1,025 research outputs found
Steady-state nucleation rate and flux of composite nucleus at saddle point
The steady-state nucleation rate and flux of composite nucleus at the saddle
point is studied by extending the theory of binary nucleation. The
Fokker-Planck equation that describes the nucleation flux is derived using the
Master equation for the growth of the composite nucleus, which consists of the
core of the final stable phase surrounded by a wetting layer of the
intermediate metastable phase nucleated from a metastable parent phase recently
evaluated by the author [J. Chem. Phys. {\bf 134}, 164508 (2011)]. The
Fokker-Planck equation is similar to that used in the theory of binary
nucleation, but the non-diagonal elements exist in the reaction rate matrix.
First, the general solution for the steady-state nucleation rate and the
direction of nucleation flux is derived. Next, this information is then used to
study the nucleation of composite nucleus at the saddle point. The dependence
of steady-state nucleation rate as well as the direction of nucleation flux on
the reaction rate in addition to the free-energy surface is studied using a
model free-energy surface. The direction of nucleation current deviates from
the steepest-descent direction of the free-energy surface. The results show the
importance of two reaction rate constants: one from the metastable environment
to the intermediate metastable phase and the other from the metastable
intermediate phase to the stable new phase. On the other hand, the gradient of
the potential or the Kramers crossover function (the commitment or
splitting probability) is relatively insensitive to reaction rates or
free-energy surface.Comment: 12 pages, 6 figures, to be published in Journal of Chemical Physic
Non-equilibrium Anisotropic Phases, Nucleation and Critical Behavior in a Driven Lennard-Jones Fluid
We describe short-time kinetic and steady-state properties of the
non--equilibrium phases, namely, solid, liquid and gas anisotropic phases in a
driven Lennard-Jones fluid. This is a computationally-convenient
two-dimensional model which exhibits a net current and striped structures at
low temperature, thus resembling many situations in nature. We here focus on
both critical behavior and details of the nucleation process. In spite of the
anisotropy of the late--time spinodal decomposition process, earlier nucleation
seems to proceed by Smoluchowski coagulation and Ostwald ripening, which are
known to account for nucleation in equilibrium, isotropic lattice systems and
actual fluids. On the other hand, a detailed analysis of the system critical
behavior rises some intriguing questions on the role of symmetries; this
concerns the computer and field-theoretical modeling of non-equilibrium fluids.Comment: 7 pages, 9 ps figures, to appear in PR
Polydispersity Effects in Colloid-Polymer Mixtures
We study phase separation and transient gelation in a mixture consisting of
polydisperse colloids and non-adsorbing polymers, where the ratio of the
average size of the polymer to that of the colloid is approximately 0.063.
Unlike what has been reported previously for mixtures with somewhat lower
colloid polydispersity, the addition of polymers does not expand the
fluid-solid coexistence region. Instead, we find a region of fluid-solid
coexistence which has an approximately constant width but an unexpected
re-entrant shape. We detect the presence of a metastable gas-liquid binodal,
which gives rise to two-stepped crystallization kinetics that can be
rationalized as the effect of fractionation. Finally, we find that the
separation into multiple coexisting solid phases at high colloid volume
fractions predicted by equilibrium statistical mechanics is kinetically
suppressed before the system reaches dynamical arrest.Comment: 11 pages, 5 figure
The stability of a crystal with diamond structure for patchy particles with tetrahedral symmetry
The phase diagram of model anisotropic particles with four attractive patches
in a tetrahedral arrangement has been computed at two different values for the
range of the potential, with the aim of investigating the conditions under
which a diamond crystal can be formed. We find that the diamond phase is never
stable for our longer-ranged potential. At low temperatures and pressures, the
fluid freezes into a body-centred-cubic solid that can be viewed as two
interpenetrating diamond lattices with a weak interaction between the two
sublattices. Upon compression, an orientationally ordered face-centred-cubic
crystal becomes more stable than the body-centred-cubic crystal, and at higher
temperatures a plastic face-centered-cubic phase is stabilized by the increased
entropy due to orientational disorder. A similar phase diagram is found for the
shorter-ranged potential, but at low temperatures and pressures, we also find a
region over which the diamond phase is thermodynamically favored over the
body-centred-cubic phase. The higher vibrational entropy of the diamond
structure with respect to the body-centred-cubic solid explains why it is
stable even though the enthalpy of the latter phase is lower. Some preliminary
studies on the growth of the diamond structure starting from a crystal seed
were performed. Even though the diamond phase is never thermodynamically stable
for the longer-ranged model, direct coexistence simulations of the interface
between the fluid and the body-centred-cubic crystal and between the fluid and
the diamond crystal show that, at sufficiently low pressures, it is quite
probable that in both cases the solid grows into a diamond crystal, albeit
involving some defects. These results highlight the importance of kinetic
effects in the formation of diamond crystals in systems of patchy particles.Comment: 15 pages, 13 figure
Homogeneous nucleation near a second phase transition and Ostwald's step rule
Homogeneous nucleation of the new phase of one transition near a second phase
transition is considered. The system has two phase transitions, we study the
nucleation of the new phase of one of these transitions under conditions such
that we are near or at the second phase transition. The second transition is an
Ising-like transition and lies within the coexistence region of the first
transition. It effects the formation of the new phase in two ways. The first is
by reducing the nucleation barrier to direct nucleation. The second is by the
system undergoing the second transition and transforming to a state in which
the barrier to nucleation is greatly reduced. The second way occurs when the
barrier to undergoing the second phase transition is less than that of the
first phase transition, and is in accordance with Ostwald's rule.Comment: 11 pages, 5 figure
On the action potential as a propagating density pulse and the role of anesthetics
The Hodgkin-Huxley model of nerve pulse propagation relies on ion currents
through specific resistors called ion channels. We discuss a number of
classical thermodynamic findings on nerves that are not contained in this
classical theory. Particularly striking is the finding of reversible heat
changes, thickness and phase changes of the membrane during the action
potential. Data on various nerves rather suggest that a reversible density
pulse accompanies the action potential of nerves. Here, we attempted to explain
these phenomena by propagating solitons that depend on the presence of
cooperative phase transitions in the nerve membrane. These transitions are,
however, strongly influenced by the presence of anesthetics. Therefore, the
thermodynamic theory of nerve pulses suggests a explanation for the famous
Meyer-Overton rule that states that the critical anesthetic dose is linearly
related to the solubility of the drug in the membranes.Comment: 13 pages, 8 figure
Diffusive Evolution of Stable and Metastable Phases II: Theory of Non-Equilibrium Behaviour in Colloid-Polymer Mixtures
By analytically solving some simple models of phase-ordering kinetics, we
suggest a mechanism for the onset of non-equilibrium behaviour in
colloid-polymer mixtures. These mixtures can function as models of atomic
systems; their physics therefore impinges on many areas of thermodynamics and
phase-ordering. An exact solution is found for the motion of a single, planar
interface separating a growing phase of uniform high density from a
supersaturated low density phase, whose diffusive depletion drives the
interfacial motion. In addition, an approximate solution is found for the
one-dimensional evolution of two interfaces, separated by a slab of a
metastable phase at intermediate density. The theory predicts a critical
supersaturation of the low-density phase, above which the two interfaces become
unbound and the metastable phase grows ad infinitum. The growth of the stable
phase is suppressed in this regime.Comment: 27 pages, Latex, eps
3D characterization of CdSe nanoparticles attached to carbon nanotubes
The crystallographic structure of CdSe nanoparticles attached to carbon
nanotubes has been elucidated by means of high resolution transmission electron
microscopy and high angle annular dark field scanning transmission electron
microscopy tomography. CdSe rod-like nanoparticles, grown in solution together
with carbon nanotubes, undergo a morphological transformation and become
attached to the carbon surface. Electron tomography reveals that the
nanoparticles are hexagonal-based with the (001) planes epitaxially matched to
the outer graphene layer.Comment: 7 pages, 8 figure
EEG-fMRI Based Information Theoretic Characterization of the Human Perceptual Decision System
The modern metaphor of the brain is that of a dynamic information processing device. In the current study we investigate how a core cognitive network of the human brain, the perceptual decision system, can be characterized regarding its spatiotemporal representation of task-relevant information. We capitalize on a recently developed information theoretic framework for the analysis of simultaneously acquired electroencephalography (EEG) and functional magnetic resonance imaging data (fMRI) (Ostwald et al. (2010), NeuroImage 49: 498–516). We show how this framework naturally extends from previous validations in the sensory to the cognitive domain and how it enables the economic description of neural spatiotemporal information encoding. Specifically, based on simultaneous EEG-fMRI data features from n = 13 observers performing a visual perceptual decision task, we demonstrate how the information theoretic framework is able to reproduce earlier findings on the neurobiological underpinnings of perceptual decisions from the response signal features' marginal distributions. Furthermore, using the joint EEG-fMRI feature distribution, we provide novel evidence for a highly distributed and dynamic encoding of task-relevant information in the human brain
Line Defects in Molybdenum Disulfide Layers
Layered molecular materials and especially MoS2 are already accepted as
promising candidates for nanoelectronics. In contrast to the bulk material, the
observed electron mobility in single-layer MoS2 is unexpectedly low. Here we
reveal the occurrence of intrinsic defects in MoS2 layers, known as inversion
domains, where the layer changes its direction through a line defect. The line
defects are observed experimentally by atomic resolution TEM. The structures
were modeled and the stability and electronic properties of the defects were
calculated using quantum-mechanical calculations based on the
Density-Functional Tight-Binding method. The results of these calculations
indicate the occurrence of new states within the band gap of the semiconducting
MoS2. The most stable non-stoichiometric defect structures are observed
experimentally, one of which contains metallic Mo-Mo bonds and another one
bridging S atoms
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