132 research outputs found
The mechanism of thickness selection in the Sadler-Gilmer model of polymer crystallization
Recent work on the mechanism of polymer crystallization has led to a proposal
for the mechanism of thickness selection which differs from those proposed by
the surface nucleation theory of Lauritzen and Hoffman and the entropic barrier
model of Sadler and Gilmer. This has motivated us to reexamine the model used
by Sadler and Gilmer. We again find a fixed-point attractor which describes the
dynamical convergence of the crystal thickness to a value just larger than the
minimum stable thickness, l_min. This convergence arises from the combined
effect of two constraints on the length of stems in a layer: it is unfavourable
for a stem to be shorter than l_min and for a stem to overhang the edge of the
previous layer. The relationship between this new mechanism and the explanation
given by Sadler and Gilmer in terms of an entropic barrier is discussed. We
also examine the behaviour of the Sadler-Gilmer model when an energetic
contribution from chain folds is included.Comment: 15 pages, 13 figures, revte
The structure of binary Lennard-Jones clusters: The effects of atomic size ratio
We introduce a global optimization approach for binary clusters that for a
given cluster size is able to directly search for the structure and composition
that has the greatest stability. We apply this approach to binary Lennard-Jones
clusters, where the strength of the interactions between the two atom types is
the same, but where the atoms have different sizes. We map out how the most
stable structure depends on the cluster size and the atomic size ratio for
clusters with up to 100 atoms and up to 30% difference in atom size. A
substantial portion of this parameter space is occupied by structures that are
polytetrahedral, both those that are polyicosahedral and those that involve
disclination lines. Such structures involve substantial strains for
one-component Lennard-Jones clusters, but can be stabilized by the
different-sized atoms in the binary clusters. These structures often have a
`core-shell' geometry, where the larger atoms are on the surface, and the
smaller atoms are in the core.Comment: 13 pages, 9 figure
CO Oxidation Catalysed by Pd-based Bimetallic Nanoalloys
Density functional theory based global geometry optimization has been used to
demonstrate the crucial influence of the geometry of the catalytic cluster on
the energy barriers for the CO oxidation reaction over Pd-based bimetallic
nanoalloys. We show that dramatic geometry change between the reaction
intermediates can lead to very high energy barriers and thus be prohibitive for
the whole process. This introduces challenges for both the design of new
catalysts, and theoretical methods employed. On the theory side, a careful
choice of geometric configurations of all reaction intermediates is crucial for
an adequate description of a possible reaction path. From the point of view of
the catalyst design, the cluster geometry can be controlled by adjusting the
level of interaction between the cluster and the dopant metal, as well as
between the adsorbate molecules and the catalyst cluster by mixing different
metals in a single nanoalloy particle. We show that substitution of a Pd atom
in the Pd cluster with a single Ag atom to form PdAg leads to
a potential improvement of the catalytic properties of the cluster for the CO
oxidation reaction. On the other hand, a single Au atom does not enhance the
properties of the catalyst, which is attributed to a weaker hybridization
between the cluster's constituent metals and the adsorbate molecules. Such
flexibility of properties of bimetallic nanoalloy clusters illustrates the
possibility of fine-tuning, which might be used for design of novel efficient
catalytic materials.Comment: 12 pages, 8 figure
Homogeneous TIP4P/2005 ice nucleation at low supercooling
We present a partial free energy profile for the homogeneous nucleation of
ice using an all-atom model of water at low supercooling, at which ice growth
dynamics are reasonably accessible to simulation. We demonstrate that the free
energy profile is well described by classical nucleation theory, and that the
nucleation barrier is entropic in origin. We also estimate to first order the
temperature dependence of the interfacial free energy
Effects of surface interactions on heterogeneous ice nucleation for a monatomic water model
Despite its importance in atmospheric science, much remains unknown about the
microscopic mechanism of heterogeneous ice nucleation. In this work, we perform
hybrid Monte Carlo simulations of the heterogeneous nucleation of ice on a
range of generic surfaces, both flat and structured, in order to probe the
underlying factors affecting the nucleation process. The structured surfaces we
study comprise one basal plane bilayer of ice with varying lattice parameters
and interaction strengths. We show that what determines the propensity for
nucleation is not just the surface attraction, but also the orientational
ordering imposed on liquid water near a surface. In particular, varying the
ratio of the surface's attraction and orientational ordering can change the
mechanism by which nucleation occurs: ice can nucleate on the structured
surface even when the orientational ordering imposed by the surface is weak, as
the water molecules that interact strongly with the surface are themselves a
good template for further growth. We also show that lattice matching is
important for heterogeneous nucleation on the structured surface we study. We
rationalise these brute-force simulation results by explicitly calculating the
interfacial free energies of ice and liquid water in contact with the
nucleating surface and their variation with surface interaction parameters
Thermodynamics of Community Structure
We introduce an approach to partitioning networks into communities that not
only determines the best community structure, but also provides a range of
characterization techniques to assess how significant that structure is. We
study the thermodynamics of community structure by producing equilibrium
ensembles of partitions, in which each partition is represented with a
well-defined statistical weight. Thus we are able to study the temperature
dependence of thermodynamic properties, namely the modularity and heat
capacity, with particular emphasis on the transition between high-temperature,
essentially random partitions and low-temperature partitions with high
modularity. We also look at frequency matrices that measure the likelihood that
two nodes belong to the same community, and introduce an order parameter to
measure the `blockiness' of the frequency matrix, and therefore the uniqueness
of the community structure. These methods have been applied to a number of
model networks in order to understand the effects of the degree distribution,
spatial embedding and randomization. Finally, we apply these methods to a
metabolic network known to have strong community structure and find
hierarchical community structure, with some communities being more robust than
others.Comment: 12 pages, 16 figure
Energy landscapes, scale-free networks and Apollonian packings
We review recent results on the topological properties of two spatial
scale-free networks, the inherent structure and Apollonian networks. The
similarities between these two types of network suggest an explanation for the
scale-free character of the inherent structure networks. Namely, that the
energy landscape can be viewed as a fractal packing of basins of attraction.Comment: 10 pages, 8 figure
Protein crystallization in vivo
Protein crystallization in vivo provides some fascinating examples of
biological self-assembly. Here, we provide a selective survey to show the
diversity of functions for which protein crystals are used, and the physical
properties of the crystals thatare exploited. Where known, we emphasize how the
nature of the protein-protein interactions leads to control of the
crystallization behaviour.Comment: 17 pages, 1 figur
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