315 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
Time-resolved microstructural changes in large amplitude oscillatory shear of model single and double component soft gels
Soft particulate gels can reversibly yield when sufficient deformation is
applied, and the characteristics of this transition can be enhanced or limited
by designing hybrid hydrogel composites. While the microscopic dynamics and
macroscopic rheology of these systems have been studied separately in detail,
the development of direct connections between the two has been difficult,
particularly with regard to the non-linear rheology. To bridge this gap, we
perform a series of large amplitude oscillatory shear (LAOS) numerical
measurements on model soft particulate gels at different volume fractions using
coarse-grained molecular dynamics simulations. We first study a particulate
network with local bending stiffness and then we combine it with a second
component that can provide additional crosslinking to obtain two-component
networks. Through the sequence of physical processes (SPP) framework we define
time-resolved dynamic moduli and, by tracking the changes in these moduli
through the period, we can distinguish transitions in the material behavior as
a function of time. This approach helps us establish the microsopic origin of
the non-linear rheology by connecting the changes in dynamics moduli to the
corresponding microstructural changes during the deformation including the
non-affine displacement of particles, and the breakage, formation, and
orientation of bonds.Comment: 17 pages, 14 figures, submitted to The Journal of Rheolog
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 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
Performance of Foundations and Retaining Structures
The design, construction, and performance of several building foundations and temporary earth retaining structures located in the downtown area of White Plains, New York are presented in this paper. High rise structures were supported on shallow mat or spread foundations bearing on erratic saturated alluvial silt and sand deposits. Additionally, the construction of two and three level underground parking structures required the use of cantilevered and braced excavation support systems to retain the adjacent streets and utilities. Several assumptions were required to design and predict the performance of the building foundations and retaining structures. The accuracy of these assumptions was verified through the use of precise field measurements during and after construction. The results of these field measurements and comparison with predicted values are presented and discussed
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
Ion specificity of confined ion-water structuring and nanoscale surface forces in clays
Ion specificity and related Hofmeister effects, ubiquitous in aqueous
systems, can have spectacular consequences in hydrated clays, where
ion-specific nanoscale surface forces can determine large scale cohesive,
swelling and shrinkage behaviors of soil and sediments. We have used a
semi-atomistic computational approach and examined sodium, calcium and aluminum
counterions confined with water between charged surfaces representative of clay
materials, to show that ion-water structuring in nanoscale confinement is at
the origin of surface forces between clay particles which are intrinsically
ion-specific. When charged surfaces strongly confine ions and water, the
amplitude and oscillations of the net pressure naturally emerge from the
interplay of electrostatics and steric effects, which can not be captured by
existing theories. Increasing confinement and surface charge densities promote
ion-water structures that increasingly deviate from the ions' bulk hydration
shells, being strongly anisotropic and persistent, and self-organizing into
optimized, nearly solid-like assemblies where hardly any free water is left. In
these conditions, strongly attractive interactions can prevail between charged
surfaces, due to the dramatically reduced dielectric screening of water and the
highly organized water-ion structures. By unravelling the ion-specific nature
of these nanoscale interactions, we provide evidence that ion-specific
solvation structures determined by confinement are at the origin of ion
specificity in clays and potentially a broader range of confined aqueous
systems.Comment: Main text: 14 pages and 6 figures. Supporting information: 5 figures.
Submitted to The Journal of Physical Chemistry
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
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
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