The Physics of Paper Cuts and Guillotines: Slicing Softly with Shear

The top two frames show how dicing deforms a material before cutting it. A slicing motion (bottom frames) allows the cutting wire to pass through easily without damaging surrounding material.

LS12904 - A soft solid is more easily sliced using a combination of pushing a blade down and slicing rather than diced by simply pushing down on it with the same knife. To explain why this is so, we experimentally probe the slicing and dicing of a soft agar gel with a wire, along with a combination of theory and numerical simulations of cutting of a highly deformable solid. We find that purely dicing leads to deformations of the soft solid, so that the blade has to penetrate deeply into the sample in order to cut it, often damaging portions that are not in the path of the cutter. In contrast, a slicing motion cuts more cleanly without damaging the surrounding material, which explains the mechanics of painful paper cuts and design of slanted guillotine blades.

Generating something from nothing: how changing boundaries generate particles

LU13319 -It is a remarkable prediction of quantum field theory that the vacuum can generate real particles (pairs of photons) when a reflecting surface suddenly accelerates. We have created an acoustic analog to this effect using a Bose-Einstein condensate. Instead of creating photons, we generate pairs of phonons by rapidly changing the speed of sound in a condensate. This process was first predicted for electromagnetism in 1970 and is known as the dynamical Casmir effect. The effect is related to Hawking radiation, a process in which particles are spontaneously produced at the horizon of a black hole. An extension of our measurement technique may permit the observation of "sonic Hawking radiation", in an appropriately tailored Bose-Einstein condensate.

Impact of subsurface water flow on shaping landscapes

ES10795 - We show that the splitting of channel heads and the evolution of surface topography is strongly influenced by water flow in the subsurface. When ground water emerges at the surface, producing a spring, the flow removes grains from the surface by erosion, progressively digging a deeper channel, which in turn can draw more water, inducing the growth of a river. Seepage erosion is said to shape many examples of valleys, canyons, and river networks and assumed to produce amphitheater-headed valleys. However, the mechanism by which seepage channels grow and form networks is far from clear.

By performing model laboratory experiments and analysis of the ground water flow, we find significant differences between the case where the groundwater comes primarily through a boundary from a far away source and the case where it is fed by uniform local rain. Our study supports the notion that a channel network can develop in a homogeneous landscape whereby groundwater flow splits as the channels grow leading the channels to split in turn. Our observations have important implications for the interpretation of field data because numerous perturbations present in nature that could influence channel dynamics. Perturbation of the erosion front due to random avalanching events is shown to not lead to channel splittings unless supported by underlying changes in groundwater flow.

Taking Pictures of Quantum Sound Waves

LV12662 - In quantum physics, all objects have a wavelike nature.  The more energy the object possesses, the faster its wave oscillates.  In this work, we study the particles of sound which exist in an ultra-cold gas called a Bose-Einstein condensate.  Previous works measured the energy of the sound particles, but did not observe the corresponding oscillations.  We look directly at these sound particles, and see that they indeed oscillate.  We are able to see the sound particles very clearly with the help of the surrounding Bose-Einstein condensate.  This occurs because the Bose-Einstein condensate is also governed by the laws of quantum physics, so it also acts like a wave.  The large wave of the Bose-Einstein condensate magnifies the small wave of the sound particle.  Upon studying the oscillations of the sound particles, we discovered a surprise.  The oscillation rate is smaller than expected.  This is because the long, narrow Bose-Einstein condensate acts as a flow channel, which only allows certain oscillation rates.  The smaller-than-expected rate implies that the speed of sound is slower than previously thought.  This implies that the Bose-Einstein condensate is less stable than was believed previously.

Tracing the explosion of a molecule

AU10815 - Intense laser can destroy molecular bonds and cause the explosion of molecule on the ultrafast time scale. Shooting movies of such processes allows understanding how the matter responds to the strong laser light and may help to develop tools to control radiation damage. In our work we have traced the explosion of an iodine molecule exposed to intense laser pulse by an ultra-short soft-X-ray pulse from free-electron laser. We were able to probe the temporal development of the electron configuration in the disintegrating molecule and could detect when the molecule gets ionized, how long the separation of atoms takes and what happens during this process: In a molecule the electrons surrounding the atomic core form the molecular bond by sharing electrons between the individual atoms. By breaking the chemical bond, the distributed electron being described by a quantum mechanical wave function has to become localized at an atomic core after a certain time in the exploding molecule. We could observe the time it takes before the electrons localize and that the probability of losing further electrons depends on the separation of the atomic fragments in the molecule.

Tractor Beam Conveyor Belt for Tiny Particles

LV13165 - A tractor beam is a traveling wave that can transport illuminated material along its length back to its source.  Here, we demonstrate one-sided optical conveyors that act as tractor beams without requiring outside assistance. The same technique we use to project a single optical conveyor also can project arrays of optical conveyors each with independently controlled transport properties.

Lift-off dynamics in a simple jumping robot

LS13692 - We study vertical jumping in a simple robot comprising an actuated mass-spring arrangement. The actuator frequency and phase are systematically varied to find optimal performance. Optimal jumps occur above and below (but not at) the robot’s resonant frequency f0. Two distinct jumping modes emerge: a simple jump which is optimal above f0 is achievable with a squat maneuver, and a peculiar stutter jump which is optimal below f0 is generated with a counter-movement. A simple dynamical model reveals how optimal lift-off results from non-resonant transient dynamics.

Effortless motion of an animal via skew tail flapping

LS13209 - One of many functions of an animal appendage, such as a tail or a feather, is to enable or enhance locomotion. In a calm stream of water or air, a flexible tail-like structure attached to a hind end of a body will spontaneously start to flap – similar to the fluttering of a flag attached to a pole – in the same direction as the incoming stream of flow. This may, however, only be true if the tail (or flag) is sufficiently long. Researchers in Italy and Sweden demonstrate on a simple model that if the flexible structure on the body is short, it flaps at an angle of 20-40 degrees either to the right or left of the incoming stream of flow, thus inducing a net force on the body which may move transversely at no additional cost. This discovery reveals how the mere presence of appendages on animals contributes to locomotion via the interaction with  the surrounding fluid flow, without any effort whatsoever from the animal. These findings may also become useful in technological applications where it is of interest to generate a side/lift force on a moving body without increasing the drag from the surrounding fluid.

Space-time crystals of trapped ions

LU13347 - Spontaneous symmetry breaking can lead to the formation of time crystals, as well as spatial crystals. Here we propose a space-time crystal of trapped ions and a method to realize it experimentally by confining ions in a ring-shaped trapping potential with a static magnetic field. The ions spontaneously form a spatial ring crystal due to Coulomb repulsion. This ion crystal can rotate persistently at the lowest quantum energy state in magnetic fields with fractional fluxes. The persistent rotation of trapped ions produces the temporal order, leading to the formation of a space-time crystal. We show that these space-time crystals are robust for direct experimental observation. We also study the effects of finite temperatures on the persistent rotation. The proposed space-time crystals of trapped ions provide a new dimension for exploring many-body physics and emerging properties of matter.

Why your boss ignores you

LQ13899 - Remember the last time you emailed your boss about something but never received an answer? We show that such behavior should not be taken personally. It is much rather the consequence of clever communication being the key to success. Analyzing the web formed by millions of email connections in the University of Oslo shows us that people constantly make cunning trade-offs – those between paying attention to some while ignoring others. Imagine climbing a pyramid of people. Reaching the top means greatest popularity. Getting ahead means talking to those above. To progress, you are required to be an active social player –  important co-workers must receive messages but unfortunately others must be ignored. In the end, even the most skilled socializers run into the dilemma that they cannot keep up with their own popularity. As more and more employees on the pyramid look up to them, they cannot properly address even important messages. This links our human social behavior to that of primates and other animals, where the number of stable social contacts was suggested to be limited by brain-capacity.

Minimizing viscous fluid fingering

LT13129 - The classical viscous fingering problem is one of the most studied among fluid dynamic systems, having a wide spectrum of applications ranging from oil recovery processes to biodynamics of living cells. Conventionally, it considers the injection of a fluid into another of higher viscosity, at a constant injection rate, in narrow channel passages. The result is the development of visually striking, highly ramified interfacial patterns. Despite their visual appeal and physical relevance, the emergence of such convoluted structures is not always desirable. In fact, the search for mechanisms to prevent the development of these complicated morphologies is of fundamental importance to a number of areas in science and technology. Now, a group of researchers has reported a remarkably simple solution for the problem of minimizing the viscous fingering instability. Using a variational technique they have shown that interfacial instabilities are dramatically restrained if a time-dependent, linearly-increasing injection rate is employed. Their analytic results are confirmed by experiments and numerical simulations. This is illustrated in the figure, where typical radial flow interfacial patterns are depicted if a constant injection rate (left), or the optimal linearly growing injection rate (right) is used.

Catching Bubble-Driven Micromotors in Action

LS13437 - Catalytic micromotors are extremely small moving particles, about the size of bacterial organisms, that convert chemical energy through a catalytic reaction involving bubble generation, to propel the particle through the fluid.  Until now direct observation of the bubble propulsion for spherical micromotors has eluded researchers since bubble formation is very difficult on convex surfaces.  By using larger micromotors with a reduced surface curvature and an ultra-fast camera, the bubble growth and burst processes that drive spherical micromotors have been resolved.  Interestingly, it was found that bubble growth tends to propel the micromotor in one direction, while the bursting of the bubble that leaves a zone of depression pulls the micromotor back. The competition between the two processes creates an almost back and forth motion of the micromotor.  However, the growth process induces a greater displacement than that in the burst process, giving the micromotor a net displacement in one direction.  A theory is developed that quantitatively describes this behavior.  These results provide further insight into the behavior of micromotors, which are expected to have various biomedical applications in the future, such as advanced drug delivery.

Weighing Cells with Pictures

Physicists determine the weight of cells with a standard optical microscope.

LK12971 - Researchers have now developed a technique to determine the mass of cells and microorganisms with a standard light microscope. Beginning in the 1950s, laboratories around the world have concocted ingenious methods to weigh the tiny organic samples. Until now, however, these methods have all relied on custom-built instruments, involved mathematical analysis, and sample preparations that are often not compatible with living tissue. A new image-processing algorithm now enables the reconstruction of the three-dimensional mass distribution inside cells from microscopic images. The method paves the way for general microscope users to perform sensitive optical measurements of organisms and individual cells at the picogram (trillionth of a gram) level.

In Search of Rogue Wave Sources

LU13570 - New experiments presented in this paper reveal that extreme wave events, or “rogue waves” on the surface of a vertically vibrated liquid are caused by the interaction between oscillating solitons. Such solitons, also known as oscillons, have been found in a variety of physical systems such as granular medium, non-Newtonian fluids, optical media and plasmon oscillons in nanoparticle arrays. New observations made in the water surface ripple and reported here suggest that oscillons interact within a lattice and that such interaction causes their horizontal mobility and merger which leads to the formation of closed regions, or craters. Rogue waves, or strong vertical jets originate from the centers of such craters. The probability of large wave events greatly increases when oscillons move faster on the surface of water. These results will be useful in understanding physics of the rogue wave generation in other nonlinear systems.

Testing Stressed Viruses

LK13179 - One of the big questions about materials at the nanoscale is whether macroscopic theories can be applied to nano-sized systems. To answer this question, we focus on self-assembling, viral nanoparticles to test the predictions of elasticity theory. Of all known viruses those with icosahedral symmetry are the most common. Their structure can be described by folding a hexagonal lattice into a sphere-like configuration. By doing so defects with 5-fold symmetry are created at the icosahedral vertices.  According to elasticity theory these vertices should be under a permanent pre-stress, a prediction which never has been verified experimentally. Here we compare naturally occurring viruses with those that have missing proteins at their vertices, and test the hypothesis that the latter particles are stress-free. Deforming the viruses by Atomic Force Microscopy and comparing our results to detailed simulations has verified our hypothesis about pre-stress. This is the first time that the predictions about pre-stressed vertices in viral particles has been demonstrated experimentally and is a huge support for the use of continuum elasticity to describe nanometer sized objects.

Experimental Violation of Heisenberg's Uncertainty Principle

LU13293 - While there is a rigorously proven relationship about uncertainties intrinsic to any quantum system, often referred to as "Heisenberg's Uncertainty Principle," Heisenberg originally formulated his ideas in terms of a relationship between the precision of a measurement and the disturbance it must create. Although this latter relationship is not rigorously proven, it is commonly believed
(and taught) as an aspect of the broader uncertainty principle. Here, we experimentally observe
a violation of Heisenberg's \measurement-disturbance relationship", using weak measurements to
characterize a quantum system before and after it interacts with a measurement apparatus. Our
experiment implements a 2010 proposal of Lund and Wiseman to con rm a revised measurement-
disturbance relationship derived by Ozawa in 2003. Its results have broad implications for the
foundations of quantum mechanics and for practical issues in quantum mechanics.

Leading the Way to Lead-Free Piezoelectrics

LQ13398 - Energy and environment are two pressing global challenges.  Piezoelectric materials enable the conversion between electrical and mechanical energies, and have been used in many important applications, such as devices for ultrasound medical imaging and for energy harvesting from vibrations.  In the past six decades, lead-containing ceramics (greater than 60% weight of lead) have dominated the piezoelectric technology even though they pose a serious threat to the environment and human health.  For best piezoelectric performances, the compositions are fine tuned to be at the so-called morphotropic phase boundaries and the ceramics go through a poling processing where they are exposed to a highest possible electric field.  A recent discovery (to be published in the September 6 issue of Physical Review Letters) breaks the ice for new lead-free piezoelectrics.  The original approach of poling to the highest possible field is incorrect and the composition is not necessary to be at the morphotropic phase boundary.  Such a fundamental alteration to the long-standing paradigm adds a new dimension to the development of high-performance lead-free piezoelectrics.  

Learning from an Eggshell

LU13685  - If you squeeze an eggshell along its major axis, the shell is strikingly rigid and it is extremely challenging to break it with our bare hands. Conversely, if the eggshell is compressed along its equator, the resulting deflections are larger and, past a critical load one is typically able to fracture it. In our paper, we have rationalized this difference in the rigidity of an eggshell depending on the shell-load orientation to be due to the local geometry near the points of indentation. We have introduced a predictive framework for the rigidity of thin elastic shells which can also account for the situation when the shell is over-pressurized. Our concept of Geometry-Induced Rigidity can be used in reverse, as a precision non-destructive tool, to measure parameters of a shell (e.g. thickness) upon knowing the geometry of the underlying surface and the local mechanical response. The scale-invariance of Geometry-Induced Rigidity suggests that our framework should find uses across length scales: from the mechanical testing of viral capsids through Atomic Force Microscopy, to ocular tonometry procedures or in the design of architectural shells. All this work was inspired by the remarkable physics of an elegant eggshell!

Jet impact on a soap film

LG14014E - We experimentally investigate the impact of a liquid jet on a soap film. We observe that the jet never breaks the film and that two  qualitatively different steady regimes may occur. The first one is a refraction-like behavior obtained at small incidence angles when the jet crosses the film and is deflected by the film-jet interaction. For larger incidence angles, the jet is absorbed by the film, giving rise to a new class of flow in which the jet undulates along the film with a characteristic wavelength. Besides its fundamental interest, this study presents a new way to guide a micro-metric flow of liquid in the inertial regime and to probe foam stability submitted to violent perturbations at the soap film scale.

Geometric Mechanics of Curved Crease Origami

LT13067 - Origami has been widely applied to fields in structural engineering, architecture, and design, contributing to solutions of many practical problems of these fields. Although the idea that a sheet of paper can be folded along an arbitrary curve seems unfamiliar, such folds have been incorporated into origami sculptures for quite some time, and are also seen in decorative and functional boxes such as the McDonald's Fries Box. Though the crease of an open closed fold lies flat on the plane, a closed crease buckles dramatically and appears mechanically stiff. Previous work has described the geometry of curved folds. For the first time, we have presented an analytical and simulation model of the mechanics of curved fold origami. We show that it is the interaction between the geometry of the fold, the mechanical tendency to avoid in-plane stretching, and the bending energy of the sheets that produce both the three-dimensional shapes observed and the mechanical stiffness.

Slime Mold Vein Networks Give Hint To Cure Cancer

LQ13471  - Slime molds are omnipresent in nature. They feed on organic material and avoid direct sunlight. The typical damp-fresh smell in the woods is emanating from them. These slimy creatures appear primitive, yet they have developed a sophisticated strategy to transport food and deliver environmental information. They use an intricate system of veins organized in a network. Most unexpectedly, slime molds give us a hint to cure cancer. Their smart mechanism of network formation is so general that it can be applied to analyze the development of blood supply via vessel in tumors. Thus, the effectiveness of different therapies in hindering tumor growth could be tested. In order to arrive at this conclusion researchers have used exact mathematical tools from topology. This branch of mathematics deals with  the ways general objects are connected independent of their shape. 

Sorting out blood flow

LT13043 - Blood contains many different types of cells, each with specific functions in the body. As blood flows, these cells segregate, with the white blood cells (WBCs) and platelets preferentially found near blood vessel walls. This phenomenon, called margination, allows for efficient immune surveillance by the WBCs and rapid response to vessel wall injuries by platelets. Our work demonstrates how the physical properties of different cells lead to this important observation. As the stiff WBCs and platelets collide with the more flexible red blood cells during flow, they are pushed aside while having little effect on the motion of the red blood cells, a process that ultimately leads to the trapping of WBCs and platelets near the vessel walls. A simple model of blood flow that accounts only for these collisions and the flow-induced repulsion of the cells from the vessel walls is able to match results from detailed numerical simulations. The understanding provided by this study will be helpful in designing effective drug delivery particles that target the vascular walls for treatment of cancer or atherosclerosis. Margination has also been employed in biomimetic microfluidic devices to separate WBCs and platelets from whole blood, which could be helpful in treating leukemia or for platelet rich plasma therapy (used by athletes for healing injuries). The principles established in our study will aid the development of these devices.

Image processing of two photons

LT13248A - The where-about of two photons along a one-dimensional line can be generally described by the probability of the two photons to have a certain combination of positions along the line. This is a two-dimensional distribution, as is, for example, the intensity distribution that makes up a classical image. There are many ways in which an image can be processed and manipulated. Could such methods be used for controlling the spatial distribution of two photons, even if it represents a highly non-classical, quantum state of light?

In this work we show that one of the most frequently used image processing techniques, namely, spatial frequency shaping, or "Fourier processing", can be adapted for the spatial manipulation of highly non-classical states of photon pairs. As an example, we demonstrate the retrieval of the two-photon quantum phase using an analog of classical phase-contrast microscopy. As many more Fourier processing algorithms can be applied, we believe that this opens up a new avenue for the manipulation of non-classical light.      

New Quantum Computer Blueprint

Physical Review X - We develop a layered quantum-computer architecture, which is a systematic framework for tackling the individual challenges of developing a quantum computer while constructing a cohesive device design. We discuss many of the prominent techniques for implementing circuit-model quantum computing and introduce several new methods, with an emphasis on employing surface-code quantum error correction. In doing  so, we propose a new quantum-computer architecture based on optical control of quantum dots. The time scales of physical-hardware operations and logical, error-corrected quantum gates differ by several orders of magnitude. By dividing functionality into layers, we can design and analyze subsystems independently, demonstrating the value of our layered architectural approach. Using this concrete hardware platform, we provide resource analysis for executing fault-tolerant quantum algorithms for integer factoring and quantum simulation, finding that the quantum-dot architecture we study could solve such problems on the time scale of days.

Light-controlled metamaterial optics

LR13197 - We suggest and verify experimentally a novel practical approach for dynamic noncontact tuning of composite structures. This approach allows metamaterials to acquire almost any desirable spatially inhomogeneous properties by interacting with visual light patterns projected onto the metamaterial, which may lead to a new generation of electromagnetic composites whose local properties can be tuned continuously. More specifically, our approach works by applying a hand-crafted light profile to an array of light-tunable magnetic meta-atoms. The illumination affects the magnetic resonances of the meta-atoms individually. Thus, for the first time we achieve a practical design of a metamaterial in which the constitutive parameters may be changed at will and gradually within a material volume. We fabricate the first reconfigurable light-tunable metamaterial that under different illumination profiles can operate as a controllable beam deflector or/and focusing or defocusing reflector.