191 research outputs found
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
Reentrant phase diagram and pH effects in cross-linked gelatin gels
Experimental results have shown that the kinetics of bond formation in
chemical crosslinking of gelatin solutions is strongly affected not only by
gelatin and reactant concentrations but also by the solution pH. We present an
extended numerical investigation of the phase diagram and of the kinetics of
bond formation as a function of the pH, via Monte Carlo simulations of a
lattice model for gelatin chains and reactant agent in solution. We find a
reentrant phase diagram, namely gelation can be hindered either by loop
formation, at low reactant concentrations, or by saturation of active sites of
the chains via formation of single bonds with crosslinkers, at high reactant
concentrations. The ratio of the characteristic times for the formation of the
first and of the second bond between the crosslinker and an active site of a
chain is found to depend on the reactant reactivity, in good agreement with
experimental data.Comment: 8 pages, 8 figure
Shear-driven solidification of dilute colloidal suspensions
We show that the shear-induced solidification of dilute charge-stabilized
(DLVO) colloids is due to the interplay between the shear-induced formation and
breakage of large non-Brownian clusters. While their size is limited by
breakage, their number density increases with the shearing-time. Upon flow
cessation, the dense packing of clusters interconnects into a rigid state by
means of grainy bonds, each involving a large number of primary colloidal
bonds. The emerging picture of shear-driven solidification in dilute colloidal
suspensions combines the gelation of Brownian systems with the jamming of
athermal systems
Kinetics of bond formation in crosslinked gelatin gels
In chemical crosslinking of gelatin solutions, two different time scales
affect the kinetics of the gel formation in the experiments. We complement the
experimental study with Monte Carlo numerical simulations of a lattice model.
This approach shows that the two characteristic time scales are related to the
formation of single bonds crosslinker-chain and of bridges between chains. In
particular their ratio turns out to control the kinetics of the gel formation.
We discuss the effect of the concentration of chains. Finally our results
suggest that, by varying the probability of forming bridges as an independent
parameter, one can finely tune the kinetics of the gelation via the ratio of
the two characteristic times.Comment: 8 pages, 9 figures, revised versio
Elasticity of arrested short-ranged attractive colloids: homogeneous and heterogeneous glasses
We evaluate the elasticity of arrested short-ranged attractive colloids by
combining an analytically solvable elastic model with a hierarchical arrest
scheme into a new approach, which allows to discriminate the microscopic
(primary particle-level) from the mesoscopic (cluster-level) contribution to
the macroscopic shear modulus. The results quantitatively predict experimental
data in a wide range of volume fractions and indicate in which cases the
relevant contribution is due to mesoscopic structures. On this basis we propose
that different arrested states of short-ranged attractive colloids can be
meaningfully distinguished as homogeneous or heterogeneous colloidal glasses in
terms of the length-scale which controls their elastic behavior.Comment: 3 figures, revised version, to appear in Physical Review Letter
Viscoelasticity near the gel-point: a molecular dynamics study
We report on extensive molecular dynamics simulations on systems of soft
spheres of functionality f, i.e. particles that are capable of bonding
irreversibly with a maximum of f other particles. These bonds are randomly
distributed throughout the system and imposed with probability p. At a critical
concentration of bonds, p_c approximately equal to 0.2488 for f=6, a gel is
formed and the shear viscosity \eta diverges according to \eta ~ (p_c-p)^{-s}.
We find s is approximately 0.7 in agreement with some experiments and with a
recent theoretical prediction based on Rouse dynamics of phantom chains. The
diffusion constant decreases as the gel point is approached but does not
display a well-defined power law.Comment: 4 pages, 4 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
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
- …