Biomedical subjects

G N Patey

Publications and source records attributed to G N Patey.

6 recordsLinked to original sources

PubMed2004

Constant-volume heat capacity in a near-critical fluid from Monte Carlo simulations.

We consider a near-critical fluid of hard spheres with short-range interactions (approximately r(-6)) and obtain its constant-volume heat capacity C(V) by means of Monte Carlo calculations in the canonical ensemble. The question addressed is whether or not the heat capacities of the finite-size systems studied in simulations can provide a reliable indication of nonclassical criticality. For the model fluid considered here this is found to be the case. The heat capacity along the critical isochore shows a peak near the critical temperature, with a system size dependence that is consistent with the known Ising universality class of the model. The relevance of our results to recent attempts to determine the universality class of ionic fluids through calculations of C(V) is briefly discussed.

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PubMed2004

Shearing of nanoscopic bridges in two-component thin liquid layers between chemically patterned walls.

Bridge phases associated with a phase transition between two liquid phases occur when a two-component liquid mixture is confined between chemically patterned walls. In the bulk the liquid mixture with components A, B undergoes phase separation into an A-rich phase and a B-rich phase. The walls bear stripes attractive to A. In the bridge phase A-rich and B-rich regions alternate. Grand canonical Monte Carlo studies are performed with the alignment between stripes on opposite walls varied. Misalignment of the stripes places the nanoscopic liquid bridges under shear strain. The bridges exert a Hookean restoring force on the walls for small displacements from equilibrium. As the strain increases there are deviations from Hooke's law. Eventually there is an abrupt yielding of the bridges. Molecular dynamics simulations show the bridges form or disintegrate on time scales which are fast compared to wall motion and transport of molecules into or from the confined space. Some interesting possible applications of the phenomena are discussed.

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PubMed2004

Surface-induced ordering of nematics in an external field: the strong influence of tilted walls.

Microscopic theory is used to investigate surface-induced order in a model nematic subjected to an external orienting field. The wall-particle interaction tends to orient particles perpendicular to the surface. It is shown that if the wall is tilted at approximately 45 degrees to the field, the reorientational effects can be an order of magnitude larger than those observed for perpendicular or parallel orientations. The surprising observation is associated with the breaking of a particular bulk symmetry. A possible practical application of the tilted geometry is briefly discussed.

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PubMed1999

Ion association in model ionic fluids.

Monte Carlo simulations are used to investigate the temperature and pressure (density) dependence of ion association in the restricted primitive model. It is shown that at temperatures below the critical temperature T(c) the vapor consists almost exclusively of strongly bound ion pairs at or near contact. Significant ion-pair dissociation begins at temperatures very near T(c). This raises the possibility that compositional fluctuations between strongly bound and free ions influence the critical behavior. We note qualitative similarities between the present results and the Kosterlitz-Thouless transition in the two-dimensional Coulomb gas.

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PubMed1999

Orientational order in model dipolar fluids.

Fluids of hard spheres each carrying two parallel point dipoles have been investigated using constant-volume Monte Carlo computer simulations. The results show that both ferroelectric and antiferroelectric fluid phases can be stabilized at high density and low temperature by dipolar interactions alone, if the separation between the dipoles on each sphere is sufficiently large. A simple lattice calculation provides some insight into the balance between dipole energy and orientational entropy which governs the polarization state.

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PubMed1999

Forces between like-charged plates in electrolyte solution: ion-solvent packing versus electrostatic effects.

The anisotropic hypernetted-chain approximation is solved numerically for mixtures of neutral hard-sphere solvent particles and divalent counterions between charged plates. A detailed analysis of the different components of force acting between the plates is given. At separations of a few solvent diameters, it is shown that even at relatively high surface charge and moderate solvent density, the ionic contribution to the force tends to be dominated by the hard-core or packing component. If the ions and solvent particles are of equal size, then the net pressure between the plates can be reasonably well approximated by adding the pressures of pure one-component ionic and solvent systems. However, if the ion and solvent diameters are significantly different the pressure curve is more complex, and the simple superposition of the ionic and solvent pressures no longer works. For this case, we show that to a good approximation it is still possible to divide the pressure into electrostatic and hard-core components, but now the appropriate hard-core system must itself be a mixture of neutral hard spheres.

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