1,552 research outputs found
Co-evolution of plumage characteristics and winter sociality in New and Old World sparrows
Understanding the evolution of phenotypic diversity, including the stunning array of avian plumage characters, is a central goal of evolutionary biology. Here, we applied a comparative analysis to test factors associated with the origin and maintenance of black chest and throat patches, which in some taxa are referred to as ‘badges-of-status’. Specifically, we tested whether the evolution of black colour patches in Old and New World sparrows is consistent with a signalling function during the nonbreeding season or breeding season. We found no positive associations between patch evolution and polygyny or summer sociality. Instead, patch evolution is significantly associated with sociality during the nonbreeding season. Additionally, unlike typical plumage characteristics under sexual selection, these patches are visible throughout the nonbreeding season. Further, the pattern of patch dimorphism uncovered in this study does not match expectations for a trait that evolved in a reproductive context. In particular, patch dimorphism is not associated with polygyny or the presence of extra-pair mating although other types of plumage dimorphism are strongly associated with nonmonogamous mating systems. Overall, patterns of patch evolution suggest that they are more strongly associated with social competition during the nonbreeding season than sexual competition during the breeding season. These results clarify why some previous work has uncovered puzzling relationships between black plumage patches and reproductive behaviour. We discuss these findings in the context of signal theory and previous work on badges-of-status.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/78723/1/j.1420-9101.2009.01861.x.pd
The lamellar-to-isotropic transition in ternary amphiphilic systems
We study the dependence of the phase behavior of ternary amphiphilic systems
on composition and temperature. Our analysis is based on a curvature elastic
model of the surfactant film with sufficiently large spontaneous curvature and
sufficiently negative saddle-splay modulus that the stable phases are the
lamellar phase and a droplet microemulsion. In addition to the curvature
energy, we consider the contributions to the free energy of the long-ranged van
der Waals interaction and of the undulation modes. We find that for bending
rigidities of order k_B T, the lamellar phase extends further and further into
the water apex of the phase diagram as the phase inversion temperature is
approached, in good agreement with experimental results.Comment: LaTeX2e, 11 pages with references and 2 eps figures included,
submitted to Europhys. Let
Solvent-free coarse-grained lipid model for large-scale simulations
A coarse-grained molecular model, which consists of a spherical particle and
an orientation vector, is proposed to simulate lipid membrane on a large length
scale. The solvent is implicitly represented by an effective attractive
interaction between particles. A bilayer structure is formed by
orientation-dependent (tilt and bending) potentials. In this model, the
membrane properties (bending rigidity, line tension of membrane edge, area
compression modulus, lateral diffusion coefficient, and flip-flop rate) can be
varied over broad ranges. The stability of the bilayer membrane is investigated
via droplet-vesicle transition. The rupture of the bilayer and worm-like
micelle formation can be induced by an increase in the spontaneous curvature of
the monolayer membrane.Comment: 13 pages, 19 figure
Capillary Waves in a Colloid-Polymer Interface
The structure and the statistical fluctuations of interfaces between
coexisting phases in the Asakura-Oosawa (AO) model for a colloid--polymer
mixture are analyzed by extensive Monte Carlo simulations. We make use of a
recently developed grand canonical cluster move with an additional constraint
stabilizing the existence of two interfaces in the (rectangular) box that is
simulated. Choosing very large systems, of size LxLxD with L=60 and D=120,
measured in units of the colloid radius, the spectrum of capillary wave-type
interfacial excitations is analyzed in detail. The local position of the
interface is defined in terms of a (local) Gibbs surface concept. For small
wavevectors capillary wave theory is verified quantitatively, while for larger
wavevectors pronounced deviations show up. For wavevectors that correspond to
the typical distance between colloids in the colloid-rich phase, the
interfacial fluctuations exhibit the same structure as observed in the bulk
structure factor. When one analyzes the data in terms of the concept of a
wavevector-dependent interfacial tension, a monotonous decrease of this
quantity with increasing wavevector is found. Limitations of our analysis are
critically discussed.Comment: 12 pages, 15 figure
Liquid drops on a surface: using density functional theory to calculate the binding potential and drop profiles and comparing with results from mesoscopic modelling
The contribution to the free energy for a film of liquid of thickness on
a solid surface, due to the interactions between the solid-liquid and
liquid-gas interfaces is given by the binding potential, . The precise
form of determines whether or not the liquid wets the surface. Note that
differentiating gives the Derjaguin or disjoining pressure. We develop a
microscopic density functional theory (DFT) based method for calculating
, allowing us to relate the form of to the nature of the molecular
interactions in the system. We present results based on using a simple lattice
gas model, to demonstrate the procedure. In order to describe the static and
dynamic behaviour of non-uniform liquid films and drops on surfaces, a
mesoscopic free energy based on is often used. We calculate such
equilibrium film height profiles and also directly calculate using DFT the
corresponding density profiles for liquid drops on surfaces. Comparing
quantities such as the contact angle and also the shape of the drops, we find
good agreement between the two methods. We also study in detail the effect on
of truncating the range of the dispersion forces, both those between the
fluid molecules and those between the fluid and wall. We find that truncating
can have a significant effect on and the associated wetting behaviour of
the fluid.Comment: 16 pages, 13 fig
Irreversibility in response to forces acting on graphene sheets
The amount of rippling in graphene sheets is related to the interactions with
the substrate or with the suspending structure. Here, we report on an
irreversibility in the response to forces that act on suspended graphene
sheets. This may explain why one always observes a ripple structure on
suspended graphene. We show that a compression-relaxation mechanism produces
static ripples on graphene sheets and determine a peculiar temperature ,
such that for the free-energy of the rippled graphene is smaller than
that of roughened graphene. We also show that depends on the structural
parameters and increases with increasing sample size.Comment: 4 pages, 4 Figure
Coupling nonpolar and polar solvation free energies in implicit solvent models
Recent studies on the solvation of atomistic and nanoscale solutes indicate
that a strong coupling exists between the hydrophobic, dispersion, and
electrostatic contributions to the solvation free energy, a facet not
considered in current implicit solvent models. We suggest a theoretical
formalism which accounts for coupling by minimizing the Gibbs free energy of
the solvent with respect to a solvent volume exclusion function. The resulting
differential equation is similar to the Laplace-Young equation for the
geometrical description of capillary interfaces, but is extended to microscopic
scales by explicitly considering curvature corrections as well as dispersion
and electrostatic contributions. Unlike existing implicit solvent approaches,
the solvent accessible surface is an output of our model. The presented
formalism is illustrated on spherically or cylindrically symmetrical systems of
neutral or charged solutes on different length scales. The results are in
agreement with computer simulations and, most importantly, demonstrate that our
method captures the strong sensitivity of solvent expulsion and dewetting to
the particular form of the solvent-solute interactions.Comment: accpted in J. Chem. Phy
Universal reduction of pressure between charged surfaces by long-wavelength surface charge modulation
We predict theoretically that long-wavelength surface charge modulations
universally reduce the pressure between the charged surfaces with counterions
compared with the case of uniformly charged surfaces with the same average
surface charge density. The physical origin of this effect is the fact that
surface charge modulations always lead to enhanced counterion localization near
the surfaces, and hence, fewer charges at the midplane. We confirm the last
prediction with Monte Carlo simulations.Comment: 8 pages 1 figure, Europhys. Lett., in pres
Attractive instability of oppositely charged membranes induced by charge density fluctuations
We predict the conditions under which two oppositely charged membranes show a
dynamic, attractive instability. Two layers with unequal charges of opposite
sign can repel or be stable when in close proximity. However, dynamic charge
density fluctuations can induce an attractive instability and thus facilitate
fusion. We predict the dominant instability modes and timescales and show how
these are controlled by the relative charge and membrane viscosities. These
dynamic instabilities may be the precursors of membrane fusion in systems where
artificial vesicles are engulfed by biological cells of opposite charge
Dynamics of Counterion Condensation
Using a generalization of the Poisson-Boltzmann equation, dynamics of
counterion condensation is studied. For a single charged plate in the presence
of counterions, it is shown that the approach to equilibrium is diffusive. In
the far from equilibrium case of a moving charged plate, a dynamical counterion
condensation transition occurs at a critical velocity. The complex dynamic
behavior of the counterion cloud is shown to lead to a novel nonlinear
force-velocity relation for the moving plate.Comment: 5 pages, 1 ps figure included using eps
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