Biomedical subjects

N M Kovalchuk

Publications and source records attributed to N M Kovalchuk.

9 recordsLinked to original sources

PubMed2006

Surfactant transfer through a liquid membrane: origin of spontaneous oscillations at the membrane/acceptor phase interface.

Instability due to surfactant redistribution in a liquid membrane system consisting of two solutions, namely source and acceptor, separated by a layer of immiscible liquid is studied theoretically and experimentally. The transfer of a surfactant from a source phase to an acceptor phase is often accompanied by spontaneous nonlinear oscillations of electrical potential and/or interfacial tension. The oscillations can be generated at each of the membrane interfaces. Here a mechanism of oscillation, which develops at the membrane/acceptor phase interface, is proposed on the basis of direct numerical simulation of the system evolution. Performed experimental studies confirm the theoretical results.

Alcohols↗

PubMed2006

Marangoni instability and spontaneous non-linear oscillations produced at liquid interfaces by surfactant transfer.

The systems producing non-linear spontaneous oscillations of the interfacial tension and electric potential are considered and the available criteria for development of convective instability by the surfactant transfer through a liquid interface are discussed. The non-linear oscillations are observed by the surfactant transfer from a point-like source situated in the bulk of liquid, by the transfer of two ionic solutes through a liquid interface in two opposite directions, and by the transfer of ionic solutes through a liquid membrane. All these systems are governed by more complicated mechanisms than merely arising oscillatory convective instability. The main experimental results obtained for these three systems as well as theoretical models proposed for their explanation are discussed.

Chemistry, Physical↗

PubMed2005

Nonlinear spontaneous oscillations at the liquid/liquid interface produced by surfactant dissolution in the bulk phase.

The results of theoretical and experimental studies of spontaneous nonlinear oscillations produced at the liquid/liquid interface by surfactant transfer from a point source situated in one of the bulk phases are presented. The theoretical analysis is based on the direct numerical simulation of the system evolution. The experiments are performed for the heptane/water interface using middle-chain aliphatic alcohols as surfactants. The results for the oil/water interface are compared with the corresponding data obtained for the air/water interface. The presented results allow the conclusion that auto-oscillations at the air/liquid and liquid/liquid interfaces are governed by very similar mechanisms but their characteristics are strongly dependent on the properties of the two contacting media, in particular, on the surfactant partition coefficient.

Models, Chemical↗

PubMed2005

Effect of buoyancy on appearance and characteristics of surface tension repeated auto-oscillations.

The effect of buoyancy on spontaneous repeated nonlinear oscillations of surface tension, which appear at the free liquid interface by dissolution of a surfactant droplet under the interface, is considered on the basis of direct numerical simulation of the model system behavior. The oscillations are the result of periodically rising and fading Marangoni instability. The buoyancy force per se cannot lead to the oscillatory behavior in the considered system, but it influences strongly both the onset and decay of the instability and therefore, affects appearance and characteristics of the oscillations. If the surfactant solution density is smaller than the density of the pure liquid, then the buoyancy force leads to a considerable decrease of the induction period and the period of oscillations. The buoyancy force affects also the dependence of the oscillation characteristics on the system dimensions. The results of the simulations are compared with the available experimental data.

Chemical Phenomena↗

PubMed2004

Effect of substance properties on the appearance and characteristics of repeated surface tension auto-oscillation driven by Marangoni force.

The effect of substance properties (solution viscosity and density, surfactant bulk and surface diffusion coefficient, activity, and solubility) on the appearance and characteristics of surface tension auto-oscillation that occurs by dissolution of a surfactant drop under the water-air interface is considered in the framework of a simple mathematical model, taking into account the convection driven by the Marangoni effect and convective diffusion together with adsorption/desorption processes at the air-water interface. Numerical simulations show that apart from the Marangoni and Schmidt number, the system behavior is governed also by the exchange number, which determines the surfactant exchange with the interface. The criterion for the instability onset in a system with both normal and tangential (with respect to the interface) concentration gradient, the correlation between the global and local Marangoni numbers, as well as a comparison with experiment are discussed.

Journal Article↗

PubMed2003

Experimental studies on the geometrical characteristics determining the system behavior of surface tension autooscillations.

Autooscillation of the surface tension is a phenomenon related to Marangoni instability periodically arising and fading by dissolution of a surfactant droplet under a water-air interface. A detailed experimental investigation was performed to clear up the influence of the system geometry on development and characteristics of autooscillations. It was found that the aspect ratio is an additional dimensionless parameter that determines the system behavior equally to the Marangoni number. The influence of the cell diameter, capillary immersion depth, and droplet radius on the autooscillation period and amplitude was studied as well.

Journal Article↗

PubMed2002

Theoretical description of repeated surface-tension auto-oscillations.

A mathematical model is proposed to follow the behavior of a system where a droplet of a surfactant with limited solubility on the tip of a capillary under the free liquid surface dissolves in a container with a unity aspect ratio. Numerical simulations show that instability repeatedly arises and fades in the system, resulting in auto-oscillations of the surface tension in agreement with the experimental data. The system evolution during the oscillation is discussed in detail. It is established that the presence of a boundary in the radial direction is a necessary precondition for the appearance of the second and following oscillations.

Journal Article↗

PubMed2001

Numerical study of the Marangoni instability resulting in surface tension auto-oscillations: general regularities of the system evolution.

Theoretical studies are performed to explain the mechanism of surface tension auto-oscillations recently found. The Marangoni instability in a system containing a surfactant droplet under the air-water interface is investigated numerically. The simulations, based on the equations of fluid mechanics, take into account convective diffusion and adsorption of the surfactant. The behavior of the system is determined by nonstationary concentration gradients that are nonuniform on the surface as well as in the normal to the surface direction. Initially a slow diffusion dissolution of the drop material takes place. The convective transfer of the surfactant is negligible, the surface tension remains nearly constant and the system parameters change rather slowly during the induction period. With the increase of the concentration gradients the system becomes unstable, resulting in a jump in the convection velocity, surface tension, and adsorption on the surface. The concentration and velocity distributions in the bulk and on the surface are obtained from the numerical solution of the problem. The contributions of different mechanisms of the mass transfer are compared in different stages of the process.

Journal Article↗

PubMed1999

Auto-oscillation of surface tension.

Long-time auto-oscillation of the surface tension can evolve when in an aqueous system a diethyl phthalate droplet is placed under the free water surface. The experimental conditions for development of surface tension auto-oscillations are described. Based on a theoretical analysis the mechanism of these auto-oscillations is proposed. The mechanism of the auto-oscillations results from a switching between diffusion and convection transfer of diethyl phthalate in the solution. A periodic Marangoni flow on the water surface resulting from a surface layer instability is discussed. The solubility of the amphiphile in the water and its surface activity are the main characteristics that determine the system behavior.

Journal Article↗