266 research outputs found
Quasi-2D dynamic jamming in cornstarch suspensions: visualization and force measurements
We report experiments investigating jamming fronts in a floating layer of
cornstarch suspension. The suspension has a packing fraction close to jamming,
which dynamically turns into a solid when impacted at a high speed. We show
that the front propagates in both axial and transverse direction from the point
of impact, with a constant ratio between the two directions of propagation of
approximately 2. Inside the jammed solid, we observe an additional compression,
which results from the increasing stress as the solid grows. During the initial
growth of the jammed solid, we measure a force response that can be completely
accounted for by added mass. Only once the jamming front reaches a boundary,
the added mass cannot account for the measured force anymore. We do not,
however, immediately see a strong force response as we would expect when
compressing a jammed packing. Instead, we observe a delay in the force response
on the pusher, which corresponds to the time it takes for the system to develop
a close to uniform velocity gradient that spans the complete system.Comment: 7 pages, 7 figure
High-speed ultrasound imaging in dense suspensions reveals impact-activated solidification due to dynamic shear jamming
A remarkable property of dense suspensions is that they can transform from
liquid-like at rest to solid-like under sudden impact. Previous work showed
that this impact-induced solidification involves rapidly moving jamming fronts;
however, details of this process have remained unresolved. Here we use
high-speed ultrasound imaging to probe non-invasively how the interior of a
dense suspension responds to impact. Measuring the speed of sound we
demonstrate that the solidification proceeds without a detectable increase in
packing fraction, and imaging the evolving flow field we find that the shear
intensity is maximized right at the jamming front. Taken together, this
provides direct experimental evidence for jamming by shear, rather than
densification, as driving the transformation to solid-like behavior. Based on
these findings we propose a new model to explain the anisotropy in the
propagation speed of the fronts and delineate the onset conditions for dynamic
shear jamming in suspensions.Comment: 9 pages, 3 figure
La Belgique et l’Europe dans la tourmente monétaire des années 1970 Entretiens avec Jacques van Ypersele = Belgium and Europe in the monetary turmoil of the 1970s. Conservations with Jacques van Yprsele. National Bank of Belgium Working Paper No. 314
Jacques van Ypersele de Strihou is a discrete person but well-known in Belgian and international
political and economic circles. After an outstanding academic career at the Universities of Namur,
Leuven and Louvain and then at Yale in the United States where he gained a PhD, he started out
his professional life as an official at the International Monetary Fund. He then returned to Belgium
where he held important posts in various Finance Ministers’ cabinets. He played a major role in
European and international monetary negotiations, notably in his capacity as President of the
European Monetary Committee when the European Monetary System was being set up. Later, as
head of Prime Minister Wilfried Martens’ cabinet, he was one of the architects of the 1982
devaluation of the Belgian franc. Discretion and modesty prevented Jacques van Ypersele from
talking about himself much. He took some convincing before agreeing to be interviewed. The end
result was these three interviews which took place at his home between June and October 2015
which he re-read the following summer. We were also given access to his private archives, the
source of the documents published in the Annex
Dynamic shear jamming in dense granular suspensions under extension
Unlike dry granular materials, a dense granular suspension like cornstarch in
water can strongly resist extensional flows. At low extension rates, such a
suspension behaves like a viscous liquid, but rapid extension results in a
response where stresses far exceed the predictions of lubrication hydrodynamics
and capillarity. To understand this remarkable mechanical response, we
experimentally measure the normal force imparted by a large bulk of the
suspension on a plate moving vertically upward at a controlled velocity. We
observe that above a velocity threshold, the peak force increases by orders of
magnitude. Using fast ultrasound imaging we map out the local velocity profiles
inside the suspension which reveal the formation of a growing jammed region
under rapid extension. This region interacts with the rigid boundaries of the
container through strong velocity gradients, suggesting a direct connection to
the recently proposed shear-jamming mechanism.Comment: Accepted for publication in Phys. Rev.
Splash wave and crown breakup after disc impact on a liquid surface
In this paper we analyze the impact of a circular disc on a free surface
using experiments, potential flow numerical simulations and theory. We focus
our attention both on the study of the generation and possible breakup of the
splash wave created after the impact and on the calculation of the force on the
disc. We have experimentally found that drops are only ejected from the rim
located at the top part of the splash --giving rise to what is known as the
crown splash-- if the impact Weber number exceeds a threshold value
\Weber_{crit}\simeq 140. We explain this threshold by defining a local Bond
number based on the rim deceleration and its radius of curvature,
with which we show using both numerical simulations and experiments that a
crown splash only occurs when , revealing that the rim
disrupts due to a Rayleigh-Taylor instability. Neglecting the effect of air, we
show that the flow in the region close to the disc edge possesses a
Weber-number-dependent self-similar structure for every Weber number. From this
we demonstrate that \Bond_{tip}\propto\Weber, explaining both why the
transition to crown splash can be characterized in terms of the impact Weber
number and why this transition occurs for .
Next, including the effect of air, we have developed a theory which predicts
the time-varying thickness of the very thin air cushion that is entrapped
between the impacting solid and the liquid. Our analysis reveals that gas
critically affect the velocity of propagation of the splash wave as well as the
time-varying force on the disc, . The existence of the air layer also
limits the range of times in which the self-similar solution is valid and,
accordingly, the maximum deceleration experienced by the liquid rim, what sets
the length scale of the splash drops ejected when We>\Weber_{crit}
Splashing onset in dense suspension droplets
We investigate the impact of droplets of dense suspensions onto a solid substrate. We show that a global hydrodynamic balance is unable to predict the splash onset and propose to replace it by an energy balance at the level of the particles in the suspension. We experimentally verify that the resulting, particle-based Weber number gives a reliable, particle size and density dependent splash onset criterion. We further show that the same argument also explains why, in bimodal systems, smaller particles are more likely to escape than larger ones
From splashing to bouncing: the influence of viscosity on the impact of suspension droplets on a solid surface
We experimentally investigated the splashing of dense suspension droplets
impacting a solid surface, extending prior work to the regime where the
viscosity of the suspending liquid becomes a significant parameter. The overall
behavior can be described by a combination of two trends. The first one is that
the splashing becomes favored when the kinetic energy of individual particles
at the surface of a droplet overcomes the confinement produced by surface
tension. This is expressed by a particle-based Weber number . The second
is that splashing is suppressed by increasing the viscosity of the solvent.
This is expressed by the Stokes number , which influences the effective
coefficient of restitution of colliding particles. We developed a phase diagram
where the splashing onset is delineated as a function of both and .
A surprising result occurs at very small Stokes number, where not only
splashing is suppressed but also plastic deformation of the droplet. This leads
to a situation where droplets can bounce back after impact, an observation we
are able to reproduce using discrete particle numerical simulations that take
into account viscous interaction between particles and elastic energy
Getting jammed in all directions: Dynamic shear jamming around a cylinder towed through a dense suspension
Experimental results of towing a cylinder through a dense suspension of
cornstarch and sucrose-water are presented. Focus is placed on the jamming
fronts that exist in such systems. The literature has concentrated on the
propagation of the jammed region under pushing, pulling or shearing conditions
independently. How the different fronts interact and if the fronts are
symmetric when generated simultaneously has remained unexplored. Investigating
this is our main goal. With the current setup, we are able to view a
continuous, quasi-2D field around the cylinder. As such, a new way of
generating jamming fronts is presented whereby pushing, pulling and shearing
can be examined simultaneously. In agreement with previous studies, the front
propagates roughly twice as fast in the longitudinal direction compared to the
transverse direction, which is attributed to a single underlying onset strain,
regardless of orientation from the cylinder. Although the jamming front shows
nearly perfect transverse symmetry, there is clear longitudinal asymmetry. This
is evident in the velocity and strain fields, and is also detectable in the
front propagation velocity and onset strain
Internal Crowd Work as a Source of Empowerment - An Empirical Analysis of the Perception of Employees in a Crowdtesting Project -
Internal crowd work has emerged as a new form of digital gainful employment that changes the nature of work. However, the possible effects of internal crowd work on the individual level have been largely neglected. In this paper, we therefore present our research in progress which is concerned with the effects of work characteristics in internal crowd work that have impact on the individual\u27s empowerment and satisfaction. Thus, we developed our research model and conducted an online survey amongst 118 internal crowd workers of a Swiss bank who were asked to test new software. Our expected contribution will increase the understanding of internal crowd work and provide important insights for organizations to (re-) design work on internal IT-platforms
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