10,161 research outputs found

### Dynamical density functional theory: phase separation in a cavity and the influence of symmetry

Consider a fluid composed of two species of particles, where the
interparticle pair potentials $u_{11} = u_{22} \neq u_{12}$. On confining an
equal number of particles from each species in a cavity, one finds that the
average one body density profiles of each species are constrained to be exactly
the same due to the symmetry, when both external cavity potentials are the
same. For a binary fluid of Brownian particles interacting via repulsive
Gaussian pair potentials that exhibits phase separation, we study the dynamics
of the fluid one body density profiles on breaking the symmetry of the external
potentials, using the dynamical density functional theory of Marconi and
Tarazona [{\it J. Chem. Phys.}, {\bf 110}, 8032 (1999)]. On breaking the
symmetry we see that the fluid one body density profiles can then show the
phase separation that is present.Comment: 7 pages, 4 figures. Accepted for the proceedings of the Liquid Matter
conference 2005, to be publication in J. Phys.: Condens. Matte

### Dynamical density functional theory and its application to spinodal decomposition

We present an alternative derivation of the dynamical density functional
theory for the one body density profile of a classical fluid developed by
Marconi and Tarazona [J. Chem. Phys., 110, 8032 (1999)]. Our derivation
elucidates further some of the physical assumptions inherent in the theory and
shows that it is not restricted to fluids composed of particles interacting
solely via pair potentials; rather it applies to general, multi-body
interactions. The starting point for our derivation is the Smoluchowski
equation and the theory is therefore one for Brownian particles and as such is
applicable to colloidal fluids. In the second part of this paper we use the
dynamical density functional theory to derive a theory for spinodal
decomposition that is applicable at both early and intermediate times. For
early stages of spinodal decomposition our non-linear theory is equivalent to
the (generalised) linear Cahn-Hilliard theory, but for later times it
incorporates coupling between different Fourier components of the density
fluctuations (modes) and therefore goes beyond Cahn-Hilliard theory. We
describe the results of calculations for a model (Yukawa) fluid which show that
the coupling leads to the growth of a second maximum in the density
fluctuations, at a wavenumber larger than that of the main peak.Comment: 23 pages, 3 figure

### 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

### Mean-field dynamical density functional theory

We examine the out-of-equilibrium dynamical evolution of density profiles of
ultrasoft particles under time-varying external confining potentials in three
spatial dimensions. The theoretical formalism employed is the dynamical density
functional theory (DDFT) of Marini Bettolo Marconi and Tarazona [J. Chem. Phys.
{\bf 110}, 8032 (1999)], supplied by an equilibrium excess free energy
functional that is essentially exact. We complement our theoretical analysis by
carrying out extensive Brownian Dynamics simulations. We find excellent
agreement between theory and simulations for the whole time evolution of
density profiles, demonstrating thereby the validity of the DDFT when an
accurate equilibrium free energy functional is employed.Comment: 8 pagers, 4 figure

### Modelling the evaporation of nanoparticle suspensions from heterogeneous surfaces

We present a Monte Carlo (MC) grid-based model for the drying of drops of a
nanoparticle suspension upon a heterogeneous surface. The model consists of a
generalised lattice-gas in which the interaction parameters in the Hamiltonian
can be varied to model different properties of the materials involved. We show
how to choose correctly the interactions, to minimise the effects of the
underlying grid so that hemispherical droplets form. We also include the
effects of surface roughness to examine the effects of contact-line pinning on
the dynamics. When there is a `lid' above the system, which prevents
evaporation, equilibrium drops form on the surface, which we use to determine
the contact angle and how it varies as the parameters of the model are changed.
This enables us to relate the interaction parameters to the materials used in
applications. The model has also been applied to drying on heterogeneous
surfaces, in particular to the case where the suspension is deposited on a
surface consisting of a pair of hydrophilic conducting metal surfaces that are
either side of a band of hydrophobic insulating polymer. This situation occurs
when using inkjet printing to manufacture electrical connections between the
metallic parts of the surface. The process is not always without problems,
since the liquid can dewet from the hydrophobic part of the surface, breaking
the bridge before the drying process is complete. The MC model reproduces the
observed dewetting, allowing the parameters to be varied so that the conditions
for the best connection can be established. We show that if the hydrophobic
portion of the surface is located at a step below the height of the
neighbouring metal, the chance of dewetting of the liquid during the drying
process is significantly reduced.Comment: 14 pages, 14 figure

### User community development for the space transportation system/Skylab

The New User Function plan for identifying beneficial uses of space is described. Critical issues such as funding, manpower, and protection of user proprietary rights are discussed along with common barriers which impede the development of a user community. Studies for developing methodologies of identifying new users and uses of the space transportation system are included

### Phase behavior of a fluid with competing attractive and repulsive interactions

Fluids in which the interparticle potential has a hard core, is attractive at
moderate separations, and repulsive at greater separations are known to exhibit
novel phase behavior, including stable inhomogeneous phases. Here we report a
joint simulation and theoretical study of such a fluid, focusing on the
relationship between the liquid-vapor transition line and any new phases. The
phase diagram is studied as a function of the amplitude of the attraction for a
certain fixed amplitude of the long ranged repulsion. We find that the effect
of the repulsion is to substitute the liquid-vapor critical point and a portion
of the associated liquid-vapor transition line, by two first order transitions.
One of these transitions separates the vapor from a fluid of spherical
liquidlike clusters; the other separates the liquid from a fluid of spherical
voids. At low temperature, the two transition lines intersect one another and a
vapor-liquid transition line at a triple point. While most integral equation
theories are unable to describe the new phase transitions, the Percus Yevick
approximation does succeed in capturing the vapor-cluster transition, as well
as aspects of the structure of the cluster fluid, in reasonable agreement with
the simulation results.Comment: 15 pages, 20 figure

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