255 research outputs found
Pair interactions between complex mesoscopic particles from Widom's particle-insertion method
We demonstrate that Widom's particle insertion technique provides a
convenient and efficient method to determine the effective pair interaction
between complex, composite soft-matter particles in the zero-density limit. By
means of three different test systems, i.e. amphiphilic dendrimers,
electrostatic polymers and colloids coated with electrostatic polymers, we
demonstrate the validity and the power of the presented method.Comment: 7 pages, 4 figures, to be published in Soft Matte
Are the energy and virial routes to thermodynamics equivalent for hard spheres?
The internal energy of hard spheres (HS) is the same as that of an ideal gas,
so that the energy route to thermodynamics becomes useless. This problem can be
avoided by taking an interaction potential that reduces to the HS one in
certain limits. In this paper the square-shoulder (SS) potential characterized
by a hard-core diameter , a soft-core diameter and a
shoulder height is considered. The SS potential becomes the HS one
if (i) , or (ii) , or (iii)
or (iv) and . The
energy-route equation of state for the HS fluid is obtained in terms of the
radial distribution function for the SS fluid by taking the limits (i) and
(ii). This equation of state is shown to exhibit, in general, an artificial
dependence on the diameter ratio . If furthermore the limit
is taken, the resulting equation of state for HS
coincides with that obtained through the virial route. The necessary and
sufficient condition to get thermodynamic consistency between both routes for
arbitrary is derived.Comment: 10 pages, 4 figures; v2: minor changes; to be published in the
special issue of Molecular Physics dedicated to the Seventh Liblice
Conference on the Statistical Mechanics of Liquids (Lednice, Czech Republic,
June 11-16, 2006
Phase coexistence of cluster crystals: beyond the Gibbs phase rule
We report a study of the phase behavior of multiple-occupancy crystals
through simulation. We argue that in order to reproduce the equilibrium
behavior of such crystals it is essential to treat the number of lattice sites
as a constraining thermodynamic variable. The resulting free-energy
calculations thus differ considerably from schemes used for single-occupancy
lattices. Using our approach, we obtain the phase diagram and the bulk modulus
for a generalized exponential model that forms cluster crystals at high
densities. We compare the simulation results with existing theoretical
predictions. We also identify two types of density fluctuations that can lead
to two sound modes and evaluate the corresponding elastic constants.Comment: 4 pages, 3 figure
Procedure to construct a multi-scale coarse-grained model of DNA-coated colloids from experimental data
We present a quantitative, multi-scale coarse-grained model of DNA coated
colloids. The parameters of this model are transferable and are solely based on
experimental data. As a test case, we focus on nano-sized colloids carrying
single-stranded DNA strands of length comparable to the colloids' size. We show
that in this regime, the common theoretical approach of assuming pairwise
additivity of the colloidal pair interactions leads to quantitatively and
sometimes even qualitatively wrong predictions of the phase behaviour of
DNA-grafted colloids. Comparing to experimental data, we find that our
coarse-grained model correctly predicts the equilibrium structure and melting
temperature of the formed solids. Due to limited experimental information on
the persistence length of single-stranded DNA, some quantitative discrepancies
are found in the prediction of spatial quantities. With the availability of
better experimental data, the present approach provides a path for the rational
design of DNA-functionalised building blocks that can self-assemble in complex,
three-dimensional structures.Comment: 17 pages, 10 figures; to be published in Soft Matte
Formation of Polymorphic Cluster Phases for Purely Repulsive Soft Spheres
We present results from density functional theory and computer simulations
that unambiguously predict the occurrence of first-order freezing transitions
for a large class of ultrasoft model systems into cluster crystals. The
clusters consist of fully overlapping particles and arise without the existence
of attractive forces. The number of particles participating in a cluster scales
linearly with density, therefore the crystals feature density-independent
lattice constants. Clustering is accompanied by polymorphic bcc-fcc
transitions, with fcc being the stable phase at high densities.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Let
Computer Assembly of Cluster-Forming Amphiphilic Dendrimers
Recent theoretical studies have predicted a new clustering mechanism for soft
matter particles that interact via a certain kind of purely repulsive, bounded
potentials. At sufficiently high densities, clusters of overlapping particles
are formed in the fluid, which upon further compression crystallize into cubic
lattices with density-independent lattice constants. In this work we show that
amphiphilic dendrimers are suitable colloids for the experimental realization
of this phenomenon. Thereby, we pave the way for the synthesis of such
macromolecules, which form the basis for a novel class of materials with
unusual properties.Comment: 4 pages, 4 figures, 1 tabl
Thermodynamically self-consistent liquid state theories for systems with bounded potentials
The mean spherical approximation (MSA) can be solved semi-analytically for
the Gaussian core model (GCM) and yields - rather surprisingly - exactly the
same expressions for the energy and the virial equations. Taking advantage of
this semi-analytical framework, we apply the concept of the self-consistent
Ornstein-Zernike approximation (SCOZA) to the GCM: a state-dependent function K
is introduced in the MSA closure relation which is determined to enforce
thermodynamic consistency between the compressibility route and either the
virial or energy route. Utilizing standard thermodynamic relations this leads
to two different differential equations for the function K that have to be
solved numerically. Generalizing our concept we propose an
integro-differential-equation based formulation of the SCOZA which, although
requiring a fully numerical solution, has the advantage that it is no longer
restricted to the availability of an analytic solution for a particular system.
Rather it can be used for an arbitrary potential and even in combination with
other closure relations, such as a modification of the hypernetted chain
approximation.Comment: 11 pages, 11 figures, submitted to J. Chem. Phy
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