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A Monte Carlo study of fluids with orientational degrees of freedom

University of British Columbia

We have modeled an electrorheological (ER) fluid as hard-sphere particles, each with smaller hard-sphere ions constrained to roll on the sphere's inside surface. When the model E R fluid is placed in an electric field, each particle becomes polarized (due to rearrangement of the ions confined within the particle) with a dipole moment depending on both the field and interactions with its neighbors. Using NVT Monte Carlo simulations, we have shown that our model can display chain formation as seen in real ER fluids. Chaining occurred at a field where the net moment no longer varied linearly with the field. This model was extended to include particles shaped as ellipsoids of revolution. In the prolate case, slightly non-spherical particles were readily ordered by the field. In the oblate case, the induced dipole is roughly perpendicular to the symmetry axis. Oblate particles may then form a biaxial phase in an applied field. NPT and Gibbs ensemble Monte Carlo simulations were performed for spherical particles modified by an anisotropic potential of the form —4A£(<r/r)6P2(cos7). We have investigated both a soft-core model of Lennard-Jones particles and a hard-core model of hard spheres. For the soft-core model at anisotropy parameter A = 0.3, we have constructed the isotropic-nematic (IN) coexistence curve using Gibbs ensemble Monte Carlo simulations (the Gibbs method). For IN coexistence, we modified the Gibbs method so that particle exchanges were rotationally biased. For the hard-core model, we have determined the isotropic fluid-nematic coexistence curve using the Gibbs method. No gas-isotropic liquid transition was found for this hard-core model. Gibbs ensemble Monte Carlo simulations were performed for a mixture of neutral and dipolar hard spheres. At fixed pressure, the coexistence curve for the demixing transition was constructed for several values of the diameter of the neutral hard spheres. We extrapolate the critical point temperature to a vanishing diameter for the neutral hard spheres. In the limit of vanishing neutral hard spheres, the demixing transition of the mixture resembles, if it exists, the gas-isotropic liquid transition for dipolar hard spheres. Our extrapolation suggests that the gas-isotropic liquid transition would occur at a very low temperature.

Thesis/Dissertation

application/pdf

2009-03-19

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http://hdl.handle.net/2429/6260

Chemistry

University of British Columbia

UBCV

DSpace