Structure and thermodynamics of associating rods solutions

Thermoreversible sol-gel transitions in solutions of rod-like associating polymers are analyzed by computer simulations and by mean field models. The sol-gel transition is determined by the divergence of the cluster weight average. The analytically determined sol-gel transition is in good agreement with the simulation results. At low temperatures we observe a peak in the heat capacity, which maximum is associated with the precipitation transition. The gelation transition is sensitive to the number of associating groups per rod but nearly insensitive to the spatial distribution of associating groups around the rod. The precipitation is strongly dependent on both the number and distribution of associating groups per rod. We find negligible nematic orientational order at the gelation and precipitation transitions

Polyelectrolytes in the presence of multivalent ions: gelation versus segregation

We analyze solutions of strongly charged chains bridged by linkers such as multivalent ions. The gelation induced by the strong short range electrostatic attractions is dramatically suppressed by the long range electrostatic correlations due to the charge along the uncrosslinked monomers and ions. A modified Debye-Huckel approach of crosslinked clusters of charged chains is used to determined the mean field gelation transition self-consistently. Highly dilute polyelectrolyte solutions tend to segregate macroscopically. Semidilute solutions can form gels if the Bjerrum length $l_B$ and the distance between neighboring charged monomers along the chain $b$ are both greater than the ion size $a$

О ПРИМЕНЕНИИ НАНОПОРОШКОВОЙ ПРОДУКЦИИ АЗИДНОЙ ТЕХНОЛОГИИ СВС ДЛЯ АРМИРОВАНИЯ И МОДИФИЦИРОВАНИЯ АЛЮМИНИЕВЫХ СПЛАВОВ

The survey of types and fabrication methods of aluminum matrix composite materials discretely reinforced with ceramic particles and nanoparticles of refractory compounds is given. It is shown that the azide technology of self-propagating high-temperature synthesis (SHS-Az), which uses sodium azide NaN3 as a solid nitriding agent, makes it possible to fabricate numerous comparatively inexpensive micropowders and nanopowders of nitrides, carbonitrides, carbides, and their compositions, which are of interest for the reinforcement and modification of aluminum alloy. Along with the mentioned target ceramic particles, the SHS-Az product contains side haloid salts (sodium and potassium chlorides and fluorides), which can play the role of fluxes with the addition of ceramic particles into the aluminum melt. The results of experiments on the introduction of the nanopowder composition into the melt of AK6M2 alloy, %: β-SiC – 48,6, α-Si3N4 – 27,0, β-Si3N4 – 5,8, Na3AlF6 – 18,6 in the composition of the compacted pseudo-master alloy with the copper powder, which evidence the modifying effect, are presented.Дан обзор видов и методов получения алюмоматричных композиционных материалов, дискретно армированных керамическими частицами и наночастицами тугоплавких соединений. Показано, что азидная технология самораспространяющегося высокотемпературного синтеза (СВС-Аз), использующая азид натрия NaN3 в качестве твердого азотирующего реагента, позволяет получать большое число сравнительно недорогих микро- и нанопорошков нитридов, карбонитридов, карбидов и их композиций, которые представляют интерес для армирования и модифицирования алюминиевых сплавов. Наряду с указанными целевыми керамическими частицами продукт СВС-Аз содержит побочные галоидные соли (хлориды и фториды натрия и калия), которые могут играть роль флюсов при добавлении керамических частиц в расплав алюминия. Представлены результаты экспериментов по вводу в расплав сплава АК6М2 нанопорошковой композиции, %: β-SiC – 48,6, α-Si3N4 – 27,0, β-Si3N4 – 5,8, Na3AlF6 – 18,6 в составе прессованной псевдолигатуры с порошком меди, свидетельствующие о модифицирующем эффекте

A Modified Random Phase Approximation of Polyelectrolyte Solutions

We compute the phase diagram of salt-free polyelectrolyte solutions using a modified Debye-Huckel Approach. We introduce the chain connectivity via the Random Phase Approximation with two important modifications. We modify the electrostatic potential at short distances to include a bound on the electrostatic attractions at the distance of closest approach between charges. This modification is shown to act as a hard core in the phase diagram of electrolyte solutions. We also introduce a cut-off on the integration of the modes of wave length smaller than the size over which the chains are strongly perturbed by the electrostatic interactions. This cut-off is shown to be essential to predict physical phase diagram in long chain solutions

Weak Segregation Theory and Non-Conventional Morphologies in the Ternary ABC Triblock Copolymers

The Leibler weak segregation theory in molten diblock copolymers is generalized with due regard for the 2nd shell harmonics contributions defined in the paper and the phase diagrams are built for the linear and miktoarm ternary ABC triblock copolymers. The symmetric linear copolymers with the middle block non-selective with respect to the side ones are shown to undergo the continuous ODT not only into the lamellar phase but also into various non-conventional cubic phases (depending on the middle block composition it could be the simple cubic, face-centered cubic or non-centrosymmetric phase revealing the symmetry of space group No.214 first predicted to appear in molten block copolymers). For asymmetric linear ABC copolymers a region of compositions is found where the weakly segregated gyroid (double gyroid) phase exists between the planar hexagonal and lamellar or one of the non-conventional cubic phases up to the very critical point. In contrast, the miktoarm ABC block copolymers with one of its arm non-selective with respect to the two others are shown to reveal a pronounced tendency towards strong segregation, which is preceded by increase of stability of the conventional BCC phase and a peculiar weakly segregated BCC phase (BCC3), where the dominant harmonics belong to the 3rd co-ordination sphere of the reciprocal lattice. The validity region of the developed theory is discussed and outlined in the composition triangles both for linear and miktoarm copolymers.Comment: 61 pages, 12 figure

The Role of Micro Breaking of Small-Scale Wind Waves in Radar Backscattering from Sea Surface

The study of the microwave scattering mechanisms of the sea surface is extremely important for the development of radar sensing methods. Some time ago, Bragg (resonance) scattering of electromagnetic waves from the sea surface was proposed as the main mechanism of radar backscattering at moderate incidence angles of microwaves. However, it has been recently confirmed that Bragg scattering is often unable to correctly explain observational data and that some other physical mechanisms should be taken into consideration. The newly introduced additional scattering mechanism was characterized as non-polarized, or non-Bragg scattering, from quasi-specular facets appearing due to breaking wave crests, the latter usually occurring in moderate and strong winds. In this paper, it was determined experimentally that such non-polarized radar backscattering appeared not only for rough sea conditions in which wave crests strongly break and “white caps” occur, but also at very low wind velocities close to their threshold values for the wave generation process. The experiments were performed using two polarized Doppler radars. The experiments demonstrated that a polarization ratio, which characterizes relative contributions of non-polarized and Bragg components to the total backscatter, changed slightly with wind velocity and wind direction. Detailed analysis of radar Doppler shifts revealed two types of scatterers responsible for the non-polarized component. One type of scatterer, moving with the velocities of decimeter-scale wind waves, determined radar backscattering at low winds. We identified these scatterers as “microbreakers” and related them to nonlinear features in the profile of decimeter-scale waves, like bulges, toes and parasitic capillary ripples. The scatterers of the second type were associated with strong breaking, moved with the phase velocities of meter-scale breaking waves and appeared at moderate winds additionally to the “microbreakers”. Along with strong breakers, the impact of microbreaking in non-polarized backscattering at moderate winds remained significant; specifically the microbreakers were found to be responsible for about half of the non-polarized component of the radar return. The presence of surfactant films on the sea surface led to a significant suppression of the small-scale non-Bragg scattering and practically did not change the non-Bragg scatterer speed. This effect was explained by the fact that the nonlinear structures associated with dm-scale waves were strongly reduced in the presence of a film due to the cascade mechanism, even if the reduction of the amplitude of dm waves was weak. At the same time, the velocities of non-Bragg scatterers remained practically the same as in non-slick areas since the phase velocity of dm waves was not affected by the film

Polarized X-band Doppler radar scatterometer for investigation of microwave scattering of the wavy water surface in laboratory conditions

The paper describes an experimental model of continuous wave X-band Doppler radar scatterometer (sine frequency modulation) designed for physics investigation of radio waves scattering from sea surface in controlled conditions. The prototype is developed and fabricated at the IAP RAS. Its main feature is adaptation to the conditions of a laboratory modeling in the wind-wave flumes to investigate the dependence of the normalized radar cross-section (NRCS) on the wind speed. The design of the microwave and antenna systems allows measurement of scattered radiation power and its Doppler spectrum both at linear co- and cross-polarizations (in a sequential switching mode). This is important from the viewpoint of studying the waves at high wind speeds. The detailed description of the design and its specifications are presented. Also the problems of calibration and results of experimental operation on the high-speed wind-wave flume of IAP RAS are discussed

On Capabilities of Tracking Marine Surface Currents Using Artificial Film Slicks

It is known that films on the sea surface can appear due to ship pollution, river and collector drains, as well as natural biological processes. Marine film slicks can indicate various geophysical processes in the upper layer of the ocean and in the atmosphere. In particular, slick signatures in SAR-imagery of the sea surface at low and moderate wind speeds are often associated with marine currents. Apart from the current itself, other factors such as wind and the physical characteristics of films can significantly influence the dynamics of slick structures. In this paper, a prospective approach aimed at measuring surface currents is developed. The approach is based on the investigation of the geometry of artificial banded slicks formed under the action of marine currents and on the retrieval of the current characteristics from this geometry. The developed approach is applied to quasi stationary slick bands under conditions when the influence of the film spreading effects can be neglected. For the stationary part of the slick band where transition processes of the band formation, e.g., methods of application of surfactants on water, film spreading processes, possible wind transformation etc., become negligible, some empirical relations between the band geometrical characteristics and the characteristics of the surface currents are obtained. The advantage of the approach is a possibility of getting information concerning the spatial structure of marine currents along the entire slick band. The suggested approach can be efficient for remote sensing data verification

Surface Waves Prediction Based on Long-Range Acoustic Backscattering in a Mid-Frequency Range

Underwater acoustic echosounding for surface roughness parameters retrieval is studied in a frequency band that is relatively new for such purposes. During the described 2-weeks sea experiment, 1–3 kHz tonal pulses were emitted from an oceanographic platform, located on the northern Black Sea shelf. Doppler spectra of the resulting reverberation were studied. The frequency band of the acoustic system, selected for this study, is chosen due to the fact that the sound propagation range is large enough for remote sensing in a coastal zone, and the resolution cell size does not limit the research. Backscattering of acoustical signals was received for distances around two nautical miles. However, it turned to be quite difficult to interpret the obtained data since backscattering spectrum shape was influenced by a series of effects, resulting in a complicated link to wind waves and currents’ parameters. Significant wave height and dominant wave frequency were estimated as the result of such signals processed with the use of machine learning tools. A decision-tree-based mathematical regression model was trained to solve the inverse problem. Wind waves prediction is in a good agreement with direct measurements, made on the platform, and machine learning results allow physical interpretation