1,233 research outputs found
Localized states in sheared electroconvection
Electroconvection in a thin, sheared fluid film displays a rich sequence of
bifurcations between different flow states as the driving voltage is increased.
We present a numerical study of an annular film in which a radial potential
difference acts on induced surface charges to drive convection. The film is
also sheared by independently rotating the inner edge of the annulus. This
simulation models laboratory experiments on electroconvection in sheared
smectic liquid crystal films. The applied shear competes with the electrical
forces, resulting in oscillatory and strongly subcritical bifurcations between
localized vortex states close to onset. At higher forcing, the flow becomes
chaotic via a Ruelle-Takens-Newhouse scenario. The simulation allows flow
visualization not available in the physical experiments, and sheds light on
previously observed transitions in the current-voltage characteristics of
electroconvecting smectic films.Comment: To be published in EuroPhysics Letters, 6 pages, 6 figures: final
versio
Weakly Nonlinear Analysis of Electroconvection in a Suspended Fluid Film
It has been experimentally observed that weakly conducting suspended films of
smectic liquid crystals undergo electroconvection when subjected to a large
enough potential difference. The resulting counter-rotating vortices form a
very simple convection pattern and exhibit a variety of interesting nonlinear
effects. The linear stability problem for this system has recently been solved.
The convection mechanism, which involves charge separation at the free surfaces
of the film, is applicable to any sufficiently two-dimensional fluid. In this
paper, we derive an amplitude equation which describes the weakly nonlinear
regime, by starting from the basic electrohydrodynamic equations. This regime
has been the subject of several recent experimental studies. The lowest order
amplitude equation we derive is of the Ginzburg-Landau form, and describes a
forward bifurcation as is observed experimentally. The coefficients of the
amplitude equation are calculated and compared with the values independently
deduced from the linear stability calculation.Comment: 26 pages, 2 included eps figures, submitted to Phys Rev E. For more
information, see http://mobydick.physics.utoronto.c
Bifurcations in annular electroconvection with an imposed shear
We report an experimental study of the primary bifurcation in
electrically-driven convection in a freely suspended film. A weakly conducting,
submicron thick smectic liquid crystal film was supported by concentric
circular electrodes. It electroconvected when a sufficiently large voltage
was applied between its inner and outer edges. The film could sustain rapid
flows and yet remain strictly two-dimensional. By rotation of the inner
electrode, a circular Couette shear could be independently imposed. The control
parameters were a dimensionless number , analogous to the Rayleigh
number, which is and the Reynolds number of the
azimuthal shear flow. The geometrical and material properties of the film were
characterized by the radius ratio , and a Prandtl-like number . Using measurements of current-voltage characteristics of a large number of
films, we examined the onset of electroconvection over a broad range of
, and . We compared this data quantitatively to
the results of linear stability theory. This could be done with essentially no
adjustable parameters. The current-voltage data above onset were then used to
infer the amplitude of electroconvection in the weakly nonlinear regime by
fitting them to a steady-state amplitude equation of the Landau form. We show
how the primary bifurcation can be tuned between supercritical and subcritical
by changing and .Comment: 17 pages, 12 figures. Submitted to Phys. Rev. E. Minor changes after
refereeing. See also http://mobydick.physics.utoronto.c
Annular electroconvection with shear
We report experiments on convection driven by a radial electrical force in
suspended annular smectic A liquid crystal films. In the absence of an
externally imposed azimuthal shear, a stationary one-dimensional (1D) pattern
consisting of symmetric vortex pairs is formed via a supercritical transition
at the onset of convection. Shearing reduces the symmetries of the base state
and produces a traveling 1D pattern whose basic periodic unit is a pair of
asymmetric vortices. For a sufficiently large shear, the primary bifurcation
changes from supercritical to subcritical. We describe measurements of the
resulting hysteresis as a function of the shear at radius ratio . This simple pattern forming system has an unusual combination of
symmetries and control parameters and should be amenable to quantitative
theoretical analysis.Comment: 12 preprint pages, 3 figures in 2 parts each. For more info, see
http://mobydick.physics.utoronto.c
Target Mass Monitoring and Instrumentation in the Daya Bay Antineutrino Detectors
The Daya Bay experiment measures sin^2 2{\theta}_13 using functionally
identical antineutrino detectors located at distances of 300 to 2000 meters
from the Daya Bay nuclear power complex. Each detector consists of three nested
fluid volumes surrounded by photomultiplier tubes. These volumes are coupled to
overflow tanks on top of the detector to allow for thermal expansion of the
liquid. Antineutrinos are detected through the inverse beta decay reaction on
the proton-rich scintillator target. A precise and continuous measurement of
the detector's central target mass is achieved by monitoring the the fluid
level in the overflow tanks with cameras and ultrasonic and capacitive sensors.
In addition, the monitoring system records detector temperature and levelness
at multiple positions. This monitoring information allows the precise
determination of the detectors' effective number of target protons during data
taking. We present the design, calibration, installation and in-situ tests of
the Daya Bay real-time antineutrino detector monitoring sensors and readout
electronics.Comment: 22 pages, 20 figures; accepted by JINST. Changes in v2: minor
revisions to incorporate editorial feedback from JINS
Knots and Random Walks in Vibrated Granular Chains
We study experimentally statistical properties of the opening times of knots
in vertically vibrated granular chains. Our measurements are in good
qualitative and quantitative agreement with a theoretical model involving three
random walks interacting via hard core exclusion in one spatial dimension. In
particular, the knot survival probability follows a universal scaling function
which is independent of the chain length, with a corresponding diffusive
characteristic time scale. Both the large-exit-time and the small-exit-time
tails of the distribution are suppressed exponentially, and the corresponding
decay coefficients are in excellent agreement with the theoretical values.Comment: 4 pages, 5 figure
Entropic Tightening of Vibrated Chains
We investigate experimentally the distribution of configurations of a ring
with an elementary topological constraint, a ``figure-8'' twist. Using vibrated
granular chains, which permit controlled preparation and direct observation of
such a constraint, we show that configurations where one of the loops is tight
and the second is large are strongly preferred. This agrees with recent
predictions for equilibrium properties of topologically-constrained polymers.
However, the dynamics of the tightening process weakly violate detailed
balance, a signature of the nonequilibrium nature of this system.Comment: 4 pages, 4 figure
Shear instabilities of freely standing thermotropic smectic-A films
In this Letter we discuss theoretically the instabilities of thermotropic
freely standing smectic-A films under shear flow\cite{re:wu}. We show that, in
Couette geometry, the centrifugal force pushes the liquid crystal toward the
outer boundary and induces smectic layer dilation close to the outer boundary.
Under strong shear, this effect induces a layer buckling instability. The
critical shear rate is proportional to , where is the thickness
of the film.Comment: 12 pages, 2 figure
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