Building towers drop by drop

LH13533

- We show an unprecedented variety of smooth symmetric, corrugated, zig-zag shaped slender structures that can be observed by simply dripping a mixture of sand and water on a liquid absorbing surface such as a dry bed of sand or blotting paper. The various shapes are in contrast with the liquid drops which can splash, spread or bounce upon hitting a surface. Successive drops are observed to freeze rapidly upon impact due to the drainage of a small fraction of liquid, literally stacking on top of each other into surprisingly slender structures named granular towers. Further, twisted pagoda dome-like structures are observed by increasing the flux into the jetting regime. We show that the towers are held together because of capillary and friction forces, and the shape of the towers depends on a subtle balance between dripping frequency, density of grains, and impact speed. Besides applications in surface patterning, this tower building technique may be a new and easy way to probe the flow properties of dense granular suspensions using the shape of the tower.

A story of colliding sea ice floes

EG10801

- As has been so often the case over the last several years, alarming news again arrived from the Arctic a few weeks ago. On 9th September, at the maximum of the summer melting season, the sea ice extent in the Arctic Ocean reached the second lowest value in the satellite record: 4.6 million square kilometers. Increasing areas of open water in the summer mean larger and larger areas where new ice forms in winter. Thick and strong perennial ice gets gradually replaced by thinner seasonal ice, more susceptible to breaking and deformation. What do we know about the dynamics of that increasingly abundant – and thus increasingly important – ice type, composed of separate floes? Incomparably less than about the dynamics of the perennial ice. In this paper, sea ice is viewed as a discontinuous medium built of floes of different sizes moving on the sea surface and colliding with each other. The model presented, deriving from analogies with other granular materials, reproduces a number of hitherto unexplained phenomena observed in sea ice at low concentrations – including the formation of floe clusters (see the attached image), important for the dynamics and freezing/melting of ice – and thus it may contribute to a better performance of state-of-the-art sea-ice and climate models.

Different mechanics of snap-trapping in the two closely related carnivorous plants, Dionaea muscipula and Aldrovanda vesiculosa

LG13128E


- The carnivorous aquatic Waterwheel Plant (Aldrovanda
vesiculosa L.) and the closely related terrestrial Venus Flytrap
(Dionaea muscipula SOL. EX J. ELLIS) both feature elaborate
snap-traps, which shut after reception of an external mechanical
stimulus by prey animals. Traditionally, Aldrovanda is considered as a
miniature, aquatic Dionaea, an assumption which was already
established by Charles Darwin. However, videos of snapping traps from
both species suggest completely different closure mechanisms. Indeed,
the well- described snapping mechanism in Dionaea comprises abrupt
curvature inversion of the two trap lobes, while the closing movement
in Aldrovanda involves deformation of the trap midrib but not of the
lobes, which do not change curvature. In this paper, we present the
first detailed mechanical models for these plants, which are based on
the theory of thin solid membranes and explain this difference by
showing that the fast snapping of Aldrovanda is due to kinematic
amplification of the bending deformation of the midrib, while that of
Dionaea unambiguously relies on the buckling instability that affects
the two lobes.

Pair Creation Constrains Superluminal Neutrino Propagation

LK12734

- The OPERA collaboration claims that muon neutrinos with mean energy of 17.5 GeV travel 730 km from CERN to the Gran Sasso at a speed exceeding that of light by about 7.5 km/s or 25 ppm. However, we show that superluminal neutrinos may lose energy rapidly via the bremsstrahlung of electron-positron pairs ( ! + e− + e+). For the claimed superluminal velocity and at the stated mean energy, we find that most of the neutrinos would have suffered several pair emissions en route, causing the beam to be depleted of higher energy neutrinos. This presents a significant challenge to the superluminal interpretation of the OPERA data. Furthermore, we appeal to Super-Kamiokande and IceCube data to establish strong new limits on the superluminal propagation of
high-energy neutrinos.

Impact craters: grains against grains

LH13515


- Nowadays it is well accepted that craters in the moons and planets were created by asteroid collisions. But, why some craters are completely flat and others show central peaks? The actual paradigm explains that central peaks arise due to the surface fluidization produced by the impact and/or the crater collapse. However, based on the granular nature of the asteroids we developed impact experiments of “granular projectiles” in sand, obtaining similar morphologies to those observed in celestial bodies. So we claim that the crater morphology is due to the granularity of the projectile: low-packed projectiles completely spread after collision producing bowl- shaped craters, high-packed projectiles confine the internal material during the impact giving rise to central peaks.

How a quintessential quantum test was fooled -- or why quantum physicists should always read the fine print

LH12960

- When it comes to pitting quantum theory against classical notions of
the world, there’s one experiment that physicists say makes quantum
theory the clear winner: a test of Bell’s inequalities. We have found
that it’s possible to fake quantum results using classical physics in
such a test, reminding us to be cautious about the assumptions in
experiments. Bell inequalities measure the strength of correlations
between two particles, or how much their behaviors are coordinated.
Quantum physics allows for stronger correlations than classical physics,
violating a Bell inequality. In our experiment, a typical apparatus for
measuring Bell violations by photons was cheated using bright pulses of
light to manipulate the output of single-photon detectors. Researchers
thinking ahead to quantum devices for communication and computation have
proposed incorporating tests of Bell inequalities in such devices to act
as safeguards. Our work highlights the practical challenges of such
schemes. It also reminds us that no Bell tests so far have been without
seemingly reasonable ‘fine print’ assumptions about how the experiments
worked. In our case, we exploited the so-called detection loophole. Our
attack was an evil intervention. We have no reason to think the Universe
conspired to trick past experiments, but one would prefer to eliminate
the need for fine print. Various experimental groups worldwide are
working towards this goal.

Switching the light fantastic

LH12759

- Quantum mechanics says that light consists of small indivisible packets of energy or quanta known as photons. In this paper, we show that the passage of a light beam through an optical fiber can be controlled by just a few quanta of energy (photons) in another light beam.

Photons are introverts by nature and usually don’t interact with each other, unless they are in large numbers. One of the holy grails of physics is single-photon all-optical switching, where one photon controls the passage of another photon through a medium. It is a critical goal for the development of future quantum information networks, promising ultrahigh speed computation and ultra-secure communication.

Our paper takes a significant step towards this goal by demonstrating all-optical switching with less than twenty photons and that too at fast timescales of around five billionths of a second. We confine the light, along with an atomic vapor, to the core of an optical fiber that has a size less than a tenth of the width of a human hair. This architecture forces light to strongly interact with the atoms, causing the absorption of photons of two different colors only if they are simultaneously present, but not if one type is absent.

BATS CHANGE EAR SHAPE AND RESHAPE THEIR SPATIAL HEARING

LC13795

- Certain bats can deform the shapes of their ears in a way that changes the animal's ultrasonic hearing spotlight. Within just one tenth of a second, these bats are able to change the configurations of their outer ear shapes from one extreme to another. As a result of these shape changes, the shape of the ultrasonic spotlight also undergoes a qualitative change. Using a combination of methods that included high-speed stereo vision and high-resolution tomography, researchers have now been able to reconstruct the three-dimensional geometries of the outer ears from live horseshoe bats as they deform on these short time scales. Using computer analysis of the deforming shapes, the researchers found ultrasonic hearing spotlights associated with the different ear configurations that could suit different hearing tasks performed by the animals. Hence, the ear deformation in horsehoe bats could be a substrate for adapting the spatial hearing of the animals on a very short time scale.

World’s fastest maze solver

LC13533E

- The authors of a recently accepted PRE paper demonstrate that a network of memristors (resistors with memory) solves the maze problem much faster than any existing supercomputer. Such an extraordinary advance in computation power is due to a massively-parallel network dynamics in which all network components are simultaneously involved in the calculation. This type of parallelism could be dubbed as “analog parallelism” which is different from that used in conventional supercomputer. It is anticipated that massively-parallel memristive processors will provide an efficient solution of many other optimization problems.

Hot electron ‘coolness’ through tunable energy transfer in nanowires

LC13992

- Harnessing and transfer of excess energy of electrons generated by light via rapid transfer of electrons across semiconductor interfaces is key to developing more efficient solar cells. In addition, hot-electron transfer is one process involved in obtaining a region of negative electrical resistance, a non-linear feature that is important in many advanced electronic devices. Researchers now report highly tunable and fast photo-excited hot electron transfer across the cylindrical interface of a co-axial core-shell semiconductor nanowire hetero-structure. The onset of the negative resistance region is shown to be highly tunable by one of three different modes, owing to the reduced dimensionality of a nanowire. Until now, the onset of negative resistance has been fixed by the selection of materials, thus requiring a network of integrated circuit components to produce tunable components important for computing, signal processing and communications. This nano-scaled multiply-tunable optically-addressed device element opens possibilities for gaining new insight into hot electron transfer processes for solar energy conversion, and for constructing complex electronic circuits using far fewer, simpler and much smaller components with the potential for requiring much less energy to operate.

An All-optical, Pain-free Method for Blood Glucose Testing

EC11052

- Pain-free blood glucose testing is highly desirable for diabetes management which requires frequent blood glucose monitoring. For many decades a search for a non-invasive blood glucose concentration detection method has been a major, yet elusive goal. Optical methods have been confined in the near-infrared where glucose has absorption peaks but is hard to identify due to multiple overlapping absorptions by other blood constituents. The development of clinically viable non-invasive glucose biosensors has been hampered by lack of specificity and sensitivity. In this paper we introduce theoretically and experimentally a new non-invasive method, Wavelength-Modulated Differential Photothermal Radiometry (WM-DPTR), for non-invasive, non-contact blood glucose monitoring. WM-DPTR features glucose specificity and sensitivity by combining laser excitation by two out-of-phase modulated beams at wavelengths near the peak and the baseline of a prominent and isolated mid-IR glucose absorption band. A theoretical photothermal model of WM-DPTR signal generation and detection has been developed. Simulation results on water-glucose phantoms within the human blood-range glucose concentration (0-300 mg/dl) demonstrated high sensitivity and resolution to meet wide clinical detection requirements. The model has also been validated by experimental data of the glucose-water system obtained using WM-DPTR.

The Fastest Way to Pour Syrup

LC13080ER

- A thick liquid flows quickest if you pour it from the corner, rather than the side, of a carton.

Pouring a fluid out of a container is an everyday process that is also relevant in a wide range of industrial situations. Despite this fundamental interest, this fundamental problem has apparently never been addressed before. The authors derive the time required for the fluid to begin discharging after tipping the container and the volume of the fluid yet to be discharged at subsequent times. The authors consider two different geometric configurations and find noticably different pouring characteristics. The theoretical predictions are compared successfully with experiments.

The results of the present study are applicable to situations where it is desirable to retrieve viscous fluids in minimal time from containers, such as buckets of detergent, cans of paint, and tanks of oil. They provide a point of reference to gauge the influence of a rich variety of effects that could be incorporated to extend the simplified model.

Deciphering the very first contact between virus particles and host cells

LD13184

- A virus replication cycle is initiated by virus binding to receptors presented on the surface of a host cell. An in-depth understanding of the very first steps of virus binding to cell membranes is therefore crucial for the development of antiviral therapies, vaccines and high-performance diagnostics. Studies of the initial interaction between a virus and the host cell membrane were so far mainly focused on identifying the binding sites on the virus and / or the cell membrane, while significantly less is known about the kinetics of the interaction. With single virus sensitivity, we quantify in this work the energy barrier for the virus-membrane association. We were able to discriminate between two structurally very similar receptors, which interaction kinetics could not be distinguished using conventional methods. We also observed that the virus release kinetics appears logarithmic over a long time span. Such interaction behavior is rarely observed in natural sciences and was earlier not reported in the context of virus binding. In our study, this feature has been explained by multiple receptor-virus contact points and membrane deformation. Such situations may occur at receptor-enriched cell-membrane regions with positive curvature, allowing us to suggest that the logarithmic kinetics may take place in vivo.



Description of the illustration:

Illustration of how surface-bound virus-like particles were used to probe their interaction with cell-membrane receptors incorporated into fluorescent-labeled lipid vesicles. By probing the rate of binding and the time individual vesicles remain bound (the residence time), we could simultaneously investigate weak (few receptor contacts) and strong (multiple receptor contacts) interactions. The virus-like particle was from the norovirus genogroup II.4 Dijon strain, being the major causative agent of acute viral gastroenteritis worldwide causing yearly up to 200, 000 children deaths, and the receptors were glycosylated sphingolipids.

X-rays vaporize water

LF13057EJ

- When ionizing radiation i s illuminated to water, it removes electrons from water molecules and ! ionizes them. This ionization can modify water surface tension (PRL 100, 217403, 2008), potentially affecting thermodynamic behavior of water: an important example is vaporization, since vaporization enthalpy depends on surface tension. The interaction between ionizing photons (x-rays or gamma-rays) and soft matter becomes important to scientists, with growing applications of x-ray techniques to soft matter systems.

In a paper in Physical Review E, researchers address for the first time (on a quantitative basis) that liquid water is vaporized by x-rays. They precisely measure the vaporization rate of water confined in a capillary tube during x-ray irradiation using high-resolution, high-speed x-ray microscopy. Intere! stingly, a reversible switching between vaporization and condensation repeatedly occurs with x-ray on-off switching.

The group finds a hidden fundamental linkage among ionization, surface tension, and vaporization: how effectively photons vaporize water depends on charging density. This finding could provide insights into topics concerning radiological or electrohydrodynamic phenomena such as radiation-induced bubble formation or cosmic-ray-induced cloud formation.

Sand physics: taking the rough with the smooth.

EE11020


- Why do the slopes of some piles of soil or sand slip very easily, while
other piles remain extremely stable? Answering this question holds the
key to the understanding of many fundamental physical questions, for
example the onset of landslides. For a long time scientists have assumed
that the surface roughness of the individual grains plays a keys role in
this process. However, this common-sense assumption has never been
verified experimentally. Here, for the first time, we provide the
necessary methodological concepts and succeed in proving this
hypothesis. We do so as follows.

We have invented a chemical recipe to increase the roughness of glass
and we use it to systematically tune the roughness of sub-mm glass spheres.

We measure the roughness of individual grains using light interference,
and the roughness of the whole granular sample using Coca-Cola.

We then measure the friction between the grains by looking at the slope
of the pile they form at the bottom of an hour glass.

We look at the correlation between roughness and friction. Our results
show that changing the roughness of grains allows us to tune the
friction between them, making friction a control parameter. And this
hands us the key to understanding real-life granular systems.

Packing of balls into a jar: first ask if the balls are all the same size

EGR1074

- How spheres pack into containers is a problem that’s been studied for decades. Recent computer simulations show that such packings appear to have large-scale density fluctuations if the spheres aren’t all exactly the same size. More significantly, the simulations show that careful consideration of each particle’s size reveals that the density fluctuations are smoother than they first appear. However, it is very hard to detect subtle particle size differences in experiments. Previously experiments had to assume all particles are the same, which is an approximation; the simulations showed that this approximation is an unfortunate one to have to make. We imaged half a million microscopic spheres in a container and developed a novel method for determination of each particle’s size. Using our data, we confirmed the prior computer simulation results. We expect our method will be quite useful for a variety of experimental studies of sand and particle suspensions.

Attosecond intramolecular electron dynamics observed

LF13695

- The current article provides an exceptional view into the microscopic,
ultrafast world of one single electron inside nature's most simple
molecule.

Our experimental and theoretical work shows how an ultrashort and strong
laser drives the only electron in the chemical bond of a hydrogen
molecular ion on an attosecond time scale (1 attosecond = 10^{-18}
seconds), makes it slosh around and finally splashes it out of the
molecule. This wild splashing electron waves come as a surprise to
scientists who previously assumed that a laser field frees electrons
gently through a narrow tunnel "drilled" into the molecule by the light.
Our work further shows the exciting prospect that the invisible dynamics
of the electron inside a molecule can be mapped onto the momenta of the
electron once it left the molecule and where they become observable.

Look Ma, No Hands!

BE11548

- During early 1960s, NASA scientists developed the first magnetic liquids (ferrofluids) as an alternative means to move liquid fuels in a gravity-free environment. Since that time, ferrofluids have found many other uses within industrial, commercial and biomedical settings, but the original goal of practical liquid manipulation in a compact system with magnetic fields alone remained elusive... Until now. In this paper, we experimentally demonstrate for the first time a general approach that allows direct pumping of ferrofluids at controllable speeds in closed-loop geometries without any mechanically moving parts. Since the pumping action involves nanoparticle rotation within the entire liquid body, the physics of this ferrohydrodynamic pumping mechanism is easily scalable to all sizes – from microfluidic devices to industrial-scale pumping systems. Here, we illustrate the simplicity of this “no-hands” pumping scheme utilizing a stereo amplifier, ordinary plumbing materials from the local hardware store and a commercially available mineral oil/magnetite ferrofluid that is easy to make and safe to handle. We believe our approach could lead to highly compact, integrated, completely quiet and very efficient liquid cooling schemes for portable, high-performance consumer electronics. With biocompatible ferrofluids, direct ferrohydrodynamic pumping could enable highly compact chambers for continuous-flow cellular perfusion and incubation, as well.

Record-Low Error Rate for Quantum Information Processing with One Qubit Achieved

LE13372AR

- Thanks to advances in experimental design, physicists have achieved a
record-low probability of error in quantum information processing with
a single quantum bit (qubit) - the first published error rate below
the theoretical threshold for building viable quantum computers. A
quantum computer could potentially solve certain problems that are
intractable using today’s technology, even supercomputers. The NIST
experiment, with a single beryllium ion qubit, is a milestone for
simple quantum logic operations, demonstrating a probability of error
of only 1 per 50,000 logic operations. For comparison, one error per
10,000 logic operations is a commonly agreed upon target for a low
enough error rate to use error correction protocols in a quantum
computer. However, a working quantum computer will also require
two-qubit logic operations with comparably low error rates. The record
low error rate was made possible by two changes in the group’s
experimental set-up. First, the scientists manipulated the ion using
microwaves instead of the usual laser beams. Second, the ion trap was
placed inside a copper vacuum chamber and cooled to 4.2 K with a
helium bath to reduce errors caused by magnetic field fluctuations in
the lab.

Primordial black holes could ring the Sun like a bell

LE13715

- Approximately 25% of the energy density of the universe consists of dark matter. One possible candidate for this dark matter is primordial black holes produced in the first second after the Big Bang. A primordial black hole in the Galaxy's dark-matter halo could pass through our Sun without accreting appreciably, but its
gravitational tidal field would cause the Sun to oscillate like a ringing bell. In this paper, we calculate for the first time the amplitude and frequencies of these solar oscillations. NASA's Solar Dynamics Observatory could detect these oscillations if the mass of the primordial black hole exceeds 10^21 g, the mass of a large
asteroid. Given the inferred local density of dark matter, the event rate for such primordial black holes passing through the Sun is about 10^-7 per year. These oscillations may also be detectable in other stars by reanalyzing the same observations used to search for extrasolar planets.

IS THE UNIVERSE A FRACTAL?

LF12878DR

- A spacetime with fractal geometry may help in unifying the gravitational
force with quantum mechanics, and bridge a gap between the observed
reality and theories postulating that Nature is discrete.

One of the greatest worries of physicists are infinities: If we hope
that Nature be described by the language of mathematics, then we expect
that everything be described by a finite set of observable quantities.
However, things go wrong when trying to merge general relativity with
quantum mechanics, and infinities arise. The author of this research
argues that a cure to this problem is to replace ordinary geometry with
fractal geometry. While in the former case concepts such as volume and
dimension have an intuitive meaning, in the latter they experience
radical transformations. At very small distances a discrete spacetime
texture emerges, thus opening up the possibility that the continuum
geometry we observe at large scales is only an effect of a coarse
resolution. Also, the dimension of spacetime is predicted to change with
the probed scale (as in multifractals), to be noninteger and smaller
than four. This phenomenon of dimensional reduction can render field
theories finite.

ATOM-LASER MAKES MEASUREMENT OF GRAVITY

LY12724A

- Compared to a thermal light source, such as a light bulb, the optical laser revolutionised the precision of optical measurements, which utilise the interference of light waves in devices known as interferometers. In the same way, atom-interferometers, which use matter waves, should benefit from the use of an atom-laser; the direct analogy of an optical laser. This paper presents the first direct comparison of a thermal and laser-like atom sources used in an atom-interferometer that measures gravity. Everyday, many people take advantage of the optical laser’s measurement precision, for example, by using a DVD player. Perhaps less obvious is the permeation of atom-interferometers into society. Atomic clocks are examples of atom-interferometers, again affecting many people daily via their use of GPS, which requires exquisite precision in the measurement of time. More recently, atom-interferometers have been used for precision inertial measurements, such as measuring gravitational acceleration. Indeed, the most precise gravity sensors include atom-interferometers, and the measurement of gravity has applications ranging from fundamental tests of physical theories, through to Earth sciences, mineral exploration, and navigation. This work represents an important step forward in the development of the next generation precision inertial sensors.

Watching Flames Spread in Microgravity

LU12255EJ

- Waves of chemical reaction spreading through a heterogeneous media are found throughout biology, chemistry, and physics. Most theories to understand how these waves spread assume that individual particles can be neglected and their effect is smoothed over the media. Our paper shows that for some systems, this assumption fails and an unusual regime of wave propagation can occur which we call the “discrete regime.” Examples of familiar systems that might exhibit this behavior include clouds of combustible dust in air, forest fires, or flames propagating through a rocket propellant. In the discrete regime, even if the particles burn infinitely fast, the reactive wave (or flame) is still limited by the time it takes the heat released by one particle to spread to the neighboring particles. Therefore, the overall process becomes statistical, being influenced by the randomized position of the particles in three-dimensional space. In addition to theoretical and computer-based solutions, we experimentally observed this discrete regime by igniting flames in suspensions of iron particles in air, where the nitrogen had been replaced by xenon in order to decrease the heat conductivity of the gas. The slow propagation speed of the flames (3 to 5 cm/s) made them sensitive to being disrupted by gravity and necessitated conducting the experiments in a reduced-gravity (freefall) environment onboard an airplane flying along a parabolic trajectory.

Metamaterial-based model of warp drive

LY11889BJ

- Electromagnetic metamaterials are capable of emulating many exotic space-time geometries, such as black holes, rotating cosmic strings, and the big bang singularity. Here we present a metamaterial-based model of the Alcubierre warp drive, and study its limitations due to available range of metamaterial parameters. It appears that the material parameter range introduces strong limitations on the achievable “warp speed”, so that ordinary magnetoelectric materials cannot be used. On the other hand, newly developed “perfect” bi-anisotropic non-reciprocal magnetoelectric metamaterials should be capable of emulating the physics of warp drive gradually accelerating up to 1/4 the speed of light.

The figure here shows an example of a metamaterial geometry, which
explicitly violates spatial and time reversal symmetries in ways that make warp drive simulation possible.

Has the black hole at our Galaxy's center been feeding lately?

LD13673

- The recent discovery of huge `bubbles' emitting gamma-rays high above
and on either side of the galactic plane has puzzled the astrophysical
community and challenged long held beliefs: Has the supermassive black
hole at the centre of the Milky Way been shooting out jets of plasma?
Just how are high-energy particles being produced so far away from all
the stars in the Galaxy? We present a model that can explain these
mysterious structures and will allow us to answer such questions in the
near future.

The Milky Way is generally perceived as a flat thin disk of stars
visible by its faint glow in the night sky. Recently, this view has been
challenged by data from the Fermi-LAT satellite experiment. High-energy
particles must be radiating in huge bubble-like structures extending far
above and below the stellar disk which are shining in gamma-rays. This
is all the more surprising as the massive black hole at the centre of
our Galaxy is believed to be quiescent, in contrast to active galaxies
where we can see jets of plasma being shot out as the central black hole
swallows up stars and gas. We show that such activity may have occurred
in our own Galaxy just a few million years ago and that high energy
electrons can be accelerated by the resulting plasma turbulence and
produce high-energy gamma-rays. Our model explains the appearance of the
bubbles and the spectrum of their emission and makes detailed
predictions for future observations which can test the model further. It
appears that our Galaxy may not be such a quiet place as we had imagined
all this time.