The physics of floating pyramids

An unexpected result is reported here.

Results just in from an experiment that levitated open-bottomed paper pyramids on gusts of air reveal a curious phenomenon: When it comes to drifting through the air, top-heavy designs are more stable than bottom-heavy ones. The finding may lead to robots that fly not like insects or birds but like jellyfish.
......
The researchers placed hollow paper pyramids inside the cylinder. The objects were about 1 to 5 centimeters high and were made of tissue paper or letter paper on carbon fiber supports, like tiny homemade kites. Physicist Bin Liu led the experiments, attaching a beadlike weight to a post running down the center of the pyramid and changing the height of the bead to give the object a different center of mass. Common sense says that the pyramid should be most stable when the bead is at the bottom of the post, like ballast in the hold of a ship. But when the team released the pyramids over the subwoofer, the opposite was true: the bottom-heavy pyramids were likely to flip over and fall, whereas the top-heavy ones remained upright and continued to hover (see first video), the group reports in an upcoming issue of Physical Review Letters.
......

The team suspected that the effect was due to swirls of air that develop along the pyramid's sides. To see the swirls in action, Zhang's group examined a two-dimensional version of the pyramid experiment in water. They placed upside-down V shapes into a pan of water and rocked it to create currents. As the water ran past the V, it created tiny whirlpools at the ends of the V's two legs (see second video). These swirls pushed away from the upside-down V, moving downward, which exerted an upward force on the V-the same mechanism that creates lift in the pyramids.

If the V was tilted, however, the swirls went in different directions: Those on the higher leg shoved it sideways, while the lower leg got a weaker upward push. This would straighten the upside-down V. Team member Leif Ristroph showed that the same sorts of swirls roll off the sides of the pyramids: They push the pyramid upright as long as the center of mass is above the tilted-up side, much in the same way that you can balance a vertical stick on the end of your finger by moving the bottom of the stick in the direction of the tilt, Zhang says. For bottom-heavy pyramids, this same mechanism causes them to flip over-it's like moving the top of the stick in the direction of the tilt, encouraging it to fall.

Vitrification vs. Crystalization

The most basic difference between the glass forming (vitrification) process and the crystallization may be seen in the figure on the left. Vitrification is actually not really a transition , because it does not involve any genuinely singular behaviors, in contrast with crystallization. A very likely implication is that, vitrification should not be due to a critical mode that features long-range correlations. Its dynamics should be essentially local, like what happens to a traffic congestion.

Fun with polymers

http://www.youtube.com/watch?v=vIQpoka8FOk&feature=related

The physics of how bubbles clean dusts

This is an interesting study published in PRL [PRL 107, 074503 (2011)], "It is now accepted that the physical forces in ultrasonic cleaning are due to strongly pulsating bubbles driven by the sound field. Here we have a detailed look at bubble induced cleaning flow by analyzing the transport of an individual particle near an expanding and collapsing bubble. The induced particulate transport is compared with a force balance model. We find two important properties of the flow which explain why bubbles are effectively cleaning: During bubble expansion a strong shear layer loosens the particle from the surface through particle spinning and secondly an unsteady boundary layer generates an attractive force, thus collecting the contamination in the bubble’s close proximity." The following is a review:

A team at the Nanyang Technological University in Singapore led by Claus-Dieter Ohl adapted a technique for creating a bubble where and when they wanted it. They focused a laser pulse up through a glass microscope slide into a strongly absorbing liquid dye. The laser heating caused the bottom layer of the dye to evaporate explosively, forming a hemispherical bubble at the glass surface that grew to tens of microns in radius and then collapsed, all within about 25 microseconds. The team stuck several-micron-diameter plastic beads on the slide surface and immersed them in the dye to mimic dirt particles adhering to a surface. They then recorded video of the beads' motion in response to the bubble.

The role of phase

This demonstration (you can watch a video there) comes from Harvard Natural Sciences Lecture Demonstrations

What it shows: Fifteen uncoupled simple pendulums of monotonically increasing lengths dance together to produce visual traveling waves, standing waves, beating, and random motion. One might call this kinetic art and the choreography of the dance of the pendulums is stunning! Aliasing and quantum revival can also be shown.

How it works: The period of one complete cycle of the dance is 60 seconds. The length of the longest pendulum has been adjusted so that it executes 51 oscillations in this 60 second period. The length of each successive shorter pendulum is carefully adjusted so that it executes one additional oscillation in this period. Thus, the 15th pendulum (shortest) undergoes 65 oscillations. When all 15 pendulums are started together, they quickly fall out of sync—their relative phases continuously change because of their different periods of oscillation. However, after 60 seconds they will all have executed an integral number of oscillations and be back in sync again at that instant, ready to repeat the dance.

How do wings work ?

This is an interesting article from the wonderful journal I posted in my last entry. It tries to poke the usual (even textbook) explanation of how wings work [2003 Phys. Educ. 38: 497].

Now Bernoulli’s equation is quoted, which states that larger velocities imply lower pressures and thus a net upwards pressure force is generated. Bernoulli’s equation is often demonstrated by blowing over a piece of paper held between both hands as demonstrated in figure 2. As air is blown along the upper surface of the sheet of paper it rises and, it is said, this is because the average velocity on the upper surface is greater (caused by blowing) than on the lower surface (where the air is more
or less at rest). According to Bernoulli’s equation this should mean that the pressure must be lower above the paper, causing lift. The above explanation is extremely widespread. It can be found in many textbooks and, to my knowledge, it is also used in the RAF’s instruction manuals. The problem is that, while it does contain a grain of truth, it is incorrect in a number of key places.

What’s wrong with the ‘popular’ explanation?
The distance argument;
The ‘equal time’ argument;
The Bernoulli demonstration.

Next, examine a particle moving along a curved streamline as shown in figure 7. For simplicity we can assume that the particle’s speed is constant3. Because the particle is changing direction there must exist a centripetal force acting normal to the direction of motion. This force can only be generated by pressure differences (all other forces are ignored), which implies that the pressure on one side of the particle is greater than that on the other. In other words, if a streamline is curved, there must be a pressure gradient across the streamline, with the pressure increasing in the direction away from the centre of curvature.

Classical Not Always Lose

It was shown that, classical physics does as efficiently as quantum physics in energy transfer in biological systems [http://physics.aps.org/synopsis-for/10.1103/PhysRevE.83.051911].

A prominent goal of quantum information and computing is to be able to exploit quantum entanglement in qualitatively new devices, such as massively parallel computers. Has biological evolution already harnessed entanglement for its own purposes? Recent studies have indeed suggested that electronic excitation transfer (EET) in photosynthesis benefits from quantum entanglement. Now, a paper appearing in Physical Review E is likely to stimulate further investigation and controversy on this question. Based on calculations, John Briggs and Alexander Eisfeld, of the Max Planck Institute for the Physics of Complex Systems in Dresden, Germany, assert that under the conditions prevailing in photosynthesis (in particular, in the so-called Fenna-Matthews-Olson complex that lies at the heart of the process), energy transfer in a classical system is just as efficient as in its quantum counterpart.

To model the photosynthesis that occurs in plants, Briggs and Eisfeld study a collection of monomers, each possessing a single electronic state and coupled to its neighboring units by a dipolar interaction. The authors find that for dipolar interactions similar to those found in real molecular aggregates, the coherences in quantum transport (from the Schrödinger equation) are identical to those occurring in classical transport according to Newton’s equation. Although their analysis neglects the influence of the environment, the authors report that calculations including dephasing processes in the quantum and classical equations lead to the same conclusion. – Ron Dickman

Light passes through without reflection

This is not an old concept: a light beam may not be reflected if the thickness of the glass it shines upon is carefully chosen so that the reflected wave from the second surface goes out of phase with the one from the first surface. Now that the proper thickness is proportional to the wavelength of the incident light, it is not possible to use a single piece of glass for reflectionless control of light with various colors. But, nature offers much more. One can make a more delicate refractive index profile of medium so that it invisible to a wide spectrum [http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.106.193903].

Abrupt interfaces disrupt wave propagation. For example, light passing from air into a sheet of glass will partially reflect backwards. When the light exits back into air, there is a second reflection that can cancel the first for light of just the right frequency, given the refractive index and thickness of the sheet. The wave nature of electrons creates similar effects when they encounter a region with a changing electrostatic potential. But early in the history of quantum mechanics, theorists realized that certain smoothly varying potential profiles could eliminate the reflection of electrons over a wide range of frequencies.

As it turns out, the same concepts work for light: intense light pulses known as solitons create precisely this kind of profile in the refractive index of the surrounding medium, eventually becoming trapped. Creating permanent versions of such “reflectionless potentials” has, however, proved difficult. In Physical Review Letters, Alexander Szameit of the Technion in Haifa, Israel, and colleagues in Germany and Australia at last implement the lack of light reflection in the laboratory.

In their experiments, a beam of light travels along an array of closely spaced, parallel waveguides created in a glass sample through direct laser-writing. By changing the spacing between some of the waveguides, the researchers construct a stripe along the length of the array that has a different refractive index modulation relative to the rest of the array. For almost any change in spacing, light traveling diagonally across the stripe is partially reflected, as usual. But a stripe having the special variation suggested by theory generates almost no reflection. The technique adds to the bag of tricks that researchers have for manipulating light. – Don Monroe

No physical signal travels faster than c

Einstein's special relativity theory stipulates that no physical signal (i.e., anything that carries energy and obeys physical laws and is measurable) can not go faster than the vacuum light speed. There have been many 'dissidents' (mostly crackpots) don't like this and want to disprove this law, but all have been defied. Now an experiment that was recently done in HKUST demonstrated that, even a single photon cannot break it.

Einstein taught us that the speed of light was the traffic law of the universe—nothing could go faster. The development of media in which atomic gases can slow down or speed up the passage of light pulses initially caused a stir, at least until the difference between phase velocity and group velocity could be carefully explained. But what about the behavior of single photons, the fundamental quanta of light? Reporting in Physical Review Letters, Shanchao Zhang and colleagues at the Hong Kong University of Science and Technology have shown that photons obey the law too.

Zhang et al. study optical precursors, which are signals preceding the main wave packet in a light pulse with a sharply rising leading edge (as in a step function pulse). Past work has shown that even in “superluminal” media where the group velocity may be faster than light speed, the precursor is always in front of the pulse. The authors extend this work to the single-photon level with the help of cold atomic gases: a photon generated in one rubidium gas traverses a second collection of rubidium atoms. With careful use of electromagnetically induced transparency, the researchers can separate the precursor from the main pulse and confirm it travels at the speed of light. The results add to our understanding of how single-photon signals propagate but also confirm the upper bound on how fast information travels. – David Voss [http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.106.243602]

How does sulcus develop ?

Sulci is a deep furrow on the surface of soft materials under pressure. It features a catching curvature that nearly breaks the smoothness. These furrows are often seen, examples including the arms of an infant. An interesting question is, how does a sulci develop when applying a compression ? According to this study, the nonlinear response is held liable, "Now, in a paper appearing in Physical Review Letters, Evan Hohlfeld from Harvard University and Lawrence
Berkeley National Laboratory and L. Mahadevan from Harvard University have proposed that the formation of a sulcus is controlled by a new type of instability dominated by nonlinearities in the elastic energy [7]. Their case is bolstered both by detailed numerics and by experiments.
Moreover, they suggest that similar nonlinear instabilities may be lurking behind the formation of many other singular structures found in materials."[physics, 4:19(2011)]

Henry's design was found an error

This interesting study has acquired attention from Nature Physics. The authors reveal an error with Henry's design on display in Princeton University.

In 1831, Henry invented a battery-powered rocking-beam motor that he later described as the first electromagnetic machine. He repeatedly modified the design over his career, but only one version of a motor actually constructed by Henry is known to exist. This version is in a collection of Henry instruments at Princeton University. We found that the Princeton motor cannot have operated in the
form that was displayed as early as 1884. We found evidence in several historical documents and in the instrument itself that the field magnet shown with the motor is a mistake. Instead of a single horizontal bar magnet, the motor was designed to use two elliptical magnets. We presume the error was made by whoever assembled the first public display. We modeled the dynamics of Henry’s vibrating motor and compared our results to the operation of a replica motor. Modeling provides
insight into how the motor is able to vibrate indefinitely even in the presence of energy loss due to friction. © 2011 American Association of Physics Teachers.
DOI: 10.1119/1.3531940

Couterflow

This Letter addresses a practical problem that is encountered in, for example, a recent oil gushing in Mexico Gulf. To 'top kill' the oil well, one may pump "mud" from above to suppress the welling from bottom. However, in this "top kill", one needs consider an issue as described in the mentioned Letter, the so-called "Helmholtz instability", namely, "Dense fluids, i.e., mineral suspensions called ‘‘mud’’ [1,2], are introduced into oil wells to provide hydrostatic pressure to offset hydrocarbon (oil and gas) fluid pressure in deep formations, stopping upward flow and reducing the fluid pressure at the surface to near ambient. If hydrocarbon is flowing upward in the well, there is a counterflow between the upwelling hydrocarbon and the descending mud. Successful top kill requires that the mud descend despite this counterflow. However, upwelling at speeds > 1m/s, as in the uncontrolled Macondo well ‘‘blowout’’ in the Gulf of Mexico in 2010, may lead to a Kelvin- Helmholtz instability [3]."[PRL 106, 058301 (2011)] How to avoid the instability ? Suggested to use viscoelastic materials.

Mpemba Effect

Water is just mundane and seems well-understood in many respects. However, there are still quite a lot of things that motivate people to find more. For example, how water molecules arrange themselves when they adsorbed on an adsorbate. Another instance is, I think more associated with the thermodynamics of water: it has been claimed that, hot water cools faster than cold water when they are placed in the same chamber. This was named after its discoverer, a middle school student Mpemba. There came a latest study on this [http://arxiv.org/ftp/arxiv/papers/1101/1101.2684.pdf]:

In this paper we have presented data confirming that water initially at higher temperature cools at a faster rate than water initially at a lower temperature and that this trend continues past the point at which the two samples reach the same temperature: the crossover temperature. Furthermore, our data indicates that the starting temperature affects the crossover temperature in a reproducible manner. We have confirmed that warmer water indeed cools faster than colder water and that, surprisingly, this trend continues past the point where the temperatures of the two samples are the same. Our results show that when using optimal initial temperature conditions, the crossover temperature is found to be 2.7 oC whereas our other set of initial conditions gave a crossover temperature of -0.07 oC. These data taken together provide a definite quantitative evidence of the Mpemba effect.

Cloak for sounds

The idea of cloaks that make objects disappear is really capturing. It has been realized at least in the lab by a theory called "transformation optics", which relates to the transformation properties of Maxwell's equations. However, this idea not only blossoms in optics but also in acoustics, where similar equations exist in 2D. A new paper in PRL designs a cloak for ultrasonic waves[Phys. Rev. Lett. 106, 024301 (2011) – Published January 10, 2011]. The difficulty lies in varying the density of materials in a desired way[Physics 4, 2 (2011)].

Invisibility devices based on coordinate transformation have opened up a new field of considerable interest. We present here the first practical realization of a low-loss and broadband acoustic cloak for underwater ultrasound. This metamaterial cloak is constructed with a network of acoustic circuit elements, namely, serial inductors and shunt capacitors. Our experiment clearly shows that the acoustic cloak can effectively bend the ultrasound waves around the hidden object, with reduced scattering and
shadow. Because of the nonresonant nature of the building elements, this low-loss ( 6 dB=m) cylindrical cloak exhibits invisibility over a broad frequency range from 52 to 64 kHz. Furthermore, our experimental study indicates that this design approach should be scalable to different acoustic frequencies and offers the possibility for a variety of devices based on coordinate transformation.

The physics in skateboarding

Here is a video that talks about how to improve skateboarding tricks by the help of simple physics,
especially the so-called "Ollie":
http://www.sciencedaily.com/videos/2007/0701-science_of_skateboarding.htm

Thermofluidic effects in nanochannels

When a fluid is subject to a temperature gradient, a velocity field can be generated, a phenomenon known as convection. It should be noticed that, such phenomena parallel what happens to electrons in a metal: temperature gradient drives electrical current under the name of thermoelectric effect. Now as things can be made smaller and smaller, it becomes an interesting subject to investigate the so-called micro- or nano-fluidic flow: liquid flow through a micro-size or nano-size channel. In this study by researchers from Hong Kong [PRL 105, 174501 (2010)], they used molecular dynamics simulations to examine a nano fluid housed in a nano-channel with particularly designed walls: the wall consists of two parts, the left and the right one, with respective surface energies, and a temperature gradient is held symmetric with respect to the border between the left and right wall. Their study showed that, an asymmetric flow can be generated with this temperature gradient provided the variance of surface energies is big enough. This is funny and many possibilities can be imagined to broaden their studies.

Wet dog shaking

This piece of study is something that should never be missed. It is perhaps one of the best examples exemplifying what science is all about: Just being curious and trying to find out how things actually happen. This work looks at how fast a wet dog wriggles its body to shake off the water sticking to its fur. These authors from Geogia Insttitute of Technology set up a model and compare the results out of this model to reality. They photographyed a range of animals and figure out the wriggling frequencies. Their model predicted the frequency should be proportional to the square root of the belly radius of the animals, close to the observations that yield an exponent of 0.75 rather than 0.5. [http://www.wired.co.uk/news/archive/2010-10/20/physicists-find-perfect-speed-for-wet-dogs-to-shake-at] In this link, there is a video that sumarizes their interesting and provoking work.

Friction not so simple

Friction is certainly a standard part of middle school physics courses. It is observed that, to move an object in contact with another one, a force must be applied larger than the static friction, which is supposed to be uniform across the interface. However, this picture is inadequate. Actually, it was perceived that non-uniformity occurs at least locally. Understanding the nature of friction and how to model it better is not only theoretically interesting but practically imperative, because friction is relevant to a plenty of phenomena, such as rampant earthquakes and snow ruptures. Friction is the force that holds those events from bursting out. On the hand, it is also desirable to gain insight into how slip occurs locally when friction fails. This is key to modeling. This latest publication investigated this problem.

The way in which a frictional interface fails is critical to our fundamental understanding of failure processes in fields ranging from engineering to the study of earthquakes. Frictional motion is initiated by rupture fronts that propagate within the thin interface that separates two sheared bodies. By measuring the shear and normal stresses along the interface, together with the subsequent rapid real-contact-area dynamics, we find that the ratio of shear stress to normal stress can locally far exceed the static-friction coefficient without precipitating slip.
Moreover, different modes of rupture selected by the system correspond to distinct regimes of the local stress ratio. These results indicate the key role of nonuniformity to frictional stability and dynamics with implications for the prediction, selection, and arrest of different modes of earthquakes.

Imaging the coffee blend with cream

This is so cool !
http://www.insidescience.org/research/coffee_cup_secrets

What does physics say about the Jobulani ?

The ball created by Adidas for this year's World Cup has its name as 'Jobulani' ? What is special about it ? Listen to the audio of this link.