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Confined colloidal crystals in and out of equilibrium
Recent studies on confined crystals of charged colloidal particles are
reviewed, both in equilibrium and out of equilibrium. We focus in particular on
direct comparisons of experiments (light scattering and microscopy) with
lattice sum calculations and computer simulations. In equilibrium we address
buckling and crystalline multilayering of charged systems in hard and soft slit
confinement. We discuss also recent crystalline structures obtained for charged
mixtures. Moreover, we put forward possibilities to apply external
perturbations, in order to drive the system out of equilibrium. These include
electrolyte gradients as well as the application of shear and electric fields.Comment: Review article, 18 pages, 5 figure
Conformation of a Polyelectrolyte Complexed to a Like-Charged Colloid
We report results from a molecular dynamics (MD) simulation on the
conformations of a long flexible polyelectrolyte complexed to a charged sphere,
\textit{both negatively charged}, in the presence of neutralizing counterions
in the strong Coulomb coupling regime. The structure of this complex is very
sensitive to the charge density of the polyelectrolyte. For a fully charged
polyelectrolyte the polymer forms a dense two-dimensional "disk", whereas for a
partially charged polyelectrolyte the monomers are spread over the colloidal
surface. A mechanism involving the \textit{overcharging} of the polyelectrolyte
by counterions is proposed to explain the observed conformations.Comment: 4 pages, 4 figures (6 EPS files
On the age of the magnetically active WW Psa and TX Psa members of the beta Pictoris association
There are a variety of different techniques available to estimate the ages of
pre-main-sequence stars. Components of physical pairs, thanks to their strict
coevality and the mass difference, such as the binary system analysed in this
paper, are best suited to test the effectiveness of these different techniques.
We consider the system WW Psa + TX Psa whose membership of the 25-Myr beta
Pictoris association has been well established by earlier works. We investigate
which age dating technique provides the best agreement between the age of the
system and that of the association. We have photometrically monitored WW Psa
and TX Psa and measured their rotation periods as P = 2.37d and P = 1.086d,
respectively. We have retrieved from the literature their Li equivalent widths
and measured their effective temperatures and luminosities. We investigate
whether the ages of these stars derived using three independent techniques are
consistent with the age of the beta Pictoris association. We find that the
rotation periods and the Li contents of both stars are consistent with the
distribution of other bona fide members of the cluster. On the contrary, the
isochronal fitting provides similar ages for both stars, but a factor of about
four younger than the quoted age of the association, or about 30% younger when
the effects of magnetic fields are included. We explore the origin of the
discrepant age inferred from isochronal fitting, including the possibilities
that either the two components may be unresolved binaries or that the basic
stellar parameters of both components are altered by enhanced magnetic
activity. The latter is found to be the more reasonable cause, suggesting that
age estimates based on the Li content is more reliable than isochronal fitting
for pre-main-sequence stars with pronounced magnetic activity.Comment: Accepted by Astronomy and Astrophysics on December 13, 2016. 13 pages
and 11 figure
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