150 research outputs found
Where does a cohesive granular heap break?
In this paper, we consider the effect of cohesion on the stability of a
granular heap. We first briefly review literature results on the cohesion force
between two rough granular beads and specifically consider the dependence of
the adhesion force on the normal load. We then compute the dependence of the
maximum angle of stability of the heap as a function of the cohesion. We point
out that the dependence of the cohesive forces on the external normal load
between grains is a key point in determining the localization of the failure
plane. While for a constant cohesive force, slip occurs deep inside the heap,
surface failure is obtained for a linear dependence of the cohesion on the
normal stress.Comment: 6 pages, 6 figures. Submitted to Phys. Rev.
A Semantic Web Annotation Tool for a Web-Based Audio Sequencer
Music and sound have a rich semantic structure which is so clear to the composer and the listener, but that remains mostly hidden to computing machinery. Nevertheless, in recent years, the introduction of software tools for music production have enabled new opportunities for migrating this knowledge from humans to machines. A new generation of these tools may exploit sound samples and semantic information coupling for the creation not only of a musical, but also of a "semantic" composition. In this paper we describe an ontology driven content annotation framework for a web-based audio editing tool. In a supervised approach, during the editing process, the graphical web interface allows the user to annotate any part of the composition with concepts from publicly available ontologies. As a test case, we developed a collaborative web-based audio sequencer that provides users with the functionality to remix the audio samples from the Freesound website and subsequently annotate them. The annotation tool can load any ontology and thus gives users the opportunity to augment the work with annotations on the structure of the composition, the musical materials, and the creator's reasoning and intentions. We believe this approach will provide several novel ways to make not only the final audio product, but also the creative process, first class citizens of the Semantic We
Slow Kinetics of Capillary Condensation in Confined Geometry: Experiment and Theory
When two solid surfaces are brought in contact, water vapor present in the
ambient air may condense in the region of the contact to form a liquid bridge
connecting the two surfaces : this is the so-called capillary condensation.
This phenomenon has drastic consequences on the contact between solids,
modifying the macroscopic adhesion and friction properties. In this paper, we
present a survey of the work we have performed both experimentally and
theoretically to understand the microscopic foundations of the kinetics of
capillary condensation. From the theoretical point of view, we have computed
the free energy barrier associated with the condensation of the liquid from the
gas in a confined system. These calculations allow to understand the existence
of very large hysteresis, which is often associated with capillary
condensation. This results are compatible with experimental results obtained
with a surface forces apparatus in a vapor atmosphere, showing a large hysteris
of the surface energy of two parallel planes as a function of their distance.
In the second part, we present some experiments on the influence of humidity on
the avalanche angle of granular media. We show that the ageing in time of this
avalanche angle can be explained by the slow kinetics of capillary condensation
in a random confined geometry.Comment: Special Volume of Colloids and Surfaces A,Proceedings of
Nanocapillarity: Wetting of Heterogeneous Surfaces and Porous Solids,June
25-27, 2001, TRI/Princeton International Workshop, Editor: Alexander V.
Neimar
Thermally Activated Dynamics of the Capillary Condensation
This paper is devoted to the thermally activated dynamics of the capillary
condensation. We present a simple model which enables us to identify the
critical nucleus involved in the transition mechanism. This simple model is
then applied to calculate the nucleation barrier from which we can obtain
informations on the nucleation time. We present a simple estimation of the
nucleation barrier in slab geometry both in the two dimensional case and in the
three dimensional case. We extend the model in the case of rough surfaces which
is closer to the experimental case and allows comparison with experimental
datas.Comment: 6 pages, 3 figures, Submitted to J. Phys. : Condens. Matter,
Proceedings of the IV Liquid Matter Conference - Grenada(Spain) july 199
A new capacitive sensor for displacement measurement in a surface force apparatus
We present a new capacitive sensor for displacement measurement in a Surface
Forces Apparatus (SFA) which allows dynamical measurements in the range of
0-100 Hz. This sensor measures the relative displacement between two
macroscopic opaque surfaces over periods of time ranging from milliseconds to
in principle an indefinite period, at a very low price and down to atomic
resolution. It consists of a plane capacitor, a high frequency oscillator, and
a high sensitivity frequency to voltage conversion. We use this sensor to study
the nanorheological properties of dodecane confined between glass surfaces.Comment: 7 pages, 8 figure
Slow dynamics and aging of a confined granular flow
We present experimental results on slow flow properties of a granular
assembly confined in a vertical column and driven upwards at a constant
velocity V. For monodisperse assemblies this study evidences at low velocities
() a stiffening behaviour i.e. the stress necessary to obtain
a steady sate velocity increases roughly logarithmically with velocity. On the
other hand, at very low driving velocity (), we evidence a
discontinuous and hysteretic transition to a stick-slip regime characterized by
a strong divergence of the maximal blockage force when the velocity goes to
zero. We show that all this phenomenology is strongly influenced by surrounding
humidity. We also present a tentative to establish a link between the granular
rheology and the solid friction forces between the wall and the grains. We base
our discussions on a simple theoretical model and independent grain/wall
tribology measurements. We also use finite elements numerical simulations to
confront experimental results to isotropic elasticity. A second system made of
polydisperse assemblies of glass beads is investigated. We emphasize the onset
of a new dynamical behavior, i.e. the large distribution of blockage forces
evidenced in the stick-slip regime
Aging in humid granular media
Aging behavior is an important effect in the friction properties of solid
surfaces. In this paper we investigate the temporal evolution of the static
properties of a granular medium by studying the aging over time of the maximum
stability angle of submillimetric glass beads. We report the effect of several
parameters on these aging properties, such as the wear on the beads, the stress
during the resting period, and the humidity content of the atmosphere. Aging
effects in an ethanol atmosphere are also studied. These experimental results
are discussed at the end of the paper.Comment: 7 pages, 9 figure
Rheology of a confined granular material
We study the rheology of a granular material slowly driven in a confined
geometry. The motion is characterized by a steady sliding with a resistance
force increasing with the driving velocity and the surrounding relative
humidity. For lower driving velocities a transition to stick-slip motion
occurs, exhibiting a blocking enhancement whith decreasing velocity. We propose
a model to explain this behavior pointing out the leading role of friction
properties between the grains and the container's boundary.Comment: 9 pages, 3 .eps figures, submitted to PR
Cognitive correlates in amyotrophic lateral sclerosis: a population-based study in Italy.
BACKGROUND:There is less data available regarding the characteristics of cognitive impairment in patients with amyotrophic lateral sclerosis (ALS) in a population-based series.
METHODOLOGY:Patients with ALS incident in Piemonte, Italy, between 2009 and 2011 underwent an extensive neuropsychological battery. Cognitive status was classified as follows: normal cognition, frontotemporal dementia (ALS-FTD), executive cognitive impairment (ALS-ECI), non-executive cognitive impairment (ALS-NECI), behavioural impairment (ALS-Bi), non-classifiable cognitive impairment. We also assessed 127 age-matched and gender-matched controls identified through patients' general practitioners.
RESULTS:Out of the 281 incident patients, 207 (71.9%) underwent the neuropsychological testing; of these, 19 were excluded from the analysis due previous conditions affecting cognition. Ninety-one (49.7%) patients were cognitively normal, 23 (12.6%) had ALS-FTD, 36 (19.7%) ALS-ECI, 10 (5.5%) ALS-NECI, 11 (6.0%) ALS-Bi and 11 (6.0%) non-classifiable cognitive impairment, 1 had comorbid Alzheimer's disease. Patients with ALS-FTD were older, had a lower education level, and had a shorter survival than any other cognitive group. Of the nine cases with C9ORF72 mutation, six had ALS-FTD, two ALS-ECI and one was cognitively normal; one of the five patients with SOD1 mutations and one of the five patients with TARBDP mutations had ALS-Bi.
CONCLUSIONS:About 50% of Italian patients with ALS had some degree of cognitive impairment, in keeping with a previous Irish study, despite the largely different genetic background of the two populations. The lower educational attainment in patients with ALS-FTD indicated a possible role of cognitive reserve in ALS-related cognitive impairment. ALS-ECI and ALS-NECI may represent discrete cognitive syndromes in the continuum of ALS and FTD
Surface Roughness and Effective Stick-Slip Motion
The effect of random surface roughness on hydrodynamics of viscous
incompressible liquid is discussed. Roughness-driven contributions to
hydrodynamic flows, energy dissipation, and friction force are calculated in a
wide range of parameters. When the hydrodynamic decay length (the viscous wave
penetration depth) is larger than the size of random surface inhomogeneities,
it is possible to replace a random rough surface by effective stick-slip
boundary conditions on a flat surface with two constants: the stick-slip length
and the renormalization of viscosity near the boundary. The stick-slip length
and the renormalization coefficient are expressed explicitly via the
correlation function of random surface inhomogeneities. The effective
stick-slip length is always negative signifying the effective slow-down of the
hydrodynamic flows by the rough surface (stick rather than slip motion). A
simple hydrodynamic model is presented as an illustration of these general
hydrodynamic results. The effective boundary parameters are analyzed
numerically for Gaussian, power-law and exponentially decaying correlators with
various indices. The maximum on the frequency dependence of the dissipation
allows one to extract the correlation radius (characteristic size) of the
surface inhomogeneities directly from, for example, experiments with torsional
quartz oscillators.Comment: RevTeX4, 14 pages, 3 figure
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