A crowd that flows like water

Dynamic response and hydrodynamics of polarized crowds
Nicolas Bain, Denis Bartolo
Modeling crowd motion is central to situations as diverse as risk prevention in mass events and visual effects rendering in the motion picture industry. The difficulty of performing quantitative measurements in model experiments has limited our ability to model pedestrian flows. We use tens of thousands of road-race participants in starting corrals to elucidate the flowing behavior of polarized crowds by probing its response to boundary motion. We establish that speed information propagates over system-spanning scales through polarized crowds, whereas orientational fluctuations are locally suppressed. Building on these observations, we lay out a hydrodynamic theory of polarized crowds and demonstrate its predictive power. We expect this description of human groups as active continua to provide quantitative guidelines for crowd management.

Read more at http://science.sciencemag.org/content/363/6422/46

Optical “Bernoulli” Forces

Light scattering from a rotating dielectric cylinder

Light scattering from a rotating dielectric cylinder

Ramis Movassagh and Steven G. Johnson
By Bernoulli’s law, an increase in the relative speed of a fluid around a body is accompanies by a decrease in the pressure.
Therefore, a rotating body in a fluid stream experiences a force perpendicular to the motion of the fluid because of the unequal relative speed of the fluid across its surface. It is well known that light has a constant speed irrespective of the relative motion.
Does a rotating body immersed in a stream of photons experience a Bernoulli-like force?
We show that, indeed, a rotating dielectric cylinder experiences such a lateral force from an electromagnetic wave.
In fact, the sign of the lateral force is the same as that of the fluid-mechanical analogue as long as the electric susceptibility is positive (ε>ε0), but for negative-susceptibility materials (e.g. metals) we show that the lateral force is in the opposite direction.
Because these results are derived from a classical electromagnetic scattering problem, Mie-resonance enhancements that occur in other scattering phenomena also enhance the lateral force.
Read more at http://arxiv.org/pdf/1305.0317v2.pdf

Reab also: Optical Bernoulli Forces Could Steer Objects Bathed in Light, Say Theorists

Humans Running in Place on Water …

… at Simulated Reduced Gravity

journal.pone.0037300.g001
Abstract

Background
On Earth only a few legged species, such as water strider insects, some aquatic birds and lizards, can run on water. For most other species, including humans, this is precluded by body size and proportions, lack of appropriate appendages, and limited muscle power. However, if gravity is reduced to less than Earth’s gravity, running on water should require less muscle power. Here we use a hydrodynamic model to predict the gravity levels at which humans should be able to run on water. We test these predictions in the laboratory using a reduced gravity simulator.

Methodology/Principal Findings
We adapted a model equation, previously used by Glasheen and McMahon to explain the dynamics of Basilisk lizard, to predict the body mass, stride frequency and gravity necessary for a person to run on water. Progressive body-weight unloading of a person running in place on a wading pool confirmed the theoretical predictions that a person could run on water, at lunar (or lower) gravity levels using relatively small rigid fins. Three-dimensional motion capture of reflective markers on major joint centers showed that humans, similarly to the Basilisk Lizard and to the Western Grebe, keep the head-trunk segment at a nearly constant height, despite the high stride frequency and the intensive locomotor effort. Trunk stabilization at a nearly constant height differentiates running on water from other, more usual human gaits.

Conclusions/Significance
The results showed that a hydrodynamic model of lizards running on water can also be applied to humans, despite the enormous difference in body size and morphology.

Citation: Minetti AE, Ivanenko YP, Cappellini G, Dominici N, Lacquaniti F (2012) Humans Running in Place on Water at Simulated Reduced Gravity. PLoS ONE 7(7): e37300.

Read more at http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0037300

Droplets bouncing over a vibrating fluid layer

Drops on Drops on Drops

Among all of the sub fields in physics, fluid dynamics research consistently generates some of the most beautiful images and videos. In honor of the upcoming APS Division of Fluid Dynamics meeting in San Diego, today we bring you one such example video.

Typically, when a droplet of water plunges into a pool of water, it will bounce slightly and then coalesce with the rest of the water. But there’s a way to keep the bounce going indefinitely.

If you put the pool of water on top of a loud speaker, you can vibrate the surface. This vibration can cause water droplets to continuously bounce and levitate atop the liquid surface.

Physicists Pablo Cabrera-Garcia and Roberto Zenit from National Autonomous University of Mexico conducted the experiments and filmed them with a high-speed camera for your viewing pleasure. Check it out!


http://youtu.be/KZ5ZLPWasrM

You can learn more about the videos from the researchers’ arXiv article. Also, check out the Division of Fluid Dynamics website for more videos and images. This year’s galleries should be up in the next month or so.

Read more: physicsbuzz.physicscentral.com

Theorem unifies superfluids and other weird materials

Collective vibrations in a crystal, called phonons, are a type of Nambu-Goldstone boson. UC Berkeley physicists have proved that counting the number of bosons in a material, whether a solid crystal, a magnet, or a superfluid, tells you what type of behavior the material will have at low temperatures where symmetry is spontaneously broken. Credit: Haruki Watanabe/UC Berkeley. Click ‘Enlarge’ for animation.

Matter exhibits weird properties at very cold temperatures. Take superfluids, for example: discovered in 1937, they can flow without resistance forever, spookily climbing the walls of a container and dripping onto the floor.
In the past 100 years, 11 Nobel Prizes have been awarded to nearly two dozen people for the discovery or theoretical explanation of such cold materials – superconductors and Bose–Einstein condensates, to name two – yet a unifying theory of these extreme behaviors has eluded theorists.
University of California, Berkeley, physicist Hitoshi Murayama and graduate student Haruki Watanabe have now discovered a commonality among these materials that can be used to predict or even design new materials that will exhibit such unusual behavior. The theory, published online June 8 by the journal Physical Review Letters, applies equally to magnets, crystals, neutron stars and cosmic strings.
“This is a particularly exciting result because it concerns pretty much all areas of physics; not only condensed matter physics, but also astrophysics, atomic, particle and nuclear physics and cosmology,” said Murayama, the MacAdams Professor of Physics at UC Berkeley, a faculty senior scientist at Lawrence Berkeley National Laboratory and director of the Kavli Institute for the Physics and Mathematics of the Universe at the University of Tokyo. “We are putting together all of them into a single theoretical framework.”
The theorem Watanabe and Murayama proved is based on the concept of spontaneous symmetry breaking, a phenomenon that occurs at low temperatures and leads to odd behavior. This produces superconductors, which allow electric currents to flow without resistance; or Bose-Einstein condensates, which have such low energy that every atom is in the same quantum state.
By describing the symmetry breaking in terms of collective behavior in the material – represented by so-called Nambu-Goldstone bosons – Murayama and Watanabe found a simple way to classify materials’ weirdness. Boson is the name given to particles with zero or integer spin, as opposed to fermions, which have half-integer spin.
“Once people tell me what symmetry the system starts with and what symmetry it ends up with, and whether the broken symmetries can be interchanged, I can work out exactly how many bosons there are and if that leads to weird behavior or not,” Murayama said. “We’ve tried it on more than 10 systems, and it works out every single time.”

Earlier theories by Nobel Laureate Yoichiro Nambu predicted that magnetic spins oscillate in two directions independently, and thus magnets have two Nambu-Goldstone bosons. The new theory shows that in ferromagnets, these two waves are not independent, so that the there is only one Nambu-Goldstone boson, a precession wave as shown above. Credit: Haruki Watanabe/UC Berkeley.

Anthony Leggett of the University of Illinois at Urbana Champaign, who won the 2003 Nobel Prize in Physics for his pioneering work on superfluids, pointed out that “it has long been appreciated that an important consequence of the phenomenon of spontaneously broken symmetry, whether occurring in particle physics or in the physics of condensed matter, is the existence of the long-wavelength collective excitations known as Nambu-Goldstone bosons.
“In their paper, Watanabe and Maruyama have now derived a beautiful general relation … (involving) Nambu Goldstone bosons … (that) reproduces the relevant results for all known cases and gives a simple framework for discussing any currently unknown form of ordering which may be discovered in the future.”
“Surprisingly, the implications of spontaneous symmetry breaking on the low energy spectrum had not been worked out, in general, until the paper by Watanabe and Murayama,” wrote Hirosi Ooguri, a professor of physics and mathematics at Caltech. “I expect that there will be a wide range of applications of this result, from condensed matter physics to cosmology. It is a wonderful piece of work in mathematical physics.”
Symmetry
Symmetry has been a powerful concept in physics for nearly 100 years, allowing scientists to find unifying principles and build theories that describe how elementary particles and forces interact now and in the early universe. The simplest symmetry is rotational symmetry in three dimensions: a sphere, for example, looks the same when you rotate it arbitrarily in any direction. A cylinder, however, has a single rotational symmetry around its axis.
Some interactions are symmetric with respect to time, that is, they look the same whether they proceed forward or backward in time. Others are symmetric if a particle is replaced by its antiparticle.
When symmetry is broken spontaneously, new phenomena occur. Following the Big Bang, the universe cooled until its symmetry was spontaneously broken, leading to a predicted Higgs boson that is now being sought at the Large Hadron Collider in Geneva, Switzerland.
With solids, liquids or gases, symmetry relates to the behavior of the spins of the atoms and electrons. In a ferromagnetic material, such as iron or nickel, the randomness of the electron spins at high temperatures makes the material symmetric in all directions. As the metal cools, however, the electron spins get locked in and force their neighbors to lock into the same direction, so that the magnet has a bulk magnetic field pointing in one direction.

A second type of vibrational wave or phonon in a crystal, identical to the second Nambu-Goldstone boson. Credit: Haruki Watanabe/UC Berkeley

Nambu-Goldstone bosons are coherent collective behavior in a material. Sound waves or phonons, for example, are the collective vibration of atoms in a crystal. Waves of excitation of the electron spin in a crystal are called magnons. During the cooling process of a ferromagnet, two symmetries were spontaneously broken, leaving only one Nambu-Goldstone boson in the material.

In Bose-Einstein condensates, for example, “you start with a thin gas of atoms, cool it to incredibly low temperature — nanokelvins — and once you get to this temperature, atoms tend to stick with each other in strange ways,” Murayama said. “They have this funny vibrational mode that gives you one Nambu-Goldstone boson, and this gas of atoms starts to become superfluid again so it can flow without viscosity forever.”

On the other hand, solid crystals, regardless of their compositions or structures, have three Nambu-Goldstone bosons, equivalent to the three vibrational modes (phonons).

“What this Nambu-Goldstone boson is, how many of them there are and how they behave decide if something becomes a superfluid or not, and how things depend on the temperature,” Murayama added. “All these properties come from how we understand the Nambu-Goldstone boson.”

Yoichiro Nambu shared the 2008 Nobel Prize in Physics, in part, for explaining that in some systems, the number of broken symmetries equals the number of Nambu-Goldstone bosons.

The new theorem expands on Nambu’s ideas to the more general case, Watanabe said, proving that in weird materials, the number of Nambu-Goldstone bosons is actually less than the number of broken symmetries.

“What Nambu showed was true, but only for specialized cases applicable to particle physics,” he said. “Now we have a general explanation for all of physics; no exceptions.”

One characteristic of states with a low Nambu-Goldstone boson number is that very little energy is required to perturb the system. Fluids flow freely in superfluids, and atoms vibrate forever in Bose-Einstein condensates with just a slight nudge.

As a student at the University of Tokyo, Watanabe had proposed a theorem to explain materials’ properties through Nambu-Goldstone bosons, but was unable to prove it until he came to UC Berkeley last year and talked with Murayama. Together, they came up with a proof in two weeks of what they call a unified theory of Nambu-Goldstone bosons.

“Those two weeks were very exciting,” Watanabe said.

More information: Unified Description of Non-Relativistic Nambu–Goldstone bosons,http://arxiv.org/p … 3.0609v2.pdf (PDF submitted to Physical Review Letters)

Read more: phys.org