Surface modifications by field induced diffusion

By applying a voltage pulse to a scanning tunneling microscope tip the surface under the tip will be modified. We have in this paper taken a closer look at the model of electric field induced surface diffusion of adatoms including the van der Waals force as a contribution in formations of a mound on a surface. The dipole moment of an adatom is the sum of the surface induced dipole moment (which is constant) and the dipole moment due to electric field polarisation which depends on the strength and polarity of the electric field. The electric field is analytically modelled by a point charge over an infinite conducting flat surface. From this we calculate the force that cause adatoms to migrate. The calculated force is small for voltage used, typical 1 pN, but due to thermal vibration adatoms are hopping on the surface and even a small net force can be significant in the drift of adatoms. In this way we obtain a novel formula for a polarity dependent threshold voltage for mound formation on the surface for positive tip. Knowing the voltage of the pulse we then can calculate the radius of the formed mound. A threshold electric field for mound formation of about 2 V/nm is calculated. In addition, we found that van der Waals force is of importance for shorter distances and its contribution to the radial force on the adatoms has to be considered for distances smaller than 1.5 nm for commonly used voltages.     

Cellulose Content in Fibers of Cottons which Differ in their Lint Lengths and Extent of Fuzz

Three cottons differing in their extent of fuzz fibers (linters) and final length of lint fibers were analyzed for amount of fiber cell walls and fiber cellulose at various times postanthesis. Cellulose determinations were performed directly on whole fibers and on fiber cell wall preparations. The data suggest that the presence of fuzz fibers does not account for a rise, followed by a plateau, followed by a rise, in cellulose content expressed as a percentage of cell wall material. It is concluded that: (1) under our greenhouse conditions, all fuzz fibers are initiated by day eight after anthesis; (2) weight per mm length of all fibers increases up to the point of secondary wall deposition and increases even more rapidly after that; (3) deposition of secondary wall cellulose in fuzz fibers probably does not begin until after similar deposition begins in lint fibers; (4) the actual amount of cellulose in primary walls of all elongating fibers (fuzz and lint) is a constant value, about 1 × 10^?16 mg/mm; and (5) secondary wall cellulose deposition in lint fibers begins very sharply, in advance of cessation of elongation, at a time closely related to final lint fiber length. It is speculated that: (1) cell wall preparation procedures may remove significant amounts of noncellulosic wall material, thus making it difficult to define all functional constituents on the basis of what is left in a cell wall residue; and/or (2) primary walls may lose to the cytoplasm some of their constituents in advance of secondary wall deposition, the extent of loss varying due to developmental age of the elongating fibers.     

Whole-cell imaging at nanometer resolutions using fast and slow focused helium ions

Observations of the interior structure of cells and subcellular organelles are important steps in unraveling organelle functions. Microscopy using helium ions can play a major role in both surface and subcellular imaging because it can provide subnanometer resolutions at the cell surface for slow helium ions, and fast helium ions can penetrate cells without a significant loss of resolution. Slow (e.g., 10–50 keV) helium ion beams can now be focused to subnanometer dimensions (∼0.25 nm), and keV helium ion microscopy can be used to image the surfaces of cells at high resolutions. Because of the ease of neutralizing the sample charge using a flood electron beam, surface charging effects are minimal and therefore cell surfaces can be imaged without the need for a conducting metallic coating. Fast (MeV) helium ions maintain a straight path as they pass through a cell. Along the ion trajectory, the helium ion undergoes multiple electron collisions, and for each collision a small amount of energy is lost to the scattered electron. By measuring the total energy loss of each MeV helium ion as it passes through the cell, we can construct an energy-loss image that is representative of the mass distribution of the cell. This work paves the way to use ions for whole-cell investigations at nanometer resolutions through structural, elemental (via nuclear elastic backscattering), and fluorescence (via ion induced fluorescence) imaging.     

Bubble formation in crustaceans following decompression from hyperbaric gas exposures

In vivo bubble formation was studied in various crustaceans equilibrated with high gas pressures and rapidly decompressed to atmospheric pressure. The species varied widely in susceptibility to bubble formation, and adults were generally more susceptible than larval stages. Bubbles did not form in early brine shrimp larvae unless equilibration pressures of at least 175 atm argon or 350 atm helium were used; for adult brine shrimp, copepods, and the larvae of crabs and shrimps, 100-125 atm argon or 175-225 atm helium were required. In contrast, bubbles formed in the leg joints of megalopa and adult crabs following decompression from only 3-10 atm argon; stimulation of limb movements increased this bubble formation, whereas inhibition of movements decreased it. High hydrostatic compressions applied before gas equilibration or slow compressions did not affect bubble formation. We concluded that circulatory systems, musculature, and storage lipids do not necessarily render organisms susceptible to bubble formation and that bubbles do not generally originate as preformed nuclei. In some cases, tribonucleation appears to be the cause of the bubbles.     

Rates of Shrinkage and Growth of Air Bubbles Entrained in Wheat Flour Dough

The rates of shrinkage at constant temperature, and growth under a temperature rise below 100°C, of bubbles entrained in wheat flour dough were analyzed and compared with those of a bubble in water. The rate of shrinkage of bubbles in flour dough was controlled by the diffusion of dissolved air from the surface of bubbles to the bulk of flour dough. The apparent diffusion coefficient of the dissolved air in wheat flour dough with the water fraction of 0.49 calculated from the shrinkage of bubbles, was (3.2 ± 1.5) × 10^1?1 m²/sec (19°C), and (6.4 ± 2.0) × 10^?11 m²/sec (42°C). However, the growth behavior of bubbles in flour dough under a temperature rise was very different from that predicted from the diffusion theory. The critical radius of bubbles to grow was larger than that estimated from the diffusion theory. The mechanism of growth of bubbles in wheat flour dough, which was different from that of a bubble in water, is a subject that needs to be clarified.     

Stable Li Metal Anode Enabled by Space Confinement and Uniform Curvature through Lithiophilic Nanotube Arrays

The application of lithium (Li) metal anodes in rechargeable batteries is primarily restricted by Li dendrite growth on the metal's surface, which leads to shortened cycle life and safety concerns. Herein, well‐spaced nanotubes with ultrauniform surface curvature are introduced as a Li metal anode structure. The ultrauniform nanotubular surface generates uniform local electric fields that evenly attract Li‐ions to the surface, thereby inducing even current density distribution. Moreover, the well‐defined nanotube spacing offers Li diffusion pathways to the electroactive areas as well as the confined spaces to host deposited Li. These structural attributes create a unique electrodeposition manner; i.e., Li metal homogenously deposits on the nanotubular wall, causing each Li nanotube to grow in circumference without obvious sign of dendritic formation. Thus, the full‐cell battery with the spaced Li nanotubes exhibits a high specific capacity of 132 mA h g^?1 at 1 C and an excellent coulombic efficiency of ≈99.85% over 400 cycles.     

Development of a novel intravascular oxygenator catheter: Oxygen mass transfer properties across nonporous hollow fiber membranes

Despite hypoxic respiratory failure representing a large portion of total hospitalizations and healthcare spending worldwide, therapeutic options beyond mechanical ventilation are limited. We demonstrate the technical feasibility of providing oxygen to a bulk medium, such as blood, via diffusion across nonporous hollow fiber membranes (HFMs) using hyperbaric oxygen. The oxygen transfer across Teflon® membranes was characterized at oxygen pressures up to 2 bars in both a stirred tank vessel (CSTR) and a tubular device mimicking intravenous application. Fluxes over 550 ml min^?1 m^?2 were observed in well‐mixed systems, and just over 350 ml min^?1 m^?2 in flow through tubular systems. Oxygen flux was proportional to the oxygen partial pressure inside the HFM over the tested range and increased with mixing of the bulk liquid. Some bubbles were observed at the higher pressures (1.9 bar) and when bulk liquid dissolved oxygen concentrations were high. High‐frequency ultrasound was applied to detect and count individual bubbles, but no increase from background levels was detected during lower pressure operation. A conceptual model of the oxygen transport was developed and validated. Model parametric sensitivity studies demonstrated that diffusion through the thin fiber walls was a significant resistance to mass transfer, and that promoting convection around the fibers should enable physiologically relevant oxygen supply. This study indicates that a device is within reach that is capable of delivering greater than 10% of a patient's basal oxygen needs in a configuration that readily fits intravascularly.     

Chemical colloids versus biological colloids: a comparative study for the elucidation of the mechanism of protein fiber formation

Fiber formation from murine serum amyloid A1 (SAA) was compared to the linear aggregation and fiber formation of colloidal gold particles. Here we report the similarities of these processes. Upon incubation with acetic acid, SAA misfolds and adopts a new conformation, which we termed saa. saa apparently is less soluble than SAA in aqueous solution; it aggregates and forms nucleation units and then fibers. The fibers appear as a string of the nucleation units. Additionally, an external electric field promotes saa fiber formation. These properties of saa are reminiscent of colloidal gold formation from gold ions and one-dimensional aggregation of the gold colloids. Colloidal gold particles were also found to be capable of aggregating one-dimensionally under an electric field or in the presence of polylysine. These gold fibers resembled in structure that of saa fibers. In summary, protein aggregation and formation of fibers appear to follow the generalized principles derived in colloidal science for the aggregation of atoms and molecules, including polymers such as polypeptides. The analysis of colloidal gold formation and of one-dimensional aggregation provides a simple model system for the elucidation of some aspects of protein fiber formation.     

On-Line Optical and Morphological Studies of Silver Nanoparticles Growth Formed by Nanosecond Laser Irradiation of Silver-Exchanged Silicate Glass

Silver-exchanged silicate glass has been irradiated by 532-nm pulsed Nd:YAG laser in order to locally form metallic nanoparticles. The particular interest of this process is to locally control the silver nanoparticles (NPs) growth. Silver ions are exchanged with sodium ions near the glass surface after dumping of a silicate glass few minutes in silver and sodium nitrates molten salt. A low-energy density laser exposure (0.239 J/cm²) chosen at the ablation threshold allows to observe the kinetics of the silver NPs growth according to the increasing shots number. An on-line optical measurement is carried out after each shot to identify the most important steps during the irradiation process. According to this measurement, we have determined four steps highlighted by UV/Visible spectrophotometry and we have identified the influence of located surface plasmon resonance. Three combined material analysis methods were used to understand the glass/laser interaction mechanism: we outlined the material volume variations by profilometric method, the element distribution by scanning electron microscopy and finally the structural distribution of the irradiated region by a local infrared investigation. The trend for NPs formation revealed by the UV/Visible spectrophotometry is thus explained by the formation of a ring expelled from a central hole. We highlight that the on-line extinction measurement can be used to data process the NPs evolution.     

Bubble formation in crabs induced by limb motions after decompression

In vivo bubble formation was studied in the megalopal stage of the crab Pachygrapsus crassipes. The animals were equilibrated with elevated argon, nitrogen, or helium pressures then rapidly decompressed to atmospheric pressure. Voluntary motions induced bubble nucleation in leg joints after exposures to as low as 2 atm nitrogen (gauge pressure). Delays of several minutes sometimes passed between decompression and bubble formation. Mechanically stimulating the animals to move their legs increased this bubble formation, whereas immobilizing the legs before gas equilibration prevented it, even in animals decompressed from 150 atm nitrogen. We conclude that preformed nuclei are not responsible for bubbles developing in the legs of this animal. Instead, tribonucleation of bubbles apparently occurs as a result of limb motions at relatively low gas supersaturations.     

Breakdown in helium in high-voltage open discharge with subnanosecond current front rise

Investigations of high-voltage open discharge in helium have shown a possibility of generation of current pulses with subnanosecond front rise, due to ultra-fast breakdown development. The open discharge is ignited between two planar cathodes with mesh anode in the middle between them. For gas pressure 6 Torr and 20 kV applied voltage, the rate of current rise reaches 500 A/(cm² ns) for current density 200 A/cm² and more. The time of breakdown development was measured for different helium pressures and a kinetic model of breakdown in open discharge is presented, based on elementary reactions for electrons, ions and fast atoms. The model also includes various cathode emission processes due to cathode bombardment by ions, fast atoms, electrons and photons of resonant radiation with Doppler shift of frequency. It is shown, that the dominating emission processes depend on the evolution of the discharge voltage during the breakdown. In the simulations, two cases of voltage behavior were considered: (i) the voltage is kept constant during the breakdown; (ii) the voltage is reduced with the growth of current. For the first case, the exponentially growing current is maintained due to photoemission by the resonant photons with Doppler-shifted frequency. For the second case, the dominating factor of current growth is the secondary electron emission. In both cases, the subnanosecond rise of discharge current was obtained. Also the effect of gas pressure on breakdown development was considered. It was found that for 20 Torr gas pressure the time of current rise decreases to 0.1 ns, which is in agreement with experimental data.     

A novel method for assessment of fragmentation and beam-material interactions in helium ion radiotherapy with a miniaturized setup

Radiotherapy with protons and carbon ions enables to deliver dose distributions of high conformation to the target. Treatment with helium ions has been suggested due to their physical and biological advantages. A reliable benchmarking of the employed physics models with experimental data is required for treatment planning. However, experimental data for helium interactions is limited, in part due to the complexity and large size of conventional experimental setups.We present a novel method for the investigation of helium interactions with matter using miniaturized instrumentation based on highly integrated pixel detectors. The versatile setup consisted of a monitoring detector in front of the PMMA phantom of varying thickness and a detector stack for investigation of outgoing particles. The ion type downstream from the phantom was determined by high-resolution pattern recognition analysis of the single particle signals in the pixelated detectors. The fractions of helium and hydrogen ions behind the used targets were determined. As expected for the stable helium nucleus, only a minor decrease of the primary ion fluence along the target depth was found. E.g. the detected fraction of hydrogen ions on axis of a 220 MeV/u ⁴He beam was below 6% behind 24.5 cm of PMMA. Monte-Carlo simulations using Geant4 reproduce the experimental data on helium attenuation and yield of helium fragments qualitatively, but significant deviations were found for some combinations of target thickness and beam energy.The presented method is promising to contribute to the reduction of the uncertainty of treatment planning for helium ion radiotherapy.     

The binding of precipitant ions in the tetragonal crystals of hen egg white lysozyme

Abstract

The bonds between lysozyme molecules and precipitant ions in single crystals grown with chlorides of several metals are analysed on the basis of crystal structure data. Crystals of tetragonal hen egg lysozyme (HEWL) were grown with chlorides of several alkali and transition metals (LiCl, NaCl, KCl, NiCl₂ and CuCl₂) as precipitants and the three-dimensional structures were determined at 1.35?Å resolution by X-ray diffraction method. The positions of metal and chloride ions attached to the protein were located, divided into three groups and analysed. Some of them, in accordance with the recently proposed and experimentally confirmed crystal growth model, provide connections in protein dimers and octamers that are precursor clusters in the crystallization lysozyme solution. The first group, including Cu⁺², Ni⁺² and Na⁺¹ cations, binds specifically to the protein molecule. The second group consists of metal and chloride ions bound inside the dimers and octamers. The third group of ions can participate in connections between the octamers that are suggested as building units during the crystal growth. The arrangement of chloride and metal ions associated with lysozyme molecule at all stages of the crystallization solution formation and crystal growth is discussed.

Communicated by Ramaswamy H. Sarma     

A comparative analysis of the contracture responses induced by acetylcholine and choline in the twitch and tonic fibers of frog skeletal muscles

A comparative analysis of the contractile responses induced by acetylcholine and replacement of the external Na⁺ ions with choline ions in the isolated twitch and tonic fibers of frog skeletal muscles was performed. The effects of extracellular Ca²⁺ concentration and several pharmacological agents modulating the activity of various systems maintaining Ca²⁺ level in the myoplasm (dantrolene, cresol, d-tubocurarine, and tetrodotoxin) were studied. It has been found that a voltage-dependent Ca²⁺ release from the sarcoplasmic reticulum depot is the main mechanism inducing the acetylcholine contracture in the fibers of both types. However, the twitch and tonic fibers differ in the properties of the α-isoform and(or) the ratio of α- to β-isoforms of ryanodine-sensitive channels. In the fibers of both types, the replacement of over 25% of Na⁺ ions with choline induces long-term contracture responses, which are also mediated by activation of acetylcholine receptors. It is assumed that an additional mechanism—accumulation of choline ions in the myoplasm and their direct action on the ryanodine-sensitive channels—is involved in the development of such contractile responses.     

The Ionic Mechanisms of Hyperpolarizing Responses in Lobster Muscle Fibers

Lobster muscle fibers develop hyperpolarizing responses when subjected to sufficiently strong hyperpolarizing currents. In contrast to axons of frog, toad, and squid, the muscle fibers produce their responses without the need for prior depolarization in high external K⁺. Responses begin at a threshold polarization (50 to 70 mv), the potential reaching 150 to 200 mv hyperpolarization while the current remains constant. The increased polarization develops at first slowly, then becomes rapid. It usually subsides from its peak spontaneously, falling temporarily to a potential less hyperpolarized than at threshold for the response. As long as current is applied there can be oscillatory behavior with sequential rise and subsidence of the polarization, repeating a number of times. Withdrawal of current leads to rapid return of the potential to the resting level and a small, brief depolarization. Associated with the latter, but of longer duration, is an increased conductance whose magnitude and duration increase with the antecedent current. Hyperpolarizing responses of lobster muscle fibers are due to increased membrane resistance caused by hyperpolarizing K inactivation. The oscillatory characteristic of the response is due to a delayed superimposed and prolonged increase in membrane permeability, probably for Na⁺ and for either K⁺ or Cl^-. The hyperpolarizing responses of other tissues also appear to result from hyperpolarizing K inactivation, on which is superimposed an increased conductance for some other ion or ions.     

Seasonal variation in methane flux from rice paddies associated with methane concentration in soil water,rice biomass and temperature,and its modelling

To attempt to develop physicochemical and physiological modelling for methane transport from the rhizosphere to the atmosphere through rice plants, methane flux, methane concentration in the soil water, and the biomass of rice were measured in lysimeter rice paddies (2.5 × 4 m, depth 2.0 m) once per week throughout the entire growing season in 1992 at Tsukuba, Japan. The addition of exogenous organic matter (rice straw) or soil amendments with the presence or absence of vegetation were also examined for their influence on methane emissions. The total methane emission over the growing season varied from 3.2 g CH₄ m^-2 y^-1 without the addition of rice straw to 49.7 g CH₄ m^-2 y^-1 with rice straw and microbiological amendment. In the unvegetated plot with the addition of rice straw, there was much ebullition of gas bubbles, particularly in the summer. The annual methane emission due to the ebullition of gas bubbles,from the unvegetated plot with the addition of rice straw was estimated to be almost the same as that from the vegetated site with the addition of rice straw. In the early growth stage, the methane flux can be analyzed by the diffusion model (Flux=Methane concentration × Conductance of rice body) using parameters for methane concentration in the soil water as a difference in concentration between the atmosphere and the rhizosphere, and for the biomass of rice as a conductance of rice body. On the other hand, although the diffusion model was inapplicable to a large extent from the middle to late growth stage, methane flux could be estimated by air temperature and concentration in the soil water. Thus, methane transport from the rhizosphere to the atmosphere through rice plants consisted of two phases: one was an explainable small part by diffusion in rice body; the other was a large part strongly, governed by air temperature. The existence of gas bubbles in the soil may be related to the transition between the two phases     

Alkaline Water Electrolysis at 25 A cm−2 with a Microfibrous Flow‐through Electrode

The generation of renewable electricity is variable, leading to periodic oversupply. Excess power can be converted to H₂ via water electrolysis, but the conversion cost is currently too high. One way to decrease the cost of electrolysis is to increase the maximum productivity of electrolyzers. This study investigates how nano‐ and microstructured porous electrodes can improve the productivity of H₂ generation in a zero‐gap, flow‐through alkaline water electrolyzer. Three nickel electrodes—foam, microfiber felt, and nanowire felt—are studied to examine the tradeoff between surface area and pore structure on the performance of alkaline electrolyzers. Although the nanowire felt with the highest surface area initially provides the highest performance, this performance quickly decreases as gas bubbles are trapped within the electrode. The open structure of the foam facilitates bubble removal, but its small surface area limits its maximum performance. The microfiber felt exhibits the best performance because it balances high surface area with the ability to remove bubbles. The microfiber felt maintains a maximum current density of 25 000 mA cm^?2 over 100 h without degradation, which corresponds to a hydrogen production rate 12.5‐ and 50‐times greater than conventional proton‐exchange membrane and alkaline electrolyzers, respectively.     

Laser spectroscopy for measuring the parameters of a plasma containing helium and argon

Laser spectroscopy diagnostics used in experiments on the PNX-U facility are described. The working gas was argon with an additive of helium. The 2³ P → 3³ D transition was excited by means of optical pumping, and helium fluorescence at wavelengths of 388 and 706.5 nm was observed. The Doppler temperature of helium atoms was determined by scanning the profile of the absorption line with the help of a tunable laser. The sum of the signals so obtained provides information on the local density of helium atoms. It was proposed to determine the local value of the electron density N _e (R) from the ratio between the fluorescence intensities at wavelengths of 388 and 706.5 nm. The ratio of these intensities as a function of N _e for He I was calculated in the collisional-radiative model, and relevant measurements of N _e in the PNX-U facility were performed. When diagnosing the argon component, the main attention was paid to measurements of the ion temperature T _i (R, t). In the course these measurements, anomalous heating of Ar II ions was revealed. The concentration of singly charged argon ions was estimated.     

Fluid-Flow Induced Wall Shear Stress and Epithelial Ovarian Cancer Peritoneal Spreading

Epithelial ovarian cancer (EOC) is usually discovered after extensive metastasis have developed in the peritoneal cavity. The ovarian surface is exposed to peritoneal fluid pressures and shear forces due to the continuous peristaltic motions of the gastro-intestinal system, creating a mechanical micro-environment for the cells. An in vitro experimental model was developed to expose EOC cells to steady fluid flow induced wall shear stresses (WSS). The EOC cells were cultured from OVCAR-3 cell line on denuded amniotic membranes in special wells. Wall shear stresses of 0.5, 1.0 and 1.5 dyne/cm² were applied on the surface of the cells under conditions that mimic the physiological environment, followed by fluorescent stains of actin and β-tubulin fibers. The cytoskeleton response to WSS included cell elongation, stress fibers formation and generation of microtubules. More cytoskeletal components were produced by the cells and arranged in a denser and more organized structure within the cytoplasm. This suggests that WSS may have a significant role in the mechanical regulation of EOC peritoneal spreading.     

An alternative explanation for cyanobacterial scum formation and persistence by oxygenic photosynthesis

The cause of persistent cyanobacteria scum formation in lakes is an unresolved subject. Scum refers to the event in which cyanobacteria are at the water surface of a lake. Factors like low turbulence levels, long day-light, high water temperatures and the buoyant capacity of cyanobacterial cells play a role in the occurrence of scums. However, they do not explain why scums are observed at periods during the day when according to theory they should have disappeared into the deeper water layers. In this study, we present an alternative explanation. The hypothesis we present here is that irreversible buoyancy of cyanobacteria colonies is created by the growth of gas bubbles on or within the mucilage of the colonies. These bubbles grow under oxygen super-saturated conditions. At low wind speed and high chlorophyll levels, the dissolved oxygen (DO) produced during photosynthesis by cyanobacteria, cannot escape sufficiently fast to the atmosphere hence a DO supersaturated condition arises in the water. At this stage, growth of oxygen bubbles may occur inside or attached to the mucilage. We present results of compression experiments to support our hypothesis. In a chamber, the pressure on lake water containing a natural cyanobacteria population is increased. At 3 × 10⁵ and 4 × 10⁵ Pa the cyanobacteria colonies were not able to float anymore and sank. This pressure is lower than the 10⁶ Pa needed to collapse all gas vacuoles inside the cyanobacteria cells (Walsby, 1994). The observed change from floating to sinking colonies due to increased water pressure suggests that gas bubbles were present inside the colonies. In lakes, these gas bubbles may lead to permanent buoyancy, i.e. a persistent scum.