2,252 research outputs found
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
Effect of topology on dynamics of knots in polymers under tension
We use computer simulations to compare the dynamical behaviour of torus and
even-twist knots in polymers under tension. The knots diffuse through a
mechanism similar to reptation. Their friction coefficients grow linearly with
average knot length for both knot types. For similar complexity, however, the
torus knots diffuse faster than the even twist knots. The knot-length
auto-correlation function exhibits a slow relaxation time that can be linked to
a breathing mode. Its timescale depends on knot type, being typically longer
for torus than for even-twist knots. These differences in dynamical behaviour
are interpreted in terms of topological features of the knots.Comment: 6 pages, 8 figure
Complex dynamics of knotted filaments in shear flow
Coarse-grained simulations are used to demonstrate that knotted filaments in
shear flow at zero Reynolds number exhibit remarkably rich dynamic behaviour.
For stiff filaments that are weakly deformed by the shear forces, the knotted
filaments rotate like rigid objects in the flow. But away from this regime the
interplay between between shear forces and the flexibility of the filament
leads to intricate regular and chaotic modes of motion that can be divided into
distinct families. The set of accessible mode families depends to first order
on a dimensionless number that relates the filament length, the elastic
modulus, the friction per unit length and the shear rate.Comment: 6 pages, 6 figure
Dynamics of sliding drops on superhydrophobic surfaces
We use a free energy lattice Boltzmann approach to investigate numerically
the dynamics of drops moving across superhydrophobic surfaces. The surfaces
comprise a regular array of posts small compared to the drop size. For drops
suspended on the posts the velocity increases as the number of posts decreases.
We show that this is because the velocity is primarily determined by the
contact angle which, in turn, depends on the area covered by posts. Collapsed
drops, which fill the interstices between the posts, behave in a very different
way. The posts now impede the drop behaviour and the velocity falls as their
density increases.Comment: 7 pages, 4 figures, accepted for publication in Europhys. Let
Jetting Micron-Scale Droplets onto Chemically Heterogeneous Surfaces
We report experiments investigating the behaviour of micron-scale fluid
droplets jetted onto surfaces patterned with lyophobic and lyophilic stripes.
The final droplet shape depends on the droplet size relative to that of the
stripes. In particular when the droplet radius is of the same order as the
stripe width, the final shape is determined by the dynamic evolution of the
drop and shows a sensitive dependence on the initial droplet position and
velocity. Numerical solutions of the dynamical equations of motion of the drop
provide a close quantitative match to the experimental results. This proves
helpful in interpreting the data and allows for accurate prediction of fluid
droplet behaviour for a wide range of surfaces.Comment: 14 pages, accepted for publication in Langmui
Rheology of cholesteric blue phases
Blue phases of cholesteric liquid crystals offer a spectacular example of
naturally occurring disclination line networks. Here we numerically solve the
hydrodynamic equations of motion to investigate the response of three types of
blue phases to an imposed Poiseuille flow. We show that shear forces bend and
twist and can unzip the disclination lines. Under gentle forcing the network
opposes the flow and the apparent viscosity is significantly higher than that
of an isotropic liquid. With increased forcing we find strong shear thinning
corresponding to the disruption of the defect network. As the viscosity starts
to drop, the imposed flow sets the network into motion. Disclinations break-up
and re-form with their neighbours in the flow direction. This gives rise to
oscillations in the time-dependent measurement of the average stress.Comment: 4 pages, 4 figure
Control of drop positioning using chemical patterning
We explore how chemical patterning on surfaces can be used to control drop
wetting. Both numerical and experimental results are presented to show how the
dynamic pathway and equilibrium shape of the drops are altered by a hydrophobic
grid. The grid proves a successful way of confining drops and we show that it
can be used to alleviate {\it mottle}, a degradation in image quality which
results from uneven drop coalescence due to randomness in the positions of the
drops within the jetted array.Comment: 3 pages, 4 figure
Polarized 3 parton production in inclusive DIS at small x
Azimuthal angular correlations between produced hadrons/jets in high energy
collisions are a sensitive probe of the dynamics of QCD at small x. Here we
derive the triple differential cross section for inclusive production of 3
polarized partons in DIS at small x using the spinor helicity formalism. The
target proton or nucleus is described using the Color Glass Condensate (CGC)
formalism. The resulting expressions are used to study azimuthal angular
correlations between produced partons in order to probe the gluon structure of
the target hadron or nucleus. Our analytic expressions can also be used to
calculate the real part of the Next to Leading Order (NLO) corrections to
di-hadron production in DIS by integrating out one of the three final state
partons.Comment: 5 pages, 6 figures; version accepted for publication in Physics
Letters
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