490 research outputs found
Coarsening rates for the dynamics of slipping droplets
We derive reduced finite dimensional ODE models starting from one dimensional
lubrication equations describing coarsening dynamics of droplets in nanometric
polymer film interacting on a hydrophobically coated solid substrate in the
presence of large slippage at the liquid/solid interface. In the limiting case
of infinite slip length corresponding in applications to free films a
collision/absorption model then arises and is solved explicitly. The exact
coarsening law is derived for it analytically and confirmed numerically.
Existence of a threshold for the decay of initial distributions of droplet
distances at infinity at which the coarsening rates switch from algebraic to
exponential ones is shown
Avalanche statistics during coarsening dynamics
We study the coarsening dynamics of a two dimensional system via lattice
Boltzmann numerical simulations. The system under consideration is a biphasic
system consisting of domains of a dispersed phase closely packed together in a
continuous phase and separated by thin interfaces. Such system is elastic and
typically out of equilibrium. The equilibrium state is attained via the
coarsening dynamics, wherein the dispersed phase slowly diffuses through the
interfaces, causing domains to change in size and eventually rearrange
abruptly. The effect of rearrangements is propagated throughout the system via
the intrinsic elastic interactions and may cause rearrangements elsewhere,
resulting in intermittent bursts of activity and avalanche behaviour. Here we
aim at quantitatively characterizing the corresponding avalanche statistics
(i.e. size, duration, inter-avalanche time). Despite the coarsening dynamics is
triggered by an internal driving mechanism, we find quantitative indications
that such avalanche statistics displays scaling-laws very similar to those
observed in the response of disordered materials to external loads
Particles with selective wetting affect spinodal decomposition microstructures
We have used mesoscale simulations to study the effect of immobile particles
on microstructure formation during spinodal decomposition in ternary mixtures
such as polymer blends. Specifically, we have explored a regime of
interparticle spacings (which are a few times the characteristic spinodal
length scale) in which we might expect interesting new effects arising from
interactions among wetting, spinodal decomposition and coarsening. In this
paper, we report three new effects for systems in which the particle phase has
a strong preference for being wetted by one of the components (say, A). In the
presence of particles, microstructures are not bicontinuous in a symmetric
mixture. An asymmetric mixture, on the other hand, first forms a
non-bicontinuous microstructure which then evolves into a bicontinuous one at
intermediate times. Moreover, while wetting of the particle phase by the
preferred component (A) creates alternating A-rich and B-rich layers around the
particles, curvature-driven coarsening leads to shrinking and disappearance of
the first A-rich layer, leaving a layer of the non-preferred component in
contact with the particle. At late simulation times, domains of the matrix
components coarsen following the Lifshitz-Slyozov-Wagner law, .Comment: Accepted for publication in PCCP on 24th May 201
Coarsening in potential and nonpotential models of oblique stripe patterns
We study the coarsening of two-dimensional oblique stripe patterns by
numerically solving potential and nonpotential anisotropic Swift-Hohenberg
equations. Close to onset, all models exhibit isotropic coarsening with a
single characteristic length scale growing in time as . Further from
onset, the characteristic lengths along the preferred directions and
grow with different exponents, close to 1/3 and 1/2, respectively. In
this regime, one-dimensional dynamical scaling relations hold. We draw an
analogy between this problem and Model A in a stationary, modulated external
field. For deep quenches, nonpotential effects produce a complicated
dislocation dynamics that can lead to either arrested or faster-than-power-law
growth, depending on the model considered. In the arrested case, small isolated
domains shrink down to a finite size and fail to disappear. A comparison with
available experimental results of electroconvection in nematics is presented.Comment: 13 pages, 13 figures. To appear in Phys. Rev.
Bubble statistics and coarsening dynamics for quasi-two dimensional foams with increasing liquid content
We report on the statistics of bubble size, topology, and shape and on their
role in the coarsening dynamics for foams consisting of bubbles compressed
between two parallel plates. The design of the sample cell permits control of
the liquid content, through a constant pressure condition set by the height of
the foam above a liquid reservoir. We find that in the scaling state, all
bubble distributions are independent not only of time but also of liquid
content. For coarsening, the average rate decreases with liquid content due to
the blocking of gas diffusion by Plateau borders inflated with liquid. By
observing the growth rate of individual bubbles, we find that von Neumann's law
becomes progressively violated with increasing wetness and with decreasing
bubble size. We successfully model this behavior by explicitly incorporating
the border blocking effect into the von Neumann argument. Two dimensionless
bubble shape parameters naturally arise, one of which is primarily responsible
for the violation of von Neumann's law for foams that are not perfectly dry
Influence of composition on the microstructure and mechanical properties of a nickel-base superalloy single crystal
The effects of cobalt, tantalum, and tungsten contents on the microstructure and mechanical properties of single crystal Mar-M247 were investigated. Elevated temperature tensile and creep-rupture properties of 001 oriented single crystals were related to microstructural features of the alloys. Substitution of Ni for Co in the high refractory metal alloys increased the lattice mismatch, which was considered to be the cause of the increases in tensile and creep strength. Substitution of Ni for Ta caused large decreases in tensile strength and creep life, consistent with decreases in gamma prime volume fraction, lattice mismatch, and solid solution hardening. Substitution of W for Ta resulted in decreased life at high stresses, which was related to small decreases in mismatch and volume fraction. However, the W substitution resulted in improved life at low stresses, which was related to solid solution strengthening by W
A phase-field study of elastic stress effects on phase separation in ternary alloys
Most of the commercially important alloys are multicomponent, producing
multiphase microstructures as a result of processing. When the coexisting
phases are elastically coherent, the elastic interactions between these phases
play a major role in the development of microstructures. To elucidate the key
effects of elastic stress on microstructural evolution when more than two
misfitting phases are present in the microstructure, we have developed a
microelastic phase-field model in two dimensions to study phase separation in
ternary alloy system. Numerical solutions of a set of coupled Cahn-Hilliard
equations for the composition fields govern the spatiotemporal evolution of the
three-phase microstructure. The model incorporates coherency strain
interactions between the phases using Khachaturyan's microelasticity theory. We
systematically vary the misfit strains (magnitude and sign) between the phases
along with the bulk alloy composition to study their effects on the
morphological development of the phases and the resulting phase separation
kinetics. We also vary the ratio of interfacial energies between the phases to
understand the interplay between elastic and interfacial energies on
morphological evolution. The sign and degree of misfit affect strain
partitioning between the phases during spinodal decomposition, thereby
affecting their compositional history and morphology. Moreover, strain
partitioning affects solute partitioning and alters the kinetics of coarsening
of the phases. The phases associated with higher misfit strain appear coarser
and exhibit wider size distribution compared to those having lower misfit. When
the interfacial energies satisfy complete wetting condition, phase separation
leads to development of stable core-shell morphology depending on the misfit
between the core (wetted) and the shell (wetting) phases
Microstructure-property relationships in directionally solidified single crystal nickel-base superalloys
Some of the microstructural features which influence the creep properties of directionally solidified and single crystal nickel-base superalloys are discussed. Gamma precipitate size and morphology, gamma-gamma lattice mismatch, phase instability, alloy composition, and processing variations are among the factors considered. Recent experimental results are reviewed and related to the operative deformation mechanisms and to the corresponding mechanical properties. Special emphasis is placed on the creep behavior of single crystal superalloys at high temperatures, where directional gamma coarsening is prominent, and at lower temperatures, where gamma coarsening rates are significantly reduced. It can be seen that very subtle changes in microstructural features can have profound effects on the subsequent properties of these materials
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