234 research outputs found
Non-linear dynamic response of glass-forming liquids to random pinning
We use large scale computer simulations of a glass-forming liquid in which a
fraction c of the particles has been permanently pinned. We find that the
relaxation dynamics shows an exponential dependence on c. This result can be
rationalized by means of a simple theoretical Ansatz and we discuss its
implication for thermodynamic theories for the glass-transition. For
intermediate and low temperatures we find that the slowing down of the dynamics
due to the pinning saturates and that the cooperativity decreases with
increasing c, results which indicate that in glass-forming liquids there is a
dynamic crossover at which the shape of the relaxing entities changes
Replica-exchange molecular dynamics simulation for supercooled liquids
We investigate to what extend the replica-exchange Monte Carlo method is able
to equilibrate a simple liquid in its supercooled state. We find that this
method does indeed allow to generate accurately the canonical distribution
function even at low temperatures and that its efficiency is about 10-100 times
higher than the usual canonical molecular dynamics simulation.Comment: 6 pages, 5 figure
The relaxation dynamics of a viscous silica melt: II The intermediate scattering functions
We use molecular dynamics computer simulations to study the relaxation
dynamics of a viscous melt of silica. The coherent and incoherent intermediate
scattering functions, F_d(q,t) and F_s(q,t), show a crossover from a nearly
exponential decay at high temperatures to a two-step relaxation at low
temperatures. Close to the critical temperature of mode-coupling theory (MCT)
the correlators obey in the alpha-regime the time temperature superposition
principle (TTSP) and show a weak stretching. We determine the wave-vector
dependence of the stretching parameter and find that for F_d(q,t) it shows
oscillations which are in phase with the static structure factor. The
temperature dependence of the alpha- relaxation times tau shows a crossover
from an Arrhenius law at low temperatures to a weaker T-dependence at
intermediate and high temperatures. At the latter temperatures the T-dependence
is described well by a power law. We find that the exponent gamma of the power
law for tau are significantly larger than the one for the diffusion constant.
The q-dependence of the alpha-relaxation times for F_d(q,t) oscillates around
tau(q) for F_s(q,t) and is in phase with the structure factor. Due to the
strong vibrational component of the dynamics at short times the TTSP is not
valid in the beta- relaxation regime. We show, however, that in this time
window the shape of the curves is independent of the correlator and is given by
a functional form proposed by MCT. We find that the value of the von Schweidler
exponent and the value of gamma for finite q are compatible with the expression
proposed by MCT. We conclude that, in the temperature regime where the
relaxation times are mesoscopic, many aspects of the dynamics of this strong
glass former can be rationalized very well by MCT.Comment: 31 pages of Latex, 14 figure
Structuring polymer gels via catalytic reactions
We use computer simulations to investigate how a catalytic reaction in a
polymer sol can induce the formation of a polymer gel. To this aim we consider
a solution of homopolymers in which freely-diffusing catalysts convert the
originally repulsive A monomers into attractive B ones. We find that at low
temperatures this reaction transforms the polymer solution into a physical gel
that has a remarkably regular mesostructure in the form of a cluster phase,
absent in the usual homopolymer gels obtained by a quench in temperature. We
investigate how this microstructuring depends on catalyst concentration,
temperature, and polymer density and show that the dynamics for its formation
can be understood in a semi-quantitative manner using the interaction
potentials between the particles as input. The structuring of the copolymers
and the AB sequences resulting from the reactions can be discussed in the
context of the phase behaviour of correlated random copolymers. The location of
the spinodal line as found in our simulations is consistent with analytical
predictions. Finally, we show that the observed structuring depends not only on
the chemical distribution of the A and B monomers but also on the mode of
formation of this distribution
Reply to the Comment of Muessel and Rieger on ``Aging Effects in a Lennard-Jones Glass''
This is a Reply to the Comment of Muessel and Rieger cond-mat/9804063 to out
paper on ``Aging Effects in a Lennerd-Jones Glass''. We show that the scaling
function proposed by Muessel and Rieger does not lead to a satisfactory scaling
of our aging data.Comment: 1 page of Revtex, 1 eps figur
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