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Biomedical subjects

A A Golovin

Publications and source records attributed to A A Golovin.

At least 19 recordsLinked to original sources
PubMed2006

Instability of a two-layer thin liquid film with surfactants: Dewetting waves.

Dewetting dynamics of a liquid film composed of two superposed ultra-thin layers of immiscible liquids resting on a solid substrate is investigated in the case when surfactants are present at the liquid-liquid interface. Two cases are considered: insoluble surfactant and surfactant soluble in the lower liquid. The dependence of Hamaker constants on the surfactant concentration is taken into account. A system of three strongly nonlinear evolution equations describing large (comparable to the layer thicknesses), long-wave perturbations of the liquid-liquid and liquid-gas interfaces, as well as the surfactant concentration, is derived for each case in the lubrication approximation. The linear stability analysis shows that in the presence of surfactants, oscillatory dewetting instability can occur. Numerical simulations of this system of nonlinear evolution equations are performed. It is found that in the case of oscillatory instabilities, the system exhibits either standing or traveling "dewetting waves." The weakly nonlinear analysis explains this phenomenon.

Journal Article↗
PubMed2006

Formation of porous metal oxides in the anodization process.

A theory of the formation of nanoscale porous structures in oxides of metals grown by anodization is developed. It is shown that a growing oxide layer can become unstable which yields the formation of a spatially irregular array of pores. The instability is shown to result from a nonlinear dependence of electrochemical kinetics at the metal-oxide and oxide-electrolyte interfaces on the overpotential which is governed by the Butler-Volmer relation. The conditions for the instability of the oxide layer are found. The dependence of the oxide conductivity on the electric field is taken into account and is shown to have a destabilizing effect. A weakly nonlinear analysis is performed and it shows that the system evolution near the instability threshold is described by the Kuramoto-Sivashinsky equation. Farther from threshold, in the long-wave approximation, a system of strongly nonlinear equations is derived and solved numerically; this system describes the formation of deep irregular pores. In a particular case, a self-similar solution describing the propagation of a pore with a paraboloidal shape is found.

Journal Article↗
PubMed2006

Nucleation and growth of droplets at a liquid-gas interface.

The nucleation of liquid droplets at a liquid-gas interface from a saturated vapor in the gas phase, as well as the droplet growth after the nucleation are studied. These two processes determine the formation of a regular hexagonal array of drops on the surface of an evaporating film of polymer solution that is used for the fabrication of polymer membranes with a regular microporous structure. The free-energy barrier for the nucleation of a droplet at a liquid-gas interface is found as a function of the droplet radius and the contact angles, and the critical nucleation radius is computed. It is shown that the heterogeneous nucleation is thermodynamically more preferable than the homogeneous one. The role of the line tension between the phases is also estimated. Further growth of a droplet nucleated at the liquid-gas interface is studied. Two growth mechanisms are considered: by the vapor diffusion flux from the gas phase and by the surface diffusion of the vapor molecules adsorbed at the liquid-gas interface outside the droplet. Two cases, corresponding to unsaturated and saturated condensation, are considered. The droplet growth is described by a free-boundary problem which is solved analytically and numerically. The droplet growth exponents at different stages of growth are found.

Journal Article↗
PubMed2006

Feedback control of subcritical oscillatory instabilities.

Feedback control of a subcritical oscillatory instability is investigated in the framework of a globally-controlled complex Ginzburg-Landau equation that describes the nonlinear dynamics near the instability threshold. The control is based on a feedback loop between the system linear growth rate and the maximum of the amplitude of the emerging pattern. It is shown that such control can suppress the blow up and result in the formation of spatially localized pulses similar to oscillons. In the one-dimensional case, depending on the values of the linear and nonlinear dispersion coefficients, several types of the pulse dynamics are possible in which the computational domain contains: (i) a single stationary pulse; (ii) several coexisting stationary pulses; (iii) competing pulses that appear one after another at random locations so that at each moment of time there is only one pulse in the domain; (iv) spatiotemporally chaotic system of short pulses; (v) spatially-synchronized pulses. Similar dynamic behavior is found also in the two-dimensional case. The effect of the feedback delay is also studied. It is shown that the increase of the delay leads to an oscillatory instability of the pulses and the formation of pulses with oscillating amplitude.

Journal Article↗
PubMed2005

Nonlinear stability analysis of a two-layer thin liquid film: dewetting and autophobic behavior.

The nonlinear stability analysis of a liquid film composed of two superposed thin layers of immiscible liquids resting on a solid substrate is performed. It is shown that the coupling of van der Waals interactions in the two layers can lead to an autophobic behavior in the form of spinodal decomposition of two planar liquid layers into a system of localized drops divided by almost planar wetting layers. The results of the weakly nonlinear analysis near the instability threshold are confirmed by the numerical solution of a system of two strongly nonlinear evolution equations for the liquid-liquid and liquid-gas interfaces. The kinetics of the drop coarsening at late stages is studied and is found to be close to that reported for a one-layer film. It is also shown that gravity effects can become significant even for very thin two-layer films.

Journal Article↗
PubMed2004

Dynamic phase separation: from coarsening to turbulence via structure formation.

We investigate some new two-dimensional evolution models belonging to the class of convective Cahn-Hilliard models: (i) a local model with a scalar order parameter, (ii) a nonlocal model with a scalar order parameter, and (iii) a model with a vector order parameter. These models are applicable to phase-separating system where concentration gradients cause hydrodynamic motion due to buoyancy or Marangoni effect. The numerical study of the models shows transition from coarsening, typical of Cahn-Hilliard systems, to spatiotemporally irregular behavior (turbulence), typical of the Kuramoto-Sivashinsky equation, which is obtained in the limit of very strong driving. The transition occurs not in a straightforward way, but through the formation of spatial patterns that emerge for intermediate values of the driving intensity. As in driven one-dimensional models studied before, the mere presence of the driving force, however small, breaks the symmetry between the two separating phases, as well as increases the coarsening rate. With increasing driving, coarsening stops. The dynamics is generally irregular at strong driving, but exhibits specific structural features.

Algorithms↗
PubMed2003

Self-organization of quantum dots in epitaxially strained solid films.

A nonlinear evolution equation for surface-diffusion-driven Asaro-Tiller-Grinfeld instability of an epitaxially strained thin solid film on a solid substrate is derived in the case where the film wets the substrate. It is found that the presence of a weak wetting interaction between the film and the substrate can substantially retard the instability and modify its spectrum in such a way that the formation of spatially regular arrays of islands or pits on the film surface becomes possible. It is shown that the self-organization dynamics is significantly affected by the presence of the Goldstone mode associated with the conservation of mass.

Journal Article↗
PubMed2003

Disclinations in square and hexagonal patterns.

We report the observation of defects with fractional topological charges (disclinations) in square and hexagonal patterns as numerical solutions of several generic equations describing many pattern-forming systems: Swift-Hohenberg equation, damped Kuramoto-Sivashinsky equation, as well as nonlinear evolution equations describing large-scale Rayleigh-Benard and Marangoni convection in systems with thermally nearly insulated boundaries. It is found that disclinations in square and hexagonal patterns can be stable when nucleated from special initial conditions. The structure of the disclinations is analyzed by means of generalized Cross-Newell equations.

Journal Article↗
PubMed2003

Faceting of a growing crystal surface by surface diffusion.

Consider faceting of a crystal surface caused by strongly anisotropic surface tension, driven by surface diffusion and accompanied by deposition (etching) due to fluxes normal to the surface. Nonlinear evolution equations describing the faceting of 1+1 and 2+1 crystal surfaces are studied analytically, by means of matched asymptotic expansions for small growth rates, and numerically otherwise. Stationary shapes and dynamics of faceted pyramidal structures are found as functions of the growth rate. In the 1+1 case it is shown that a solitary hill as well as periodic hill-and-valley solutions are unique, while solutions in the form of a solitary valley form a one-parameter family. It is found that with the increase of the growth rate, the faceting dynamics exhibits transitions from the power-law coarsening to the formation of pyramidal structures with a fixed average size and finally to spatiotemporally chaotic surfaces resembling the kinetic roughening.

Journal Article↗
PubMed2002

Fingering instability of thin evaporating liquid films.

The fingering instability of growing dry patches in an evaporating film of a polar liquid placed on a solid substrate is investigated. The instability manifests itself as fingering of mobile fronts between growing "dry" (thin) and shrinking "wet" (thick) regions of the film corresponding to two stable states of the evaporating film in contact with its vapor. The boundaries of the fingering instability are found through linear stability analysis of numerical solutions of the nonlinear evolution equation defining the film profile, and the influence of the evaporation rate, polar intermolecular forces, and chemical heterogeneity of the substrate is investigated.

Journal Article↗
PubMed2001

Convective Cahn-Hilliard models: from coarsening to roughening.

In this paper we demonstrate that convective Cahn-Hilliard models, describing phase separation of driven systems (e.g., faceting of growing thermodynamically unstable crystal surfaces), exhibit, with the increase of the driving force, a transition from the usual coarsening regime to a chaotic behavior without coarsening via a pattern-forming state characterized by the formation of various stationary and traveling periodic structures as well as structures with localized oscillations. Relation of this phenomenon to a kinetic roughening of thermodynamically unstable surfaces is discussed.

Journal Article↗
PubMed1993

[The involvement of the components of the immune system in iron deficiency].

As exemplified by iron deficiency anemia, the study was made of effect of iron deficiency on interdependence of immunity parameters. It is noted that patients with definite forms of the anemia showed less numerous correlations than in controls. The emergence of complications increased the number of persistent correlation pleiad compared to controls. Moreover, correlation and variance analyses suggested a correlation between immunity and iron deficiency parameters.

Adolescent↗
PubMed1992

[Psychosomatic disorders in patients with neurocirculatory dystonia and their correction in the day hospital].

Clinical and psychological tests were used to examine 77, 31 and 32 patients with normotonic, hypertonic and hypotonic neurocirculatory asthenia, respectively. Compared to control subjects, 80% of the patients responded to stress with psychic and social dysadaptation (psychic asthenia, frustration, emotional lability, hypothymism) as well as depression with visceral manifestations: cardialgia, extrasystole, lability of arterial pressure. Chemotherapy, physical and psychological treatment in day hospital produced a response in 89.2% of neurocirculatory asthenia sufferers.

Adult↗
PubMed1991

[Lipid peroxidation in patients with iron deficiency anemia complicated by frequent acute respiratory diseases].

Lipid peroxidation (diene conjugates and lipofuscinoid pigments) and activity of antiperoxidation defense enzymes (catalase, glutathione reductase, glucose 6-phosphate dehydrogenase) were studied in 55 patients with iron deficiency anemia. It was found that in uneventful iron deficiency anemia the above parameters change insignificantly in contrast with anemic patients prone to acute respiratory infections. The latter demonstrate enhancement of lipid peroxidation in spite of high activity of glutathione reductase and glucose 6-phosphate dehydrogenase.

Acute Disease↗
PubMed1989

[Importance of leukograms for the evaluation of nonspecific body resistance in patients with iron-deficiency anemia and prognosis of the course of the disease].

The possibility of stage evaluation of body resistance in patients with iron-deficiency anemia (IDA) was studied by isolation of adaptation reactions basing on the leukogram analysis. The results of the studies have permitted suggestion that changes in leukograms of IDA patients can be used for evaluation of non-specific body resistance. A detailed analysis of the leucocytic formula enables prognosing the course of the disease and can be used as an additional criterion of the evaluation of the conducted therapy effectiveness.

Adolescent↗
PubMed1989

[Metabolic and immunologic disorders in the pathogenesis of iron deficiency anemia and its complications].

Sixty-five patients with iron deficiency anemia have been examined for lipid peroxidation, antiperoxidation protection and immunoreactivity. It was noted that progression of the disease was associated with developing hypoxia followed by enhancement of lipid peroxidation and derangement of immunity. Origin of complications may be related to abnormal lipid peroxidation and defects in immunity.

Adolescent↗