ADAPTIVE NETWORKS, OR HANGING OUT WITH HEALTHY PEOPLE
A novel analytic formalism is introduced to track epidemic dynamics in populations featuring an adaptive contact network. Under the threat of an emerging disease, people naturally tend to avoid interactions with the infectious in order to reduce their own chances of getting sick. This behavioral response to the disease can be captured in the framework of adaptive networks, where the contact network and the state of the nodes evolve in an intricate manner, allowing healthy individuals to cut ties with those who are infectious: in other words, hang out with healthy people. Being the first to correctly reproduce the time evolution of both dynamical elements, disease and topology, the approach presented in this paper is able to make accurate predictions on the conditions under which a disease will invade a population under the adaptive contact network hypothesis. This represents an important step forward in the inclusion of more realistic features in existing epidemic models. This work is part of a collective effort made by network epidemiologists to provide policy makers with an analytic toolbox that will help guiding the development of future prevention and intervention strategies.
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LT12730
Physics of peeling
The phenomenon of peeling or delamination i.e., a spatial separation
of rigid bodies at an interface occurs commonly in nature. It ranges
from geological processes, such as lithospheres separating from
tectonic plates to biological ones, involving blastoderms in cells
forming a gastrula. It is also a common failure-mode in many everyday
processes such as peeling of paint etc.. In a single imaging
experiment using colloidal crystals of varying rigidity, we have
captured this entire process in a laboratory scale measurement. We
found that the origin of the observed diversity of the process is in
the competing effects of the rigidity of the medium, the strength of
adhesion to the substrate from which it peels and the externally
applied stress. The images obtained in the experiment - vapor-like
evaporation of individual particles occurs in the soft films, while
solid-like collective delamination of large chunks, triggered by
nucleation and propagation of cracks occurs for rigid films - provide
a simple and intuitive understanding of the otherwise extremely
complex phenomenon. This understanding is important for applications
such as adhesion, patterning, lithography and soft electronics, in
addition to the prevention of catastrophic mechanical failure in
materials.
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EU10655
Tunable solid-like behavior in fluids
Yield stress is one of the distinguishing properties of solids, and
ordinary (Newtonian) fluids are not known to exhibit this feature.
However, some complex (nonNewtonian) fluids under external driving do
exhibit a solid-like behavior, so that they can support a shear stress
without flowing. A group of Brazilian scientists has investigated the
influence of a tunable magnetic yield stress on the morphology of fluid
droplets subjected to external magnetic and centrifugal forces. A sizable
magnetic field-dependent yield stress appears on highly nonNewtonian fluid
suspensions of magnetic micronsized particles, known as magnetorheological
(MR) fluids. In contrast, colloidal suspensions of magnetic nanoparticles
called ferrofluids (FF) present negligible yield stress. By confining
magnetic fluid droplets between narrowly spaced parallel glass plates, an
exact balance between the forces involved is achieved, leading to the
emergence of novel pattern forming structures. One particularly noteworthy
aspect is the material dependence that clearly enters into the resulting
shapes. The manipulation of yield stress properties via magnetic means
opens up the possibility of unveiling a number of still unexplored pattern
morphologies and new dynamic behaviors. This offers a stimulating
challenge to soft matter researchers.
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LT12324E
Crowding or Sticking: How do breast cancer cells move?
Understanding how breast cancer cells interact with each other and move through host tissues
is the first critical step in understanding how breast cancer can spread through our body.
To examine the motion of breast cancer cells we present new data describing cell movement in a series
of carefully designed experiments. Various experimental conditions lead to a variety of responses
which cannot be interpreted intuitively. We anticipate that the experimental results could be explained
by cell-to-cell adhesion (stickiness) or cell-to-cell crowding effects. By replicating the experimental
data with an appropriate discrete random walk model, we show that a low value of cell-to-cell adhesion
strength provides the best explanation of the experimental data suggesting that cell crowding effects plays
a more important role than cell-to-cell adhesion. This is a critical result since cell crowding effects are
typically neglected in standard models describing the motion of cell populations
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LS12522
Positrons Spy on Defects
The worlds most intense positron beam was used to track
down fast processes of atomic defects in solids.
Novel materials often gain their advanced superior
properties not only by specific compositions but also by
structural modifications. Processing metallic materials,
e.g., by extreme plastic deformation produces structures
on a nanoscale which lead to superior mechanical properties.
It may seem like a paradox that the enhanced mechanical
properties are based on defects of the lattice. However,
these defects are on an atomic scale whereas defects on
a micro scale such as cracks or pores often deteriorate
materials.
Now, for the first time, fast kinetic processes of
atomic defects in materials were investigated by making
use of the worlds most intense positron beam provided by
NEPOMUC at the research reactor FRM II of the Technische
Universität München at Garching (Germany) in collaboration
with Austrian researchers from the Technical University
of Graz, the Montan University Leoben and the University
of Vienna. These studies are essential for the development
of novel structural materials and the antiparticle-particle
annihilation of the positron-electron reaction is ideally
suited for such kind of studies.
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LS12130
Optical control of the refractive index of a single atom
The effects of matter on light are known from our everyday life: It passes through transparent air, is refracted by glass or is absorbed by soot. These optical properties usually don't change for a certain material. However, in some cases, they can be controlled and drastically changed using laser light with the right color. In this paper, we have demonstrated that a single cesium atom can be tuned continuously from absorbing to transparent. Because the effect of a single atom on a light beam is very small, we have placed the atom inside an optical cavity using optical tweezers. The cavity is formed by two highly-reflecting mirrors, such that our probing light passes the atom up to 300,000 times. With a control-laser beam shone on it from the side, the absorption of the probe light passing the atom has been strongly suppressed. An optical switch based on these mechanisms could prove useful in quantum communication. Surprisingly, we observed that the atoms are also cooled when they are rendered transparent. Since colder atoms stay longer in optical tweezers, the new cooling effect enabled us to experiment with one atom about twenty times longer than before.
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LU12566
Simple crystals show exotic negative refraction effects in the far infrared
In this letter we describe how a simple quartz crystal may be used to achieve negative refraction for all angles of incidence, positive or negative. Research on refraction, the change of direction suffered by a light ray as it passes from one medium to another, can be traced back as far as the ancient Greeks. In conventional (positive) refraction, the ray has to cross the normal, an imaginary line drawn perpendicular to the interface where the incident ray hits it. However, in the case of negative refraction, a phenomenon which has come to prominence over the last decade, the change of direction is sufficient for the ray to bend back to the same side of the normal. Most studies of negative refraction have concentrated on materials formed from intricate artificial structures. However, in this work we show how, at certain far infrared frequencies, all-angle negative refraction may be achieved using a natural anisotropic crystal such as quartz. The secret
lies in the interaction of the far infrared radiation with phonons – natural vibrations of the crystal lattice. Reflection and transmission measurements both point to the occurrence of negative refraction and show that, although absorption exists, transmission efficiency is relatively high.
