7,336 research outputs found
Microscopic theory of solvent mediated long range forces: influence of wetting
We show that a general density functional approach for calculating the force
between two big particles immersed in a solvent of smaller ones can describe
systems that exhibit fluid-fluid phase separation: the theory captures effects
of strong adsorption (wetting) and of critical fluctuations in the solvent. We
illustrate the approach for the Gaussian core model, a simple model of a
polymer mixture in solution and find extremely attractive, long ranged solvent
mediated potentials between the big particles for state points lying close to
the binodal, on the side where the solvent is poor in the species which is
favoured by the big particles.Comment: 7 pages, 3 figures, submitted to Europhysics Letter
Solidification in soft-core fluids: disordered solids from fast solidification fronts
Using dynamical density functional theory we calculate the speed of
solidification fronts advancing into a quenched two-dimensional model fluid of
soft-core particles. We find that solidification fronts can advance via two
different mechanisms, depending on the depth of the quench. For shallow
quenches, the front propagation is via a nonlinear mechanism. For deep
quenches, front propagation is governed by a linear mechanism and in this
regime we are able to determine the front speed via a marginal stability
analysis. We find that the density modulations generated behind the advancing
front have a characteristic scale that differs from the wavelength of the
density modulation in thermodynamic equilibrium, i.e., the spacing between the
crystal planes in an equilibrium crystal. This leads to the subsequent
development of disorder in the solids that are formed. For the one-component
fluid, the particles are able to rearrange to form a well-ordered crystal, with
few defects. However, solidification fronts in a binary mixture exhibiting
crystalline phases with square and hexagonal ordering generate solids that are
unable to rearrange after the passage of the solidification front and a
significant amount of disorder remains in the system.Comment: 18 pages, 14 fig
Phase separation in fluids exposed to spatially periodic external fields
We consider the liquid-vapor type phase transition for fluids confined within
spatially periodic external fields. For a fluid in d=3 dimensions, the periodic
field induces an additional phase, characterized by large density modulations
along the field direction. At the triple point, all three phases (modulated,
vapor, and liquid) coexist. At temperatures slightly above the triple point and
for low (high) values of the chemical potential, two-phase coexistence between
the modulated phase and the vapor (liquid) is observed. We study this
phenomenon using computer simulations and mean-field theory for the Ising
model. The theory shows that, in order for the modulated phase to arise, the
field wavelength must exceed a threshold value. We also find an extremely low
tension of the interface between the modulated phase and the vapor/liquid
phases. The tension is of the order 10^{-4} kB T per squared lattice spacing,
where kB is the Boltzmann constant, and T the temperature. In order to detect
such low tensions, a new simulation method is proposed. We also consider the
case of d=2 dimensions. The modulated phase then does not survive, leading to a
radically different phase diagram.Comment: 11 pages, 14 figure
Intrinsic point defects and volume swelling in ZrSiO4 under irradiation
The effects of high concentration of point defects in crystalline ZrSiO4 as
originated by exposure to radiation, have been simulated using first principles
density functional calculations. Structural relaxation and vibrational studies
were performed for a catalogue of intrinsic point defects, with different
charge states and concentrations. The experimental evidence of a large
anisotropic volume swelling in natural and artificially irradiated samples is
used to select the subset of defects that give similar lattice swelling for the
concentrations studied, namely interstitials of O and Si, and the anti-site
Zr(Si), Calculated vibrational spectra for the interstitials show additional
evidence for the presence of high concentrations of some of these defects in
irradiated zircon.Comment: 9 pages, 7 (color) figure
Criticality and phase separation in a two-dimensional binary colloidal fluid induced by the solvent critical behavior
We present an experimental and theoretical study of the phase behavior of a
binary mixture of colloids with opposite adsorption preferences in a critical
solvent. As a result of the attractive and repulsive critical Casimir forces,
the critical fluctuations of the solvent lead to a further critical point in
the colloidal system, i.e. to a critical colloidal-liquid--colloidal-liquid
demixing phase transition which is controlled by the solvent temperature. Our
experimental findings are in good agreement with calculations based on a simple
approximation for the free energy of the system.Comment: 5 pages, 5 figures, to be published in Europhysics Letter
Australia's first fossil marsupial mole (Notoryctemorphia) resolves controversies about their evolution and palaeoenvironmental origins
Fossils of a marsupial mole (Marsupialia, Notoryctemorphia, Notoryctidae) are described from early Miocene deposits in the Riversleigh World Heritage Area, northwestern Queensland, Australia. These represent the first unequivocal fossil record of the order Notoryctemorphia, the two living species of which are among the world's most specialized and bizarre mammals, but which are also convergent on certain fossorial placental mammals (most notably chrysochlorid golden moles). The fossil remains are genuinely ‘transitional', documenting an intermediate stage in the acquisition of a number of specializations and showing that one of these—the dental morphology known as zalambdodonty—was acquired via a different evolutionary pathway than in placentals. They, thus, document a clear case of evolutionary convergence (rather than parallelism) between only distantly related and geographically isolated mammalian lineages—marsupial moles on the island continent of Australia and placental moles on most other, at least intermittently connected continents. In contrast to earlier presumptions about a relationship between the highly specialized body form of the blind, earless, burrowing marsupial moles and desert habitats, it is now clear that archaic burrowing marsupial moles were adapted to and probably originated in wet forest palaeoenvironments, preadapting them to movement through drier soils in the xeric environments of Australia that developed during the Neogene
Solidification fronts in supercooled liquids: how rapid fronts can lead to disordered glassy solids
We determine the speed of a crystallisation (or more generally, a
solidification) front as it advances into the uniform liquid phase after the
system has been quenched into the crystalline region of the phase diagram. We
calculate the front speed by assuming a dynamical density functional theory
model for the system and applying a marginal stability criterion. Our results
also apply to phase field crystal (PFC) models of solidification. As the
solidification front advances into the unstable liquid phase, the density
profile behind the advancing front develops density modulations and the
wavelength of these modulations is a dynamically chosen quantity. For shallow
quenches, the selected wavelength is precisely that of the crystalline phase
and so well-ordered crystalline states are formed. However, when the system is
deeply quenched, we find that this wavelength can be quite different from that
of the crystal, so that the solidification front naturally generates disorder
in the system. Significant rearrangement and ageing must subsequently occur for
the system to form the regular well-ordered crystal that corresponds to the
free energy minimum. Additional disorder is introduced whenever a front
develops from random initial conditions. We illustrate these findings with
results obtained from the PFC.Comment: 14 pages, 7 figure
Intralocus sexual conflict can resolve the male-female health-survival paradox
This is the final version. Available on open access from Springer Nature via the DOI in this recordAt any given age, men are more likely to die than women, but women have poorer health at
older ages. This is referred to as the “male-female, health-survival paradox”, which is not
fully understood. Here, we provide a general solution to the paradox that relies on intralocus
sexual conflict, where alleles segregating in the population have late-acting positive effects
on male fitness, but negative effects on female health. Using an evolutionary modelling
framework we show that male-benefit, female-detriment alleles can spread if they are
expressed after female reproduction stops. We provide support for our conflict based
solution using experimental Drosophila data. Our results show that selecting for increased
late-life male reproductive effort can increase male fitness but have a detrimental effect on
female fitness. Furthermore, we show that late-life male fertility is negatively genetically
correlated with female health. Our study suggests that intralocus sexual conflict could
resolve the health-survival paradoxWe thank the National Science Center
(Poland: 2013/09/N/NZ/NZ8/03231) and the Leverhulme Trust (UK: RF-2015-01) for funding
which partially supported this work, and the University of Exeter’s Dean’s Fellowship for
additional support
Generation of defects and disorder from deeply quenching a liquid to form a solid
We show how deeply quenching a liquid to temperatures where it is linearly
unstable and the crystal is the equilibrium phase often produces crystalline
structures with defects and disorder. As the solid phase advances into the
liquid phase, the modulations in the density distribution created behind the
advancing solidification front do not necessarily have a wavelength that is the
same as the equilibrium crystal lattice spacing. This is because in a deep
enough quench the front propagation is governed by linear processes, but the
crystal lattice spacing is determined by nonlinear terms. The wavelength
mismatch can result in significant disorder behind the front that may or may
not persist in the latter stage dynamics. We support these observations by
presenting results from dynamical density functional theory calculations for
simple one- and two-component two-dimensional systems of soft core particles.Comment: 25 pages, 11 figure
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