63 research outputs found
Anomalous phase behavior of a soft-repulsive potential with a strictly monotonic force
We study the phase behavior of a classical system of particles interacting
through a strictly convex soft-repulsive potential which, at variance with the
pairwise softened repulsions considered so far in the literature, lacks a
region of downward or zero curvature. Nonetheless, such interaction is
characterized by two length scales, owing to the presence of a range of
interparticle distances where the repulsive force increases, for decreasing
distance, much more slowly than in the adjacent regions. We investigate, using
extensive Monte Carlo simulations combined with accurate free-energy
calculations, the phase diagram of the system under consideration. We find that
the model exhibits a fluid-solid coexistence line with multiple re-entrant
regions, an extremely rich solid polymorphism with solid-solid transitions, and
water-like anomalies. In spite of the isotropic nature of the interparticle
potential, we find that, among the crystal structures in which the system can
exist, there are also a number of non-Bravais lattices, such as cI16 and
diamond.Comment: 21 pages, 7 figures, in press on Phys. Rev.
Statistical entropy and clustering in absence of attractive terms in the interparticle potential
Recently a new intriguing class of systems has been introduced, the so-called generalized exponential models, which exhibit clustering phenomena even if the attractive term is missing in their interaction potential. This model is characterized by a index n which tunes the repulsive penetrability of the potential. This family of potentials can represent the effective interactions for a large number of soft matter systems. In this paper we study the structural and thermodynamic properties in the fluid regime of the generalized exponential model with a value of index n suggested by Mladek et al. [B. M. Mladek, G. Kahl, and C. N. Likos, Phys. Rev. Lett. (2008)] to fit the effective potential of a typical amphiphilic dendrimers. We use the conventional approach of the liquid state theory based on the hypernetted chain closure of the Ornstein-Zernike equation together with some Monte Carlo numerical simulations. Moreover, we try to detect qualitatively the freezing line exploiting the predictive properties of a one-phase rule based on the expansion of the statistical entropy
Hexatic phase and water-like anomalies in a two-dimensional fluid of particles with a weakly softened core
We study a two-dimensional fluid of particles interacting through a
spherically-symmetric and marginally soft two-body repulsion. This model can
exist in three different crystal phases, one of them with square symmetry and
the other two triangular. We show that, while the triangular solids first melt
into a hexatic fluid, the square solid is directly transformed on heating into
an isotropic fluid through a first-order transition, with no intermediate
tetratic phase. In the low-pressure triangular and square crystals melting is
reentrant provided the temperature is not too low, but without the necessity of
two competing nearest-neighbor distances over a range of pressures. A whole
spectrum of water-like fluid anomalies completes the picture for this model
potential.Comment: 26 pages, 14 figures; printed article available at
http://link.aip.org/link/?jcp/137/10450
Residual Multiparticle Entropy for a Fractal Fluid of Hard Spheres
The residual multiparticle entropy (RMPE) of a fluid is defined as the
difference, , between the excess entropy per particle (relative to an
ideal gas with the same temperature and density), , and the
pair-correlation contribution, . Thus, the RMPE represents the net
contribution to due to spatial correlations involving three,
four, or more particles. A heuristic `ordering' criterion identifies the
vanishing of the RMPE as an underlying signature of an impending structural or
thermodynamic transition of the system from a less ordered to a more spatially
organized condition (freezing is a typical example). Regardless of this, the
knowledge of the RMPE is important to assess the impact of non-pair
multiparticle correlations on the entropy of the fluid. Recently, an accurate
and simple proposal for the thermodynamic and structural properties of a
hard-sphere fluid in fractional dimension has been proposed [Santos,
A.; L\'opez de Haro, M. \emph{Phys. Rev. E} \textbf{2016}, \emph{93}, 062126].
The aim of this work is to use this approach to evaluate the RMPE as a function
of both and the packing fraction . It is observed that, for any given
dimensionality , the RMPE takes negative values for small densities, reaches
a negative minimum at a packing fraction
, and then rapidly increases, becoming positive beyond a
certain packing fraction . Interestingly, while both
and monotonically decrease as dimensionality
increases, the value of exhibits a nonmonotonic
behavior, reaching an absolute minimum at a fractional dimensionality . A plot of the scaled RMPE shows a
quasiuniversal behavior in the region .Comment: 10 pages, 3 figures; v2: minor change
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