306 research outputs found
Ferromagnetic phase transition in a Heisenberg fluid: Monte Carlo simulations and Fisher corrections to scaling
The magnetic phase transition in a Heisenberg fluid is studied by means of
the finite size scaling (FSS) technique. We find that even for larger systems,
considered in an ensemble with fixed density, the critical exponents show
deviations from the expected lattice values similar to those obtained
previously. This puzzle is clarified by proving the importance of the leading
correction to the scaling that appears due to Fisher renormalization with the
critical exponent equal to the absolute value of the specific heat exponent
. The appearance of such new corrections to scaling is a general
feature of systems with constraints.Comment: 12 pages, 2 figures; submitted to Phys. Rev. Let
Onset of chaotic advection in open flows
Non peer reviewedPublisher PD
Highly permeable and mechanically robust silicon carbide hollow fiber membranes
Silicon carbide (SiC) membranes have shown large potential for applications in water treatment. Being able to make these membranes in a hollow fiber geometry allows for higher surface-to-volume ratios. In this study, we present a thermal treatment procedure that is tuned to produce porous silicon carbide hollow fiber membranes with sufficient mechanical strength. Thermal treatments up to 1500 °C in either nitrogen or argon resulted in relatively strong fibers, that were still contaminated with residual carbon from the polymer binder. After treatment at a higher temperature of 1790 °C, the mechanical strength had decreased as a result of carbon removal, but after treatments at even higher temperature of 2075 °C the SiC-particles sinter together, resulting in fibers with mechanical strengths of 30â40 MPa and exceptionally high water permeabilities of 50,000 L mâ2 hâ1 barâ1. Combined with the unique chemical and thermal resistance of silicon carbide, these properties make the fibers suitable microfiltration membranes or as a membrane support for application under demanding condition
Monte Carlo study of the magnetic critical properties of the two-dimensional Ising fluid
A two-dimensional fluid of hard spheres each having a spin and
interacting via short-range Ising-like interaction is studied near the second
order phase transition from the paramagnetic gas to the ferromagnetic gas
phase. Monte Carlo simulation technique and the multiple histogram data
analysis were used. By measuring the finite-size behaviour of several different
thermodynamic quantities,we were able to locate the transition and estimate
values of various static critical exponents. The values of exponents
and are close to the ones for the two-dimensional
lattice Ising model. However, our result for the exponent is very
different from the one for the Ising universality class.Comment: 6 pages, 8 figures. To appear in Phys. Rev.
Conservation laws for invariant functionals containing compositions
The study of problems of the calculus of variations with compositions is a
quite recent subject with origin in dynamical systems governed by chaotic maps.
Available results are reduced to a generalized Euler-Lagrange equation that
contains a new term involving inverse images of the minimizing trajectories. In
this work we prove a generalization of the necessary optimality condition of
DuBois-Reymond for variational problems with compositions. With the help of the
new obtained condition, a Noether-type theorem is proved. An application of our
main result is given to a problem appearing in the chaotic setting when one
consider maps that are ergodic.Comment: Accepted for an oral presentation at the 7th IFAC Symposium on
Nonlinear Control Systems (NOLCOS 2007), to be held in Pretoria, South
Africa, 22-24 August, 200
Wetting of a symmetrical binary fluid mixture on a wall
We study the wetting behaviour of a symmetrical binary fluid below the
demixing temperature at a non-selective attractive wall. Although it demixes in
the bulk, a sufficiently thin liquid film remains mixed. On approaching
liquid/vapour coexistence, however, the thickness of the liquid film increases
and it may demix and then wet the substrate. We show that the wetting
properties are determined by an interplay of the two length scales related to
the density and the composition fluctuations. The problem is analysed within
the framework of a generic two component Ginzburg-Landau functional
(appropriate for systems with short-ranged interactions). This functional is
minimized both numerically and analytically within a piecewise parabolic
potential approximation. A number of novel surface transitions are found,
including first order demixing and prewetting, continuous demixing, a
tricritical point connecting the two regimes, or a critical end point beyond
which the prewetting line separates a strongly and a weakly demixed film. Our
results are supported by detailed Monte Carlo simulations of a symmetrical
binary Lennard-Jones fluid at an attractive wall.Comment: submitted to Phys. Rev.
On Quantum State Observability and Measurement
We consider the problem of determining the state of a quantum system given
one or more readings of the expectation value of an observable. The system is
assumed to be a finite dimensional quantum control system for which we can
influence the dynamics by generating all the unitary evolutions in a Lie group.
We investigate to what extent, by an appropriate sequence of evolutions and
measurements, we can obtain information on the initial state of the system. We
present a system theoretic viewpoint of this problem in that we study the {\it
observability} of the system. In this context, we characterize the equivalence
classes of indistinguishable states and propose algorithms for state
identification
Nonequilibrium molecular dynamics simulation of rapid directional solidification
We present the results of non-equilibrium molecular dynamics simulations for
the growth of a solid binary alloy from its liquid phase. The regime of high
pulling velocities, , for which there is a progressive transition from
solute segregation to solute trapping, is considered. In the segregation
regime, we recover the exponential form of the concentration profile within the
liquid phase. Solute trapping is shown to settle in progressively as is
increased and our results are in good agreement with the theoretical
predictions of Aziz [J. Appl. Phys. {\bf 53}, 1158 (1981)]. In addition, the
fluid advection velocity is shown to remain directly proportional to , even
at the highest velocities considered here (ms).Comment: Submitted to Phys. Rev.
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