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Biomedical subjects

A D Nikolov

Publications and source records attributed to A D Nikolov.

4 recordsLinked to original sources
PubMed2005

Structural transitions in two-dimensional hard-sphere systems.

We spread randomly noncharged steel particles (diameter, 1.59 mm) on a silicon wafer to form a two-dimensional hard-sphere system. The particle structure versus the particle coverage was monitored. We observed the particle structural transition from liquidlike to triangular-lattice crystal-like with increasing particle coverage by analyzing the particle structure factor. The particle coverage at which the structural transition occurs was quantified by the curves of S(max) (A) and G6 (A); S(max) is the amplitude of the first peak of the structure factor (depicting the particle positional order), and G6 is the bond orientation order parameter. We also conducted a Monte Carlo simulation study. The Monte Carlo simulation results show good agreement with the experimental results at low particle area fractions. However, at high area fractions, the experimentally observed particle structure is less organized than that generated by simulations.

Journal Article↗
PubMed2004

Texture and stability of emulsions and suspensions: role of oscillatory structural forces.

The stability of macro-dispersions, such as emulsions and particle suspensions, is characterized in different ways-creaming or sedimentation, flocculation of drops/particles, coalescence between drops or phase separation. Several novel experimental techniques have been developed in our laboratory to examine both the texture and stability of emulsions and suspensions. These methods include direct image analysis to extract the emulsion radial distribution function and to determine the effective inter-droplet interaction, and the Kossel diffraction technique, which is used to obtain the structural factors. The film thinning interferometric technique employing our capillary force balance is used to study the role of the surfactant micelles/colloidal particle-layering phenomenon and the in-layer structure formation. Monte Carlo simulations and a theoretical model based on the Ornstein-Zernike equation of statistical mechanics are used to discern the effects of the micelle/particle structuring and layering phenomenon in confined films between two droplets/particles. These experiments and theoretical calculations are used to gain a fundamental understanding of the role of long-range oscillatory (repulsion/attraction) structural interactions on the stability of both mono- and polydispersed systems. During the past 10 years, our research group has worked on several problems of interest to industry in which structural forces in emulsions and suspensions appear to play an important role. This paper is an overview of some of these relevant examples.

Journal Article↗
PubMed2001

Foaming in simulated radioactive waste.

Radioactive waste treatment process usually involves concentration of radionuclides before waste can be immobilized by storing it in stable solid form. Foaming is observed at various stages of waste processing like SRAT (sludge receipt and adjustment tank) and melter operations. This kind of foaming greatly limits the process efficiency. The foam encountered can be characterized as a three-phase foam that incorporates finely divided solids (colloidal particles). The solid particles stabilize foaminess in two ways: by adsorption of biphilic particles at the surfaces of foam lamella and by layering of particles trapped inside the foam lamella. During bubble generation and rise, solid particles organize themselves into a layered structure due to confinement inside the foam lamella, and this structure provides a barrier against the coalescence of the bubbles, thereby causing foaming. Our novel capillary force balance apparatus was used to examine the particle-particle interactions, which affect particle layer formation in the foam lamella. Moreover, foaminess shows a maximum with increasing solid particle concentration. To explain the maximum in foaminess, a study was carried out on the simulated sludge, a non-radioactive simulant of the radioactive waste sludge at SRS, to identify the parameters that affect the foaming in a system characterized by the absence of surface-active agents. This three-phase foam does not show any foam stability unlike surfactant-stabilized foam. The parameters investigated were solid particle concentration, heating flux, and electrolyte concentration. The maximum in foaminess was found to be a net result of two countereffects that arise due to particle-particle interactions: structural stabilization and depletion destabilization. It was found that higher electrolyte concentration causes a reduction in foaminess and leads to a smaller bubble size. Higher heating fluxes lead to greater foaminess due to an increased rate of foam lamella generation in the sludge system.

Chemical Phenomena↗