734 research outputs found
The structure of colloid-polymer mixtures
We investigate the structure of colloid-polymer mixtures by calculating the
structure factors for the Asakura-Oosawa model in the PY approximation. We
discuss the role of potential range, polymer concentration and polymer-polymer
interactions on the colloid-colloid structure. Our results compare reasonably
well with the recent experiments of Moussa\"{i}d et. al. for small wavenumber
, but we find that the Hansen-Verlet freezing criterion is violated when the
liquid phase becomes marginal.Comment: 7 pages, 4 figures, to appear in EuroPhys. Let
Crystallization and phase-separation in non-additive binary hard-sphere mixtures
We calculate for the first time the full phase-diagram of an asymmetric
non-additive hard-sphere mixture. The non-additivity strongly affects the
crystallization and the fluid-fluid phase-separation. The global topology of
the phase-diagram is controlled by an effective size-ratio y_{eff}, while the
fluid-solid coexistence scales with the depth of the effective potential well.Comment: 4 pages, 4 figures, to appear in Phys. Rev.
Coarse-graining polymers as soft colloids
We show how to coarse grain polymers in a good solvent as single particles,
interacting with density-independent or density-dependent interactions. These
interactions can be between the centres of mass, the mid-points or end-points
of the polymers. We also show how to extend these methods to polymers in poor
solvents and mixtures of polymers. Treating polymers as soft colloids can
greatly speed up the simulation of complex many-polymer systems, including
polymer-colloid mixtures.Comment: to appear in Physica A, special STATPHYS 2001 edition. Content of
invited talk by AA
Wrinkling of microcapsules in shear flow
Elastic capsules can exhibit short wavelength wrinkling in external shear
flow. We analyse this instability of the capsule shape and use the length scale
separation between the capsule radius and the wrinkling wavelength to derive
analytical results both for the threshold value of the shear rate and for the
critical wave-length of the wrinkling. These results can be used to deduce
elastic parameters from experiments.Comment: 4 pages, 2 figures, submitted to PR
Micro-Capsules in Shear Flow
This paper deals with flow-induced shape transitions of elastic capsules. The
state of the art concerning both theory and experiments is briefly reviewed
starting with dynamically induced small deformation of initially spherical
capsules and the formation of wrinkles on polymerized membranes. Initially
non-spherical capsules show tumbling and tank-treading motion in shear flow.
Theoretical descriptions of the transition between these two types of motion
assuming a fixed shape are at variance with the full capsule dynamics obtained
numerically. To resolve the discrepancy, we expand the exact equations of
motion for small deformations and find that shape changes play a dominant role.
We classify the dynamical phase transitions and obtain numerical and analytical
results for the phase boundaries as a function of viscosity contrast, shear and
elongational flow rate. We conclude with perspectives on timedependent flow, on
shear-induced unbinding from surfaces, on the role of thermal fluctuations, and
on applying the concepts of stochastic thermodynamics to these systems.Comment: 34 pages, 15 figure
Phase transitions in simple and not so simple binary fluids
Compared to pure fluids, binary mixtures display a very diverse phase
behavior, which depends sensitively on the parameters of the microscopic
potential. Here we investigate the phase diagrams of simple model mixtures by
use of a microscopic implementation of the renormalization group technique.
First, we consider a symmetric mixture with attractive interactions, possibly
relevant for describing fluids of molecules with internal degrees of freedom.
Despite the simplicity of the model, slightly tuning the strength of the
interactions between unlike species drastically changes the topology of the
phase boundary, forcing or inhibiting demixing, and brings about several
interesting features such as double critical points, tricritical points, and
coexistence domains enclosing `islands' of homogeneous, mixed fluid.
Homogeneous phase separation in mixtures can be driven also by purely repulsive
interactions. As an example, we consider a model of soft particles which has
been adopted to describe binary polymer solutions. This is shown to display
demixing (fluid-fluid) transition at sufficiently high density. The nature and
the physical properties of the corresponding phase transition are investigated.Comment: 6 pages + 3 figures, presented at the 5th EPS Liquid Matter
Conference, Konstanz, 14-18 September 200
Phase separation of a multiple occupancy lattice gas
A binary lattice gas model that allows for multiple occupancy of lattice
sites, inspired by recent coarse-grained descriptions of solutions of
interacting polymers, is investigated by combining the steepest descent
approximation with an exploration of the multidimensional energy landscape, and
by Gibbs ensemble Monte Carlo simulations. The one-component version of the
model, involving on site and nearest neighbour interactions, is shown to
exhibit microphase separation into two sub-lattices with different mean
occupation numbers. The symmetric two-component version of the multiple
occupancy lattice gas is shown to exhibit a demixing transition into two phases
above a critical mean occupation number.Comment: submitted to Journal of Physics
Long-term stability of cortisol production and metabolism throughout adolescence: longitudinal twin study
Life-course experiences have been postulated to program hypothalamus-pituitary-adrenal (HPA) axis activity, suggesting that HPA axis activity is, at least partially, stable over time. Yet, there is paucity of data on the long-term stability of cortisol production and metabolism. We performed a prospective follow-up study in twins recruited from a nationwide register to estimate the stability of cortisol production and metabolism over time, and the contribution of genetic and environmental factors to this stability. In total, 218 healthy mono- and dizygotic twins were included. At the ages of 9, 12 and 17 years, morning urine samples were collected for assessment (by gas chromatography-tandem mass spectrometry) of cortisol metabolites, enabling the calculation of cortisol metabolite excretion rate and cortisol metabolism activity. Our results showed a low stability for both cortisol metabolite excretion rate (with correlations <.20) and cortisol metabolism activity indices (with correlations of .25 to .46 between 9 and 12 years, -.02 to .15 between 12 and 17 years and .09 to .28 between 9 and 17 years). Because of the low stability over time, genetic and environmental contributions to this stability were difficult to assess, although it seemed to be mostly determined by genetic factors. The low stability in both cortisol production and metabolism between ages 9 and 17 years reflects the dynamic nature of the HPA axis
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