9,213 research outputs found
Dynamics of shallow impact cratering
We present data for the time-dependence of wooden spheres penetrating into a
loose non-cohesive packing of glass beads. The stopping time is a factor of
three longer than the time needed to travel the total penetration
distance at the impact speed . The acceleration decreases
monotonically throughout the impact. These kinematics are modelled by a
position- and velocity-dependent stopping force that is constrained to
reproduce prior observations for the scaling of the penetration depth with the
total drop distance.Comment: 4 pages, experimen
Speckle visibility spectroscopy and variable granular fluidization
We introduce a dynamic light scattering technique capable of resolving motion
that changes systematically, and rapidly, with time. It is based on the
visibility of a speckle pattern for a given exposure duration. Applying this to
a vibrated layer of glass beads, we measure the granular temperature and its
variation with phase in the oscillation cycle. We observe several transitions
involving jammed states, where the grains are at rest during some portion of
the cycle. We also observe a two-step decay of the temperature on approach to
jamming.Comment: 4 pages, 4 figures, experimen
Avalanche statistics and time-resolved grain dynamics for a driven heap
We probe the dynamics of intermittent avalanches caused by steady addition of
grains to a quasi-two dimensional heap. To characterize the time-dependent
average avalanche flow speed v(t), we image the top free surface. To
characterize the grain fluctuation speed dv(t), we use Speckle-Visibility
Spectroscopy. During an avalanche, we find that the fluctuation speed is
approximately one-tenth the average flow speed, and that these speeds are
largest near the beginning of an event. We also find that the distribution of
event durations is peaked, and that event sizes are correlated with the time
interval since the end of the previous event. At high rates of grain addition,
where successive avalanches merge into smooth continuous flow, the relationship
between average and fluctuation speeds changes to dv Sqrt[v]
Penetration depth for shallow impact cratering
We present data for the penetration of a variety of spheres, dropped from
rest, into a level non-cohesive granular medium. We improve upon our earlier
work [Uehara {\it et al.} Phys. Rev. Lett. {\bf 90}, 194301 (2003)] in three
regards. First, we explore the behavior vs sphere diameter and density more
systematically, by holding one of these parameters constant while varying the
other. Second, we prepare the granular medium more reproducibly and, third, we
measure the penetration depth more accurately. The new data support our
previous conclusion that the penetration depth is proportional to the 1/2 power
of sphere density, the 2/3 power of sphere diameter, and the 1/3 power of total
drop distance
Liquid crystals from poly(4,4'-methoxybiphenylyl methacrylate)
Side-chain liquid crystalline poly(4,4'-biphenylyl methacrylate) was studied by X-ray diffraction. Three smectic phases were described using a ribbon-like structural model for the polymer chain. In the smectic SP phase, ribbons lie flat on their sides to form layers with a spacing equal to the thickness of the ribbons. In the smectic SA phase, the ribbons stand upright giving layers with a thickness corresponding to the length of one extended monomeric unit. In the ordered smectic So phase, the layer thickness also corresponds to one extended repeat unit, and the pendant groups are arranged according to a two-dimensional oblique lattice. The Schlieren texture observed in the SP phase was analysed in terms of symmetry.Les phases liquides cristallines d'un poly(methacrylate) de 4,4'-biphénylyle ont été étudiées par diffraction des rayons X. Trois phases smectiques ont été décrites à l'aide d'un même modèle de chaîne macromoléculaire sous forme de ruban. Dans la phase smectique Sp, les rubans sont disposés à plat dans des couches avec un espacement égal à l'épaisseur des rubans. Dans la phase smectique SA, les rubans sont arrangés en simples couches avec un espacement égal à la largeur des rubans. Dans la phase smectique ordonnée So, les rubans sont arrangés comme dans la phase SA mais avec les groupes mésogènes assemblés suivant un réseau bidimensionnel oblique. La texture de type Schlieren observée pour la phase Sp a été analysée en termes de symétrie
Geometry dependence of the clogging transition in tilted hoppers
We report the effect of system geometry on the clogging of granular material
flowing out of flat-bottomed hoppers with variable aperture size D. For such
systems, there exists a critical aperture size Dc at which there is a
divergence in the time for a flow to clog. To better understand the origins of
Dc, we perturb the system by tilting the hopper an angle Q and mapping out a
clogging phase diagram as a function of Q and D. The clogging transition
demarcates the boundary between the freely-flowing (large D, small Q) and
clogging (small D, large Q) regimes. We investigate how the system geometry
affects Dc by mapping out this phase diagram for hoppers with either a circular
hole or a rectangular narrow slit. Additionally, we vary the grain shape,
investigating smooth spheres (glass beads), compact angular grains (beach
sand), disk-like grains (lentils), and rod-like grains (rice). We find that the
value of Dc grows with increasing Q, diverging at pi-Qr where Qr is the angle
of repose. For circular apertures, the shape of the clogging transition is the
same for all grain types. However, this is not the case for the narrow slit
apertures, where the rate of growth of the critical hole size with tilt angle
depends on the material
Statistical characterization of the forces on spheres in an upflow of air
The dynamics of a sphere fluidized in a nearly-levitating upflow of air were
previously found to be identical to those of a Brownian particle in a
two-dimensional harmonic trap, consistent with a Langevin equation [Ojha {\it
et al.}, Nature {\bf 427}, 521 (2004)]. The random forcing, the drag, and the
trapping potential represent different aspects of the interaction of the sphere
with the air flow. In this paper we vary the experimental conditions for a
single sphere, and report on how the force terms in the Langevin equation scale
with air flow speed, sphere radius, sphere density, and system size. We also
report on the effective interaction potential between two spheres in an upflow
of air.Comment: 7 pages, experimen
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