218 research outputs found
Experiments on Corn Pressure in Silo Cells -- Translation and Comment of Janssen's Paper from 1895
The German engineer H.A. Janssen gave one of the first accounts of the often
peculiar behavior of granular material in a paper published in German in 1895.
From simple experiments with corn he inferred the saturation of pressure with
height in a granular system. Subsequently, Janssen derived the equivalent of
the barometric formula for granular material from the main assumption that the
walls carry part of the weight. The following is a translation of this article.
The wording is chosen as close as possible to the original. While drawings are
copied from the original, figures displaying data are redone for better
readability. The translation is complemented by some bibliographical notes and
an assessment of earlier work, wherein Hagen predicted the saturation of
pressure with depth in 1852, and Huber-Burnand demonstrated that saturation
qualitatively as early as in 1829. We conclude with a brief discussion of more
recent developments resting on Janssen's work.Comment: 7 pages, 15 figures, accepted for publication in Granular Matter,
original article (German) can be found under
http://www.phy.duke.edu/~msperl/Janssen
Velocity Distribution of a Homogeneously Cooling Granular Gas
In contrast to molecular gases, granular gases are characterized by inelastic
collisions and require therefore permanent driving to maintain a constant
kinetic energy. The kinetic theory of granular gases describes how the average
velocity of the particles decreases after the driving is shut off. Moreover it
predicts that the rescaled particle velocity distribution will approach a
stationary state with overpopulated high-velocity tails as compared to the
Maxwell-Boltzmann distribution. While this fundamental theoretical result was
reproduced by numerical simulations, an experimental confirmation is still
missing. Using a microgravity experiment which allows the spatially homogeneous
excitation of spheres via magnetic fields, we confirm the theoretically
predicted exponential decay of the tails of the velocity distribution.Comment: 11 pages, 14 figure
Thinning and thickening in active microrheology
When pulling a probe particle in a many-particle system with fixed velocity,
the probe's effective friction, defined as average pulling force over its
velocity, , first keeps constant (linear
response), then decreases (thinning) and finally increases (thickening). We
propose a three-time-scales picture (TTSP) to unify thinning and thickening
behaviour. The points of the TTSP are that there are three distinct time scales
of bath particles: diffusion, damping, and single probe-bath (P-B) collision;
the dominating time scales, which are controlled by the pulling velocity,
determine the behaviour of the probe's friction. We confirm the TTSP by
Langevin dynamics simulation. Microscopically, we find that for computing the
effective friction, Maxwellian distribution of bath particles' velocities works
in low Reynolds number (Re) but fails in high Re. It can be understood based on
the microscopic mechanism of thickening obtained in the limit. Based on
the TTSP, we explain different thinning and thickening observations in some
earlier literature
Kinetic theory for strong uniform shear flow of granular media at high density
We discuss the uniform shear flow of a fluidized granular bed composed of
monodisperse Hertzian spheres. Considering high densities around the glass
transition density of inelastic Hertzian spheres, we report kinetic theory
expressions for the Newtonian viscosity as well as the Bagnold coefficient. We
discuss the dependence of the transport coefficients on density and coefficient
of restitution.Comment: Powders & Grains 201
Active Microrheology of Driven Granular Particles
When pulling a particle in a driven granular fluid with constant force
, the probe particle approaches a steady-state average velocity .
This velocity and the corresponding friction coefficient of the probe
are obtained within a schematic model of mode-coupling theory
and compared to results from event-driven simulations. For small and moderate
drag forces, the model describes the simulation results successfully for both
the linear as well as the nonlinear region: The linear response regime
(constant friction) for small drag forces is followed by shear thinning
(decreasing friction) for moderate forces. For large forces, the model
demonstrates a subsequent increasing friction in qualitative agreement with the
data. The square-root increase of the friction with force found in [Fiege et
al., Granular Matter , 247 (2012)] is explained by a simple
kinetic theory.Comment: 5 pages, 4 figure
Particle characterization using THz spectroscopy
THz extinction spectroscopy extends UV-Vis and NIR-spectroscopy to
characterize particles from fine powders and dust to sand, grains and
granulated materials. We extract particle sizes from the spectral position of
the first peak of the interference structure and size distributions from the
visibility of the fine ripple structure in the measured extinction spectra. As
such, we can demonstrate a route for a quick determination of these parameters
from single measurements.Comment: 6 pages, 6 figure
Arbitrary Controlled Re-Orientation of a Spinning Body by Evolving its Tensor of Inertia
Bodies with the nonspherical tensor of inertia exhibit a variety of
rotational motion patterns, including chaotic motion, stable periodic
(quasi-periodic) rotation, unstable rotation around the direction close to the
body's second principal axis, featuring a well-known tennis-racket (also known
as Garriott-Dzhanibekov) effect -- series of seemingly spontaneous 180 degrees
flips. These patterns are even more complex if the body's tensor of inertia
(TOI) is changing with time. Changing a body's TOI has been discussed recently
as a tool to perform controllable Garriott-Dzhanibekov flips and similar
maneuvers. In this work, the optimal control of the TOI of the body
(spacecraft, or any other device that admits free rotation in three dimensions)
is used as a means to perform desirable re-orientations of a body with respect
to its angular velocity. Using the spherical TOI as the initial and final point
of the maneuver, we optimize the parameters of the maneuver to achieve and
stabilize the desired orientation of the body's principal axes with respect to
spin angular velocity. It appears that such a procedure allows for finding
arbitrarily complex maneuver trajectories of a spinning body. In particular,
intermediate axis instability can be used to break the alignment of the body's
principal axis and the axis of rotation. Such maneuvers do not require
utilization of propellants and could be straightforwardly used for attitude
control of a spin-stabilized spacecraft. The capabilities of such a method of
angular maneuvering are demonstrated in numerical simulations
Monitoring Three-Dimensional Packings in Microgravity
We present results from experiments with granular packings in three
dimensions in microgravity as realized on parabolic flights. Two different
techniques are employed to monitor the inside of the packings during
compaction: (1) X-ray radiography is used to measure in transmission the
integrated fluctuations of particle positions. (2) Stress-birefringence in
three dimensions is applied to visualize the stresses inside the packing. The
particle motions below the transition into an arrested packing are found to
produce a well agitated state. At the transition, the particles lose their
energy quite rapidly and form a stress network. With both methods, non-arrested
particles (rattlers) can be identified. In particular, it is found that
rattlers inside the arrested packing can be excited to appreciable dynamics by
the rest-accelerations (g-jitter) during a parabolic flight without destroying
the packings. At low rates of compaction, a regime of slow granular cooling is
identified. The slow cooling extends over several seconds, is described well by
a linear law, and terminates in a rapid final collapse of dynamics before
complete arrest of the packing.Comment: 8 pages, 8 figure
The Cole-Cole Law for Critical Dynamics in Glass-Forming Liquids
Within the mode-coupling theory (MCT) for glassy dynamics, the asymptotic
low-frequency expansions for the dynamical susceptibilities at critical points
are compared to the expansions for the dynamic moduli; this shows that the
convergence properties of the two expansions can be quite different. In some
parameter regions, the leading-order expansion formula for the modulus
describes the solutions of the MCT equations of motion outside the transient
regime successfully; at the same time, the leading- and next-to-leading order
expansion formulas for the susceptibility fail. In these cases, one can derive
a Cole-Cole law for the susceptibilities; and this law accounts for the
dynamics for frequencies below the band of microscopic excitations and above
the high-frequency part of the alpha-peak. It is shown that this scenario
explains the optical-Kerr-effect data measured for salol and benzophenone
(BZP). For BZP it is inferred that the depolarized light-scattering spectra
exhibit a wing for the alpha-peak within the Gigahertz band. This wing results
from the crossover of the von Schweidler-law part of the alpha-peak to the
high-frequency part of the Cole-Cole peak; and this crossover can be described
quantitatively by the leading-order formulas of MCT for the modulus.Comment: 15 pages, 9 figure
Granular Structure Determined by Terahertz Scattering
Light-scattering in the terahertz region is demonstrated for granular matter.
A quantum-cascade laser is used in a benchtop setup to determine the
angle-dependent scattering of spherical grains as well as coffee powder and
sugar grains. For the interpretation of the form factors for the scattering
from single particles one has to go beyond the usual Rayleigh-Gans-Debye theory
and apply calculations within Mie theory. In addition to single scattering also
collective correlations can be identified and extracted as a static structure
factor.Comment: 7 pages, 12 figure
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