8,019 research outputs found
Cubatic phase for tetrapods
We investigate the phase behavior of tetrapods, hard non-convex bodies formed
by 4 rods connected under tetrahedral angles. We predict that, depending on the
relative lengths of the rods these particles can form a uniaxial nematic phase,
and more surprisingly they can exhibit a cubatic phase, a special case of the
biaxial nematic phase. These predictions may be experimentally testable, as
experimental realizations of tetrapods have recently become available.Comment: 8 pages ReVTeX 4, including 3 EPS figure
Sampling along reaction coordinates with the Wang-Landau method
The multiple range random walk algorithm recently proposed by Wang and Landau
[Phys. Rev. Lett. 86, 2050 (2001)] is adapted to the computation of free energy
profiles for molecular systems along reaction coordinates. More generally, we
show how to extract partial averages in various statistical ensembles without
invoking simulations with constraints, biasing potentials or unknown
parameters. The method is illustrated on a model 10-dimensional potential
energy surface, for which analytical results are obtained. It is then applied
to the potential of mean force associated with the dihedral angle of the butane
molecule in gas phase and in carbon tetrachloride solvent. Finally,
isomerization in a small rocksalt cluster, Na4F4, is investigated in the
microcanonical ensemble, and the results are compared to those of parallel
tempering Monte Carlo.Comment: 6 pages, 5 figure
The role of long-range forces in the phase behavior of colloids and proteins
The phase behavior of colloid-polymer mixtures, and of solutions of globular
proteins, is often interpreted in terms of a simple model of hard spheres with
short-ranged attraction. While such a model yields a qualitative understanding
of the generic phase diagrams of both colloids and proteins, it fails to
capture one important difference: the model predicts fluid-fluid phase
separation in the metastable regime below the freezing curve. Such demixing has
been observed for globular proteins, but for colloids it appears to be
pre-empted by the appearance of a gel. In this paper, we study the effect of
additional long-range attractions on the phase behavior of spheres with
short-ranged attraction. We find that such attractions can shift the
(metastable) fluid-fluid critical point out of the gel region. As this
metastable critical point may be important for crystal nucleation, our results
suggest that long-ranged attractive forces may play an important role in the
crystallization of globular proteins. However, in colloids, where refractive
index matching is often used to switch off long-ranged dispersion forces,
gelation is likely to inhibit phase separation.Comment: EURO-LATEX, 6 pages, 2 figure
Vapor-liquid surface tension of strong short-range Yukawa fluid
The thermodynamic properties of strong short-range attractive Yukawa fluids,
k=10, 9, 8, and 7, are determined by combining the slab technique with the
standard and the replica exchange Monte Carlo (REMC) methods. A good agreement
was found among the coexistence curves of these systems calculated by REMC and
those previously reported in the literature. However, REMC allows exploring the
coexistence at lower temperatures, where dynamics turns glassy. To obtain the
surface tension we employed, for both methods, a procedure that yields the
pressure tensor components for discontinuous potentials. The surface tension
results obtained by the standard MC and REMC techniques are in good agreement.Comment: 6 pages, 4 figure
Anomalous Dynamic Arrest in a Mixture of Big and Small Particles
We present molecular dynamics simulations on the slow dynamics of a mixture
of big and small soft-spheres with a large size disparity. Dynamics are
investigated in a broad range of temperature and mixture composition. As a
consequence of large size disparity, big and small particles exhibit very
different relaxation times. As previously reported for simple models of
short-ranged attractive colloids and polymer blends, several anomalous dynamic
features are observed: i) sublinear behavior for mean squared displacements,
ii) concave-to-convex crossover for density-density correlators, by varying
temperature or wavevector, iii) logarithmic decay for specific wavevectors of
density-density correlators. These anomalous features are observed over time
intervals extending up to four decades, and strongly resemble predictions of
the Mode Coupling Theory (MCT) for state points close to higher-order MCT
transitions, which originate from the competition between different mechanisms
for dynamic arrest. For the big particles we suggest competition between
soft-sphere repulsion and depletion effects induced by neighboring small
particles. For the small particles we suggest competition between bulk-like
dynamics and confinement, respectively induced by neighboring small particles
and by the slow matrix of big particles. By increasing the size disparity, a
new relaxation scenario arises for the small particles. Self-correlators decay
to zero at temperatures where density-density correlations are frozen. The
behavior of the latters resembles features characteristic of type-A MCT
transitions, defined by a zero value of the critical non-ergodicity parameter.Comment: Version 2. Added major new result
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