The simulation of dendrite growth and partial discharges in epoxy resin

A self-consistent model of the development of dendrites and partial discharges in a dielectric solid under a variable voltage is suggested. Dendrites originate at sites with the enhanced local field strength and also where the dielectric is broken down under the action of partial discharges. The numerical simulation is used to quantitatively describe the space-time dynamics of the dendrites and partial discharges in epoxy resin for the tip-plane electrode configuration. The simulated data are compared with electrical measurements of partial discharges and with optical images of dendrite growth under the same conditions.     

Optical bistability of planar metal/dielectric nonlinear nanostructures

We study theoretically a nonlinear response of the planar metal/dielectric nanostructures constituted from periodical array of ultra thin silver layers and the layers of Kerr-like nonlinear dielectric. We predict hysteresis-type dependences of the components of the tensor of effective dielectric permittivity on the field intensity allowing the change in material transmission properties from transparent to opaque and back at extremely low intensities of the light. It makes possible to control the light by light in all-optical nanoscale devices and circuits.     

Femtosecond quantum dynamics of diatomic molecules in optically dense gaseous media

A consistent quantum theory of a two-pulse pump-probe experiment in the femto-and picosecond spectroscopy of diatomic molecules is developed. The theory aims at the use of weak quantum femtosecond light pulses to excite molecular coherence. It is suggested that the experiment be realized using a double-frequency excitation scheme that is feasible for Na₂ molecules. On the basis of the theory developed, the luminescence signals are calculated at different temperatures and different optical densities of the medium for the time range from 100 fs to ~ 100 ps. The results obtained show the presence of a strong dependence of the time and frequency spectra of a luminescence signal on the optical density of the medium at temperatures from 50 to 300 K. The proposed approach makes it possible to use the quantum coherence properties of optically dense media for more detailed study of the vibronic dynamics of diatomic molecules, in particular, detection of weak optical transitions.     

Kinetics and Mechanism of a Reaction of Phenylacetylene with Carbon Monoxide and Butanol in Toluene Catalyzed by a Palladium(0) Complex in the Presence of Trifluoroacetic Acid and Triphenylphosphine

The reaction of phenylacetylene with CO and n-butanol in toluene (363 K) catalyzed by the Pd(dba)₂/m(CF₃COOH)/n(Ph₃P) system (dba is dibenzylideneacetone; 2 m 8; 10 n 30) is studied. The initial rate of the main product (butyl 2-phenylpropenoate) buildup is found to depend on the pressure of CO and the concentrations of reactants and system components. The state of the catalyst under reaction conditions is studied in situby IR spectroscopy. A kinetic model is developed based on the experimental results. This model corresponds to the mechanism that resembles the hydride mechanism in the type of main intermediates in the catalytic cycle.     

Dynamic elasticity of triblock copolymer of poly(ethylene oxide) and poly(propylene oxide) on a water surface

Dilatational viscoelasticity of adsorbed and spread films of the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer at the air-water interface is studied by the capillary waves and oscillating barrier techniques. At the surface pressure below 10 mN/m, dynamic surface properties of these films coincide with those of poly(ethylene oxide). At higher surface pressures, the results obtained indicate the desorption of poly(propylene oxide) segments from the monolayer and their interaction with poly(ethylene oxide) segments in an aqueous phase. At a surface pressure close to 19 mN/m, the behavior of adsorbed and spread poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) films becomes different. The real part of dynamic surface elasticity of spread films tends toward its maximum value (20 mN/m) and, upon further compression, films begin to dissolve. At the same time, the surface elasticity of adsorbed films decreases nearly twofold upon the achievement of the maximum value that testifies the formation of looser structure of the surface layer.     

Unsteady-state performance of NOx catalytic reduction by NH3

Experimental results of studying NO_x catalytic reduction by NH₃ under periodic reversals in the direction of filtration of the mixture purified in a catalyst bed are discussed.

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NO_x .

     

Dynamic surface elasticity of polymer solutions

The surfaces of surfactant solutions exhibit viscoelastic dilational behavior which may be investigated by modern optical and mechanical methods. The present study focuses on the dynamic dilational properties of the polymer solution — gas interface. Linear flexible polymer chains at the surface are considered as consisting of trains, lying on the surface, and loops and tails that protrude into the liquid. The Rouse model is used to describe the inner dynamics of the trains, whereas their motion relative to their neighbors on the surface can be described with the help of the reptation conception. An expression for the complex dynamic dilational surface elasticity has been derived. Although, in general, the relaxation of the surface tension is characterized by an infinite series of relaxation times, it is shown that for many systems the dynamics of the surface layer can be described approximately by only two main relaxation times. The dispersion equation, which was obtained earlier for solutions of conventional surfactants, is shown to be fulfilled for polymer solutions also.     

Molecular dynamics study of hydration in ethanol-water mixtures using a polarizable force field

The abnormal physicochemical characteristics of ethanol solvation in water are commonly attributed to the phenomenon of hydrophobic hydration. To investigate the structural organization of hydrophobic hydration in water-ethanol mixtures, we use molecular dynamics simulations based on detailed atomic models. Induced polarization is incorporated into the potential function on the basis of the classical Drude oscillator model. Water-ethanol mixtures are simulated at 11 ethanol molar fractions, from 0.05 to 0.9. Although the water and ethanol models are parametrized separately to reproduce the vaporization enthalpy, static dielectric constant, and self-diffusion constant of neat liquids at ambient conditions, they also reproduce the energetic and dynamical properties of the mixtures accurately. Furthermore, the calculated dielectric constant for the various water-alcohol mixtures is in excellent agreement with experimental data. The simulations provide a detailed structural characterization of the mixtures. A depletion of water-water hydrogen bonding in the first hydration shell of ethanol is compensated by an enhancement in the second hydration shell. The structuring effect from the second solvation shell gives rise to a net positive hydrogen-bonding excess for ethanol molar fractions up to approximately 0.5. For larger molar fractions, the second hydration shell is not sufficiently populated to overcome the net H-bond depletion from the first shell.     

Dynamic Surface Properties of Poly(vinylpyrrolidone) Solutions

The dynamic surface tension and the complex dynamic surface elasticity of poly(vinylpyrrolidone) (PVP) solutions were measured in the concentration range 10(-5) wt% up to about 1 wt%. The surface tension changed slowly with time at low (<10(-4) wt%) and high concentrations (>0.1 wt%). At low concentrations this is a consequence of the slow transport by diffusion of PVP molecules from the depth of the bulk phase to the surface. At high concentrations the time effect is unexpected and probably the result of PVP contamination of high surface activity. The dynamic surface elasticity of PVP solutions gradually decreases with increasing concentration up to the range of high concentrations (>0.1 wt%) where an abrupt increase in the elasticity caused by the adsorbed impurity is observed. At low and medium concentrations the viscoelastic behavior of PVP adsorbed films is similar to that of the previously investigated poly(ethylene oxide) and poly(ethylene glycol) films and is determined by the number of loops and tails protruding into the bulk phase.