2,903 research outputs found
Effect of shear on droplets in a binary mixture
In this article we use a lattice-Boltzmann simulation to examine the effects
of shear flow on a equilibrium droplet in a phase separated binary mixture. We
find that large drops break up as the shear is increased but small drops
dissolve. We also show how the tip-streaming, observed for deformed drops,
leads to a state of dynamic equilibrium.Comment: 10 pages, LaTex, to appear in International Journal of Physics
The Role of Spin Anisotropy in the Unbinding of Interfaces
We study the ground state of a classical X-Y model with -fold spin
anisotropy in a uniform external field, . An interface is introduced
into the system by a suitable choice of boundary conditions. For large , as
, we prove using an expansion in that the interface unbinds
from the surface through an infinite series of layering transitions. Numerical
work shows that the transitions end in a sequence of critical end points.Comment: 7 pages RevTeX, plus 1 postscript figure available from the authors
OUTP-94-41
Low temperature phase diagram and critical behaviour of the four-state chiral clock model
The low temperature behaviour of the four-state chiral clock () model
is reexamined using a systematic low temperature series expansion of the free
energy. Previously obtained results for the low temperature phases are
corrected and the low temperature phase diagram is derived. In addition, the
phase transition from the modulated region to the high temperature paraphase is
shown to belong to the universality class of the 3d-XY model.Comment: 17 pages in ioplppt style, 3 figure
Superhydrophobicity on hairy surfaces
We investigate the wetting properties of surfaces patterned with fine elastic
hairs, with an emphasis on identifying superhydrophobic states on hydrophilic
hairs. We formulate a two dimensional model of a large drop in contact with a
row of equispaced elastic hairs and, by minimising the free energy of the
model, identify the stable and metastable states. In particular we concentrate
on "partially suspended" states, where the hairs bend to support the drop --
singlet states where all hairs bend in the same direction, and doublet states
where neighbouring hairs bend in opposite directions -- and find the limits of
stability of these configurations in terms of material contact angle, hair
flexibility, and system geometry. The drop can remain suspended in a singlet
state at hydrophilic contact angles, but doublets exist only when the hairs are
hydrophobic. The system is more likely to evolve into a singlet state if the
hairs are inclined at the root. We discuss how, under limited circumstances,
the results can be modified to describe an array of hairs in three dimensions.
We find that now both singlets and doublets can exhibit superhydrophobic
behaviour on hydrophilic hairs. We discuss the limitations of our approach and
the directions for future work
Dissipative particle dynamics: the equilibrium for finite time steps
Dissipative particle dynamics (DPD) is a relatively new technique which has
proved successful in the simulation of complex fluids. We caution that for the
equilibrium achieved by the DPD simulation of a simple fluid the temperature
depends strongly on the time step. An analytic expression for the dependence is
obtained and shown to agree well with simulation results.Comment: 5 pages, LaTeX, 1 Postscript figure, submitted to Europhys.Letts.,
Algebraic corrections made to final resul
Capillary filling in microchannels patterned by posts
We investigate the capillary filling of three dimensional micro-channels with
surfaces patterned by posts of square cross section. We show that pinning on
the edges of the posts suppresses, and can halt, capillary filling. We stress
the importance of the channel walls in controlling whether filling can occur.
In particular for channels higher than the distance between adjacent posts,
filling occurs for contact angles less than a threshold angle \sim 55 deg.,
independent of the height of the channel.Comment: To appear in Phys. Rev.
Wetting on Lines and Lattices of Cylinders
This paper discusses wetting and capillary condensation transitions on a line
and a rectangular array of cylinders using an interface potential formalism.
For a line of cylinders, there is a capillary condensation transition followed
by complete wetting if the cylinders are sufficiently close together. Both
transitions disappear as the cylinder separation is increased. The dependence
of the wetting phase diagram of a rectangular array of cylinders is discussed
as a function of the chemical potential, substrate--fluid interaction strength
and surface tension.Comment: 17 pages in total: 11 pages of Latex document and 6 pages of figures,
Latex Version 2.09, OUTP-93-40
Modelling receding contact lines on superhydrophobic surfaces
We use mesoscale simulations to study the depinning of a receding contact
line on a superhydrophobic surface patterned by a regular array of posts. In
order that the simulations are feasible, we introduce a novel geometry where a
column of liquid dewets a capillary bounded by a superhydrophobic plane which
faces a smooth hydrophilic wall of variable contact angle. We present results
for the dependence of the depinning angle on the shape and spacing of the
posts, and discuss the form of the meniscus at depinning. We find, in agreement
with [17], that the local post concentration is a primary factor in controlling
the depinning angle, and show that the numerical results agree well with recent
experiments. We also present two examples of metastable pinned configurations
where the posts are partially wet.Comment: Revised version accepted for publication in Langmui
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