270 research outputs found
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
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
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
Pacman percolation: a model for enzyme gel degradation
We study a model for the gel degradation by an enzyme, where the gel is
schematized as a cubic lattice, and the enzyme as a random walker, that cuts
the bonds over which it passes. The model undergoes a (reverse) percolation
transition, which for low density of enzymes falls in a universality class
different from random percolation. In particular we have measured a gel
fraction critical exponent beta=1.0+-0.1, in excellent agreement with
experiments made on the real system.Comment: 4 pages, 7 eps 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
Oxidative stress related to chlorpyrifos exposure in rainbow trout: Acute and medium term effects on genetic biomarkers
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
Patient-reported outcome measures in patients with familial cerebral cavernous malformations: results from the Treat_CCM trial
Experimental system to displace radioisotopes from upper to deeper soil layers: chemical research
BACKGROUND: Radioisotopes are introduced into the environment following nuclear power plant accidents or nuclear weapons tests. The immobility of these radioactive elements in uppermost soil layers represents a problem for human health, since they can easily be incorporated in the food chain. Preventing their assimilation by plants may be a first step towards the total recovery of contaminated areas. METHODS: The possibility of displacing radionuclides from the most superficial soil layers and their subsequent stabilisation at lower levels were investigated in laboratory trials. An experimental system reproducing the environmental conditions of contaminated areas was designed in plastic columns. A radiopolluted soil sample was treated with solutions containing ions normally used in fertilisation (NO(3)(-), NH(4)(+), PO(4)(--- )and K(+)). RESULTS: Contaminated soils treated with an acid solution of ions NO(3)(-), PO(4)(--- )and K(+), undergo a reduction of radioactivity up to 35%, after a series of washes which simulate one year's rainfall. The capacity of the deepest soil layers to immobilize the radionuclides percolated from the superficial layers was also confirmed. CONCLUSION: The migration of radionuclides towards deeper soil layers, following chemical treatments, and their subsequent stabilization reduces bioavailability in the uppermost soil horizon, preventing at the same time their transfer into the water-bearing stratum
- …