98 research outputs found
High speed imaging of traveling waves in a granular material during silo discharge
We report experimental observations of sound waves in a granular material
during resonant silo discharge called silo music. The grain motion was tracked
by high speed imaging while the resonance of the silo was detected by
accelerometers and acoustic methods. The grains do not oscillate in phase at
neighboring vertical locations, but information propagates upward in this
system in the form of sound waves. We show that the wave velocity is not
constant throughout the silo, but considerably increases towards the lower end
of the system, suggesting increased pressure in this region, where the flow
changes from cylindrical to converging flow. In the upper part of the silo the
wave velocity matches the sound velocity measured in the same material when
standing (in the absence of flow). Grain oscillations show a stick-slip
character only in the upper part of the silo.Comment: 5 pages, 5 figures, accepted to Phys. Rev.
Dynamics of a faceted nematic-smectic B front in thin-sample directional solidification
We present an experimental study of the directional-solidification patterns
of a nematic - smectic B front. The chosen system is C_4H_9-(C_6H_{10})_2CN (in
short, CCH4) in 12 \mu m-thick samples, and in the planar configuration
(director parallel to the plane of the sample). The nematic - smectic B
interface presents a facet in one direction -- the direction parallel to the
smectic layers -- and is otherwise rough, and devoid of forbidden directions.
We measure the Mullins-Sekerka instability threshold and establish the
morphology diagram of the system as a function of the solidification rate V and
the angle theta_{0} between the facet and the isotherms. We focus on the
phenomena occurring immediately above the instability threshold when theta_{0}
is neither very small nor close to 90^{o}. Under these conditions we observe
drifting shallow cells and a new type of solitary wave, called "faceton", which
consists essentially of an isolated macroscopic facet traveling laterally at
such a velocity that its growth rate with respect to the liquid is small.
Facetons may propagate either in a stationary, or an oscillatory way. The
detailed study of their dynamics casts light on the microscopic growth
mechanisms of the facets in this system.Comment: 12 pages, 19 figures, submitted to Phys. Rev.
Two scenarios for avalanche dynamics in inclined granular layers
We report experimental measurements of avalanche behavior of thin granular
layers on an inclined plane for low volume flow rate. The dynamical properties
of avalanches were quantitatively and qualitatively different for smooth glass
beads compared to irregular granular materials such as sand. Two scenarios for
granular avalanches on an incline are identified and a theoretical explanation
for these different scenarios is developed based on a depth-averaged approach
that takes into account the differing rheologies of the granular materials.Comment: 4 pages, 4 figures, accepted to Phys. Rev. Let
Regular dendritic patterns induced by non-local time-periodic forcing
The dynamic response of dendritic solidification to spatially homogeneous
time-periodic forcing has been studied. Phase-field calculations performed in
two dimensions (2D) and experiments on thin (quasi 2D) liquid crystal layers
show that the frequency of dendritic side-branching can be tuned by oscillatory
pressure or heating. The sensitivity of this phenomenon to the relevant
parameters, the frequency and amplitude of the modulation, the initial
undercooling and the anisotropies of the interfacial free energy and molecule
attachment kinetics, has been explored. It has been demonstrated that besides
the side-branching mode synchronous with external forcing as emerging from the
linear Wentzel-Kramers-Brillouin analysis, modes that oscillate with higher
harmonic frequencies are also present with perceptible amplitudes.Comment: 15 pages, 23 figures, Submitted to Phys. Rev.
Carrier-envelope offset stable, coherently combined ytterbium-doped fiber CPA delivering 1 kW of average power
We present a carrier-envelope offset (CEO) stable ytterbium-doped fiber chirped-pulse amplification system employing the technology of coherent beam combining and delivering more than 1 kW of average power at a pulse repetition rate of 80 MHz. The CEO stability of the system is 220 mrad rms, characterized out-of-loop with an f -to-2f interferometer in a frequency offset range of 10 Hz to 20 MHz. The high-power amplification system boosts the average power of the CEO stable oscillator by five orders of magnitude while increasing the phase noise by only 100 mrad. No evidence of CEO noise deterioration due to coherent beam combining is found. Low-frequency CEO fluctuations at the chirped-pulse amplifier are suppressed by a “slow loop” feedback. To the best of our knowledge, this is the first demonstration of a coherently combined laser system delivering an outstanding average power and high CEO stability at the same time. © 2020 Optical Society of Americ
Modulated structures in electroconvection in nematic liquid crystals
Motivated by experiments in electroconvection in nematic liquid crystals with
homeotropic alignment we study the coupled amplitude equations describing the
formation of a stationary roll pattern in the presence of a weakly-damped mode
that breaks isotropy. The equations can be generalized to describe the planarly
aligned case if the orienting effect of the boundaries is small, which can be
achieved by a destabilizing magnetic field. The slow mode represents the
in-plane director at the center of the cell. The simplest uniform states are
normal rolls which may undergo a pitchfork bifurcation to abnormal rolls with a
misaligned in-plane director.We present a new class of defect-free solutions
with spatial modulations perpendicular to the rolls. In a parameter range where
the zig-zag instability is not relevant these solutions are stable attractors,
as observed in experiments. We also present two-dimensionally modulated states
with and without defects which result from the destabilization of the
one-dimensionally modulated structures. Finally, for no (or very small)
damping, and away from the rotationally symmetric case, we find static chevrons
made up of a periodic arrangement of defect chains (or bands of defects)
separating homogeneous regions of oblique rolls with very small amplitude.
These states may provide a model for a class of poorly understood stationary
structures observed in various highly-conducting materials ("prechevrons" or
"broad domains").Comment: 13 pages, 13 figure
Surface effects in nucleation and growth of smectic B crystals in thin samples
We present an experimental study of the surface effects (interactions with
the container walls) during the nucleation and growth of smectic B crystals
from the nematic in free growth and directional solidification of a mesogenic
molecule () called CCH4 in thin (of thickness in the 10
m range) samples. We follow the dynamics of the system in real time with a
polarizing microscope. The inner surfaces of the glass-plate samples are coated
with polymeric films, either rubbed polyimid (PI) films or monooriented
poly(tetrafluoroethylene) (PTFE) films deposited by friction at high
temperature. The orientation of the nematic and the smectic B is planar. In
PI-coated samples, the orientation effect of SmB crystals is mediated by the
nematic, whereas, in PTFE-coated samples, it results from a homoepitaxy
phenomenon occurring for two degenerate orientations. A recrystallization
phenomenon partly destroys the initial distribution of crystal orientations. In
directional solidification of polycrystals in PTFE-coated samples, a particular
dynamics of faceted grain boundary grooves is at the origin of a dynamical
mechanism of grain selection. Surface effects also are responsible for the
nucleation of misoriented terraces on facets and the generation of lattice
defects in the solid.Comment: 15 pages, 24 figures, submitted to PR
Nucleation and Bulk Crystallization in Binary Phase Field Theory
We present a phase field theory for binary crystal nucleation. In the
one-component limit, quantitative agreement is achieved with computer
simulations (Lennard-Jones system) and experiments (ice-water system) using
model parameters evaluated from the free energy and thickness of the interface.
The critical undercoolings predicted for Cu-Ni alloys accord with the
measurements, and indicate homogeneous nucleation. The Kolmogorov exponents
deduced for dendritic solidification and for "soft-impingement" of particles
via diffusion fields are consistent with experiment.Comment: 4 pages, 4 figures, accepted to PR
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