16 research outputs found
Static and dynamic heterogeneities in irreversible gels and colloidal gelation
We compare the slow dynamics of irreversible gels, colloidal gels, glasses
and spin glasses by analyzing the behavior of the so called non-linear
dynamical susceptibility, a quantity usually introduced to quantitatively
characterize the dynamical heterogeneities. In glasses this quantity typically
grows with the time, reaches a maximum and then decreases at large time, due to
the transient nature of dynamical heterogeneities and to the absence of a
diverging static correlation length. We have recently shown that in
irreversible gels the dynamical susceptibility is instead an increasing
function of the time, as in the case of spin glasses, and tends asymptotically
to the mean cluster size. On the basis of molecular dynamics simulations, we
here show that in colloidal gelation where clusters are not permanent, at very
low temperature and volume fractions, i.e. when the lifetime of the bonds is
much larger than the structural relaxation time, the non-linear susceptibility
has a behavior similar to the one of the irreversible gel, followed, at higher
volume fractions, by a crossover towards the behavior of glass forming liquids.Comment: 9 pages, 3 figure
Dynamic heterogeneities in attractive colloids
We study the formation of a colloidal gel by means of Molecular Dynamics
simulations of a model for colloidal suspensions. A slowing down with gel-like
features is observed at low temperatures and low volume fractions, due to the
formation of persistent structures. We show that at low volume fraction the
dynamic susceptibility, which describes dynamic heterogeneities, exhibits a
large plateau, dominated by clusters of long living bonds. At higher volume
fraction, where the effect of the crowding of the particles starts to be
present, it crosses over towards a regime characterized by a peak. We introduce
a suitable mean cluster size of clusters of monomers connected by "persistent"
bonds which well describes the dynamic susceptibility.Comment: 4 pages, 4 figure
Static and dynamic heterogeneities in a model for irreversible gelation
We study the structure and the dynamics in the formation of irreversible gels
by means of molecular dynamics simulation of a model system where the gelation
transition is due to the random percolation of permanent bonds between
neighboring particles. We analyze the heterogeneities of the dynamics in terms
of the fluctuations of the intermediate scattering functions: In the sol phase
close to the percolation threshold, we find that this dynamical susceptibility
increases with the time until it reaches a plateau. At the gelation threshold
this plateau scales as a function of the wave vector as , with
being related to the decay of the percolation pair connectedness
function. At the lowest wave vector, approaching the gelation threshold it
diverges with the same exponent as the mean cluster size. These
findings suggest an alternative way of measuring critical exponents in a system
undergoing chemical gelation.Comment: 4 pages, 4 figure
Dynamical heterogeneity in a model for permanent gels: Different behavior of dynamical susceptibilities
We present a systematic study of dynamical heterogeneity in a model for
permanent gels, upon approaching the gelation threshold. We find that the
fluctuations of the self intermediate scattering function are increasing
functions of time, reaching a plateau whose value, at large length scales,
coincides with the mean cluster size and diverges at the percolation threshold.
Another measure of dynamical heterogeneities, i.e. the fluctuations of the
self-overlap, displays instead a peak and decays to zero at long times. The
peak, however, also scales as the mean cluster size. Arguments are given for
this difference in the long time behavior. We also find that non-Gaussian
parameter reaches a plateau in the long time limit. The value of the plateau of
the non-Gaussian parameter, which is connected to the fluctuations of
diffusivity of clusters, increases with the volume fraction and remains finite
at percolation threshold.Comment: 11 pages, 14 figure
Columnar and lamellar phases in attractive colloidal systems
In colloidal suspensions, the competition between attractive and repulsive
interactions gives rise to a rich and complex phenomenology. Here, we study the
equilibrium phase diagram of a model system using a DLVO interaction potential
by means of molecular dynamics simulations and a thermodynamical approach. As a
result, we find tubular and lamellar phases at low volume fraction. Such
phases, extremely relevant for designing new materials, may be not easily
observed in the experiments because of the long relaxation times and the
presence of defects.Comment: 5 pages, 5 figure
Length scale dependence of the dynamical heterogeneities in colloidal gelation at low volume fraction
Colloidal gelation, percolation and structural arrest
new version, 5 pages, 9 figuresBy means of molecular dynamics, we study a model system for colloidal suspensions where the interaction is based on a competition between attraction and repulsion. At low temperatures the relaxation time first increases as a power law as a function of the volume fraction and then, due to the finite lifetime of the bonded structures, it deviates from this critical behavior. We show that colloidal gelation at low temperatures and low volume fractions is crucially related to the formation of spanning long living cluster. Besides agreeing with experimental findings in different colloidal systems, our results shed new light on the different role played by the formation of long living bonds and the crowding of the particles in colloidal structural arrest