3,389 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
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.
Large Language Models Can Infer Psychological Dispositions of Social Media Users
As Large Language Models (LLMs) demonstrate increasingly human-like abilities
in various natural language processing (NLP) tasks that are bound to become
integral to personalized technologies, understanding their capabilities and
inherent biases is crucial. Our study investigates the potential of LLMs like
ChatGPT to infer psychological dispositions of individuals from their digital
footprints. Specifically, we assess the ability of GPT-3.5 and GPT-4 to derive
the Big Five personality traits from users' Facebook status updates in a
zero-shot learning scenario. Our results show an average correlation of r = .29
(range = [.22, .33]) between LLM-inferred and self-reported trait scores.
Furthermore, our findings suggest biases in personality inferences with regard
to gender and age: inferred scores demonstrated smaller errors for women and
younger individuals on several traits, suggesting a potential systematic bias
stemming from the underlying training data or differences in online
self-expression
Resistance and Education: An Exploration of Anti-Colonial Struggles and Implications for Critical, Reflexive Pedagogy
On the basis of knowledge and learning as social and subjective in nature, this paper explores contemporary notions of identity and discourse to inform an argument on how sites of formal education can confine students in oppressive subject positions but also potentially allow students to exert agency in the constructions and performances of their own identity. This paper argues that if education is to incite self-empowerment and social change, discursive understandings of identity formation and socialization must be reconciled with pedagogical conceptions of agency and social justice. To this end, postcolonial arguments on resistance and criticality are drawn upon to posit that identity is important in struggles against oppression, and as such is a central concern for critical pedagogy
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
Line-scanning microphotolysis for diffraction-limited measurements of lateral diffusion
Fluorescence microphotolysis was combined with confocal laser-scanning microscopy to yield a method, herein referred to as line-scanning microphotolysis (LINESCAMP), for the measurement of molecular transport at a lateral resolution of approximately 0.34 microns and a temporal resolution of approximately 0.5 ms. A confocal microscope was operated in the line scan mode, while the laser beam power could be switched during scanning between low monitoring and high photolysing levels in less then a microsecond. The number and location of line segments to be photolysed could be freely determined. The length of the photolysed segments could be also chosen and was only limited by diffraction. Together with instrumentation a new, completely general, theoretical framework for the evaluation of diffusion measurements was developed. Based on the numerical simulation of diffusion processes employing a modified Crank-Nicholson scheme, the theory could be applied to any photobleaching geometry and profile as the initial condition and took into account the convolution with the microscope point spread function. With small diffraction-limited areas, the method yielded accurate values for diffusion coefficients in the range between approximately 10(-4) and 1 micron2 s-1. A first application of the method to the diffusion of a fluorescently labeled tracer inside the cell nucleus showed the potential of the method for the study of complex biological systems
Mathematische und phänomenologische Strenge: Distributionen in der Quantenmechanik und -feldtheorie
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