1,361 research outputs found
A Continuum,O(N) Monte-Carlo algorithm for charged particles
We introduce a Monte-Carlo algorithm for the simulation of charged particles
moving in the continuum. Electrostatic interactions are not instantaneous as in
conventional approaches, but are mediated by a constrained, diffusing electric
field on an interpolating lattice. We discuss the theoretical justifications of
the algorithm and show that it efficiently equilibrates model polyelectrolytes
and polar fluids. In order to reduce lattice artifacts that arise from the
interpolation of charges to the grid we implement a local, dynamic subtraction
algorithm. This dynamic scheme is completely general and can also be used with
other Coulomb codes, such as multigrid based methods
Understanding plastic deformation in thermal glasses from single-soft-spot dynamics
By considering the low-frequency vibrational modes of amorphous solids,
Manning and Liu [Phys. Rev. Lett. 107, 108302 (2011)] showed that a population
of "soft spots" can be identified that are intimately related to plasticity at
zero temperature under quasistatic shear. In this work we track individual soft
spots with time in a two-dimensional sheared thermal Lennard Jones glass at
temperatures ranging from deep in the glassy regime to above the glass
transition temperature. We show that the lifetimes of individual soft spots are
correlated with the timescale for structural relaxation. We additionally
calculate the number of rearrangements required to destroy soft spots, and show
that most soft spots can survive many rearrangements. Finally, we show that
soft spots are robust predictors of rearrangements at temperatures well into
the super-cooled regime. Altogether, these results pave the way for mesoscopic
theories of plasticity of amorphous solids based on dynamical behavior of
individual soft spots.Comment: 9 pages, 6 figure
Local Simulation Algorithms for Coulombic Interactions
We consider dynamically constrained Monte-Carlo dynamics and show that this
leads to the generation of long ranged effective interactions. This allows us
to construct a local algorithm for the simulation of charged systems without
ever having to evaluate pair potentials or solve the Poisson equation. We
discuss a simple implementation of a charged lattice gas as well as more
elaborate off-lattice versions of the algorithm. There are analogies between
our formulation of electrostatics and the bosonic Hubbard model in the phase
approximation. Cluster methods developed for this model further improve the
efficiency of the electrostatics algorithm.Comment: Proceedings Statphys22 10 page
Local Molecular Dynamics with Coulombic Interaction
We propose a local, O(N) molecular dynamics algorithm for the simulation of
charged systems. The long ranged Coulomb potential is generated by a
propagating electric field that obeys modified Maxwell equations. On coupling
the electrodynamic equations to an external thermostat we show that the
algorithm produces an effective Coulomb potential between particles. On
annealing the electrodynamic degrees of freedom the field configuration
converges to a solution of the Poisson equation much like the electronic
degrees of freedom approach the ground state in ab-initio molecular dynamics.Comment: 4 pages with 3 figure
Unified Description of Aging and Rate Effects in Yield of Glassy Solids
The competing effects of slow structural relaxations (aging) and deformation
at constant strain rate on the shear yield stress of simple model
glasses are examined using molecular simulations. At long times, aging leads to
a logarithmic increase in density and . The yield stress also rises
logarithmically with rate, but shows a sharp transition in slope at a rate that
decreases with increasing age. We present a simple phenomenological model that
includes both intrinsic rate dependence and the change in properties with the
total age of the system at yield. As predicted by the model, all data for each
temperature collapse onto a universal curve.Comment: 4 pages, 3 figure
Controlling crystal symmetries in phase-field crystal models
We investigate the possibility to control the symmetry of ordered states in
phase-field crystal models by tuning nonlinear resonances. In two dimensions,
we find that a state of square symmetry as well as coexistence between squares
and hexagons can be easily obtained. In contrast, it is delicate to obtain
coexistence of squares and liquid. We develop a general method for constructing
free energy functionals that exhibit solid-liquid coexistence with desired
crystal symmetries. As an example, we develop a free energy functional for
square-liquid coexistence in two dimensions. A systematic analysis for
determining the parameters of the necessary nonlinear terms is provided. The
implications of our findings for simulations of materials with simple cubic
symmetry are discussed.Comment: 19 pages, 6 figure
Jamming under tension in polymer crazes
Molecular dynamics simulations are used to study a unique expanded jammed
state. Tension transforms many glassy polymers from a dense glass to a network
of fibrils and voids called a craze. Entanglements between polymers and
interchain friction jam the system after a fixed increase in volume. As in
dense jammed systems, the distribution of forces is exponential, but they are
tensile rather than compressive. The broad distribution of forces has important
implications for fibril breakdown and the ultimate strength of crazes.Comment: 4 pages, 4 figure
Tensile Fracture of Welded Polymer Interfaces: Miscibility, Entanglements and Crazing
Large-scale molecular simulations are performed to investigate tensile
failure of polymer interfaces as a function of welding time . Changes in the
tensile stress, mode of failure and interfacial fracture energy are
correlated to changes in the interfacial entanglements as determined from
Primitive Path Analysis. Bulk polymers fail through craze formation, followed
by craze breakdown through chain scission. At small welded interfaces are
not strong enough to support craze formation and fail at small strains through
chain pullout at the interface. Once chains have formed an average of about one
entanglement across the interface, a stable craze is formed throughout the
sample. The failure stress of the craze rises with welding time and the mode of
craze breakdown changes from chain pullout to chain scission as the interface
approaches bulk strength. The interfacial fracture energy is calculated
by coupling the simulation results to a continuum fracture mechanics model. As
in experiment, increases as before saturating at the average
bulk fracture energy . As in previous simulations of shear strength,
saturation coincides with the recovery of the bulk entanglement density. Before
saturation, is proportional to the areal density of interfacial
entanglements. Immiscibiltiy limits interdiffusion and thus suppresses
entanglements at the interface. Even small degrees of immisciblity reduce
interfacial entanglements enough that failure occurs by chain pullout and
"Wet-to-Dry" Conformational Transition of Polymer Layers Grafted to Nanoparticles in Nanocomposite
The present communication reports the first direct measurement of the
conformation of a polymer corona grafted around silica nano-particles dispersed
inside a nanocomposite, a matrix of the same polymer. This measurement
constitutes an experimental breakthrough based on a refined combination of
chemical synthesis, which permits to match the contribution of the neutron
silica signal inside the composite, and the use of complementary scattering
methods SANS and SAXS to extract the grafted polymer layer form factor from the
inter-particles silica structure factor. The modelization of the signal of the
grafted polymer on nanoparticles inside the matrix and the direct comparison
with the form factor of the same particles in solution show a clear-cut change
of the polymer conformation from bulk to the nanocomposite: a transition from a
stretched and swollen form in solution to a Gaussian conformation in the matrix
followed with a compression of a factor two of the grafted corona. In the
probed range, increasing the interactions between the grafted particles (by
increasing the particle volume fraction) or between the grafted and the free
matrix chains (decreasing the grafted-free chain length ratio) does not
influence the amplitude of the grafted brush compression. This is the first
direct observation of the wet-to-dry conformational transition theoretically
expected to minimize the free energy of swelling of grafted chains in
interaction with free matrix chains, illustrating the competition between the
mixing entropy of grafted and free chains, and the elastic deformation of the
grafted chains. In addition to the experimental validation of the theoretical
prediction, this result constitutes a new insight for the nderstanding of the
general problem of dispersion of nanoparticles inside a polymer matrix for the
design of new nanocomposites materials
- …