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

Helical structures through simple mechanical rules

Structures of this kind commonly occurs in nature. How do we generate them artificially? Here is a review of a work that gives a simple thumb rule.

Macroscopic helical structures formed by organisms include seashells, horns, plant tendrils, and seed pods (see the figure, panel A). The helices that form are chiral; like wood screws, they have a handedness. Some are helicoids, twisted helices with saddle-like curvature and a straight centerline; others are cylindrical helices with cylindrical curvature and a helical centerline. Studies of the mechanisms underlying the formation of helicoid or helical ribbons and of the transitions between these structures (1–4) have left an important question unanswered: How do the molecular organization of the material and its global geometrical features interact to create a diversity of helical shapes? On page 1726 of this issue, Armon et al. (5) explore the rich phenomenology associated with slender strips made of mutually opposing “molecular” layers, taking a singular botanical structure—the Bauhinia seed pod—as their inspiration. They show that a single component, namely a flat strip with a saddle-like intrinsic curvature, is sufficient to generate a wide variety of helical shapes.

Morphology

Recently there have appeared some interesting works on biological morphology development. They help us understanding how a particular pattern, e.g., fingerprints and intestine, comes about under basic mechanical laws [http://www.nature.com/nphys/journal/v7/n9/full/nphys2088.html?WT.ec_id=NPHYS-201109].

Thierry Savin and colleagues refer to Thompson's tome in their investigation, published in Nature, of the elaborate looped morphology that arises in the vertebrate gut (Nature 476, 57–62; 2011). Using experiment, simulation, and an innovative physical mock-up comprising rubber tubing stitched to latex, they have examined the forces arising from relative growth between the gut tube and a neighbouring sheet of tissue known as the dorsal mesentery. The study reveals a mechanism for the formation of loops based on differential strain between the two tissues.

© SPL

This is a timely nod to Thompson's century-old ideas, given the recent surge of physicists and mathematicians into the biological sciences, problem-solving artillery engaged. In another paper, published in Physical Review Letters, Edouard Hannezo, Jacques Prost and Jean-Francçois Joanny adopt a similarly mechanical approach to understanding the complex structures seen lining the small intestine (pictured), invoking an analogy with the buckling of metallic plates under compression (Phys. Rev. Lett. 107, 078104; 2011). They have developed a model that implicates cellular division and death as sources of internal stress, which in turn influences morphology and induces mechanical feedback on organ and tissue development.

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.

Cool Water Fountain

Here is a show of a very cute water fountain in Japan.

Think about the physics in it !

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.

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.

Photon generates upthrust

I think this is a very funny work [Nature Photon. doi:10.1038/nphoton.2010.266 (2010)], which demonstrates that photons carry energy and momentum, just as other forms of matter. This working mechanism is quite straightforward, much the way as air supports air crafts.

Grover Swartzlander at the Rochester Institute of Technology in New York and his colleagues shone a weakly focused laser beam through the roughly semi-cylindrical rods, which refracted the light rays. This refraction changed the direction of the rays' momentum, causing an equal and opposite momentum change on the rods themselves. Because of the rods' asymmetrical shape, the momentum shift was directed more towards one side, driving the rods upwards at around 2.5 micrometres per second.
[Nature, 468:734]

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.

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.

Fluid close to a wall

The turbulence phenomena have remained a puzzle for hundreds of years. Efforts to resolve it never stop.

The behavior of turbulent fluid motion, particularly in the thin chaotic fluid layers immediately adjacent to solid boundaries, can be difficult to understand or predict. These layers account for up to 50% of the aerodynamic drag on modern airliners and occupy the first 100 meters or so of the atmosphere, thus governing wider meteorological phenomena. The physics of these layers is such that the most important processes occur very close to the solid boundary—the region where accurate measurements and simulations are most challenging. We propose a mathematical model to predict the near-wall turbulence given only large-scale information from the outer boundary layer region. This predictive capability may enable new strategies for the control of turbulence and may provide a basis for improved engineering and weather prediction simulations.

Science 9 July 2010:
Vol. 329. no. 5988, pp. 193 - 196
DOI: 10.1126/science.1188765

Flight Of The Bumble Bee Is Based More On Brute Force Than Aerodynamic Efficiency

In recent years scientists have modelled how insect wings interact with the air around them to generate lift by using computational models that are relatively simple, often simplifying the motion or shape of the wings.

"We decided to go back to the insect itself and use smoke, a wind tunnel and high-speed cameras to observe in detail how real bumblebee wings work in free flight," said Dr Richard Bomphrey of the Department of Zoology, co-author of a report of the research published this month in Experiments in Fluids. ‘We found that bumblebee flight is surprisingly inefficient – aerodynamically-speaking it’s as if the insect is ‘split in half’ as not only do its left and right wings flap independently but the airflow around them never joins up to help it slip through the air more easily.’

Such an extreme aerodynamic separation between left and right sets the bumblebee [Bombus terrestris] apart from most other flying animals.

"Our observations show that, instead of the aerodynamic finesse found in most other insects, bumblebees have a adopted a brute force approach powered by a huge thorax and fuelled by energy-rich nectar," said Dr Bomphrey. "This approach may be due to its particularly wide body shape, or it could have evolved to make bumblebees more manoeuvrable in the air at the cost of a less efficient flying style."

Professor Adrian Thomas of Oxford’s Department of Zoology, co-author of the report, said: "a bumblebee is a tanker-truck, its job is to transport nectar and pollen back to the hive. Efficiency is unlikely to be important for that way of life."

Observing insects in free – as opposed to tethered – flight is a considerable challenge. The Oxford team trained bumblebees to commute from their hive to harvest pollen from cut flowers at one end of a wind tunnel. They then used the wind tunnel to blow streams of smoke passed the flying bees, to reveal vortices in the air, and recorded the results with high-speed cameras taking up to 2000 images per second. From these images the team were able to visualise the airflow over flapping bumblebee wings.

The old myth that "bumblebees shouldn’t be able to fly" was based on calculations using the aerodynamic theory of 1918-19, just 15 years after the Wright brothers made the first powered flight. These early theories suggested that bumblebee wings were too small to create sufficient lift but since then scientists have made huge advances in understanding aerodynamics and how different kinds of airflow can generate lift.

Email or share this story:

| More

---

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Oxford.

---

Journal Reference:

1. Richard James Bomphrey, Graham K. Taylor and Adrian L. R. Thomas. Smoke visualization of free-flying bumblebees indicates independent leading-edge vortices on each wing pair. Experiments in Fluids, 2009; 46 (5): 811 DOI: 10.1007/s00348-009-0631-8

tipping time of quantum rod

Here is physicist who wrote a blog on a very pedagogical question on quantum mechanics, which is calculating the tipping time of a vertical rod.

Tipping Time of a Quantum Pencil

I ran across this article in Eur. J. of Phys. and it reminded me of several other articles that I've read on this very topic. This is, of course, a rather familiar problem to many physics students. It involves the a pencil balanced vertically on its tip. So classically, it is in an unstable equilibrium. The problem is to use quantum mechanics, or the Heisenberg Uncertainty principle in particular, to find the tipping time for the pencil. The application of the HUP invokes the fact that the exact position of the top of the pencil can have a natural fluctuation that will tip it off the vertical axis.

The latest paper that I'm aware of on this topic deals with a very detailed calculation of calculating the tipping time of a quantum rod[1]. In this calculation, the author showed that the classical problem can be recovered when the Planck constant goes to zero, and draws the conclusion that:

.. the tipping of the quantum rod can be understood as having been triggered by the uncertainty in angular momentum engendered by localization of the initial state...

The article is a bit difficult to follow, and I didn't get any direct value of the tipping time.

The more interesting papers that I've found earlier on the same topic are much more illuminating than this one. A paper by Don Easton presents a caution for people who tries to apply QM as the basis of the tipping time[2]. His calculation of the tipping time, using QM, gives a humongous number: 0.6 million years. He examined why some posted solutions actually gave a balancing time of the order of 3 seconds, and why those treatment may be faulty.

Another paper that cautioned the use of the HUP in calculating the tipping time is a paper by Shegelski et al.[3] Here, they caution that one can't just use the HUP alone, and they also compared this to the faulty application of the WKB approximation to this problem.

Fascinating! Certainly something that I read in bed before going to sleep! :)