10,345 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 . 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
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
Dynamical density functional theory analysis of the laning instability in sheared soft matter
Using dynamical density functional theory (DDFT) methods we investigate the
laning instability of a sheared colloidal suspension. The nonequilibrium
ordering at the laning transition is driven by non-affine particle motion
arising from interparticle interactions. Starting from a DDFT which
incorporates the non-affine motion, we perform a linear stability analysis that
enables identification of the regions of parameter space where lanes form. We
illustrate our general approach by applying it to a simple one-component fluid
of soft penetrable particles
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
Solvent mediated interactions close to fluid-fluid phase separation: microscopic treatment of bridging in a soft core fluid
Using density functional theory we calculate the density profiles of a binary
solvent adsorbed around a pair of big solute particles. All species interact
via repulsive Gaussian potentials. The solvent exhibits fluid-fluid phase
separation and for thermodynamic states near to coexistence the big particles
can be surrounded by a thick adsorbed `wetting' film of the coexisting solvent
phase. On reducing the separation between the two big particles we find there
can be a `bridging' transition as the wetting films join to form a fluid
bridge. The potential between the two big particles becomes long ranged and
strongly attractive in the bridged configuration. Within our mean-field
treatment the bridging transition results in a discontinuity in the solvent
mediated force. We demonstrate that accounting for the phenomenon of bridging
requires the presence of a non-zero bridge function in the correlations between
the solute particles when our model fluid is described within a full mixture
theory based upon the Ornstein-Zernike equations.Comment: 28 pages, 8 figure
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
Dynamical density functional theory for the dewetting of evaporating thin films of nanoparticle suspensions exhibiting pattern formation
Recent experiments have shown that the striking structure formation in
dewetting films of evaporating colloidal nanoparticle suspensions occurs in an
ultrathin `postcursor' layer that is left behind by a mesoscopic dewetting
front. Various phase change and transport processes occur in the postcursor
layer, that may lead to nanoparticle deposits in the form of labyrinthine,
network or strongly branched `finger' structures. We develop a versatile
dynamical density functional theory to model this system which captures all
these structures and may be employed to investigate the influence of
evaporation/condensation, nanoparticle transport and solute transport in a
differentiated way. We highlight, in particular, the influence of the subtle
interplay of decomposition in the layer and contact line motion on the observed
particle-induced transverse instability of the dewetting front.Comment: 5 pages, 5 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
Soft core fluid in a quenched matrix of soft core particles: A mobile mixture in a model gel
We present a density-functional study of a binary phase-separating mixture of
soft core particles immersed in a random matrix of quenched soft core particles
of larger size. This is a model for a binary polymer mixture immersed in a
crosslinked rigid polymer network. Using the replica `trick' for
quenched-annealed mixtures we derive an explicit density functional theory that
treats the quenched species on the level of its one-body density distribution.
The relation to a set of effective external potentials acting on the annealed
components is discussed. We relate matrix-induced condensation in bulk to the
behaviour of the mixture around a single large particle. The interfacial
properties of the binary mixture at a surface of the quenched matrix display a
rich interplay between capillary condensation inside the bulk matrix and
wetting phenomena at the matrix surface.Comment: 20 pages, 5 figures. Accepted for Phys. Rev.
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