27 research outputs found
Ultrasonic tracking of a sinking ball in a vibrated dense granular suspension
Observing and understanding the movement of an intruder through opaque dense
suspensions such as quicksand remains a practical and conceptual challenge.
Here we use an ultrasonic probe to investigate the dynamics of a steel ball
sinking in a 3D dense glass bead packing saturated by water. We show that the
frictional model developed for dry granular media can be used to describe the
ball motion induced by horizontal vibration. From this rheology we infer the
static friction coefficient and effective viscosity that decrease when
increasing the vibration intensity. Our main finding is that the
vibration-induced reduction of the yield stress and increase of the sinking
depth are presumably due to induced slipping at the grain contacts but without
visible plastic rearrangements of grains, in contrast to dry granular packings.
To explain these results, we propose a mechanism of acoustic lubrication that
reduces the inter-particle friction and leads to a decrease of the yield
stress. This scenario is different from the mechanism of liquefaction usually
invoked in loosely packed quicksands where the vibration-induced compaction
increases the pore pressure and decreases the confining pressure on the solid
skeleton, thus reducing the granular resistance to external load.Comment: 9 pages and 5 figures, plus the supplemental information (1 page, 2
movies, 1 figure
Control of drop positioning using chemical patterning
We explore how chemical patterning on surfaces can be used to control drop
wetting. Both numerical and experimental results are presented to show how the
dynamic pathway and equilibrium shape of the drops are altered by a hydrophobic
grid. The grid proves a successful way of confining drops and we show that it
can be used to alleviate {\it mottle}, a degradation in image quality which
results from uneven drop coalescence due to randomness in the positions of the
drops within the jetted array.Comment: 3 pages, 4 figure
Probing viscoelastic properties of a thin polymer film sheared between a beads layer and quartz crystal resonator
We report measurements of viscoelastic properties of thin polymer films of
10-100 nm at the MHz range. These thin films are confined between a quartz
crystal resonator and a millimetric bead layer, producing an increase of both
resonance frequency and dissipation of the quartz resonator. The shear modulus
and dynamic viscosity of thin films extracted from these measurements are
consistent with the bulk values of the polymer. This modified quartz resonator
provides an easily realizable and effective tool for probing the rheological
properties of thin films at ambient environment.Comment: submitted to ap
FINE: Fisher Information Non-parametric Embedding
We consider the problems of clustering, classification, and visualization of
high-dimensional data when no straightforward Euclidean representation exists.
Typically, these tasks are performed by first reducing the high-dimensional
data to some lower dimensional Euclidean space, as many manifold learning
methods have been developed for this task. In many practical problems however,
the assumption of a Euclidean manifold cannot be justified. In these cases, a
more appropriate assumption would be that the data lies on a statistical
manifold, or a manifold of probability density functions (PDFs). In this paper
we propose using the properties of information geometry in order to define
similarities between data sets using the Fisher information metric. We will
show this metric can be approximated using entirely non-parametric methods, as
the parameterization of the manifold is generally unknown. Furthermore, by
using multi-dimensional scaling methods, we are able to embed the corresponding
PDFs into a low-dimensional Euclidean space. This not only allows for
classification of the data, but also visualization of the manifold. As a whole,
we refer to our framework as Fisher Information Non-parametric Embedding
(FINE), and illustrate its uses on a variety of practical problems, including
bio-medical applications and document classification.Comment: 30 pages, 21 figure
Sliding friction perturbed by shear ultrasound vibrations: dynamic lubrication and overaging
Reduction of the effective static coefficient of friction by shear ultrasound has been shown recently to be due to the partial lubrication of the solid–solid contact. Here, we study the effect of ultrasound perturbation on a multicontact interface at imposed drive velocity. We show that, together with the partial lubrication similar to the static case, ultrasound vibrations enhance structural aging and the emergence of stick–slip
Coalescence of droplets on chemical boundaries
We show that emulsion droplets sedimenting toward a chemically patterned substrate can self-assemble and coalesce onto chemical boundaries, leading to a 2D growth pattern. After
discussing the various features characteristic of the self-assembly process, we focus on the behaviour of a single droplet when sliding from a high contact angle area to a low contact angle
area. The adsorption on the low contact angle area shall occur via two main mechanisms, depending on the interplay between droplet dynamics and wettability contrast
Triggering granular avalanches with ultrasound
International audienceGranular flows triggered by vibration below the avalanche angle are ubiquitous in nature. However, the mechanism of triggering and the nature of the resulting flow are not fully understood. Here we investigate the triggering of the shear instability of granular layers by nanometer-amplitude ultrasound close to the static threshold. We find that such small-amplitude and high-frequency sound waves provoke unjamming, resulting in a self-accelerated inertial flow or a creeplike regime which stops flowing after the removal of ultrasound. We show that these effects are due to the reduction of interparticle friction at grain contacts by the shear acoustic lubrication. Our observations are consistent with the bistability inherent to velocity-weakening friction models [e.g., Jaeger et al., Europhys. Lett. 11, 619 (1990), 10.1209/0295-5075/11/7/007]. This work should help to understand the local and remote triggering of landslides and earthquakes by seismic waves