548 research outputs found
Magnetic traveling-stripe-forcing: enhanced transport in the advent of the Rosensweig instability
A new kind of contactless pumping mechanism is realized in a layer of
ferrofluid via a spatio-temporally modulated magnetic field. The resulting
pressure gradient leads to a liquid ramp, which is measured by means of X-rays.
The transport mechanism works best if a resonance of the surface waves with the
driving is achieved. The behavior can be understood semi-quantitatively by
considering the magnetically influenced dispersion relation of the fluid.Comment: 6 Pages, 8 Figure
Impact of noise on domain growth in electroconvection
The growth and ordering of striped domains has recently received renewed
attention due in part to experimental studies in diblock copolymers and
electroconvection. One surprising result has been the relative slow dynamics
associated with the growth of striped domains. One potential source of the slow
dynamics is the pinning of defects in the periodic potential of the stripes. Of
interest is whether or not external noise will have a significant impact on the
domain ordering, perhaps by reducing the pinning and increasing the rate of
ordering. In contrast, we present experiments using electroconvection in which
we show that a particular type of external noise decreases the rate of domain
ordering
Direct observation of twist mode in electroconvection in I52
I report on the direct observation of a uniform twist mode of the director
field in electroconvection in I52. Recent theoretical work suggests that such a
uniform twist mode of the director field is responsible for a number of
secondary bifurcations in both electroconvection and thermal convection in
nematics. I show here evidence that the proposed mechanisms are consistent with
being the source of the previously reported SO2 state of electroconvection in
I52. The same mechanisms also contribute to a tertiary Hopf bifurcation that I
observe in electroconvection in I52. There are quantitative differences between
the experiment and calculations that only include the twist mode. These
differences suggest that a complete description must include effects described
by the weak-electrolyte model of electroconvection
Fluid pumped by magnetic stress
A magnetic field rotating on the free surface of a ferrofluid layer is shown
to induce considerable fluid motion toward the direction the field is rolling.
The measured flow velocity i) increases with the square of the magnetic field
amplitude, ii) is proportional to the thickness of the fluid layer, and iii)
has a maximum at a driving frequency of about 3 kHz. The pumping speed can be
estimated with a two-dimensional flow model.Comment: 3 pages, 4 figure
Optimal Control of the Thermistor Problem in Three Spatial Dimensions
This paper is concerned with the state-constrained optimal control of the
three-dimensional thermistor problem, a fully quasilinear coupled system of a
parabolic and elliptic PDE with mixed boundary conditions. This system models
the heating of a conducting material by means of direct current. Local
existence, uniqueness and continuity for the state system are derived by
employing maximal parabolic regularity in the fundamental theorem of Pr\"uss.
Global solutions are addressed, which includes analysis of the linearized state
system via maximal parabolic regularity, and existence of optimal controls is
shown if the temperature gradient is under control. The adjoint system
involving measures is investigated using a duality argument. These results
allow to derive first-order necessary conditions for the optimal control
problem in form of a qualified optimality system. The theoretical findings are
illustrated by numerical results
Strangeness Production Incorporating Chiral Symmetry
Chiral symmetry is known to be decisive for an understanding of the low
energy sector of strong interactions. It is thus important for a model of
relativistic heavy ion collisions to incorporate the dynamical breaking and
restoration of chiral symmetry. Thus we study an expansion scenario for a
quark-meson plasma using the Nambu--Jona-Lasinio (NJL) model in its three
flavor version. The equations of motion for light and strange quarks as well as
for pions, kaons and etas are solved using a QMD type algorithm, which is based
on a parametrization of the Wigner function. The scattering processes
incorporated into this calculation are of the types qq qq, q\bar q
q\bar q, q\bar q MM and M q\bar q.Comment: Talk presented at the 4th International Symposium on Strangeness in
Quark Matter, Padova, Italy, July 20--2
Energetics of positron states trapped at vacancies in solids
We report a computational first-principles study of positron trapping at
vacancy defects in metals and semiconductors. The main emphasis is on the
energetics of the trapping process including the interplay between the positron
state and the defect's ionic structure and on the ensuing annihilation
characteristics of the trapped state. For vacancies in covalent semiconductors
the ion relaxation is a crucial part of the positron trapping process enabling
the localization of the positron state. However, positron trapping does not
strongly affect the characteristic features of the electronic structure, e.g.,
the ionization levels change only moderately. Also in the case of metal
vacancies the positron-induced ion relaxation has a noticeable effect on the
calculated positron lifetime and momentum distribution of annihilating
electron-positron pairs.Comment: Submitted to Physical Review B on 17 April 2007. Revised version
submitted on 6 July 200
Modeling the momentum distributions of annihilating electron-positron pairs in solids
Measuring the Doppler broadening of the positron annihilation radiation or
the angular correlation between the two annihilation gamma quanta reflects the
momentum distribution of electrons seen by positrons in the
material.Vacancy-type defects in solids localize positrons and the measured
spectra are sensitive to the detailed chemical and geometric environments of
the defects. However, the measured information is indirect and when using it in
defect identification comparisons with theoretically predicted spectra is
indispensable. In this article we present a computational scheme for
calculating momentum distributions of electron-positron pairs annihilating in
solids. Valence electron states and their interaction with ion cores are
described using the all-electron projector augmented-wave method, and atomic
orbitals are used to describe the core states. We apply our numerical scheme to
selected systems and compare three different enhancement (electron-positron
correlation) schemes previously used in the calculation of momentum
distributions of annihilating electron-positron pairs within the
density-functional theory. We show that the use of a state-dependent
enhancement scheme leads to better results than a position-dependent
enhancement factor in the case of ratios of Doppler spectra between different
systems. Further, we demonstrate the applicability of our scheme for studying
vacancy-type defects in metals and semiconductors. Especially we study the
effect of forces due to a positron localized at a vacancy-type defect on the
ionic relaxations.Comment: Submitted to Physical Review B on September 1 2005. Revised
manuscript submitted on November 14 200
The selective V1a receptor agonist FE 202158 does not cause von Willebrand factor release in sheep unlike arginine vasopressin
Dislocation Dynamics in an Anisotropic Stripe Pattern
The dynamics of dislocations confined to grain boundaries in a striped system
are studied using electroconvection in the nematic liquid crystal N4. In
electroconvection, a striped pattern of convection rolls forms for sufficiently
high driving voltages. We consider the case of a rapid change in the voltage
that takes the system from a uniform state to a state consisting of striped
domains with two different wavevectors. The domains are separated by domain
walls along one axis and a grain boundary of dislocations in the perpendicular
direction. The pattern evolves through dislocation motion parallel to the
domain walls. We report on features of the dislocation dynamics. The kinetics
of the domain motion are quantified using three measures: dislocation density,
average domain wall length, and the total domain wall length per area. All
three quantities exhibit behavior consistent with power law evolution in time,
with the defect density decaying as , the average domain wall length
growing as , and the total domain wall length decaying as .
The two different exponents are indicative of the anisotropic growth of domains
in the system.Comment: 8 figures: 7 jpeg and 1 pd
- …