45 research outputs found
Forces between elongated particles in a nematic colloid
Using molecular dynamics simulations we study the interactions between elongated colloidal particles (length to breath ratio ≫1) in a nematic host. The simulation results are compared to the results of a Landau–de Gennes elastic free energy. We find that depletion forces dominate for the sizes of the colloidal particles studied. The tangential component of the force, however, allows us to resolve the elastic contribution to the total interaction. We find that this contribution differs from the quadrupolar interaction predicted at large separations. The difference is due to the presence of nonlinear effects, namely, the change in the positions and structure of the defects and their annihilation at small separations
Simulating Particle Dispersions in Nematic Liquid-Crystal Solvents
A new method is presented for mesoscopic simulations of particle dispersions
in nematic liquid crystal solvents. It allows efficient first-principle
simulations of the dispersions involving many particles with many-body
interactions mediated by the solvents. A simple demonstration is shown for the
aggregation process of a two dimentional dispersion.Comment: 5 pages, 5 figure
A Smooth Interface Method for Simulating Liquid Crystal Colloid Dispersions
A new method is presented for mesoscopic simulations of particle dispersions
in liquid crystal solvents. It allows efficient first-principle simulations of
the dispersions involving many particles with many-body interactions mediated
by the solvents. Demonstrations have been performed for the aggregation of
colloid dispersions in two-dimensional nematic and smectic-C* solvents
neglecting hydrodynamic effects, which will be taken into account in the near
future.Comment: 13 pages, 4 figure
Effective forces in colloidal mixtures: from depletion attraction to accumulation repulsion
Computer simulations and theory are used to systematically investigate how
the effective force between two big colloidal spheres in a sea of small spheres
depends on the basic (big-small and small-small) interactions. The latter are
modeled as hard-core pair potentials with a Yukawa tail which can be both
repulsive or attractive. For a repulsive small-small interaction, the effective
force follows the trends as predicted by a mapping onto an effective
non-additive hard-core mixture: both a depletion attraction and an accumulation
repulsion caused by small spheres adsorbing onto the big ones can be obtained
depending on the sign of the big-small interaction. For repulsive big-small
interactions, the effect of adding a small-small attraction also follows the
trends predicted by the mapping. But a more subtle ``repulsion through
attraction'' effect arises when both big-small and small-small attractions
occur: upon increasing the strength of the small-small interaction, the
effective potential becomes more repulsive. We have further tested several
theoretical methods against our computer simulations: The superposition
approximation works best for an added big-small repulsion, and breaks down for
a strong big-small attraction, while density functional theory is very accurate
for any big-small interaction when the small particles are pure hard-spheres.
The theoretical methods perform most poorly for small-small attractions.Comment: submitted to PRE; New version includes an important quantitative
correction to several of the simulations. The main conclusions remain
unchanged thoug
Emigration rates and population turnover of teal Anas crecca in two major wetlands of western Europe
Spring migration dates of Teal (Anas crecca) ringed in the Camargue
The policy of the
European Commission prohibits hunting of migratory birds while they travel to
their breeding grounds. To date, spring migration dates of ducks have mainly
been determined using bird counts, but the validity of this sometimes disputed
method has never been tested. We used ring-recovery data from close to 9,000
teal Anas crecca ringed in the Camargue, southern France, to determine the
onset of spring migration. This method makes it possible to avoid potential
biases linked to duck counts, and was used to test the validity of spring migration
dates inferred from such counts. Depending on the type of analysis (intra- or
inter-annual recoveries), teal appeared to start migrating from the Camargue
during the first or second 10-day period of January, with no significant differences
between years, and no effect of the bird’s age or sex. However, when taking
potential winter dispersion into account, we suggest that a conservative estimate
for the onset of spring migration is the first 10-day period of February. Migration
dates inferred from ring-recovery analyses were consistent with earlier results
from duck counts, and provide a firm basis for policy making related to
hunting. Though ringing data should be preferred when available, our study suggests
that determining migration dates from bird counts may be a reliable method for
teal, and potentially for other dabbling and diving ducks as well.</p
Recensements d'anatides et foulques hivernant en Afrique du Nord et de l'Ouest. Janvier 1984, 1985 et 1986
SIGLECNRS RP 185 (1916) / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc