Szabolcs Varga

Bio: Szabolcs Varga is an academic researcher from University of Pannonia. The author has contributed to research in topics: Liquid crystal & Phase transition. The author has an hindex of 21, co-authored 76 publications receiving 1254 citations. Previous affiliations of Szabolcs Varga include Autonomous University of Madrid & Brigham Young University.

Nematic phase transitions in mixtures of thin and thick colloidal rods.

Abstract: We report experimental measurements of the phase behavior of mixtures of thin (charged semiflexible fd virus) and thick (fd-PEG, fd virus covalently coated with polyethylene glycol) rods with diameter ratio varying from 3.7 to 1.1. The phase diagrams of the rod mixtures reveal isotropic-nematic, isotropicnematic-nematic, and nematic-nematic coexisting phases with increasing concentration. In stark contrast to predictions from earlier theoretical work, we observe a nematic-nematic coexistence region bound by a lower critical point. Moreover, we show that a rescaled Onsager-type theory for binary hard-rod mixtures qualitatively describes the observed phase behavior.

Study of the demixing transition in model athermal mixtures of colloids and flexible self-excluding polymers using the thermodynamic perturbation theory of Wertheim

Abstract: Fluid phase separation in model athermal mixtures of colloids and polymers is examined by means of the first-order thermodynamic perturbation theory of Wertheim [M. S. Wertheim, J. Chem. Phys. 87, 7323 (1987); W. G. Chapman, G. Jackson, and K. E. Gubbins, Mol. Phys. 65, 1057 (1988)]. The colloidal particles are modeled simply as hard spheres, while the polymers are represented as chains formed from tangent hard-sphere segments. In this study the like (colloid–colloid, polymer–polymer) and unlike (polymer–colloid) repulsive interactions are treated at the same level of microscopic detail; we do not employ the common Asakura–Oosawa (AO) approximations which essentially involve treating the polymer as an ideal (noninteracting) chain. The effect of varying both the chain length and the diameter of the hard-sphere segments of the polymer on the fluid phase behavior of the model polymer–colloid system is investigated. We focus our attention on the stability of the fluid phase relative to a demixing transition into colloid-rich and polymer-rich fluid phases by using a spinodal instability analysis and determine the full coexistence boundaries (binodal). The colloid–polymer system represents the limit where the diameter of the colloid is much larger than the diameter of the segments making up the polymer chain. The precise segment/colloid diameter ratio at which liquid–liquid demixing first occurs is examined in detail as a function of the chain length of the polymer. In the case of moderately short chains the addition of polymer induces the “colloidal vapor–liquid” transition found in polymer–colloid systems, while for long chains a “polymeric vapor–liquid” transition is found. The diameter of the polymeric segments must lie between the AO limit (minimum diameter) and the so-called protein limit (maximum diameter) in order for the system to exhibit fluid–fluid phase separation. The maximum value of the segment diameter which induces phase separation is determined from a simple approximate stability analysis. The critical density of the demixing transitions is not found to tend to be zero for infinitely long polymers, but has a limiting value which depends on the diameter of the segment. An examination of the thermodynamic properties of mixing indicates that the fluid–fluid phase separation in such systems is driven by a large positive enthalpy of mixing which is induced by a large positive volume of mixing due to the unfavorable polymer–colloid excluded volume interactions. The enthalpy of mixing makes an unfavorable contribution to the overall Gibbs free energy (which is seen to counter the favorable entropy of mixing), giving rise to fluid–fluid immiscibility.

Generalized equation of state for square-well potentials of variable range

Abstract: An equation of state for square-well fluids of short and long potential range λ is presented and compared with Gibbs ensemble and canonical Monte Carlo simulation data; vapour–liquid coexistence densities, vapour pressures, internal energies and contact radial distribution functions are examined. The equation is an extension of that presented in previous work for the reference monomer fluid in the SAFT-VR approach [GIL-VILLEGAS et al., 1997, J. chem. Phys., 106, 4168]. The Helmholtz free energy is written as a high-temperature expansion up to second order, where simple expressions are obtained for the mean attractive energy and the fluctuation term using the mean-value theorem and a mapping of radial distribution functions. In previous work the range of the square-well potential was limited to λ ≤ 1.8. In this work we show that the phase behaviour of such a fluid is far from the expected long-range limits given by the mean-field and van der Waals approximations. We extend the applicability of the equation...

The phase behavior of a binary mixture of rodlike and disclike mesogens: Monte Carlo simulation, theory, and experiment

Abstract: The phase behavior of a binary mixture of rodlike and disclike hard molecules is studied using Monte Carlo NVT (constant number of particles N, volume V, and temperature T) computer simulation. The rods are modeled as hard spherocylinders of aspect ratio LHSC/DHSC=5, and the discs as hard cut spheres of aspect ratio LCS/DCS=0.12. The diameter ratio DCS/DHSC=3.62 is chosen such that the molecular volumes of the two particles are equal. The starting configuration in the simulations is a mixed isotropic state. The phase diagram is mapped by changing the overall density of the system. At low densities stabilization of the isotropic phase relative to the ordered states is seen on mixing, and at high densities nematic–columnar and smectic A–columnar phase coexistence is observed. Biaxiality in the nematic phase is not seen. The phase diagram of the mixture is also calculated using the second virial theory of Onsager for nematic ordering, together with the scaling of Parsons and Lee to take into account the high...

Nematic-nematic phase separation in binary mixtures of thick and thin hard rods: Results from Onsager-like theories

Abstract: The fundamental nature of the nematic-nematic N-N phase separation in binary mixtures of rigid hard rods is analyzed within the Onsager second-virial theory and the extension of Parsons and Lee which includes a treatment of the higher-body contributions. The particles of each component are modeled as hard spherocylinders of different diameter D1D2, but equal length L1 =L2 =L. In the case of a system which is restricted to be fully aligned parallel rods, we provide an analytical solution for the spinodal boundary for the limit of stability of N-N demixing; only a single region of N-N coexistence bounded at lower pressures densities by a N-N critical point is possible for such a system. The full numerical solution with the Parsons-Lee extension also indicates that, depending on the length of the particles, there is a range of values of the diameter ratio d =D2 /D1 where the N-N phase coexistence is closed off by a critical point at lower pressure. A second region of N-N coexistence can be found at even lower pressures for certain values of the parameters; this region is bounded by an “upper” N-N critical point. The two N-N coexistence regions can also merge to give a single region of N-N coexistence extending to very high pressure without a critical point. By including the higher-order contributions to the excluded volume end effects in the Onsager theory, we prove analytically that the existence of the N-N lower critical point is a direct consequence of the finite size of the particles. A new analytical equation of state is derived for the nematic phase using the Gaussian approximation. In the case of Onsager limit infinite aspect ratio, we show that the N-N phase behavior obtained using the Parsons-Lee approach substantially deviates from that with the Onsager theory for the N-N transition due to the nonvanishing third and higher order virial coefficients. We also provide a detailed discussion of the N-N phase behavior of recent experimental results for mixtures of thin and thick rods of the same length, for which the Onsager and Parsons-Lee theories can provide a qualitative description.

Proceedings of the National Academy of Sciences

Abstract: where A represents the magnetic vector potential, is an integral of the hydromagnetic equations. This -integral made it possible to formulate a variational principle for the force-free magnetic fields. The integral expresses the fact that motions cannot transform a given field in an entirely arbitrary different field, if the conductivity of the medium isconsidered infinite. In this paper we shall show that the full set of hydromagnetic equations admit five more integrals, besides the energy integral, if dissipative processes are absent. These integrals, as we shall presently verify, are I2 =fbHvdV, (2)

Theory Of Simple Liquids

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Fluid phase equilibria

Abstract: ING AND INDEXING