Biomedical subjects

Kostya Shundyak

Publications and source records attributed to Kostya Shundyak.

5 recordsLinked to original sources

PubMed2006

Theory of the isotropic-nematic transition in dispersions of compressible rods.

We theoretically study the nematic ordering transition of rods that are able to elastically adjust their mutually excluded volumes. The model rods, which consist of a hard core surrounded by a deformable shell, mimic the structure of polymer-coated, rodlike fd virus particles that have recently been the object of experimental study [K. Purdy, Phys. Rev. Lett. 94, 057801 (2005)]. We find that fluids of such soft rods exhibit an isotropic-nematic phase transition at a density higher than that of the corresponding hard-rod system of identical diameter, and that at coexistence the order parameter of the nematic phase depends nonmonotonically on the elastic properties of the polymer coating. For binary mixtures of hard and soft rods, the topology of the phase diagram turns out to depend sensitively on the elasticity of a shell. The lower nematic-nematic critical point, discovered in mixtures of bare and polymer-coated fd virus particles, is not reproduced by the theory.

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PubMed2005

Phase behavior and interfacial properties of nonadditive mixtures of Onsager rods.

Within a second virial theory, we study bulk phase diagrams as well as the free planar isotropic-nematic interface of binary mixtures of nonadditive thin and thick hard rods. For species of the same type, the excluded volume is determined only by the dimensions of the particles, whereas for dissimilar ones it is taken to be larger or smaller than that, giving rise to a nonadditivity that can be positive or negative. We argue that such a nonadditivity can result from modeling of soft interactions as effective hard-core interactions. The nonadditivity enhances or reduces the fractionation at isotropic-nematic (IN) coexistence and may induce or suppress a demixing of the high-density nematic phase into two nematic phases of different composition (N(1) and N(2)), depending on whether the nonadditivity is positive or negative. The interfacial tension between coexisting isotropic and nematic phases shows an increase with increasing fractionation at the IN interface, and complete wetting of the IN(2) interface by the N(1) phase upon approach of the triple-point coexistence. In all explored cases bulk and interfacial properties of the nonadditive mixtures exhibit a striking and quite unexpected similarity with the properties of additive mixtures of different diameter ratio.

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PubMed2004

Isotropic-nematic transition in hard-rod fluids: relation between continuous and restricted-orientation models.

We explore models of hard-rod fluids with a finite number of allowed orientations, and construct their bulk phase diagrams within Onsager's second virial theory. For a one-component fluid, we show that the discretization of the orientations leads to the existence of an artificial (almost) perfectly aligned nematic phase, which coexists with the (physical) nematic phase if the number of orientations is sufficiently large, or with the isotropic phase if the number of orientations is small. Its appearance correlates with the accuracy of sampling the nematic orientation distribution within its typical opening angle. For a binary mixture this artificial phase also exists, and a much larger number of orientations is required to shift it to such high densities that it does not interfere with the physical part of the phase diagram.

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PubMed2003

Free planar isotropic-nematic interfaces in binary hard-rod fluids.

Within the Onsager theory we study free planar isotropic-nematic interfaces in binary mixtures of hard rods. For sufficiently different particle shapes the bulk phase diagrams of these mixtures exhibit a triple point, where an isotropic (I) phase coexists with two nematic phases (N1 and N2) of different composition. For all explored mixtures we find that upon approach of the triple point the I-N2 interface shows complete wetting by an intervening N1 film. We compute the surface tensions of isotropic-nematic interfaces, and find a remarkable increase with fractionation, similar to the effect in polydisperse hard-rod fluids.

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PubMed2002

Entropy-driven triple point wetting in hard-rod mixtures.

We theoretically study binary mixtures of thin and thick hard rods with diameter ratio more extreme than 1:4. The bulk phase diagram of these systems exhibits a triple point, where an isotropic (I) phase coexists with two nematic phases ( N1 and N2) of different composition. Using density functional theory, we predict that the I-N2 interface is completely wet by N1 upon approach of the the I-N1-N2 triple point. This entropic triple point wetting should be experimentally observable in colloidal suspensions of rodlike particles.

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