Charge screening in a plasma with an external ionization source

An asymptotic theory for the screening of the electric field of a dust particle or a spherical probe in a plasma with an external steady and/or internal (proportional to the electron density) gas ionization source has been developed for the first time. It has been established that the screening of the charge of a spherical body adsorbing the charge of the incident plasma particles is described by a superposition of two exponentials with different screening constants. The two exponentials are retained even in the absence of nonequilibrium fluxes on the macroparticle and only in the special case of an isothermal plasma does the screening become Debye one. The screening length is determined by the ratio of the electron-ion, β_ei, and Langevin, β_L = 4πeμ_i (where μ_i is the ion mobility), recombination coefficients. If β_L ? β_ei, then it is much larger than the electron Debye length. The ions in an isothermal plasma have been found to give the same contribution to the screening as the electrons if the electron-ion recombination coefficient exceeds the Langevin ion recombination coefficient by a factor of 2 or more, β_ei ≥ 2β_L. The Vlasov equation is used to analyze the asymptotic behavior of the macroparticle potential in a collisionless plasma.     

Screening of a moving charge in a nonequilibrium plasma

The problem of screening a moving charged dust particle is analyzed in the model of point sinks. Typical time scales for the formation of a polarization cloud around the moving macroscopic particle are found using the three-dimensional integral transform with respect to the spatial coordinates and the Laplace transform in time. It is shown that the stationary potential of a moving charge has a dipole component dominating at sufficiently large distances. The force exerted on a moving charged macroscopic particle by the electric field of induced charges is calculated. It is shown that, in general, the direction of this force depends on the ratio between the transfer coefficient and the decay rate of plasma particles in the plasma. In the presence of sinks, a dust particle is accelerated by this force if the Langevin recombination rate for ions, β _iL = 4πeμ _i , exceeds the electron-ion recombination rate β _ei . In the absence of sinks or if β _ei > β _iL this force is antiparallel to the dust-particle velocity.     

Inhomogeneous NMR line shape as a probe of microscopic organization of bicontinuous cubic phases

NMR line shapes of the lipid and aqueous species in bicontinuous cubic phase (BCP) samples prepared by centrifugation are inhomogeneously broadened. The broadening of the lipid peaks is removed by magic-angle spinning (MAS). In this work, we studied the mechanism of this broadening using (1)H and (13)C NMR spectroscopy of a myverol/water BCP. It is demonstrated that the inhomogeneity possesses an intrinsic contribution that is independent of instrumental or setup factors and can be attributed to the microscopic organization of the BCP bilayer. A mechanism of the inhomogeneous broadening is proposed, which involves a spatially nonuniform diamagnetically induced magnetic field determined by the mesoscopic structure and the diamagnetic susceptibilities of the two BCP domains. The proposed mechanism does not require that molecular reorientation of the lipid be slow for the inhomogeneous broadening to survive. We discuss how this inhomogeneous broadening can be employed as a probe of compositional uniformity and microscopic organization of BCP samples.     

Geometric Form of the Hahn–Banach Theorem for Generalized Convexity

We investigate a class of compact sets convex with respect to a certain family of planes. For compact sets that satisfy the condition of acyclicity of sections by a certain collection of two-dimensional planes, we prove their generalized convexity.     

Diagnostics of coated fuel particles by neutron and synchrotron radiography

The nondestructive monitoring of coated fuel particles has been performed using contact neutron radiography and refraction radiography based on synchrotron radiation. It is shown that these methods supplement each other and have a high potential for determining the sizes, densities, and isotopic composition of the particle components.     

Properties of poly(vinyl alcohol)-graft-polyacrylamide copolymers depending on the graft length. 3. Benzene solubilization by solutions of the copolymer

The peculiarities of poly(vinyl alcohol)-graft-polyacrylamide copolymers (PVA-g-PAA), which are characterized by the equal average number (N=9), but various molecular weight (or length) of graft chains, in comparison with individual PAA and PVA, were investigated in aqueous medium. Sharp rise in benzene solubilization in PVA-g-PAA solutions at M_PAA higher than 4.3·10⁵ has been established. It was shown that such effect is stipulated by the destruction of intramolecular polymer-polymer complex in the copolymer and increasing the benzene binding to separate PVA-g-PAA groups by means of hydrogen bonds. The changes in the PVA-g-PAA solubilizing ability as the function of temperature were also investigated. The obtained results are discussed from the point of view of conformational transitions of intramolecular polymer-polymer complexes (intraPPC), which exist in copolymers, in dependence on the length of graft chains.     

Acquisition of pure-phase diffusion spectra using oscillating-gradient spin echo

Oscillating-gradient spin echo (OGSE) diffusion experiments have long been used to measure the short-time apparent diffusion coefficient, D(app)(t), in the presence of restricted diffusion, as well as the spectrum of the slow-motion velocity autocorrelation function. In this work, we focus on two previously unexplored aspects of OGSE experiments: convection compensation and acquisition of pure-phase diffusion spectra in the presence of homonuclear scalar couplings. We demonstrate that convection compensation afforded by single-echo OGSE compares well with that in double-echo convection-compensated PGSE experiments. We also show that, in the presence of homonuclear scalar couplings, setting the OGSE echo time to 1/2J enables acquisition of pure-phase diffusion spectra and yields more reliable D estimates than mixed-phase PGSE or OGSE spectra. Pure-phase OGSE acquisition is also compatible with measurements of the apparent diffusion coefficient at an arbitrary diffusion time. These features of OGSE can be valuable in diffusion measurements of scalar-coupled small-molecule probes in cellular and other heterogeneous systems.     

Enhancement of Na+ diffusion in a bicontinuous cubic phase by the ionophore monensin

Pulsed field gradient spin-echo NMR diffusion and relaxation measurements were used to investigate how the Na+ ionophore monensin affected the dynamics of sodium ions in a Myverol 18-99/saline bicontinuous Ia3d cubic phase (BCP). The monensin Na+ binding number was estimated from 23Na line widths to be between 3 and 6. The dependence of the apparent Na+ diffusion coefficient on the concentration of monensin revealed monensin-induced Na+ transport. At high monensin concentrations, the enhancement of D(Na+) was offset by Na+-monensin binding. The greatest enhancement was measured at short diffusion times (delta < or = 5 ms), which we explain in terms of the bicontinuous topology of the cubic phase and a combination of tortuosity and bilayer permeability effects. We also propose numerical simulations which would enable the separation of the two effects. To our knowledge, this is the first study of ionophore-mediated cation diffusion in a bicontinuous cubic phase. The approach could be used to study the dynamics of hydrophilic species in the aqueous channels of BCPs and similar structures, as well as to measure the ion-transporting efficiency of ionophores.     

A New Technique for Determining Coproporphyrin III Content in Culture Broth

A new technique for spectrophotometric determination of coproporphyrin III in culture broth, based on its preliminary separation from impurities, was developed. This technique ensures higher accuracy of coproporphyrin III determination than the commonly used method based on sorption of coproporphyrin III on talc.     

Simple preparation of diamondoid 1,3-dienes via oxetane ring opening

The transformations of apical mono- and bisacetyl diamondoids to the respective oxetanes and subsequent acid-catalyzed ring opening/dehydration lead to diamondoidyl mono- and bis-1,3-dienes in high preparative yields.