208,377 research outputs found
Non-equilibrium stochastic dynamics of open ion channels
No full textWe present and discuss a modified version of reaction rate theory (RRT) to describe the passage of a positive ion through a biological ion channel. It takes explicit account of the non-equilibrium nature of the permeation process. Unlike traditional RRT, it allows for the non-constant transition rates that arise naturally in an archetypal model of an ion channel. In particular, we allow for the fact that the average escape time of an ion trapped at the selectivity filter (SF) can be reduced substantially by the pair correlations between ions: the arrival of a second ion at the channel entrance significantly reduces the potential barrier impeding the escape of the ion from the SF. The effects of this rate modulation on the current- voltage and current-concentration characteristics of the channel are studied parametrically. Stochastic amplification of the channel conductivity by charge fluctuations is demonstrated and compared with the results of Brownian dynamics simulations
Self-organized enhancement of conductivity in biological ion channels
No full textWe discuss an example of self-organization in a biological system. It arises from long-range ion–ion interactions, and it leads us to propose a new kind of enhanced conduction in ion channels. The underlying mechanism involves charge fluctuations near the channel mouth, amplified by the mismatch between the relative permittivities of water and the protein of the channel walls. We use Brownian dynamics simulations to show that, as in conventional 'knock on' permeation, these interactions can strongly enhance the channel current; but unlike the conventional mechanism, the enhancement occurs without the instigating bath ion entering the channel. The transition between these two mechanisms is clearly demonstrated, emphasizing their distinction. A simple model accurately reproduces the observed phenomena. We point out that electrolyte plus protein of low relative permittivity are universal in living systems, so that long-range ion–ion correlations of the kind considered must be common
Stochastic dynamics of remote knock-on permeation in biological ion channels
No full textBrownian dynamics simulations provide evidence for a remote knock-on mechanism facilitating the permeation of a biological ion channel by an ion that is initially trapped at the selectivity filter (SF). Unlike the case of conventional direct knock-on, the second ion that instigates permeation does not need to enter the channel. Nor does it necessarily take the place of the permeating ion at the SF, and it can even be of a different ionic species. The study is based on the simultaneous, self-consistent, solution of the coupled Poisson and Langevin equations for a simple generic model, taking account of all the charges present. The new permeation mechanism involves electrostatic amplification attributable to the permittivity mismatch between water and protein: the arrival of the instigating ion at the channel entrance reduces the exit barrier for the ion trapped at the SF, facilitating escape
Formation of silicon nanodots via ion beam sputtering of ultrathin gold thin film coatings on Si
No full textIon beam sputtering of ultrathin film Au coatings used as a physical catalyst for self-organization of Si nanostructures has been achieved by tuning the incident particle energy. This approach holds promise as a scalable nanomanufacturing parallel processing alternative to candidate nanolithography techniques. Structures of 11- to 14-nm Si nanodots are formed with normal incidence low-energy Ar ions of 200 eV and fluences above 2 x 10(17) cm(-2). In situ surface characterization during ion irradiation elucidates early stage ion mixing migration mechanism for nanodot self-organization. In particular, the evolution from gold film islands to the formation of ion-induced metastable gold silicide followed by pure Si nanodots formed with no need for impurity seeding
The wear behaviour of ion implanted biomaterials
No full textThe tribological performance of biomaterials used for artificial joints is of much importance, and require low coefficients of friction, resistance to wear and the ability to withstand many millions of cycles under a multitude of loading regimes. Currently used material combinations include Ti6A14V, 316L stainless steel and Co-Cr-Mo articulating against UHMWPE. Although typical wear rates are low (60 mm(^3)/10(^6) cycles), the UHMWPE wear debris produced during articulation has been linked to osteolysis, leading to loosening of prostheses and necessitating revision surgery. This study aimed to characterise the surfaces and quantitatively assess the tribological performance of such biomaterials when surface modified by N(^+) ion implantation. Beyond this, investigation of the physical effects of the N(^+) ion implantations were carried out with a view to determination of an optimum ion implantation protocol. The tribological performance of the materials, were quantitatively assessed using multidirectional pin-on-plate wear testing. Surface characterisation of the materials, were studied using a combination of optical microscopy, AFM, non-contacting interferometry, SEM, and XPS. A significant increase in the surface microhardness of the modified materials was measured post ion implantation. This was attributed to the formation of ion implantation induced lattice disorder and hard phase nitride precipitates on the metallic surfaces, and cross-linking incorporating new formed chemical bonds on the polymeric surfaces. N(^+) ion implantation with 5 x 10(^15) N(^+)ions/cm(^2) significantly enhanced the wear resistance of UHMWPE by ≈ 55 % when articulated against 2 x lO(^17) N(^+) ions/cm(^2) implanted Ti6A14V; by ≈ 48 % when articulated against 2 x lO(^17) N(^+) ions/cm(^2) implanted stainless steel; and by ≈ 48 % when articulated against 2 x 10(^17) N(^+) ions/cm^ implanted Co-Cr-Mo. The technique of ion implantation offers potential as a modification method, to improve wear resistance of these biomaterials for articulating applications such as in total joint replacement
Kinetics and equilibria of ion-molecule association reactions : studied using temperature variable high pressure ion sources
No full textInterest in termolecular association reactions of the type shown below, stems from their
importance in the chemistry of planetary atmospheres, gas-cooled nuclear reactors and
gas-phase cluster ions. This study is concerned with evaluating the rate constants of such
X+ + X + M ------> X2+ + M k3 (1)
reactions as both a function of temperature and of the third body M. The values of the third
order rate constant k3 are expressed conventionally in terms of k3 = CT-m where T is the
temperature and C and m are constants characteristic of the reaction which depend also on the
nature of M. Literature now shows a general measure of agreement on values of C and m in
several studies for which X=M, however, inconsistent values have been reported on the M=He system. This thesis describes an investigation of the two systems X=N2, CO and M= the
reactant or a rare gas.
Experiments were conducted in a conventional high pressure ion source and a pulsed drift
ion source fitted to an updated Kratos MS9 mass spectrometer.
Results obtained for the one component studies show good agreement with other literature
values for the temperature dependence, m. In general, for both N2 and CO systems, He was
found to have the same efficiency as the parent molecule as a third body at 300K, but the
temperature dependence of k3 is markedly lower. Ar was found to behave very similarly to the
parent molecule in both systems. For the CO system, although good agreement is found for the
temperature dependence result with literature, there is still an uncertainty of about a factor of 2
in the room temperature values of k3
From structural colors to super-hydrophobicity and achromatic transparent protective coatings: Ion plating plasma assisted TiO2 and SiO2 nano-film deposition
No full textThe implementation of the Ion Plating Plasma Assisted technology in the area of surface functionalization for structural color and relic preservation applications is presented. Interferometric structural colors on irregular bumped Titanium surfaces and transparent and achromatic nano films on ancient ceramic artifact have been investigated. Titanium metal and ceramic supports have been utilized for the surface functionalization tests: A metallic electron beam additive manufactured Titanium component and an ancient tile of the XIX century, which was characterized by strong chromatic valence and by a mixed porous and glazed surfaces, have been considered. A reactive magnetron sputtering Ion Plating Plasma Assisted apparatus operating in Argon or Oxygen atmospheres for TiO2 and SiO2 deposition has been utilized. Preliminary tests with two plasma treatments were carried out for optimal processing conditions definition. TiO2 nano-film deposition on irregular Ti surfaces has generated light direction depending color-changing surfaces while good achromatic and transparent coatings were obtained by using SiO2 coating. The high processing flexibility of the Ion plating technology is discussed. The SiO2 IPPA surfaces treatment resulted more convenient for restorative and preservation ancient historical tile was used to finally test the optimized process with Ion Beam Electron Microscopy, which was carried out on the tile porous structure, confirmed the high flexibility and efficiency of the innovative IPPA technology
Ion trapping with fast-response ion-selective microelectrodes enhances detection of extracellular ion channel gradients
No full textPreviously, functional mapping of channels has been achieved by measuring the passage of net charge and of specific ions with electrophysiological and intracellular fluorescence imaging techniques. However, functional mapping of ion channels using extracellular ion-selective microelectrodes has distinct advantages over the former methods. We have developed this method through measurement of extracellular K+ gradients caused by efflux through Ca2+-activated K+ channels expressed in Chinese hamster ovary cells. We report that electrodes constructed with short columns of a mechanically stable K+-selective liquid membrane respond quickly and measure changes in local [K+] consistent with a diffusion model. When used in close proximity to the plasma membrane (<4 µm), the ISMs pose a barrier to simple diffusion, creating an ion trap. The ion trap amplifies the local change in [K+] without dramatically changing the rise or fall time of the [K+] profile. Measurement of extracellular K+ gradients from activated rSlo channels shows that rapid events, 10–55 ms, can be characterized. This method provides a noninvasive means for functional mapping of channel location and density as well as for characterizing the properties of ion channels in the plasma membrane
High-frequency Alfven waves in multi-ion coronal plasma : observational implications
No full textWe investigate the effects of high-frequency (of order ion gyrofrequency) Alfvén and ion-cyclotron waves on ion emission lines by studying the dispersion of these waves in a multi-ion coronal plasma. For this purpose we solve the dispersion relation of the linearized multifluid and Vlasov equations in a magnetized multi-ion plasma with coronal abundances of heavy ions. We also calculate the dispersion relation using nonlinear one-dimensional hybrid kinetic simulations of the multi-ion plasma. When heavy ions are present the dispersion relation of parallel propagating Alfvén cyclotron waves exhibits the following branches (in the positive Ω − k quadrant): right-hand polarized nonresonant and left-hand polarized resonant branch for protons and each ion. We calculate the ratio of ion to proton velocities perpendicular to the direction of the magnetic field for each wave modes for typical coronal parameters and find strong enhancement of the heavy ion perpendicular fluid velocity compared with proton perpendicular fluid velocity. The linear multifluid cold plasma results agree with linear warm plasma Vlasov results and with the nonlinear hybrid simulation model results. In view of our findings we discuss how the observed nonthermal line broadening of minor ions in coronal holes may relate to the high-frequency wave motions
Fundamental Ion-Surface Interactions in Plasma Thrusters
No full textIon thrusters offer the potential to enable many future interplanetary robotic missions presently under consideration by NASA. To realize the benefits offered by these low thrust devices, the sputtering mechanisms that are responsible for the degradation of thruster components over time must be well understood. Predictions of thruster life depend directly on the material removal rates from thruster electrodes such as the ion optics and hollow cathodes. To better understand the conditions encountered at these surfaces, this study includes an investigation of low energy sputtering at glancing incidence. Relevant ion–target combinations that were considered included Xe⁺ incident on Mo, C, and Cu, as well as Ar⁺ incident on W, C, and Cu.
To characterize the sputtering yield angular dependence experimentally, an ion beam was used to etch a coated quartz crystal microbalance. This required the development of techniques to accurately measure the incident low energy ion flux to the target and the use of surface diagnostics to investigate the properties of target materials. Measurements of C and Mo sputtering yields were obtained for Xe⁺ incidence angles up to 80° from the surface normal and for energies ranging from 80 eV–1 keV. In addition, existing transport theory models were used to examine projectile scattering within the different target media. The models also indicate that the sputtering behavior as a function of angle of incidence is not a strong function of energy, a conclusion that is supported by the experimental results. The surface roughness of the targets was investigated using atomic force microscopy to obtain local incidence angle distributions.
A surface layer activation technique served as an alternate method of evaluating the sputtering rates of thruster components for situations where the ion bombardment conditions are not well known. In this study, a radioactive tracer was produced in the surfaces of a number of laboratory model ion thruster cathode assemblies by high energy proton bombardment. The cathodes were tested in a 30 cm diameter xenon ion thruster to provide insight into the relevant wear mechanisms at different thruster operating points. Methods for combating cathode degradation are proposed based on the experimental results
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