How stone monuments deteriorate in the presence of salts

LU13997 - Crystallization in confined spaces, such as pores, is relevant to nearly every field of science. Salts crystallizing within porous materials exert a pressure that contributes to damage, but the quantification of this in-pore pressure is very difficult to achieve. Salt-induced physical weathering has been documented as a substantial damage mechanism in nearly every environment all over the world from pharaoh’s tombs in Egypt, the cathedrals of Europe, decorated caves in China, Mayan pyramids in Mexico and Central America and historic buildings in the United States and England. In this paper, we calculate the solution supersaturation and resulting crystallization pressure and unequivocally identify the precipitating phase in a multi-phase salt system (containing hydrous and anhydrous, metastable and stable phases). These results help to explain why salts, such as Na2SO4 with various hydrated phases are more damaging than single phase salts such as NaCl. We also show that damage associated with meta-stable phase transitions can be suppressed by the use of crystallization promoters. These results point to new ways to prevent salt damage to building materials by bypassing metastable phase formation.

Understanding astrophysical jets with magnetized laser experiments

LX13024 - Objects as diverse as proto-stars, dying-stars and black-holes eject  powerful beams of matter (jets) that are believed to be shaped by magnetic fields. In this letter we show how their formation can be studied in the laboratory by coupling high-power lasers with magnetic fields. In the proposed experiments, an intense laser source irradiates a thin-foil generating a spherically expanding, million-degrees plasma. By imposing a strong magnetic field, this flow can be shaped into an elongated, drop-like bubble which generates a centimetre-long  jet propagating at more than 300 km/s. We speculate that astrophysical jets may be similarly produced from stellar winds expanding in a magnetic field.

Physical Limits to Leaf Size in Tall Trees

LV13534 - Leaf size in angiosperm trees vary by more than three orders of magnitude, from a few millimeters to over 1 meter. This large morphological freedom is, however, only expressed in small trees and the observed leaf size range declines with tree height, forming well-defined upper and lower boundaries. The vascular system of tall trees that distributes the products of photosynthesis connects distal arts of the plant and forms one of the largest known continuous microfluidic distribution networks. In biological systems, intrinsic properties of vascular systems are known to constrain the morphological freedom of the organism. We show that the limits to leaf size can be understood by physical constraints imposed by intrinsic properties of the carbohydrate transport network. The lower boundary is set by a minimum energy flux, the upper boundary by a diminishing gain in transport efficiency.

Cooling by Heating

PRX - When a bulk piece of material is heated at its surface, does its center become hotter or cooler? It turns out that the answer to this seemingly banal question is not obvious at all at a fundamental level. In this paper, we report, first through a theoretical analysis and prediction and then with experimental confirmation, an extraordinary phenomenon of cooling at the center of a supercooled liquid drop heated at its free surface.

Friends of friends key to job and mate searches

Physical Review E - Imagine that you are searching for a job, a date or an expert opinion on a social network. What do you do? Until recently, you had to rely on your friends or acquaintances, but nowadays, using social network sites (like Facebook, Twitter or LinkedIn) you can go beyond that and start to search not only using your direct friends but also the friends of your friends, or the friends of your friends of your friends, and so on. That is, you can find out not only people that is one degree of separation from you in the social network, but people that is two or three degrees from you on the social network. Since, we live in a small world perhaps with only six degrees we can span the whole network. In a recent paper, published on November 19 in Physical Review E, researchers found a navigation strategy that does precisely that: to jump directly to more than one degree of separation on a network. They found that this strategy, named Levy walks, akin to the one used by foraging animals, can be very efficient to search and navigate the social network.

Pedestrian Experiments

Physical Review E - In human crowds, interactions among individuals give rise to a variety of self-organized collective motions that help the group to effectively solve the problem of coordination. However, it is still not known exactly how humans adjust their behavior locally, nor what are the direct consequences on the emergent organization. One of the underlying mechanisms of adjusting individual motions is the stepping dynamics. In this paper, we present first quantitative analysis on the stepping behavior in a one-dimensional pedestrian flow studied under controlled laboratory conditions.We find that the step length is proportional to the velocity of the pedestrian, and is directly related to the space available in front of him, while the variations of the step duration are much smaller. This is in contrast with locomotion studies performed on isolated pedestrians and shows that the local density has a direct influence on the stepping characteristics. Furthermore, we study the phenomena of synchronization—walking in lock step—and show its dependence on flow densities. We show that the synchronization of steps is particularly important at high densities, which has direct impact on the studies of optimizing pedestrians’ flow in congested situations. However, small synchronization and antisynchronization effects are found also at very low densities, showing the natural tendency to synchronize according to perceived visual signals.

Cellular Tug-of-War: Measuring the Forces as Cells Divide

LS13909 - New generations of cells are generated when existing cells divide: how do physical forces drive this process? In this Letter, we quantify for the first time the force exerted by dividing cells to their surroundings. We observed dividing Dictyostelium cells, a simple model organism for the study of cell division, and measured the force acting between cells and their substrate by Traction Force Microscopy. We found that two sister cells have two force spots for each and using these spots as anchoring points, they apply strong pulling force to their connecting bridge. The pulling force gets stronger until the last step of division and then suddenly drops when the middle structure breaks. This result indicates that dividing cells pull each other using two footholds to separate just like a tug-of-war between two people. This study revealing the unexpected role of forces on cell division highlights the power of physical approach to understand this biologically important phenomenon.

Two-footed nanowalker powered by light

LT13770 - Artificial nanowalkers are inspired by biomolecular counterparts from living cells, but remain far from comparable to the latter in design principles. The walkers reported to date mostly rely on chemical mechanisms to gain a direction; they all produce chemical wastes. This paper reports a light-powered DNA bipedal walker based on a design principle derived from cellular walkers. The walker has two identical feet and the track has equal binding sites; yet the walker gains a direction by pure physical mechanisms that autonomously amplify an intra-site asymmetry into a ratchet effect without producing any chemical waste. It has a distinct thermodynamic feature that it possesses the same equilibrium before and after operation, but generates a truly non-equilibrium distribution during operation. The demonstrated design principle exploits mechanical effects for symmetry breaking and direction rectification, hence is adaptable for use in other nanomachines. The walker is advantageous for certain applications, especially biomedical ones, as it is free of chemical wastes, remotely controlled by light, and requires a low-level irradiation within the safety limit of biological tissues.

Secure cloud quantum computation with light

LV13623 - A first-generation quantum computer must be implemented in the ``cloud" style, since only governments or huge industries will be able to possess  such a super-expensive and high-maintenance object. Can we guarantee the security of client's privacy  in such a cloud quantum computing? The protocol of blind quantum computation can solve that problem. It is a new secure quantum computing protocol where a client, who does not have enough quantum technology, can

delegate her quantum computation to a server, who has a fully-fledged quantum computer, in such a way that the server cannot learn anything about client's input, output, and program. This protocol was recently experimentally realized in an optical system by Zeillinger's group at Vienna. In this experiment, the discrete degrees of freedom (polarization) of photons was used.  In this paper, we have shown that such a blind quantum computation is also possible by using the continuous degrees of freedom of photons. Quantum computation by using such a continuous degrees of freedom is one of the most central research subjects in today's quantum optics, and in fact plenty of experiments have been done. Hence our result will push the blind quantum computation to a new domain, and will open a door to more practical secure cloud quantum computation.

Teletransporting magnetic energy

LV13317 - Electromagnetic cloaks and perfect lenses have passed from imagination to actual realizations thanks to transformation optics. This elegant technique allows to make electromagnetic waves to follow a desired path in real space. Here we apply it to static magnetic fields in order to design a perfect magnetic concentrator, which can largely enhance the ambient magnetic field value, therefore improving the sensitivity of magnetic sensors. By using two or more of these concentrators, themagnetic energy of a source like a magnet can be transferred to a desired distant point through free space. The concentrator can be constructed with readily available materials -superconductors and ferromagnets. Because magnetic energy is responsible for generating energy in power plants, for saving information in magnetic memories or for moving our devices with motors, the new possibilities of magnetic energy manipulation brought by our device may offer improvements in these fields and thus in our everyday life.

Shape-shifting red blood cells

LV12899 - In this work, we demonstrate that the red blood cells (RBC) circulating in the body can develop surprising shapes under the simple  action of the forces generated by blood flow. They divide into two  portions, connected by a tube. These new shapes appear if the cohesion of the RBC membrane is  relatively low and the level of cohesion can vary significantly from one person to the next. The circulation of RBCs in the capillaries is  affected by such shape changes, interfering with the process of  supplying the body with oxygen. Furthermore, the formation of tubes is  accompanied by an increase in the mechanical stress on the membrane,  which may favor the development of micropores in the membrane,  ultimately resulting in the loss of RBC haemoglobin (haemolysis). This  may cause pathologies such as kidney failure or pulmonary  hypertension. This discovery will guide future experimental research  focused on tracking the morphological evolution of RBCs in the circulatory system in artificial architectures representing human blood vessels. This approach will ultimately enable better understanding of the therapeutic prevention of cardiovascular diseases and may prevent possible blood disorders from developing in  asymptomatic individuals

Sculpting sandcastles grain by grain: Self-assembled sand towers

LS13910E - We study the spontaneous formation of granular towers produced when dry sand is poured on a wet sand bed. When the liquid content of the bed exceeds a threshold value, the impacting grains have a non-zero probability to stick on the wet grains due to instantaneous liquid bridges created during the impact. The trapped grains become wet by the capillary ascension of water and the process continues, giving rise to stable narrow towers. The growth velocity is determined by the surface liquid content which decreases exponentially as the tower height grows. This self-assembly mechanism could theoretically as long as the capillary rise of water is possible, however the structure collapses before reaching this limit. The collapse occurs when the weight of the tower surpasses the cohesive stress at its base. The cohesive stress increases as the liquid content of the bed is reduced. Consequently, the highest towers are found just above the sticking threshold value.

Linking Nanoparticles with DNA "Velcro"

LU14111 - We have discovered a new type of interaction between micro/nanoscale particles that results from the entanglement of DNA strands attached to their surfaces. Self-complementary DNA single strands on a particle can be induced to form loops. When loops are formed on adjacent particles, they can form mechanical links similar to the ones macroscopic hook-and-loop materials like Velcro rely on. The interactions can be created and broken by a combination of forces, temperature, light sensitive crosslinking and enzymatic unwinding of the topological links. This novel topological interaction may lead to new materials and phenomena such as particles strung on nano-necklaces.

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.