464 research outputs found
Dynamics of Low Anisotropy Morphologies in Directional Solidification
We report experimental results on quasi-two-dimensional diffusion limited
growth in directionally solidified succinonitrile with small amounts of
poly(ethylene oxide), acetone, or camphor as a solute. Seaweed growth, or dense
branching morphology, is selected by growing grains close to the
plane, where the in-plane surface tension is nearly isotropic. The observed
growth morphologies are very sensitive to small anisotropies in surface tension
caused by misorientations from the plane. Different seaweed
morphologies are found, including the degenerate, the stabilized, and the
strongly tilted seaweeds. The degenerate seaweeds show a limited fractal
scaling range and, with increased undercooling, suggests a transition from
"fractal" to "compact" seaweed. Strongly tilted seaweeds demonstrate a
significant twofold anisotropy. In addition, seaweed-dendrite transitions are
observed in low anisotropy growth.Comment: 12 pages, 17 figures, submitted to Phys. Rev. E, reduced image
quality for smaller file siz
Statistics of defect motion in spatiotemporal chaos in inclined layer convection
We report experiments on defect-tracking in the state of undulation chaos
observed in thermal convection of an inclined fluid layer. We characterize the
ensemble of defect trajectories according to their velocities, relative
positions, diffusion, and gain and loss rates. In particular, the defects
exhibit incidents of rapid transverse motion which result in power law
distributions for a number of quantitative measures. We examine connections
between this behavior and L\'evy flights and anomalous diffusion. In addition,
we describe time-reversal and system size invariance for defect creation and
annihilation rates.Comment: (21 pages, 17 figures
Inertial effects on two-particle relative dispersion in turbulent flows
We report experimental results on the relative motion of pairs of solid
spheric particles with initial separations in the inertial range of fully
developed turbulence in water. The particle densities were in the range of , \textit{i.e.}, from neutrally
buoyant to highly inertial; and their sizes were of the Kolmogorov scale. For
all particles, we observed a Batchelor like regime, in which particles
separated ballistically. Similar to the Batchelor regime for tracers, this
regime was observed in the early stages of the relative separation for times with determined by the turbulence energy dissipation
rate and the initial separation between particle pairs. In this time interval
heavier particles separated faster than fluid tracers. The second order
Eulerian velocity structure functions was found to increase with density. In
other words, both observations show that the relative velocity between inertial
particles was larger than that between tracers. Based on the widely used,
simplified equation of motion for inertial point-particles, we derived a model
that shows an increase in relative velocity between inertial particles. In its
scale dependence, however, it disagrees quantitatively with the experimental
results. This we attribute to the preferential sampling of the flow field by
inertial particles, which is not captured by the model.Comment: 6 pages, 5 figures, 2 tables, epl2.cls, submitted to EP
Pattern formation in inclined layer convection
We report experiments on thermally driven convection in an inclined layer of
large aspect ratio in a fluid of Prandtl number . We observed
a number of new nonlinear, mostly spatio-temporally chaotic, states. At small
angles of inclination we found longitudinal rolls, subharmonic oscillations,
Busse oscillations, undulation chaos, and crawling rolls. At larger angles, in
the vicinity of the transition from buoyancy- to shear-driven instability, we
observed drifting transverse rolls, localized bursts, and drifting bimodals.
For angles past vertical, when heated from above, we found drifting transverse
rolls and switching diamond panes.Comment: For MPEG movies, see http://milou.msc.cornell.edu/ILCmovie
Competition and bistability of ordered undulations and undulation chaos in inclined layer convection
Experimental and theoretical investigations of undulation patterns in
high-pressure, inclined layer gas convection at a Prandtl number near unity are
reported. Particular focus is given to the competition between the
spatiotemporal chaotic state of undulation chaos and stationary patterns of
ordered undulations. In experiments a competition and bistability between the
two states is observed, with ordered undulations most prevalent at higher
Rayleigh number. The spectral pattern entropy, spatial correlation lengths, and
defect statistics are used to characterize the competing states. The
experiments are complemented by a theoretical analysis of the
Oberbeck-Boussinesq equations. The stability region of the ordered undulation
as a function of their wavevectors and the Rayleigh number is obtained with
Galerkin techniques. In addition, direct numerical simulations are used to
investigate the spatiotemporal dynamics. In the simulations both ordered
undulations and undulation chaos were observed dependent on initial conditions.
Experiment and theory are found to agree well.Comment: Reduced-resolution figure
On the Swimming of \textit{Dictyostelium} amoebae
Traditionally, the primary mode for locomotion of amoeboid cells was thought
to be crawling on a substrate. Recently, it has been experimentally shown that
\textit{Dictostelium} amoeba and neutrophils can also swim in a directed
fashion. The mechanisms for amoeboid crawling and swimming were hypothesized to
be similar. In this letter, we show that the shape changes generated by a
crawling \textit{D. discoideum} cell are consistent with swimming.Comment: letter submitted to PNA
Defect turbulence and generalized statistical mechanics
We present experimental evidence that the motion of point defects in thermal
convection patterns in an inclined fluid layer is well-described by Tsallis
statistics with an entropic index . The dynamical properties of
the defects (anomalous diffusion, shape of velocity distributions, power law
decay of correlations) are in good agreement with typical predictions of
nonextensive models, over a range of driving parameters
Turbulence attenuation by large neutrally buoyant particles
Turbulence modulation by inertial-range-size, neutrally-buoyant particles is
investigated experimentally in a von K\'arm\'an flow. Increasing the particle
volume fraction , maintaining constant impellers Reynolds
number attenuates the fluid turbulence. The inertial-range energy transfer rate
decreases as , suggesting that only particles
located on a surface affect the flow. Small-scale turbulent properties, such as
structure functions or acceleration distribution, are unchanged. Finally,
measurements hint at the existence of a transition between two different
regimes occurring when the average distance between large particles is of the
order of the thickness of their boundary layers.Comment: 7 pages, 4 figure
Impact of noise on domain growth in electroconvection
The growth and ordering of striped domains has recently received renewed
attention due in part to experimental studies in diblock copolymers and
electroconvection. One surprising result has been the relative slow dynamics
associated with the growth of striped domains. One potential source of the slow
dynamics is the pinning of defects in the periodic potential of the stripes. Of
interest is whether or not external noise will have a significant impact on the
domain ordering, perhaps by reducing the pinning and increasing the rate of
ordering. In contrast, we present experiments using electroconvection in which
we show that a particular type of external noise decreases the rate of domain
ordering
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