Smooth change from liquid to solid in granular media

LQ13873 - A shaken confined granular system presents a very unusual solid-liquid-like phase transition; it occurs above a certain heating threshold and, as this paper shows, it can be either abrupt or continuous. For equilibrium systems it is believed that a liquid-solid transition is always abrupt: a disordered liquid state cannot change smoothly to an ordered solid state because of their different symmetries. However, recent studies show that confined equilibrium systems, like water nano-films, can behave differently. In this paper, we show that a shaken non-equilibrium confined granular system also differs from the classical image. The transition can be either abrupt or smooth depending on the vertical height and filling density, meaning that one can go smoothly from one state to the other by changing a control parameter. In the experiments, density fluctuations do not present strong variations at the transition, whereas the way grains are locally ordered varies strongly, either abruptly or continuously. In the second case, grains smoothly order in a square symmetry in highly fluctuating domains. The associated fluctuations, characteristic size and relaxation time seem to diverge at the transition in a way described by the non-equilibrium theory of phase transitions.

What will happen when a bubble is close to a flexible soft boundary?

ER10845 -The interaction between an oscillating bubble and a flexible soft boundary is an important phenomenon, which is commonly found in nature, marine industrial applications, and medical treatments. The behavior of an oscillating bubble is greatly dependent on the characteristics of a boundary that it is placed near to.  If the boundary is rigid, the bubble moves towards it; whereas if the boundary is a free surface, the bubble migrates away from it. The behavior of the bubble near a flexible soft boundary would fall in-between these mentioned limiting cases and is more complex.  We investigate the physical behavior of the interaction between a bubble and a flexible soft boundary numerically and experimentally. The results from this study may provide physical insights into the complex physics of bubble-rubber interaction. The understanding is possibly applicable in biomedicine for drug delivery to tissue, which is a soft material. It is also probably useful in the marine industry where ultrasonic bubbles are generated for the defouling of the ship surfaces which has been coated with an elastic material. There is also potential interest in underwater explosion near an elastic structure. 

With whom do you want to share rumors, a blabbermouth or a person of few words?

EG10922 - On a social network, it seems better to transfer information preferentially toward those who have many social links ("blabbermouths") because they have larger impact on the network.  However, at the same time they can be bottlenecks for spreading information since information tends to concentrate on these blabbermouths and becomes redundant.  This paper shows that to achieve a faster spread over the network, it is indeed better to spread information preferentially toward those who have few social links (reverse preferential).

The Abraham force: the end of a century in hiding?

ASJ1069 - A century after Abraham predicted his elusive force we propose a conceptually simple way to see it in the laboratory. The rival theories of Abraham and Minkowski, both presented around 1910, seemed to give slightly different predictions for the physics of light travelling inside, say, glass or water. In particular, Abraham says that fields that change with time will give a little extra push to the material compared to what Minkowski predicts: the Abraham force. It's proved very hard to think of ways to detect this force, however, it existence is still in question. Using a series of intense and very short laser pulses, we show theoretically that the Abraham force could be detected in a way that is, at least conceptually, very simple. When a light pulse enters an optical fiber, the Abraham force would give a small "kick", setting the fiber in motion. As the pulse exits, a "kick" in the opposite direction makes it stop, but in the meantime the fiber has moved a tiny distance. Repeat this process enough times, and the distance becomes measurable. By winding a long fiber onto a cylinder and using a rapid repetition of short laser pulses, the Abraham force makes the cylinder rotate if it hangs by a thin thread. The rotation could be made so large as to be visible to the naked eye. And if the Abraham force isn't there, the cylinder will rotate in the opposite direction.

Quantum wires: prime numbers make the difference

LP12825 - Imagine a bunch of people forming a line. By talking to each of the their neighbours, they help Alice, at one end, in passing a message to Bob, at the other end. Like a sort of bucket brigade. If all do their job correctly, the message should reach Bob independently of the number of people in line. However, if we substitute people with (interacting) “talking” quantum particles, a new analysis shows that this is not the case, and that the number of people is crucial. Lines of this type are called quantum wires: they model the transfer of information between quantum particles and appear in a variety of nanodevices engineered and natural. It is known since a decade that perfect communication in quantum wires is impossible beyond three particles. But now we know that if we include an arbitrarily small imperfection, and if we wait long enough, then the message is very likely to reach Bob, exactly when the length of the wire is very special: a power of two, a prime or twice a prime. This peculiar phenomenon is due to interference effects. So, if the wire is composed by 9 particles, which is not one of these numbers, Alice better invites an extra particle to join the line and then get to 10 – which is twice a prime. The result opens up the possibility of employing quantum dynamics to design “natural algorithms” for probing the arithmetic structure of numbers

Asymmetric Higgsino Dark Matter

LP12845 - In the supersymmetric framework, prior to the electroweak phase transition, the existence of a baryon asymmetry implies the existence of a higgsino asymmetry. We investigate whether the higgsino could be a viable asymmetric dark matter candidate. We find that this is indeed possible. Thus, supersymmetry can provide the observed dark matter abundance and, furthermore, relate it with the baryon asymmetry, in which case the puzzle of why the baryonic and dark matter mass densities are similar would be explained. To accomplish this task, two conditions are required. First, the gauginos, squarks and sleptons must all be very heavy, such that the only electroweak-scale superpartners are the higgsinos. With this spectrum, supersymmetry does not solve the fi ne-tuning problem. Second, the temperature of the electroweak phase transition must be low, in the (1-10) GeV range. This condition requires an extension of the minimal supersymmetric standard model.

Zippy harmonic oscillators

LT13019 AND LT13077 - How fast can one transport a particle in a harmonic well from one place to another and come to a dead stop? With precise control, the particle can be transported arbitrarily fast and still return to its initial quantum mechanical state. So far, most experiments have been performed adiabatically, on time-scales much longer than one oscillation period. Now two research groups have cooled trapped ions to their quantum mechanical ground state and transported them over hundreds of micrometers (10,000s of  times the extent of the particle wave packet), in as few as 5 oscillation cycles. The rapid acceleration leads to excitation during transport, but the experiments show that the deceleration can be controlled well enough to return the ions to their ground state. Besides exploring a new regime of quantum transport, this work also has practical implications: Some proposals for scaling quantum information processing with trapped ions require ion transport inside a complicated array structure. So far, transport durations were long compared to those of quantum logic operations on the ions. The recent diabatic experiments  put these time scales on the same level, thereby significantly reducing the processing overhead.

Spread of microorganisms in the soil labyrinth

A network of soil pore space.

LQ13682 - Soil hosts a stunning wealth of biological activity of microbes -bacteria, fungi, protozoa...- that plays an essential role in processes such as plant growth, climate change, or soil-borne epidemics. Such microorganisms inhabit the soil pore space which is a complex maze-like network of interconnected microchannels of different shapes and lengths (see the attached figure). Despite the qualitative insight provided by experiments dealing with 2D thin sections of soil, the effect of soil structure on microbial spread is nowadays poorly understood. For instance, it is not known to what an extent the soil structural heterogeneity affects the ability of microbes to invade large portions of soil . In our Letter, we use 3D radiographs of soil and mathematical network models to show that the extreme complexity of the soil labyrinth has a prominent effect on microbial invasion. One of the most remarkable results is that neglecting the structural complexity of soil would typically lead to a substantial underestimation of microbial invasion. Invasions in complex soil labyrinths are typically larger than in more homogeneous habitats because of the presence of relatively long channels that may act as bridges for microbial transmission between distant parts of soil.

The weakest link

LR12838 - How do arches break under vibration? It turns out that arches sporting a neat, clean shape resist better than those with geometrical imperfections. In a recent experiment, physicists first create arches in a vertical layer of small spheres and then submit them to controlled perturbations. After considering several geometrical features of the arches such as size or aspect ratio, they conclude that the danger is concentrated at places where the arch departs from a perfect, uniform shape. Worse still, if one of the beads is hanging from its neighbors due to friction, odds are high that the arch will break just there, as observed in high-speed recordings. Moreover, the more prominent the defect, the easier it is to shatter the arch. Other variables, such as the number of beads, only affect the robustness to the extent that they influence the probability of finding a defect. The researchers also point to the forces between the spheres and the friction coefficient of the material as issues deserving a closer look. Apart from the obvious hits for designing stronger structures, this could also help in getting weaker ones - which could be interesting, for instance, to avoid clogging in particulate flows.

Acoustic mirage: Do you believe what you hear?

BJ12067 - In China, there is a famous saying "Words are but wind, but seeing is believing." This means that only the eyes see that is the fact while the ear heard is not the truth. Here, an intriguing acoustic mirage is present by using an acoustic concentrator, in which what you hear is not the actual, but an illusion. We design the acoustic concentrator made of gradient negative-refraction medium, which is a class of composite materials with anomalous acoustic properties varying along the radial direction. The concentrator shell functions as a magnifying superlens for sound, and projects zoom-in acoustic image at a shifted position. Thus the human auditory system will be misled and we cannot identify the location or origin of a heard sound. Based on the mirage effect, we go a step further and describe how to acoustically transform a particular object into another: the object can be made heard as another object at will. Then, do you believe what you hear?

Uncovering the Source of Epidemics, Influence, and Ideas

LN12765 - How can we localize the source of a virus, a contamination, or even an idea  in a complex network? Due to the tremendous size of many real networks---such  as the Internet or the human social graph---it is usually infeasible to observe  the state of all nodes in a network. In this paper, we demonstrate why and how  it is fundamentally possible to infer the location of the source   by observing  the information flow at only  a few nodes in the network. We find that  the proposed  technique can perform source localization in a remarkable variety of real networks,  such as the Thukela river basin in South Africa, which was affected by a large cholera outbreak in 2000.

Tracking down the source of swirl patterns on the moon

We present in-situ satellite data, theory and laboratory validation that show how small scale collisionless shocks and mini-magnetospheres can form on the electron inertial scale length. The resulting retardation and detection of the solar wind ions could be responsible for the unusual "lunar swirl" patterns seen on the surface of the Moon.

A Toy Invisibility Cloak

LR13453 - We present the first experimental demonstration of a D. C. electric cloak for steady current fields. Using the analogy between electrically conducting materials and resistor networks, a D. C. invisibility cloak is designed, fabricated and tested using the circuit theory. We show that the D. C. cloak can guide electric currents around the cloaked region smoothly and keep perturbations only inside the cloak. Outside the cloak, the current lines return to their original directions as if nothing happens. The measurement data agree exceptionally well with the theoretical prediction and simulation result, with nearly perfect cloaking performance. The proposed method can be directly used to realize other D. C. electric devices with anisotropic conductivities designed by the transformation optics. Manipulation of steady currents with the control of anisotropic conductivities has a lot of potential applications, such as electric impedance tomography, graphene, natural resource exploration, and military hiding. The described invisibility cloak can be fabricated and measured easily in an ordinary university lab, which makes the device particularly suitable for the education of young students, besides its value in science.

Gold nanoparticles may help producing greener and faster LCD

LQ13486 - Can liquid crystal displays (LCD) be greener and faster? Physicists are now developing a new approach to do both. In a LCD, LC molecules are well aligned, with the first layer of LC molecules anchored on a substrate. Strong anchoring is important for high contrast display, but higher voltage is necessary to get the display to work, and the display responds slowly. People used to believe that anchoring strength is fixed once a device is packaged. Physicists have now found that, by placing gold nanoparticles on substrate and using an effect called localized surface plasmon resonance, it is possible to manipulate the anchoring by orders of magnitude with a weak light beam, because the light excites gold nanoparticles and boosts the light intensity in its neighborhood. The strong local field effectively lowers the anchoring strength at will. The dynamic reduction of anchoring strength also helps in speeding-up the response of LC display.

Water jets always break with the same conical shape

LQ13656E - Water trickling from a faucet, drops splashing from a puddle, the fringe of a breaking wave or the jet squirted by the archer fish: all involve a stream of water which breaks into drops. It turns out that underlying this process is a fascinating curiosity of nature. The precise conical shape of the liquid thread just before it breaks, whatever its origin and history, is always exactly the same, with an angle of 36 degrees. This phenomenon is known as self-similarity, and the angle was predicted theoretically in 1998. Now for the first time researchers have measured that angle in accurate experiments, for a wide range of starting conditions, and have found that the theory is indeed correct. They used a high-speed camera to study what happens in the last few millionths of a second before the jet breaks, and studied the behavior for both water and alcohol (ethanol). They also verified another prediction of the theory, about the speed with which the threads thins down. Theories for the behavior of liquid jets and drops are vital for improved technologies from ink-jet printing to crop spraying, and this new experimental work proves how accurate the current theories are.

Flow hydrodynamics revealed by MRI

LP13351 - In this work, new advances in magnetic resonance imaging (MRI) are used to give fresh insight into the complex behavior of multiphase flows, such as the reason bubbles wobble from side to side as they rise. Physicists have long sought to understand the complex dynamics of these systems, which are of fundamental importance throughout physics and engineering, in establishing predictive models for systems as diverse as bubbles of gas rising in a glass of beer, to oil droplets rising from the sea floor. While conventional MRI is too slow to image these rapidly changing systems, MRI acquisitions are here accelerated using the science behind image compression to allow the measurement of effectively instantaneous velocity fields around rising single bubbles and swarms of bubbles. These measurements, which have not been previously possible, reveal that vortices generated in the horizontal plane behind rising bubbles are coupled with oscillations of the bubble rise path, and instigate the generation of turbulence in multi-bubble systems.

Liquid projections originate in bubble distortion

LM12997 - Liquid projections and bubbles are often intimately linked, as witnessed by our everyday experience: tiny jets from sparkling wines, moisturizing droplets from ocean spray, and splashes from boiling soup.  Occurring in very ordinary situations, liquid projections also play a key role in the gaseous balance between ocean and atmosphere, in the fabrication process of glassy or metallic melts and even in aroma diffusion from champagne fizz. Although of prime importance for this wide variety of applications, projections are still poorly understood and their occurrence appears somewhat erratic and random. In this paper, we have been able to track down the origin of large projections to the very initial formation and distortion of large bubbles. Studying the formation process of such bubbles, we have uncovered a surprisingly violent deformation of bubbles just after detachment that systematically sparks a fast liquid jet, possible prelude of a liquid projection. This dramatic experimental observation appears to exhibit an unexpected dynamics, and is likely to trigger new research in the field.

Fast liquid jet forming inside a large disconnecting bubble

Experiments demonstrate technique for improved optical gyroscopes

AC10917 - Accurate navigation in an aircraft or other vehicle requires sensitive measurements of rotation of the vehicle. For example, ring laser gyroscopes are commonplace in navigation systems used today. Experiments reported in PRA, [link], demonstrate that the sensitivity of such devices may be increased substantially. These gyroscopes use an optical cavity, a series of mirrors in which laser light bounces in a closed path, which  effectively changes length when the gyroscope undergoes rotation. The frequency of light which may be sustained in the optical cavity changes correspondingly, and a measurement of the change in frequency gives the rotation rate of the cavity. By adding a rubidium vapor cell inside the optical cavity, the new results demonstrate that the frequency change of the cavity can be enhanced by a factor of as much as 15, the largest increase achieved in an experiment to date. The "dispersion-enhanced cavity" can be used for measuring rotation rates with substantially greater sensitivity, leading to more accurate measurements of rotation, and thereby, improving the accuracy of navigation systems.

Rolled-up hyperlens goes broadband

BM11761 - We demonstrate that a rolled-up hyperlens fabricated from thin self-rolling metallic and semi-conducting layers can be used to image sub-wavelength details over a broad frequency range in the visible. We perform transmission and reflection measurements on rolled-up hyperlenses and characterize their optical properties. By means of simulations we demonstrate that two dipoles with a sub-wavelength separation placed at the inner perimeter of the rolled-up hyperlens create a magnified image at the outer perimeter. For our devices this hyperlensing behavior is present in a broad wavelength range in the visible and near-infrared regime making them particularly interesting, e.g., for sub-wavelength imaging of biological cells in a microfluidic system.

Galaxy Cluster Motions Detected for the First Time

LQ13028 - A team of astronomers and physicists have for the first time detected motions
of galaxy clusters, the largest objects in the universe. In 1972, Rashid Sunyaev and
Yakov Zel'dovich predicted that a moving cluster of galaxies should slightly
shift the temperature of microwave background radiation passing
through it, in analogy with a Doppler shift. This effect has now been
observed statistically by combining a catalog of 27000 luminous galaxies
from the Sloan Digital Sky Survey's Baryon Oscillation Spectroscopic
Survey (which trace cluster locations on the sky) with overlapping
maps of the microwave background temperature from the Atacama
Cosmology Telescope. The collaboration finds that pairs of galaxy
clusters are slightly more likely to be moving towards each other
rather than away from each other, as expected from the attractive
nature of the gravitational force. This is the first direct measurement
of motions of objects at cosmological distances. Future improvements
of this measurement can give precisely characterize the growth of
structure in the universe and the nature of dark energy. The result also
demonstrates the power of combining large astronomical surveys,
allowing new statistical measurements of subtle physical effects that no
single survey could detect.

Dying stars have a Halo

LQ13224 - We have discovered that within the heart of an exploding star (core-collapse supernova) there is a "Halo" of neutrinos.  The neutrinos in this halo, although very few in number, may nevertheless control the way that neutrinos change their flavors, e.g., converting from "electron-type neutrinos" into, say, "muon-type neutrinos".  This will force a new paradigm in the way that neutrino flavor is  handled in supernovae, and, in turn, could lead to a new understanding of supernova explosions and the origin of the elements.

Uncovering the secrets of splashing

LP12846 - Drop impact splashing has fascinated and challenged scientists for over a century. In a surprise development, super-high-resolution numerical simulations and ultra-high-speed video imaging suggest a new mechanism responsible for splashing. In a similar way as telephone lines hum in the wind, the liquid jet below a drop starts to oscillate at higher velocities, thus breaking into micro-droplets and shedding vortices into the drop. Systematic experiments showed the diversity and complexity of drop impacts, while numerical results were used to investigate the flow inside the drop. Direct comparison between experimental results and numerical simulations showed the high accuracy achieved to reproduce some of the finest details. Familiar to photographers for their aesthetic appeal, drop splashing has applications as diverse as combustion, inkjet printing, coating, crop spraying and aerosol production. Understanding these dynamics could be important to improve or control splashes

Cooper pairs of an electron and a hole

LL13172B - Superconductors conduct current without resistance, because many-body effects create pairs of two electrons, called “Cooper pairs.” Now, a paper appearing in Physical Review B reports the observation of similar pairs in a bulk semiconductor, but now of an electron and a hole. Rather than causing superconductivity, these electron-hole Cooper pairs give rise to strong light emission.

Physicists used ultrashort laser pulses to highly excite a bulk zinc oxide crystal. Upon cooling down the crystal to 4 kelvin, they discovered a strong new peak in the light emission spectrum. Both wavelength and intensity of the light excellently agreed with predictions from theory. Their conclusion is that the observed light emanates from electron-hole Cooper pairs in an uncondensed state. Since there are indications that high-temperature superconductivity might also be related to uncondensed Cooper pairs, these so-called “preformed electron-hole Cooper pairs” can be important in the quest to understand high-temperature superconductivity.

Quantum mechanics on a Mobius ring: Energy levels, symmetry, optical transitions, and level splitting in a magnetic field

BQ12069 - In this paper, the authors explore the quantum mechanical properties of an electron constrained to move on the one-sided surface of a nanoscale Möbius ring. The results are of more than theoretical interest, as recent advances in the production of graphene sheets suggest that it may be only a matter of time before it will be possible to create twisted graphene ribbons. By solving the Schrödinger equation on the Möbius surface, the authors show that the quantum numbers for the energy spectra correspond to those for cylindrical rings of the same dimension while their degeneracies are lifted. Rings with odd numbers of twists have quantum numbers with both whole and half integer values. Because the twisted rings lack the rotational symmetry of the cylindrical rings, the values for the orbital angular momentum component vary slightly from the integer and half-integer values seen in the cylindrical ring. Also, the non-zero variance in angular momentum permits the transition of photons from half-integral to integral angular momentum states, something normally prohibited by the requirement for the conservation of angular momentum. The Zeeman splitting in an external magnetic field shows level anti-crossing in the lowest two levels. Using high-accuracy finite element methods, the authors investigate rings with 1, 2, ...5 twists to identify a pattern in the level splitting that they explain using group representation theory. Beautiful wavefunctions with 2p and also 4p periodicity are also shown.

Magnetic control of Leidenfrost drops

LQ12932E - Oxygen is a common gas but its liquid state has interesting properties. Since its boiling point is at –183°C, a drop of liquid oxygen placed on a solid at room temperature levitates on a cushion of its own vapour (the so-called Leidenfrost effect, also observed with water on a hot pan at 300°C), which makes it ultra-mobile since it does not contact its substrate. In addition, oxygen is attracted by the poles of a magnet so that, as shown in our paper, magnets properly placed can be used to deviate, capture or accelerate these ultra-mobile and elusive drops. Figure 1 shows top views of trajectories observed when a drop of liquid oxygen travels across a horizontal glass plate below which a magnet is placed (grey circle). Beyond the understanding of these trajectories, this simple system allows to probe the dynamics of a liquid drop without touching it and it provides new possibilities for controlled experiments with levitating liquids.