8,362 research outputs found
Dual Fronts Propagating into an Unstable State
The interface between an unstable state and a stable state usually develops a
single confined front travelling with constant velocity into the unstable
state. Recently, the splitting of such an interface into {\em two} fronts
propagating with {\em different} velocities was observed numerically in a
magnetic system. The intermediate state is unstable and grows linearly in time.
We first establish rigorously the existence of this phenomenon, called ``dual
front,'' for a class of structurally unstable one-component models. Then we use
this insight to explain dual fronts for a generic two-component
reaction-diffusion system, and for the magnetic system.Comment: 19 pages, Postscript, A
Revealing the pure confinement effect in glass-forming liquids by dynamic mechanical analysis
Many molecular glass forming liquids show a shift of the glass transition Tg
to lower temperatures when the liquid is confined into mesoporous host
matrices. Two contrary explanations for this effect are given in literature:
First, confinement induced acceleration of the dynamics of the molecules leads
to an effective downshift of Tg increasing with decreasing pore size. Secondly,
due to thermal mismatch between the liquid and the surrounding host matrix,
negative pressure develops inside the pores with decreasing temperature, which
also shifts Tg to lower temperatures. Here we present novel dynamic mechanical
analysis measurements of the glass forming liquid salol in Vycor and Gelsil
with pore sizes of d = 2.6, 5.0 and 7.5 nm. The dynamic complex elastic
susceptibility data can be consistently described with the assumption of two
relaxation processes inside the pores: A surface induced slowed down relaxation
due to interaction with rough pore interfaces and a second relaxation within
the core of the pores. This core relaxation time is reduced with decreasing
pore size d, leading to a downshift of Tg in perfect agreement with recent DSC
measurements
Stick-Slip Motion and Phase Transition in a Block-Spring System
We study numerically stick slip motions in a model of blocks and springs
being pulled slowly. The sliding friction is assumed to change dynamically with
a state variable. The transition from steady sliding to stick-slip is
subcritical in a single block and spring system. However, we find that the
transition is continuous in a long chain of blocks and springs. The size
distribution of stick-slip motions exhibits a power law at the critical point.Comment: 8 figure
Amplitude equations near pattern forming instabilities for strongly driven ferromagnets
A transversally driven isotropic ferromagnet being under the influence of a
static external and an uniaxial internal anisotropy field is studied. We
consider the dissipative Landau-Lifshitz equation as the fundamental equation
of motion and treat it in ~dimensions. The stability of the spatially
homogeneous magnetizations against inhomogeneous perturbations is analyzed.
Subsequently the dynamics above threshold is described via amplitude equations
and the dependence of their coefficients on the physical parameters of the
system is determined explicitly. We find soft- and hard-mode instabilities,
transitions between sub- and supercritical behaviour, various bifurcations of
higher codimension, and present a series of explicit bifurcation diagrams. The
analysis of the codimension-2 point where the soft- and hard-mode instabilities
coincide leads to a system of two coupled Ginzburg-Landau equations.Comment: LATeX, 25 pages, submitted to Z.Phys.B figures available via
[email protected] in /pub/publications/frank/zpb_95
(postscript, plain or gziped
Time-delayed feedback control in astrodynamics
In this paper we present time-delayed feedback control (TDFC) for the purpose of autonomously driving trajectories of nonlinear systems into periodic orbits. As the generation of periodic orbits is a major component of many problems in astodynamics we propose this method as a useful tool in such applications. To motivate the use of this method we apply it to a number of well known problems in the astrodynamics literature. Firstly, TDFC is applied to control in the chaotic attitude motion of an asymmetric satellite in an elliptical orbit. Secondly, we apply TDFC to the problem of maintaining a spacecraft in a periodic orbit about a body with large ellipticity (such as an asteroid) and finally, we apply TDFC to eliminate the drift between two satellites in low Earth orbits to ensure their relative motion is bounded
Direct Hopf Bifurcation in Parametric Resonance of Hybridized Waves
We study parametric resonance of interacting waves having the same wave
vector and frequency. In addition to the well-known period-doubling instability
we show that under certain conditions the instability is caused by a Hopf
bifurcation leading to quasiperiodic traveling waves. It occurs, for example,
if the group velocities of both waves have different signs and the damping is
weak. The dynamics above the threshold is briefly discussed. Examples
concerning ferromagnetic spin waves and surface waves of ferro fluids are
discussed.Comment: Appears in Phys. Rev. Lett., RevTeX file and three postscript
figures. Packaged using the 'uufiles' utility, 33 k
Stick-slip motion of solids with dry friction subject to random vibrations and an external field
We investigate a model for the dynamics of a solid object, which moves over a
randomly vibrating solid surface and is subject to a constant external force.
The dry friction between the two solids is modeled phenomenologically as being
proportional to the sign of the object's velocity relative to the surface, and
therefore shows a discontinuity at zero velocity. Using a path integral
approach, we derive analytical expressions for the transition probability of
the object's velocity and the stationary distribution of the work done on the
object due to the external force. From the latter distribution, we also derive
a fluctuation relation for the mechanical work fluctuations, which incorporates
the effect of the dry friction.Comment: v1: 23 pages, 9 figures; v2: Reference list corrected; v3: Published
version, typos corrected, references adde
Confinement effects on glass forming liquids probed by DMA
Many molecular glass forming liquids show a shift of the glass transition T-g
to lower temperatures when the liquid is confined into mesoporous host
matrices. Two contrary explanations for this effect are given in literature:
First, confinement induced acceleration of the dynamics of the molecules leads
to an effective downshift of T-g increasing with decreasing pore size. Second,
due to thermal mismatch between the liquid and the surrounding host matrix,
negative pressure develops inside the pores with decreasing temperature, which
also shifts T-g to lower temperatures. Here we present dynamic mechanical
analysis measurements of the glass forming liquid salol in Vycor and Gelsil
with pore sizes of d=2.6, 5.0 and 7.5 nm. The dynamic complex elastic
susceptibility data can be consistently described with the assumption of two
relaxation processes inside the pores: A surface induced slowed down relaxation
due to interaction with rough pore interfaces and a second relaxation within
the core of the pores. This core relaxation time is reduced with decreasing
pore size d, leading to a downshift of T-g proportional to 1/d in perfect
agreement with recent differential scanning calorimetry (DSC) measurements.
Thermal expansion measurements of empty and salol filled mesoporous samples
revealed that the contribution of negative pressure to the downshift of T-g is
small (<30%) and the main effect is due to the suppression of dynamically
correlated regions of size xi when the pore size xi approaches
Resonant steps and spatiotemporal dynamics in the damped dc-driven Frenkel-Kontorova chain
Kink dynamics of the damped Frenkel-Kontorova (discrete sine-Gordon) chain
driven by a constant external force are investigated. Resonant steplike
transitions of the average velocity occur due to the competitions between the
moving kinks and their radiated phasonlike modes. A mean-field consideration is
introduced to give a precise prediction of the resonant steps. Slip-stick
motion and spatiotemporal dynamics on those resonant steps are discussed. Our
results can be applied to studies of the fluxon dynamics of 1D
Josephson-junction arrays and ladders, dislocations, tribology and other
fields.Comment: 20 Plain Latex pages, 10 Eps figures, to appear in Phys. Rev.
Imbibition in mesoporous silica: rheological concepts and experiments on water and a liquid crystal
We present, along with some fundamental concepts regarding imbibition of
liquids in porous hosts, an experimental, gravimetric study on the
capillarity-driven invasion dynamics of water and of the rod-like liquid
crystal octyloxycyanobiphenyl (8OCB) in networks of pores a few nanometers
across in monolithic silica glass (Vycor). We observe, in agreement with
theoretical predictions, square root of time invasion dynamics and a sticky
velocity boundary condition for both liquids investigated.
Temperature-dependent spontaneous imbibition experiments on 8OCB reveal the
existence of a paranematic phase due to the molecular alignment induced by the
pore walls even at temperatures well beyond the clearing point. The ever
present velocity gradient in the pores is likely to further enhance this
ordering phenomenon and prevent any layering in molecular stacks, eventually
resulting in a suppression of the smectic phase in favor of the nematic phase.Comment: 18 pages, 8 figure
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