415 research outputs found
Superadiabatic dynamical density functional study of Brownian hard-spheres in time-dependent external potentials
Superadiabatic dynamical density functional theory (superadiabatic-DDFT), a
first-principles approach based on the inhomogeneous two-body correlation
functions, is employed to investigate the response of interacting Brownian
particles to time-dependent external driving. Predictions for the
superadiabatic dynamics of the one-body density are made directly from the
underlying interparticle interactions, without need for either adjustable fit
parameters or simulation input. The external potentials we investigate have
been chosen to probe distinct aspects of structural relaxation in dense,
strongly interacting liquid states. Nonequilibrium density profiles predicted
by the superadiabatic theory are compared with those obtained from both
adiabatic DDFT and event-driven Brownian dynamics simulation. Our findings show
that superadiabatic-DDFT accurately predicts the time-evolution of the one-body
density
Mean-Field Theory of Inhomogeneous Fluids
The Barker-Henderson perturbation theory is a bedrock of liquid-state
physics, providing quantitative predictions for the bulk thermodynamic
properties of realistic model systems. However, this successful method has not
been exploited for the study of inhomogeneous systems. We develop and implement
a first-principles 'Barker-Henderson density functional', thus providing a
robust and quantitatively accurate theory for classical fluids in external
fields. Numerical results are presented for the hard-core Yukawa model in three
dimensions. Our predictions for the density around a fixed test particle and
between planar walls are in very good agreement with simulation data. The
density profiles for the free liquid vapour interface show the expected
oscillatory decay into the bulk liquid as the temperature is reduced towards
the triple point, but with an amplitude much smaller than that predicted by the
standard mean-field density functional
Dense colloidal suspensions under time-dependent shear
We consider the nonlinear rheology of dense colloidal suspensions under a
time-dependent simple shear flow. Starting from the Smoluchowski equation for
interacting Brownian particles advected by shearing (ignoring fluctuations in
fluid velocity) we develop a formalism which enables the calculation of
time-dependent, far-from-equilibrium averages. Taking shear-stress as an
example we derive exactly a generalized Green-Kubo relation, and an equation of
motion for the transient density correlator, involving a three-time memory
function. Mode coupling approximations give a closed constitutive equation
yielding the time-dependent stress for arbitrary shear rate history. We solve
this equation numerically for the special case of a hard sphere glass subject
to step-strain.Comment: 4 page
Lorentz forces induce inhomogeneity and flux in active systems
We consider the dynamics of a charged active Brownian particle in three dimensions subjected to an external magnetic field. We show that, in the presence of a field gradient, a macroscopic flux emerges from a flux-free system and the density distribution becomes inhomogeneous. The flux is induced by the gradient of the magnetic field only and does not require additional symmetry breaking such as density or potential gradients. This stands in marked contrast to similar phenomena in condensed matter such as the classical Hall effect. We further demonstrate that passive tracer particles can be used to measure the essential effects caused by the Lorentz force on the active particle bath, and we discuss under which conditions this diffusive Hall-like effect might be observed experimentally
Phase behavior and structure of model colloid-polymer mixtures confined between two parallel planar walls
Using Gibbs ensemble Monte Carlo simulations and density functional theory we
investigate the fluid-fluid demixing transition in inhomogeneous
colloid-polymer mixtures confined between two parallel plates with separation
distances between one and ten colloid diameters covering the complete range
from quasi two-dimensional to bulk-like behavior. We use the
Asakura-Oosawa-Vrij model in which colloid-colloid and colloid-polymer
interactions are hard-sphere like, whilst the pair potential between polymers
vanishes. Two different types of confinement induced by a pair of parallel
walls are considered, namely either through two hard walls or through two
semi-permeable walls that repel colloids but allow polymers to freely
penetrate. For hard (semi-permeable) walls we find that the capillary binodal
is shifted towards higher (lower) polymer fugacities and lower (higher) colloid
fugacities as compared to the bulk binodal; this implies capillary condensation
(evaporation) of the colloidal liquid phase in the slit. A macroscopic
treatment is provided by a novel symmetric Kelvin equation for general binary
mixtures, based on the proximity in chemical potentials of statepoints at
capillary coexistence and the reference bulk coexistence. Results for capillary
binodals compare well with those obtained from the classic version of the
Kelvin equation due to Evans and Marini Bettolo Marconi [J. Chem. Phys. 86,
7138 (1987)], and are quantitatively accurate away from the fluid-fluid
critical point, even at small wall separations. For hard walls the density
profiles of polymers and colloids inside the slit display oscillations due to
packing effects for all statepoints. For semi-permeable walls either similar
structuring or flat profiles are found, depending on the statepoint considered.Comment: 15 pages, 13 figure
Green-Kubo approach to the average swim speed in active Brownian systems
We develop an exact Green-Kubo formula relating nonequilibrium averages in
systems of interacting active Brownian particles to equilibrium
time-correlation functions. The method is applied to calculate the
density-dependent average swim speed, which is a key quantity entering coarse
grained theories of active matter. The average swim speed is determined by
integrating the equilibrium autocorrelation function of the interaction force
acting on a tagged particle. Analytical results are validated using Brownian
dynamics simulations
Plants assemble species specific bacterial communities from common core taxa in three arcto-alpine climate zones
Evidence for the pivotal role of plant-associated bacteria to plant health and
productivity has accumulated rapidly in the last years. However, key questions related
to what drives plant bacteriomes remain unanswered, among which is the impact of
climate zones on plant-associated microbiota. This is particularly true for wild plants
in arcto-alpine biomes. Here, we hypothesized that the bacterial communities
associated with pioneer plants in these regions have major roles in plant health
support, and this is reflected in the formation of climate and host plant specific
endophytic communities. We thus compared the bacteriomes associated with the
native perennial plants Oxyria digyna and Saxifraga oppositifolia in three arcto-alpine
regions (alpine, low Arctic and high Arctic) with those in the corresponding bulk
soils. As expected, the bulk soil bacterial communities in the three regions were
significantly different. The relative abundances of Proteobacteria decreased
progressively from the alpine to the high-arctic soils, whereas those of Actinobacteria
increased. The candidate division AD3 and Acidobacteria abounded in the low Arctic
soils. Furthermore, plant species and geographic region were the major determinants
of the structures of the endophere communities. The plants in the alpine region had
higher relative abundances of Proteobacteria, while plants from the low- and high-
arctic regions were dominated by Firmicutes. A highly-conserved shared set of
ubiquitous bacterial taxa (core bacteriome) was found to occur in the two plant
species. Burkholderiales, Actinomycetales and Rhizobiales were the main taxa in this
core, and they were also the main contributors to the differences in the endosphere
bacterial community structures across compartments as well as regions. We postulate
that the composition of this core is driven by selection by the two plants.peerReviewe
Critical behavior in colloid-polymer mixtures: theory and simulation
We extensively investigated the critical behavior of mixtures of colloids and
polymers via the two-component Asakura-Oosawa model and its reduction to a
one-component colloidal fluid using accurate theoretical and simulation
techniques. In particular the theoretical approach, hierarchical reference
theory [Adv. Phys. 44, 211 (1995)], incorporates realistically the effects of
long-range fluctuations on phase separation giving exponents which differ
strongly from their mean-field values, and are in good agreement with those of
the three-dimensional Ising model. Computer simulations combined with
finite-size scaling analysis confirm the Ising universality and the accuracy of
the theory, although some discrepancy in the location of the critical point
between one-component and full-mixture description remains. To assess the limit
of the pair-interaction description, we compare one-component and two-component
results.Comment: 15 pages, 10 figures. Submitted to Phys. Rev.
Critical phenomena in colloid-polymer mixtures: interfacial tension, order parameter, susceptibility and coexistence diameter
The critical behavior of a model colloid-polymer mixture, the so-called AO
model, is studied using computer simulations and finite size scaling
techniques. Investigated are the interfacial tension, the order parameter, the
susceptibility and the coexistence diameter. Our results clearly show that the
interfacial tension vanishes at the critical point with exponent 2\nu ~ 1.26.
This is in good agreement with the 3D Ising exponent. Also calculated are
critical amplitude ratios, which are shown to be compatible with the
corresponding 3D Ising values. We additionally identify a number of subtleties
that are encountered when finite size scaling is applied to the AO model. In
particular, we find that the finite size extrapolation of the interfacial
tension is most consistent when logarithmic size dependences are ignored. This
finding is in agreement with the work of Berg et al.[Phys. Rev. B, V47 P497
(1993)]Comment: 13 pages, 16 figure
Dynamics of localized particles from density functional theory
A fundamental assumption of the dynamical density functional theory (DDFT) of
colloidal systems is that a grand-canonical free energy functional may be
employed to generate the thermodynamic driving forces. Using one-dimensional
hard-rods as a model system we analyze the validity of this key assumption and
show that unphysical self-interactions of the tagged particle density fields,
arising from coupling to a particle reservoir, are responsible for the
excessively fast relaxation predicted by the theory. Moreover, our findings
suggest that even employing a canonical functional would not lead to an
improvement for many-particle systems, if only the total density is considered.
We present several possible schemes to suppress these effects by incorporating
tagged densities. When applied to confined systems we demonstrate, using a
simple example, that DDFT neccessarily leads to delocalized tagged particle
density distributions, which do not respect the fundamental geometrical
contraints apparent in Brownian dynamics simulation data. The implication of
these results for possible applications of DDFT to treat the glass transition
are discussed
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