Roumen Tsekov

EFFECT OF THICKNESS NON-HOMOGENEITY ON THE KINETIC BEHAVIOUR OF MICROSCOPIC FOAM FILM

Abstract The effect of thickness non-homogeneity on the drainage of microscopic horizontal foam films was studied experimentally and theoretically. Quasi-static and asymmetrical distribution of thickness irregularities over the film surface was established in the experiment. This type of dimpling was modelled theoretically and employed to derive a new equation for the film thinning. It differs from the theoretical expressions known from the literature and is in better agreement with the experimentally determined functional dependence of thinning rate on film size.

Peculiarity of the liquid/vapour interface of an ionic liquid: study of surface tension and viscoelasticity of liquid BMImPF6 at various temperatures

We have measured the surface tension and the capillary wave spectra at the liquid/vapour interface of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate at various temperatures up to 400 K. From the weak temperature dependence of the surface tension a low value of the surface excess entropy of approximately 3.5 x 10(5) J K(-1) m(-2) results which is consistent with a strongly aligned surface layer of imidazolium cations previously predicted by MD-calculations. The capillary wave spectra recorded at different wave numbers in the range 170 cm(-1) < or = q < or =500 cm(-1) exhibit strong deviations from the behaviour expected for the free surface of simple liquids. With an extended dispersion relation including the contributions of surface dipole moment density gamma and shear surface excess viscosity mu the spectra have been analyzed. It is found that mu is negligibly small, whereas gamma substantially influences the capillary wave spectra. The electrostatic potential across the interface, which corresponds to the measured dipole moment densities, qualitatively agrees with simulation calculations. The distinct temperature dependence of gamma suggests that with increasing temperature an order-disorder transformation occurs in the surface layer.

Anomalous ion effects on rupture and lifetime of aqueous foam films formed from monovalent salt solutions up to saturation concentration

We report the effects of ions on rupture and lifetime of aqueous foam films formed from sodium chloride (NaCl), lithium chloride (LiCl), sodium acetate (NaAc), and sodium chlorate (NaClO 3) using microinterferometry. In the case of NaCl and LiCl, the foam films prepared from the salt solutions below 0.1 M were unstable they thinned until rupturing. The film lifetime measured from the first interferogram (appearing at a film thickness on the order of 500 nm) until the film rupture was only a second or so. However, relatively long lasting and nondraining films prepared from salt solutions above 0.1 M were observed. The film lifetime was significantly longer by 1 to 2 orders of magnitude, i.e., from 10 to 100 s. Importantly, both the film lifetime and the (average) thickness of the nondraining films increased with increasing salt concentration. This effect has not been observed with foam films stabilized by surfactants. The film lifetime and thickness also increased with increasing film radius. The films exhibited significant surface corrugations. The films with large radii often contained standing dimples. There was a critical film radius below which the films thinned until rupturing. In the cases of NaAc and NaClO 3, the films were unstable at all radii and salt concentrations they thinned until rupturing, ruling out the effect of solution viscosity on stabilizing the films.

The R4/5-problem in the drainage of dimpled thin liquid films

Abstract A new formula describing the thinning rate of dimpled thin liquid films is derived. In contrast to the Reynolds law, the predicted rate of drainage is inversely proportional to the film radius R raised to the power of 4/5 and is in excellent agreement with the known experimental results. It is demonstrated that the drainage of small films obeys the Reynolds formula and a criterion for the transition between the two regimes of thinning is proposed. An estimation of the thickness inhomogeneity in dimpled films is also obtained.

Stochastic dynamics of a subsystem interacting with a solid body with application to diffusive processes in solids

In this paper the dynamics of a mechanical subsystem interacting with a solid body is studied. The Newton equations are transformed to a set of stochastic generalized Langevin equations describing the evolution of the coordinates of the subsystem particles. The solid is modeled as a bath of interacting harmonic oscillators, and the effect of their spatial correlations on the statistical properties of the Langevin forces is accounted for. The most important result is the relation established between the static interaction of the subsystem with the solid body and the dissipative and fluctuation forces. In the particular case of a subsystem consisting of a single particle, an expression is derived for the friction tensor in terms of the static interaction potential and Debye cutoff frequency of the solid. The analysis is applied in the latter case to some simple processes occurring in solids, such as adsorption, desorption, and diffusion.

Bohmian Mechanics versus Madelung Quantum Hydrodynamics

It is shown that the Bohmian mechanics and the Madelung quantum hydrodynamics are different theories and the latter is a better ontological interpretation of quantum mechanics. A new stochastic interpretation of quantum mechanics is proposed, which is the background of the Madelung quantum hydrodynamics. Its relation to the complex mechanics is also explored. A new complex hydrodynamics is proposed, which eliminates completely the Bohm quantum potential. It describes the quantum evolution of the probability density by a convective diffusion with imaginary transport coefficients.

Two-dimensional connective nanostructures of electrodeposited Zn on Au (111) induced by spinodal decomposition

Phase formation of surface alloying by spinodal decomposition has been studied at an electrified interface. For this aim Zn was electrodeposited on Au(111) from the ionic liquid AlCl(3)-MBIC (58:42) containing 1 mM Zn(II) at different potentials in the underpotential range corresponding to submonolayer up to monolayer coverage. Structure evolution was observed by in situ electrochemical scanning tunneling microscopy (STM) at different times after starting the deposition via potential jumps and at temperatures of 298 and 323 K. Spinodal or labyrinth two-dimensional structures predominate at middle coverage, both in deposition and in dissolution experiments. They are characterized by a length scale of typically 5 nm which has been determined from the power spectral density of STM images. Structure formation and surface alloying are governed by slow kinetics with a rate constant k with activation energy of 120 meV and preexponential factor of 0.17 s(-1). The evolution of the structural features is described by a continuum model and is found to be in good agreement with the STM observations. From the experimental and model calculation results we conclude that the two-dimensional phase formation in the Zn on Au(111) system is dominated by surface alloying. The phase separation of a Zn-rich and a Zn-Au alloy phase is governed by two-dimensional spinodal decomposition.

Experiments on surface waves in thin wetting films

Abstract A new method for excitation and measurement of time stationary, free running waves on wetting films is described. The waves in the frequency range of 10–140 Hz are generated by an oscillating bubble against a quartz plate in a Derjaguin–Scheludko film balance. The results from the experimentally measured dispersion equation are compared with a theoretical model based on the lubrication approximation. It is shown that there are significant discrepancies between experiment and theory which cannot be explained by the classical DLVO-theory as well as by the assumption of a non-slip boundary condition.

Radiation field in continuous annular photocatalytic reactors: role of the lamp finite size

A model for the radiation field generated by the lamp of a continuous annular photoreactor was developed. The present model is applied to the case where the length of the lamp is significantly smaller than that of the reactor, and therefore the radiation field varies strongly along the axis of the reactor. The basic assumption of the model is that the radiation is emitted by spherical sources which are placed one on top of the other. The predicted radiation field describes satisfactorily experimental observations (Fotou and Pratsinis, 1996). Analytical solution for the concentration profile of continuous heterogeneous photocatalytic reactors, which carry out first-order reaction, was derived. The proposed model of radiation was included in the estimation of the reactant concentration.

Electro-osmotic equilibria for a semipermeable shell filled with a solution of polyions

The authors study theoretically the electrostatic equilibria for a shell filled with a suspension of polyions (e.g., colloids, polyelectrolytes, etc.) and immersed in an infinite salt-free reservoir. The shell is treated as impermeable for polyions, but allowing free diffusion of counterions. From the solution of the linearized Poisson-Boltzmann equation we obtain the distribution of the potential and concentration profiles for polyions. The authors then derive explicit formulas for the excess electro-osmotic pressure of a polyion solution exerted by the shell. This is shown to be due to a concentration of polyions at the inner shell boundary and can be very different from the pressure of a corresponding bulk polyion solution.