How can you compact a thin sheet of paper?

LB13270

- You can crumple this sheet as a paper ball but the growth of a network of high energy ridges and vertices (d-cones) will hinder the formation of a dense object. It is indeed very difficult to attain 50% of compaction even by applying very large forces !

On the contrary, you can try to make with great care regular folds following a complex origami scheme to get the most compact morphology, i.e., the smallest volume for a given sheet but this is rather time consuming !

Why not using self-organization of folds ?

Here, we show that you can produce very easily optimal self-similar patterns of fold by constraining a sheet at one edge, i.e., a hanging curtain. By exploring these self-organized curtains, we uncover an universal law governing the shape of the sheet whatever the used materials, from graphene to fabrics. In addition, we show that these spontaneous patterns can be manipulated by adding a simple tensile force, regularizing the complex hierarchy of folds.

BOOMERANG BEHAVIOUR OF MASSIVE PARTICLES IN BLACK-HOLE RADIATION

DB10717

- The traditional view on Hawking radiation is that black holes emit (massless)
photons as well as massive particles, provided that the latter have an energy
at least equal to their rest mass. We have demonstrated that this image is
actually not correct for the massive particles: these are radiated from the
black hole horizon independently of their energy. However, the particles with
an energy smaller than their rest mass only arrive at a finite distance from
the black hole before bouncing back in a boomerang-like fashion and being
re-absorbed by the black hole. We have also suggested several examples of
analogue gravity systems (systems where sound waves or other perturbations
behave as if they were moving in a black-hole spacetime) where such an effect
could realistically be detectable in a laboratory setting.

Optically induced crystals of submicron particles

EY10506

- Laser beams sent through microscope lenses are widely used in
physics and biology to trap and manipulate small particles in a solution.
In these so-called optical tweezers, micron-sized particles floating in the
microscope's field of view become trapped at the focal spots of the laser
beams, allowing mechanical control of tiny objects under the microscope.
Using several laser beams, multi-site optical tweezers have been realized
that enable the assembly and control of ensembles of such particles. In the
present work, we use optical tweezers formed by four interfering laser beams
to grow and control large, optically induced crystals of submicron particles
in aqueous solution. In contrast to crystals in the usual sense, these are
non-frozen, periodic particle assemblies held together by the optical forces
generated by the lasers. Several thousands of particles can be arranged into
nearly defect-free, three-dimensional crystals with high packing density.
The crystal structure is controlled by laser beam polarizations and angles,
while the crystal size can be controlled by laser beam diameters and powers.
Future applications of the work may include studies of artificially
crystallized biological matter (bacteria, viruses, proteins) using soft
x-ray Bragg scattering. The research may also lead to the creation of novel
tools for photonics applications, such as materials with a tunable photonic
bandgap.

Break-Up in Granular Jets

LW12658
- Attractive forces between the intimate particles of matter (atoms
or molecules) are at the heart of the conception of cohesion of matter.
This is why matter stick together : gases condense to liquids,and
liquids freeze to solids. Granular jets falling out of a funnel shaped
container, subjected to small vertical vibrations, under the action of
gravity display a strikingly liquid-like appearance. Such jets start out
spatially uniform and break up into clusters farther downstream as may
happen for ordinary liquids under the action of surface tension forces.
This is surprising since attractive forces between grains (at the heart
of surface tension or capillary forces in liquids) are much weaker than
other mechanical forces at play (gravity, friction, inelasticity).
From an analysis of the long wavelength variations of the jet
radius (induced by the vertical vibration), it turns out that these
modes are unstable and produce a long wavelength break up of the jet
which is reminiscent of a "Rayleigh-Plateau" capillary instability
(that leads to drop formation for liquids). This instability and the
break up of the jet can be inhibited when the effect of the surrounding
medium (air) is reduced by enclosing the jet in an evacuated chamber,
showing that the effective surface tension measured is the result of a
strong interaction with the surrounding air.

Two telescope views on drifting constants in the early universe

LC12995

- Since the days of Dirac scientists have been wondering about the possibility that fundamental constants may have varied over the history of the universe. In the past decade this question has been made operational through the comparison of spectral lines observed in objects, known to be old, in comparison with spectral lines observed in the laboratory, i.e. in the present epoch. The mass ratio between a proton and an electron is such an important fundamental constant, which may be tested by looking at hydrogen molecules. Now very detailed observations of the largest set of hydrogen lines so far (over 90) of the brightest know quasar system (J2123-005 at redshift =2.05) with a lookback time of over 10 billion years from the two largest optical telescopes in the world (the Very Large Telescope in Paranal, Chile and the Keck Telescope in Hawaii) show that such observations on "old hydrogen" can be made, and are not overwhelmed by systematic effects. The results from both telescopes perfectly agree with each other and give a slight indication of a change of a fundamental constant. However, the evidence is too small to call proof. More observations will be needed (of other quasar systems), but we know now better about the trustworthyness of such observations.

Quantum “Tricks” in the Biochemical Reactions of the Avian Compass Mechanism

LW12015E

-“Quantum” and “Bio” are two phrases rarely seen together in a scientific context. The idea that quantum physics is underlying biological systems has been entertained for a long time, with evidence, however, being scarce. We here show that a sort of biochemical reactions, namely radical-ion-pair reactions, at the heart of the avian magnetic compass mechanism and central in photosynthesis, are full of the counter-intuitive quantum-mechanical traits usually encountered in experiments dealing with the simplest of quantum objects, atoms or photons. One of the central themes of quantum physics is the infamous Young's double slit experiment, which beautifully manifests the particle-versus-wave duality and the principle of quantum interference, both cornerstones of quantum physics. In this paper we show that radical-ion-pair reactions are governed by the same principles, only now it is the spin of the electrons that is the main actor. Electrons are spinning little magnets, and their magnetic orientation determines the fate of these reactions, as the electrons hop between neighboring molecules. We have unraveled the rich quantum dynamical behavior of these reactions that has been overlooked for more than 40 years, providing further evidence that Nature has invented quantum physics well ahead of quantum physicists and has genuinely applied it in large dangling biomolecules living in the “wet and warm” biological environment.