C. Ebner

Molecular-dynamics simulation of compressible fluid flow in two-dimensional channels

We study compressible fluid flow in narrow two-dimensional channels using a molecular-dynamics simulation method. In the simulation area, an upstream source is maintained at constant density and temperature while a downstream reservoir is kept at vacuum. The channel is sufficiently long in the direction of the flow that the finite length has little effect on the properties of the fluid in the central region. The simulated system is represented by an efficient data structure, whose internal elements are created and manipulated dynamically in a layered fashion. Consequently the computer code is highly efficient and manifests completely linear performance in simulations of large systems. We obtain the steady-state velocity, temperature, and density distributions in the system. The velocity distribution across the channel is very nearly a quadratic function of the distance from the center of the channel and reveals velocity slip at the boundaries; the temperature distribution is only approximately a quartic function of this distance from the center to the channel. The density distribution across the channel is nonuniform. We attribute this nonuniformity to the relatively high Mach number, approximately 0.5, in the fluid flow. An equation for the density distribution based on simple compressibility arguments is proposed; its predictions agree well with the simulation results. The validity of the concept of local dynamic temperature and the variation of the temperature along the channel are discussed.

Micro-emulsions in oil-water-surfactant mixtures: an Ising lattice gas model

An Ising lattice gas model is constructed for oil-water-surfactant mixtures. The phase diagram of this model is obtained by using mean-field theory and Monte Carlo simulations. The paramagnetic phase displays microstructures similar to those found in laboratory micro-emulsions. Also oil-rich, water-rich, and micro-emulsion phases coexist in certain regions and long-lived, metastable phases are common at low temperatures. Low-period, modulated phases are found. The experimental relevance of these phases and the limitations of the authors model are discussed.

The glass state in weak-coupled superconductors

Abstract We review the work of the Ohio State group on the equilibrium and non-equilibrium properties of disordered granular superconductors in a magnetic field, pointing out the connections between the behavior of these materials and that of more conventional spin glasses, and considering briefly the possible connections to the behavior of high-Tc materials in a magnetic field.

Critical micelle concentration from a lattice gas model

The temperature dependence of the critical micelle concentration (CMC) and a closed-loop coexistence curve are obtained, via Monte Carlo simulations, in the water surfactant limit of a two-dimensional version of a statistical mechanical model for micro-emulsions, The CMC and the coexistence curve reproduce various experimental trends as functions of the couplings. In the oil-surfactant limit, there is a conventional coexistence cure with an upper consolute point that allows for a region of three-phase coexistence between oil-rich, water-rich and microemulsion phases.

Fluctuation specific heat and vortex configurations in model high-temperature superconductors

Abstract We calculate the fluctuation specific heat and diamagnetic susceptibility of a single layer of model high-temperature superconductor with parameters resembling YBa 2 Cu 3 O 7−δ , as a function of perpendicular magnetic field, assuming an s-wave order parameter and fluctuations about a Ginzburg-Landau mean-field theory. Snapshots of the vortex configurations in a perpendicular field are also shown.