92 research outputs found
Structural transitions in the 309-atom magic number Lennard-Jones cluster
The thermal behaviour of the 309-atom Lennard-Jones cluster, whose structure
is a complete Mackay icosahedron, has been studied by parallel tempering Monte
Carlo simulations. Surprisingly for a magic number cluster, the heat capacity
shows a very pronounced peak before melting, which is attributed to several
coincident structural transformation processes. The main transformation is
somewhat akin to surface roughening, and involves a cooperative condensation of
vacancies and adatoms that leads to the formation of pits and islands one or
two layers thick on the Mackay icosahedron. The second transition in order of
importance involves a whole scale transformation of the cluster structure, and
leads to a diverse set of twinned structures that are assemblies of
face-centred-cubic tetrahedra with 6 atoms along their edges, i.e., one atom
more than the edges of the 20 tetrahedra that make up the 309-atom Mackay
icosahedron. A surface reconstruction of the icosahedron from a Mackay to an
anti-Mackay overlayer is also observed, but with a lower probability.Comment: 7 pages, 4 figure
Surface phase transitions and crystal habits of ice in the atmosphere
With climate modeling predicting a raise of at least 2°C by year 2100, the fate of ice has become a serious concern, but we still do not understand how ice grows (or melts). In the atmosphere, crystal growth rates of basal and prism facets exhibit an enigmatic temperature dependence and crossover up to three times in a range between 0° and −40°. Here, we use large-scale computer simulations to characterize the ice surface and identify a sequence of previously unidentified phase transitions on the main facets of ice crystallites. Unexpectedly, we find that as temperature is increased, the crystal surface transforms from a disordered phase with proliferation of steps to a smooth phase with small step density. This causes the anomalous increase of step free energies and provides the long sought explanation for the enigmatic crossover of snow crystal growth rates found in the atmosphere
Programming patchy particles to form three-dimensional dodecagonal quasicrystals
Model patchy particles have been shown to be able to form a wide variety of
structures, including symmetric clusters, complex crystals and even
two-dimensional quasicrystals. Here, we investigate whether we can design
patchy particles that form three-dimensional quasicrystals, in particular
targeting a quasicrystal with dodecagonal symmetry that is made up of stacks of
two-dimensional quasicrystalline layers. We obtain two designs that are able to
form such a dodecagonal quasicrystal in annealing simulations. The first is a
one-component system of 7-patch particles but with wide patches that allow them
to adopt both 7- and 8-coordinated environments. The second is a ternary system
that contains a mixture of 7- and 8-patch particles, and is likely to be more
realizable in experiments, for example, using DNA origami. One interesting
feature of the first system is that the resulting quasicrystals very often
contain a screw dislocation.Comment: 10 pages, 6 figure
Reversible self-assembly of patchy particles into monodisperse icosahedral clusters
We systematically study the design of simple patchy sphere models that
reversibly self-assemble into monodisperse icosahedral clusters. We find that
the optimal patch width is a compromise between structural specificity (the
patches must be narrow enough to energetically select the desired clusters) and
kinetic accessibility (they must be sufficiently wide to avoid kinetic traps).
Similarly, for good yields the temperature must be low enough for the clusters
to be thermodynamically stable, but the clusters must also have enough thermal
energy to allow incorrectly formed bonds to be broken. Ordered clusters can
form through a number of different dynamic pathways, including direct
nucleation and indirect pathways involving large disordered intermediates. The
latter pathway is related to a reentrant liquid-to-gas transition that occurs
for intermediate patch widths upon lowering the temperature. We also find that
the assembly process is robust to inaccurate patch placement up to a certain
threshold, and that it is possible to replace the five discrete patches with a
single ring patch with no significant loss in yield.Comment: 12 pages, 12 figure
A three dimensional integral equation approach for fluids under confinement: Argon in zeolites
Peer Reviewe
The phase diagram of water from quantum simulations
The phase diagram of water has been calculated for the TIP4PQ/2005 model, an
empirical rigid non-polarisable model. The path integral Monte Carlo technique
was used, permitting the incorporation of nuclear quantum effects. The
coexistence lines were traced out using the Gibbs-Duhem integration method,
once having calculated the free energies of the liquid and solid phases in the
quantum limit, which were obtained via thermodynamic integration from the
classical value by scaling the mass of the water molecule. The resulting phase
diagram is qualitatively correct, being displaced to lower temperatures by
15-20K. It is found that the influence of nuclear quantum effects are
correlated to the tetrahedral order parameter.Comment: 10 pages, 6 figures, 1 tabl
Self-assembly scenarios of patchy colloidal particles
The rapid progress in precisely designing the surface decoration of patchy
colloidal particles offers a new, yet unexperienced freedom to create building
entities for larger, more complex structures in soft matter systems. However,
it is extremely difficult to predict the large variety of ordered equilibrium
structures that these particles are able to undergo under the variation of
external parameters, such as temperature or pressure. Here we show that, by a
novel combination of two theoretical tools, it is indeed possible to predict
the self-assembly scenario of patchy colloidal particles: on one hand, a
reliable and efficient optimization tool based on ideas of evolutionary
algorithms helps to identify the ordered equilibrium structures to be expected
at T = 0; on the other hand, suitable simulation techniques allow to estimate
via free energy calculations the phase diagram at finite temperature. With
these powerful approaches we are able to identify the broad variety of emerging
self-assembly scenarios for spherical colloids decorated by four patches and we
investigate and discuss the stability of the crystal structures on modifying in
a controlled way the tetrahedral arrangement of the patches.Comment: 11 pages, 7 figures, Soft Matter Communication (accepted
Rounded layering transitions on the surface of ice
Understanding the wetting properties of premelting films requires knowledge of the film’s equation
of state, which is not usually available. Here we calculate the disjoining pressure curve of premelting
films, and perform a detailed thermodynamic characterization of premelting behavior on ice. Analysis of the density profiles reveals the signature of weak layering phenomena, from one to two and
from two to three water molecular layers. However, disjoining pressure curves, which closely follow
expectations from a renormalized mean field liquid state theory, show that there are no layering
phase transitions in the thermodynamic sense along the sublimation line. Instead, we find that
transitions at mean field level are rounded due to capillary wave fluctuations. We see signatures
that true first order layering transitions could arise at low temperatures, for pressures between the
metastable line of water/vapor coexistence and the sublimation line. The extrapolation of the disjoining pressure curve above water vapor saturation displays a true first order phase transition from
a thin to a thick film consistent with experimental observations
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