127 research outputs found
Positive streamers in ambient air and a N2:O2-mixture (99.8 : 0.2)
Photographs show distinct differences between positive streamers in air or in
a nitrogen-oxygen mixture (0.2% O2). The streamers in the mixture branch more
frequently, but the branches also extinguish more easily. Probably related to
that, the streamers in the mixture propagate more in a zigzag manner while they
are straighter in air. Furthermore, streamers in the mixture can become longer;
they are thinner and more intense.Comment: 2 pages, 4 figures, paper is accepted for IEEE Trans. Plasma Sci. and
scheduled to appear in June 200
Translational and rotational friction on a colloidal rod near a wall
We present particulate simulation results for translational and rotational
friction components of a shish-kebab model of a colloidal rod with aspect ratio
(length over diameter) in the presence of a planar hard wall.
Hydrodynamic interactions between rod and wall cause an overall enhancement of
the friction tensor components. We find that the friction enhancements to
reasonable approximation scale inversely linear with the closest distance
between the rod surface and the wall, for in the range between and
. The dependence of the wall-induced friction on the angle between
the long axis of the rod and the normal to the wall is studied and fitted with
simple polynomials in .Comment: 8 pages, 8 figure
Mesoscale modeling of the rheology of pressure sensitive adhesives through inclusion of transient forces
For optimal application, pressure-sensitive adhesives must have rheological
properties in between those of a viscoplastic solid and those of a viscoelastic
liquid. Such adhesives can be produced by emulsion polymerisation, resulting in
latex particles which are dispersed in water and contain long-chain acrylic
polymers. When the emulsion is dried, the latex particles coalesce and an
adhesive film is formed. The rheological properties of the dried samples are
believed to be dominated by the interface regions between the original latex
particles, but the relation between rheology and latex particle properties is
poorly understood. In this paper we show that it is possible to describe the
bulk rheology of a pressure-sensitive adhesive by means of a mesoscale
simulation model. To reach experimental time and length scales, each latex
particle is represented by just one simulated particle. The model is subjected
to oscillatory shear flow and extensional flow. Simple order of magnitude
estimates of the model parameters already lead to semi-quantitative agreement
with experimental results. We show that inclusion of transient forces in the
model, i.e. forces with memory of previous configurations, is essential to
correctly predict the linear and nonlinear properties.Comment: 29 pages, 8 figure
Force calculation on walls and embedded particles in multiparticle collision dynamics simulations
Colloidal solutions posses a wide range of time and length scales, so that it
is unfeasible to keep track of all of them within a single simulation. As a
consequence some form of coarse-graining must be applied. In this work we use
the Multi-Particle Collision Dynamics scheme. We describe a particular
implementation of no-slip boundary conditions upon a solid surface, capable of
providing correct force s on the solid bypassing the calculation of the
velocity profile or the stre ss tensor in the fluid near the surface. As an
application we measure the friction on a spherical particle, when it is placed
in a bulk fluid and when it is confined in a slit. We show that the
implementation of the no-slip boundary conditions leads to an enhanced Ensko g
friction, which can be understood analytically. Because of the long-range
nature of hydrodynamic interactions, the Stokes friction obtained from the
simulations is sensitive of the simulation box size. We address this topic for
the slit geometry, showing that that the dependence on the system size differs
very much from what is expected in a 3D system, where periodic boundary
conditions are used in all directions.Comment: To appear in Physical Review
Positive and negative streamers in ambient air: measuring diameter, velocity and dissipated energy
Positive and negative streamers are studied in ambient air at 1 bar; they
emerge from a needle electrode placed 40 mm above a planar electrode. The
amplitudes of the applied voltage pulses range from 5 to 96 kV; most pulses
have rise times of 30 ns or shorter. Diameters, velocities and energies of the
streamers are measured. Two regimes are identified; a low voltage regime where
only positive streamers appear and a high voltage regime where both positive
and negative streamers exist. Below 5 kV, no streamers emerge. In the range
from 5 to 40 kV, positive streamers form, while the negative discharges only
form a glowing cloud at the electrode tip, but no streamers. For 5 to 20 kV,
diameters and velocities of the positive streamers have the minimal values of
d=0.2 mm and v \approx 10^5 m/s. For 20 to 40 kV, their diameters increase by a
factor 6 while the voltage increases only by a factor 2. Above the transition
value of 40 kV, streamers of both polarities form; they strongly resemble each
other, though the positive ones propagate further; their diameters continue to
increase with applied voltage. For 96 kV, positive streamers attain diameters
of 3 mm and velocities of 4*10^6 m/s, negative streamers are about 20 % slower
and thinner. An empirical fit formula for the relation between velocity v and
diameter d is v=0.5 d^2/(mm ns) for both polarities. Streamers of both
polarities dissipate energies of the order of several mJ per streamer while
crossing the gap.Comment: 20 pages, 9 figures, accepted for J. Phys.
Inception and propagation of positive streamers in high-purity nitrogen: effects of the voltage rise-rate
Controlling streamer morphology is important for numerous applications. Up to
now, the effect of the voltage rise rate was only studied across a wide range.
Here we show that even slight variations in the voltage rise can have
significant effects. We have studied positive streamer discharges in a 16 cm
point-plane gap in high-purity nitrogen 6.0, created by 25 kV pulses with a
duration of 130 ns. The voltage rise varies by a rise rate from 1.9 kV/ns to
2.7 kV/ns and by the first peak voltage of 22 to 28 kV. A structural link is
found between smaller discharges with a larger inception cloud caused by a
faster rising voltage. This relation is explained by the greater stability of
the inception cloud due to a faster voltage rise, causing a delay in the
destabilisation. Time-resolved measurements show that the inception cloud
propagates slower than an earlier destabilised, more filamentary discharge.
This explains that the discharge with a faster rising voltage pulse ends up to
be shorter. Furthermore, the effect of remaining background ionisation in a
pulse sequence has been studied, showing that channel thickness and branching
rate are locally affected, depending on the covered volume of the previous
discharge.Comment: 16 pages, 9 figure
Positive streamers in air and nitrogen of varying density: experiments on similarity laws
Positive streamers in ambient air at pressures from 0.013 to 1 bar are
investigated experimentally. The voltage applied to the anode needle ranges
from 5 to 45 kV, the discharge gap from 1 to 16 cm. Using a "slow" voltage rise
time of 100 to 180 ns, the streamers are intentionally kept thin. For each
pressure p, we find a minimal diameter d_{min}. To test whether streamers at
different pressures are similar, the minimal streamer diameter d_{min} is
multiplied by its pressure p; we find this product to be well approximated by
p*d_{min}=0.20 \pm 0.02 mm*bar over two decades of air pressure at room
temperature. The value also fits diameters of sprite discharges above
thunderclouds at an altitude of 80 km when extrapolated to room temperature (as
air density rather than pressure determines the physical behavior). The minimal
velocity of streamers in our measurements is approximately 0.1 mm/ns = 10^5
m/s. The same minimal velocity has been reported for tendrils in sprites. We
also investigate the size of the initial ionization cloud at the electrode tip
from which the streamers emerge, and the streamer length between branching
events. The same quantities are also measured in nitrogen with a purity of
approximately 99.9 %. We characterize the essential differences with streamers
in air and find a minimal diameter of p*d_{min}=0.12 \pm 0.02 mm*bar in our
nitrogen.Comment: 24 pages, 11 figures, accepted for J. Phys.
Positive and negative streamers in ambient air: modeling evolution and velocities
We simulate short positive and negative streamers in air at standard
temperature and pressure. They evolve in homogeneous electric fields or emerge
from needle electrodes with voltages of 10 to 20 kV. The streamer velocity at
given streamer length depends only weakly on the initial ionization seed,
except in the case of negative streamers in homogeneous fields. We characterize
the streamers by length, head radius, head charge and field enhancement. We
show that the velocity of positive streamers is mainly determined by their
radius and in quantitative agreement with recent experimental results both for
radius and velocity. The velocity of negative streamers is dominated by
electron drift in the enhanced field; in the low local fields of the present
simulations, it is little influenced by photo-ionization. Though negative
streamer fronts always move at least with the electron drift velocity in the
local field, this drift motion broadens the streamer head, decreases the field
enhancement and ultimately leads to slower propagation or even extinction of
the negative streamer.Comment: 18 pages, 10 figure
Systematic coarse-graining of the dynamics of entangled polymer melts: the road from chemistry to rheology
For optimal processing and design of entangled polymeric materials it is
important to establish a rigorous link between the detailed molecular
composition of the polymer and the viscoelastic properties of the macroscopic
melt. We review current and past computer simulation techniques and critically
assess their ability to provide such a link between chemistry and rheology. We
distinguish between two classes of coarse-graining levels, which we term
coarse-grained molecular dynamics (CGMD) and coarse-grained stochastic dynamics
(CGSD). In CGMD the coarse-grained beads are still relatively hard, thus
automatically preventing bond crossing. This also implies an upper limit on the
number of atoms that can be lumped together and therefore on the longest chain
lengths that can be studied. To reach a higher degree of coarse-graining, in
CGSD many more atoms are lumped together, leading to relatively soft beads. In
that case friction and stochastic forces dominate the interactions, and actions
must be undertaken to prevent bond crossing. We also review alternative methods
that make use of the tube model of polymer dynamics, by obtaining the
entanglement characteristics through a primitive path analysis and by
simulation of a primitive chain network. We finally review super-coarse-grained
methods in which an entire polymer is represented by a single particle, and
comment on ways to include memory effects and transient forces.Comment: Topical review, 31 pages, 10 figure
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