A light source less ordinary
Fluorescent molecules are efficient light sources that can be found in
many applications, ranging from energy efficient lighting and laptop
screens to medical imaging apparatus and microscopes. In clear
materials, identical molecules will always emit light at the exact same
rate. In many practical situations the molecules are located in opaque
materials such as paint or biological tissue that strongly scatter
light: a "maze for photons". A team of scientists from the Netherlands
and France has discovered that fluorescent molecules inside such a maze
emit photons at a strongly variable rate that strongly deviates from the
average. The emission of light is determined by a molecule's close
surroundings, in particular the closest scatterer. Understanding this
process allows one to design materials for energy‑efficient lamps,
powerful microscopes and efficient solar cells.
***
LP12012
Looking into the molecular origins of viscoelasticity
When a molten polymer liquid is cooled it sets into the shape
of the mould in which it was blown or injected. This is why
e.g. plastic bottles comes in so many useful shapes.
Polymer liquids displays complex flow properties, that
differs vastly from simple liquids such as water. The reason is
to be found on the molecular level, simple liquids are made of
small molecules, that can easily move between each other.
The liquid flows effordlessly. Polymers on the other hand are
long string like molecules, that are highly entangled with each
other like strings of spaghetti, this vastly restricts their freedom
to move. This not only makes polymer liquids highly viscous,
but it even displays elastic properties like a soft solid. Computer
simulations allows us to obtain new and unique insights into
polymer liquids, since we can simultaneously look at the
dynamics of individual polymer molecules (see figure)
and the viscoelastic flow properties of the material.
***
EQJ1049
New Surprises in a Classic Experiment
Inverse Chladni patterns, i.e., grains collecting at the anti-nodes of a resonating horizontal plate, were traditionally believed to occur only for particles light enough to be carried along by the air currents induced by the vibrating plate. Thus it comes as a surprise that there is yet a second mechanism leading to inverse Chladni patterns: Dutch physicists show – in a brief report in Physical Review E – that when the acceleration of the resonating plate remains below g, all grains spontaneously roll towards the anti-nodes, irrespective of their size and even in the absence of air. The authors stress that the new mechanism is a subtle one, and explain why it has escaped detection for more than two centuries, in thousands of demonstrations of this classic experiment.
***
LP12418ER
Bending to Fly: Why Elasticity is The Key To Flapping Flight
Birds, bats and insects flap their wings to fly, and they do it so
skillfully that engineers have gone a long way trying to imitate the
flapping motion to power flying machines. Both biologists and engineers
have since long observed that the amazing propulsive and maneuvering
possibilities offered by flapping flight come at a price: the perpetual
cycle of moving wings back and forth costs a lot of energy. All
flapping flyers in nature have found out a way to minimize this cost and
it always involves the fact that animal wings are flexible. This paper
uses a simplified experimental model of a flapping‑wing flyer to
investigate the problem of the effect of wing flexibility on flight
performance.
***
LM12086
Imaging atoms and molecules with a nanoscale force sensor.
Scanning tunnelling microscopy (STM) changed our perception of the nanoworld because it has made single atoms and molecules visible to us. This “vision” is, unfortunately, quite indirect: A density of valent electronic states, imaged by the STM, may sometimes reflect positions of the atoms, but in the cases when the atoms are bound chemically to each other and share their valence electrons the STM fails to resolve the individual atomic positions without the support from time consuming theoretical simulations. In this work we extended the imaging capabilities of the STM by equipping it with a nanoscale sensor composed of a single hydrogen/deuterium molecule. The sensor probes atomic short-range forces and thus maps the density of the core electronic states, which in contrast to the valence ones are not affected by the by chemical interactions and thus reveal the positions of the individual atoms within the studied chemical compound.
***
LR11957
Close encounters of the third kind between cosmic strings
Magnetic flux tubes, known as Abrikosov vortices, are routinely observed
in superconductors. Yet their relativistic cousins -- known as cosmic strings,
widely predicted in particle physics and superstring theory models-- have
never been observed in the sky. The stakes are high: if found, strings
could provide a "fossil record" for the forces among particles immediately
after the Big Bang. A promising search strategy is to try to detect the
gravitational radiation emitted by the wildly oscillating strings, but this
requires very accurate theoretical predictions of the emitted radiation.
These predictions, in turn, require detailed knowledge of what strings do
when they collide, the object of this study. "Abrikosov" cosmic strings
always reconnect but, occasionally, when the collision happens at
near-luminal speed, they reconnect a second time and effectively pass
through with some deformation. Surprisingly, the present study shows that if
the strings have strongly repulsive cores, double reconnections are much
less uncommon, but also that the strings may even dance around and reconnect
three or four times before parting company. The effect of these close
encounters on the strings' detectability is now under investigation.
***
LG12153E
Consensus is not always driven by the majority
Consensus is a concept which relates to the agreement of a set of
entities. It is very much studied in contemporary physics due to its
widespread presence in interdisciplinary scenarios. Examples of
consensus reaching systems include magnetic materials, animal moving
groups and human systems driven by opinion formation. It is usually
believed that consensus is easily reached when it accommodates the
opinion of the majority. But however things do not necessarily happen
this way. In our work we have considered a simple model inspired in
collective animal motion. Two merging groups of animals, or more
generally entities, travelling with opposite directions form a new
group which selects its direction of motion according to some rule. If
this rule indicates that the direction of motion is chosen randomly,
then a large number of groups initially travelling with different
directions will in short time reach consensus and a unique direction
of travel will be globally selected. If, alternatively, the direction
of motion of the majority is chosen with a higher probability then
consensus is not reached and the initial disordered pattern of
velocities is maintained for all times. This shows that consensus is
not necessarily a consequence of the opinion of the majority and might
contrarily arise out of a multiplicity of random interactions.
