38 research outputs found
A self-similar model for shear flows in dense granular materials
We propose a model to describe the quasistatic shearing of dry granular
materials, which notably captures the differences in velocity profiles recently
observed in 2 and 3-D Couette flow experiments. In our scheme, the steady-state
flow is due to the intermittent motion of particle clusters moving together
with the wall. The motion of a cluster is associated with the transient
formation of a fracture inside the sheared pack. The model is based on the
existence of a persistence length for the fractures, which imposes a
self-similar structure on the clusters. Through a probabilistic approach, we
can evaluate the rate of appearance of a cluster of a given size and obtain a
prediction for the average velocity profiles. We also predict the existence of
large stress fluctuations at the moving wall, which characteristics are in good
agreement with experimental data.Comment: 7 pages, 2 figures, correction of the tex
A 2-D asymmetric exclusion model for granular flows
A 2-D version of the asymmetric exclusion model for granular sheared flows is
presented. The velocity profile exhibits two qualitatively different behaviors,
dependent on control parameters. For low friction, the velocity profile follows
an exponential decay while for large friction the profile is more accurately
represented by a Gaussian law. The phase transition occurring between these two
behavior is identified by the appearance of correlations in the cluster size
distribution. Finally, a mean--field theory gives qualitative and quantitative
good agreement with the numerical results.Comment: 13 pages, 5 figures; typos added, one definition change
The role of fingerprints in the coding of tactile information probed with a biomimetic sensor
In humans, the tactile perception of fine textures (spatial scale <200
micrometers) is mediated by skin vibrations generated as the finger scans the
surface. To establish the relationship between texture characteristics and
subcutaneous vibrations, a biomimetic tactile sensor has been designed whose
dimensions match those of the fingertip. When the sensor surface is patterned
with parallel ridges mimicking the fingerprints, the spectrum of vibrations
elicited by randomly textured substrates is dominated by one frequency set by
the ratio of the scanning speed to the interridge distance. For human touch,
this frequency falls within the optimal range of sensitivity of Pacinian
afferents, which mediate the coding of fine textures. Thus, fingerprints may
perform spectral selection and amplification of tactile information that
facilitate its processing by specific mechanoreceptors.Comment: 25 pages, 11 figures, article + supporting materia
A Hydrodynamic model for a dynamical jammed-to-flowing transition in gravity driven granular media
Granular material on an inclined plane will flow like a fluid if the angle
the plane makes with the horizontal is large enough. We employ a
modification of a hydrodynamic model introduced previously to describe Couette
flow experiments to describe chute flow down a plane. In this geometry, our
model predicts a jammed-to-flowing transition as is increased even
though it does not include solid friction, which might seem necessary to
stabilize a state without flow. The transition is driven by coupling between
mean and fluctuating velocity. In agreement with experiments and simulations,
it predicts flow for layers with a thickness H larger than a critical value
and absence of flow for
Self-diffusion in dense granular shear flows
Diffusivity is a key quantity in describing velocity fluctuations in granular
materials. These fluctuations are the basis of many thermodynamic and
hydrodynamic models which aim to provide a statistical description of granular
systems. We present experimental results on diffusivity in dense, granular
shear in a 2D Couette geometry. We find that self-diffusivities are
proportional to the local shear rate with diffusivities along the mean flow
approximately twice as large as those in the perpendicular direction. The
magnitude of the diffusivity is D \approx \dot\gamma a^2 where a is the
particle radius. However, the gradient in shear rate, coupling to the mean
flow, and drag at the moving boundary lead to particle displacements that can
appear sub- or super-diffusive. In particular, diffusion appears superdiffusive
along the mean flow direction due to Taylor dispersion effects and subdiffusive
along the perpendicular direction due to the gradient in shear rate. The
anisotropic force network leads to an additional anisotropy in the diffusivity
that is a property of dense systems with no obvious analog in rapid flows.
Specifically, the diffusivity is supressed along the direction of the strong
force network. A simple random walk simulation reproduces the key features of
the data, such as the apparent superdiffusive and subdiffusive behavior arising
from the mean flow, confirming the underlying diffusive motion. The additional
anisotropy is not observed in the simulation since the strong force network is
not included. Examples of correlated motion, such as transient vortices, and
Levy flights are also observed. Although correlated motion creates velocity
fields qualitatively different from Brownian motion and can introduce
non-diffusive effects, on average the system appears simply diffusive.Comment: 13 pages, 20 figures (accepted to Phys. Rev. E
Size limits the formation of liquid jets during bubble bursting
A bubble reaching an air–liquid interface usually bursts and forms a liquid jet. Jetting is relevant to climate and health as it is a source of aerosol droplets from breaking waves. Jetting has been observed for large bubbles with radii of R≫100 μm. However, few studies have been devoted to small bubbles (R<100 μm) despite the entrainment of a large number of such bubbles in sea water. Here we show that jet formation is inhibited by bubble size; a jet is not formed during bursting for bubbles smaller than a critical size. Using ultrafast X-ray and optical imaging methods, we build a phase diagram for jetting and the absence of jetting. Our results demonstrate that jetting in bubble bursting is analogous to pinching-off in liquid coalescence. The coalescence mechanism for bubble bursting may be useful in preventing jet formation in industry and improving climate models concerning aerosol production
Capillary Bridge Formation and Breakage: A Test to Characterize Antiadhesive Surfaces
In order to characterize very weak adhesive surfaces, we have developed a
quantitative test inspired by the Johnson, Kendall, and Roberts adhesion test
for soft adhesives, which relies on the formation and then the rupture of a
capillary bridge between the surface to be tested and a liquid bath. Both the
shape and the kinetics of breakage of the capillary bridge for various coatings
put into contact with liquids of various viscosities and surface tensions have
been studied. Several pull off regimes can be distinguished. For low pull off
velocities, a quasi-static regime is observed, well described by capillary
equations and sensitive to the hysteresis of the contact angle of the fluid on
the coating. Above a critical pull off velocity that depends on the fluid
viscosity, a dynamic regime is observed, characterized by the formation of a
flat pancake of fluid on the coating that recedes more slowly than the
capillary bridge itself. After the breakage of the capillary bridge, a small
drop can remain attached to the surface. The volume of this drop depends on the
dynamical regime and is strongly affected by very small differences between the
coatings. The aptitude of this test in characterizing very weakly adhesive
surfaces is exemplified by a comparison between three different perfluorinated
coatings
An introduction to InP-based generic integration technology
Photonic integrated circuits (PICs) are considered as the way to make photonic systems or subsystems cheap and ubiquitous. PICs still are several orders of magnitude more expensive than their microelectronic counterparts, which has restricted their application to a few niche markets. Recently, a novel approach in photonic integration is emerging which will reduce the R&D and prototyping costs and the throughput time of PICs by more than an order of magnitude. It will bring the application of PICs that integrate complex and advanced photonic functionality on a single chip within reach for a large number of small and larger companies and initiate a breakthrough in the application of Photonic ICs. The paper explains the concept of generic photonic integration technology using the technology developed by the COBRA research institute of TU Eindhoven as an example, and it describes the current status and prospects of generic InP-based integration technology
An introduction to InP-based generic integration technology
Photonic integrated circuits (PICs) are considered as the way to make photonic systems or
subsystems cheap and ubiquitous. PICs still are several orders of magnitude more expensive than their microelectronic counterparts, which has restricted their application to a few niche markets.Recently, a novel approach in photonic integration is emerging which will reduce the R&D and prototyping costs and the throughput time of PICs by more than an order of magnitude. It will bring the application of PICs that integrate complex and advanced photonic functionality on a single chip within reach for a large number of small and larger companies and initiate a breakthrough in the application of Photonic ICs. The paper explains the concept of generic photonic integration technology using the technology developed by the COBRA research institute of TU Eindhoven as an example, and it describes the current status and prospects of generic InP-based integration technology.Funding is acknowledged by the EU-projects ePIXnet, EuroPIC and PARADIGM and the Dutch projects NRC Photonics, MEMPHIS, IOP Photonic Devices and STW GTIP. Many others have contributed and the authors would like to thank other PARADIGM and EuroPIC partners for their help in discussions, particularly Michael Robertson (CIP).This is the final published version distributed under a Creative Commons Attribution License. It can also be viewed on the publisher's website at: http://iopscience.iop.org/0268-1242/29/8/08300
A MEMS-based tactile sensor to study human digital touch: mechanical transduction of the tactile information and role of fingerprints
We present recent results showing that human epidermal ridges (fingerprints) could
play a central role in fine texture discrimination tasks by spatially modulating
the contact stress field between the fingertip and the substrate. Using an
original biomimetic finger whose surface is patterned with parallel ridges, we
demonstrate that the subsurface stress signals elicited by continuous rubbing of
randomly textured substrates is dominated by fluctuations at a frequency defined
by the inter-ridge distance divided by the rubbing velocity. In natural
exploratory conditions, this frequency matches the best frequency of one type of
mechanoreceptors, namely the Pacinian corpuscles, which are specifically involved
in the tactile coding of fine textures. The use of white-noise patterned stimuli
has alloowed us to extract, using a reverse-correlation analysis, the
stimulus-signal response function associated with roughness modality. Its shape
could provides spectral, spatial and directional selectivity to the digital
tactile system. It offers a physiological basis for the recently proposed
hypothesis of a dual-coding (spatio-temporal and vibratory) of tactile
information