

Soft, noisy silicon avalanche photodiodes were studied using the SEM electron beam induced current technique and transmission electron microscopy. They were found to contain varying concentrations of (i) diffusion-induced misfit dislocations, (ii) precipitates and (iii) cusps (lines of shallower penetration) in ragged p-n⁺ junctions due to dislocation-retarded diffusion. The noise was of classical microplasma form but the sites of this breakdown did not correlate with the precipitates as in most previous cases, but occurred at favoured points along the cusps in the p-n⁺ junction. Low and intermediate densities of misfit dislocations were found to produce microplasmas with lower breakdown voltages than very high densities. The shallowest misfit dislocations produced the greatest diffusion retardation, suggesting that removal of atoms from the diffusing flux by segregation to the dislocation is the mechanism responsible.[/p]     

Test for YBJ-ARGO RPC working in avalanche mode

The measurement of YBJ-ARGO Resistive Plate Chamber (RPC), working in avalanche mode was performed. With different component of i-C₄H₁₀ in C₂H₂F₄-based gas mixtures C₂H₂F₄/i-C₄H₁₀/SF₆, the behavior of the detector with respect to the high voltage was studied. The experiment confirms that it is possible to operate YBJ-ARGO RPC in avalanche mode. The results show that with the gas mixtures containing 10% of i-C₄H₁₀ the detector achieves its optimum performance.     

Properties and in vitro biological evaluation of nano-hydroxyapatite/chitosan membranes for bone guided regeneration

Nano-hydroxyapatite(n-HA)/chitosan(CS) composite membranes were prepared by solvent casting and evaporation methods for the function of guided bone regeneration (GBR). The effect of n-HA content and solvent evaporation temperature on the properties of the composite membranes was studied. The n-HA/CS membranes were analyzed by scanning electron microscopy, Fourier transformed infrared spectroscopy, X-ray diffraction, swelling measurement, mechanical test, cell culture and MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenylte-2H-tetrazolium bromide) assay. The results show that the surface roughness and micropores of the composite membranes increase with the rise of n-HA content, suitable for adhesion, crawl and growth of cells. The hydroxyapatite holds nano size and distributes uniformly in the composite membranes. Chemical bond interaction exists between Ca ions and –OH groups of n-HA and –NH₂ or –OH groups of CS. The n-HA content and solvent evaporation temperature have obvious influence on the swelling ratio, tensile strength and elongation rate of the composite membranes. Cell culture and MTT assays show that n-HA and its content can affect the proliferation of cells. The n-HA/CS composite membranes have no negative effect on the cell morphology, viability and proliferation and possess good biocompatibility. This study makes the n-HA/CS composite membrane be a prospective biodegradable GBR membrane for future applications.     

Particle simulation of snow avalanche motion

The continuum model of a snow avalanche is abandoned, and instead an avalanche is modeled as a collection of $\text{～10}{}^3$ particles that move randomly and independently subject to gravity and resistive forces which have a random fluctuation computed by Monte-Carlo simulation. The model includes entrainment at the avalanche front and the possibility of varying resistive parameters with speed and slope position. Particle statistics computed for an avalanche event in Norway, April 1982, provide a reasonable simulation of recorded speeds and debris distribution.     

Accurate model for single-photon avalanche diodes

An accurate model useful for simulating single-photon avalanche diodes including biasing circuits is presented. The authors developed the model using Verilog-A codes to describe both static and dynamic behaviours. The derived model fits experimental results extracted from practical devices better than the one used in the open literature. SPECTRE simulations confirmed the validity of the proposed model, which avoids convergence problems and also shows a higher accuracy than traditional models.     

Fractal in Fatigue Fractography

Fracture profile roughnesses and fractal dimensionsof fracture traces were measured on a fatigued Ti-6Al-4Valloy.It is found that although fractal dimension can wellreflect the variation of fracture traces with the measuringunits,it is difficult to apply it to quantitative analysis offractured surfaces because of the dependence of the meas-ured profile roughness on the measuring units.Based onfractal concept.an alternative equationlgR_l(η)=lgR_o-(D-1)lgη was obtained.in which we intro-duced a parameter of intrinsic profile roughness to evalu-ate fracture profile roughness without restriction of themeasuring units employed.