137 research outputs found
Numerical modelling of non-ionic microgels: an overview
Microgels are complex macromolecules. These colloid-sized polymer networks
possess internal degrees of freedom and, depending on the polymer(s) they are
made of, can acquire a responsiveness to variations of the environment
(temperature, pH, salt concentration, etc.). Besides being valuable for many
practical applications, microgels are also extremely important to tackle
fundamental physics problems. As a result, these last years have seen a rapid
development of protocols for the synthesis of microgels, and more and more
research has been devoted to the investigation of their bulk properties.
However, from a numerical standpoint the picture is more fragmented, as the
inherently multi-scale nature of microgels, whose bulk behaviour crucially
depends on the microscopic details, cannot be handled at a single level of
coarse-graining. Here we present an overview of the methods and models that
have been proposed to describe non-ionic microgels at different length-scales,
from the atomistic to the single-particle level. We especially focus on
monomer-resolved models, as these have the right level of details to capture
the most important properties of microgels, responsiveness and softness. We
suggest that these microscopic descriptions, if realistic enough, can be
employed as starting points to develop the more coarse-grained representations
required to investigate the behaviour of bulk suspensions
Unveiling the complex glassy dynamics of square shoulder systems: simulations and theory
We performed extensive molecular dynamics (MD) simulations, supplemented by
Mode Coupling Theory (MCT) calculations, for the Square Shoulder (SS) model, a
purely repulsive potential where the hard-core is complemented by a finite
shoulder. For the one-component version of this model, MCT predicted [Sperl
{\it et al.} Phys. Rev. Lett. {\bf 104}, 145701 (2010)] the presence of
diffusion anomalies both upon cooling and upon compression and the occurrence
of glass-glass transitions. In the simulations, we focus on a non-crystallising
binary mixture, which, at the investigated shoulder width, shows a
non-monotonic behaviour of the diffusion upon cooling but not upon isothermal
compression. In addition, we find the presence of a disconnected glass-glass
line in the phase diagram, ending in two higher-order singularities. These
points generate a logarithmic dependence of the density correlators as well as
a subdiffusive behaviour of the mean squared displacement, although with the
interference of the nearby liquid-glass transition. We also perform novel MCT
calculations using as input the partial structure factors obtained within MD,
confirming the simulation results. The presence of two hard sphere glasses,
differing only in their hard core length, is revealed, showing that the simple
competition between the two is sufficient for creating a rather complex
dynamical behaviour
Tuning effective interactions close to the critical point in colloidal suspensions
We report a numerical investigation of two colloids immersed in a critical
solvent, with the aim of quantifying the effective colloid-colloid interaction
potential. By turning on an attraction between the colloid and the solvent
particles we follow the evolution from the case in which the solvent density
close to the colloids changes from values smaller than the bulk to values
larger than the bulk. We thus effectively implement the so-called and
boundary conditions defined in field theoretical approaches focused on
the description of critical Casimir forces. We find that the effective
potential at large distances decays exponentially, with a characteristic decay
length compatible with the bulk critical correlation length, in full agreement
with theoretical predictions. We also investigate the case of boundary
condition, where the effective potential becomes repulsive. Our study provides
a guidance for a design of the interaction potential which can be exploited to
control the stability of colloidal systems
Crystal-to-crystal transition of ultrasoft colloids under shear
Ultrasoft colloids typically do not spontaneously crystallize, but rather
vitrify, at high concentrations. Combining in-situ rheo-SANS experiments and
numerical simulations we show that shear facilitates crystallization of
colloidal star polymers in the vicinity of their glass transition. With
increasing shear rate well beyond rheological yielding, a transition is found
from an initial bcc-dominated structure to an fcc-dominated one. This
crystal-to-crystal transition is not accompanied by intermediate melting but
occurs via a sudden reorganization of the crystal structure. Our results
provide a new avenue to tailor colloidal crystallization and crystal-to-crystal
transition at molecular level by coupling softness and shear
Modelling microgels with controlled structure across the volume phase transition
Thermoresponsive microgels are soft colloids that find widespread use as
model systems for soft matter physics. Their complex internal architecture,
made of a disordered and heterogeneous polymer network, has been so far a major
challenge for computer simulations. In this work we put forward a
coarse-grained model of microgels whose structural properties are in
quantitative agreement with results obtained with small-angle X-ray scattering
experiments across a wide range of temperatures, encompassing the volume phase
transition. These results bridge the gap between experiments and simulations of
individual microgel particles, paving the way to theoretically address open
questions about their bulk properties with unprecedented nano and microscale
resolution
Aging effects manifested in the potential energy landscape of a model glass former
We present molecular dynamics simulations of a model glass-forming liquid
(the binary Kob-Anderson Lennard-Jones model) and consider the distributions of
inherent energies and metabasins during aging. In addition to the typical
protocol of performing a temperature jump from a high temperature to a low
destination temperature, we consider the temporal evolution of the
distributions after an 'up-jump', i.e. from a low to a high temperature. In
this case the distribution of megabasin energies exhibits a transient two-peak
structure. Our results can qualitatively be rationalized in terms of a trap
model with a Gaussian distribution of trap energies. The analysis is performed
for different system sizes. A detailed comparison with the trap model is
possible only for a small system because of major averging effects for larger
systems.Comment: 16 pages, 14 figure
Tuning the rheological behavior of colloidal gels through competing interactions
We study colloidal gels formed by competing electrostatic repulsion and short-range attraction by means of extensive numerical simulations under external shear. We show that, upon varying the repulsion strength, the gel structure and its viscoelastic properties can be largely tuned. In particular, the gel fractal dimension can be either increased or decreased with respect to mechanical equilibrium conditions. Unexpectedly, gels with stronger repulsion, despite being mechanically stiffer, are found to be less viscous with respect to purely attractive ones. We provide a microscopic explanation of these findings in terms of the influence of an underlying phase separation. Our results allow for the design of colloidal gels with desired structure and viscoelastic response by means of additional electrostatic interactions, easily controllable in experiments
Predicting the Effective Temperature of a Glass
We explain the findings by Di Leonardo et al. [Phys. Rev. Lett. 84, 6054
(2000)] that the effective temperature of a Lennard-Jones glass depends only on
the final value of the density in the volume and/or temperature jump that
produces the glass phase. This is not only a property of the Lennard-Jones
liquid, but a feature of all strongly correlating liquids. For such liquids
data from a single quench simulation provides enough information to predict the
effective temperature of any glass produced by jumping from an equilibrium
state. This prediction is validated by simulations of the Kob-Andersen binary
Lennard-Jones liquid and shown not to apply for the non-strongly correlating
monatomic Lennard-Jones Gaussian liquid.Comment: 5 pages, 6 figure
Properties of patchy colloidal particles close to a surface: a Monte Carlo and density functional study
We investigate the behavior of a patchy particle model close to a hard-wall
via Monte Carlo simulation and density functional theory (DFT). Two DFT
approaches, based on the homogeneous and inhomogeneous versions of Wertheim's
first order perturbation theory for the association free energy are used. We
evaluate, by simulation and theory, the equilibrium bulk phase diagram of the
fluid and analyze the surface properties for two isochores, one of which is
close to the liquid side of the gas-liquid coexistence curve. We find that the
density profile near the wall crosses over from a typical high-temperature
adsorption profile to a low-temperature desorption one, for the isochore close
to coexistence. We relate this behavior to the properties of the bulk network
liquid and find that the theoretical descriptions are reasonably accurate in
this regime. At very low temperatures, however, an almost fully bonded network
is formed, and the simulations reveal a second adsorption regime which is not
captured by DFT. We trace this failure to the neglect of orientational
correlations of the particles, which are found to exhibit surface induced
orientational order in this regime
Pressure-energy correlations in liquids. V. Isomorphs in generalized Lennard-Jones systems
This series of papers is devoted to identifying and explaining the properties
of strongly correlating liquids, i.e., liquids with more than 90% correlation
between their virial W and potential energy U fluctuations in the NVT ensemble.
Paper IV [N. Gnan et al., J. Chem. Phys. v131, 234504 (2009)] showed that
strongly correlating liquids have "isomorphs", which are curves in the phase
diagram along which structure, dynamics, and some thermodynamic properties are
invariant in reduced units. In the present paper, using the fact that
reduced-unit radial distribution functions are isomorph invariant, we derive an
expression for the shapes of isomorphs in the WU phase diagram of generalized
Lennard-Jones systems of one or more types of particles. The isomorph shape
depends only on the Lennard-Jones exponents; thus all isomorphs of standard
Lennard-Jones systems (with exponents 12 and 6) can be scaled onto to a single
curve. Two applications are given. One is testing the prediction that the
solid-liquid coexistence curve follows an isomorph by comparing to recent
simulations by Ahmed and Sadus [J. Chem. Phys. v131, 174504 (2009)]. Excellent
agreement is found on the liquid side of the coexistence, whereas the agreement
is worse on the solid side. A second application is the derivation of an
approximate equation of state for generalized Lennard-Jones systems by
combining the isomorph theory with the Rosenfeld-Tarazona expression for the
temperature dependence of potential energy on isochores. It is shown that the
new equation of state agrees well with simulations.Comment: 12 pages, 14 figures, Section on solid-liquid coexistence expande
- …