Weighted ping-pong balls can fall endlessly through a granular medium (w/ video)

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This partial image taken from the video below shows a projectile near the beginning of its trajectory as it falls through a tube filled with granular matter. Image credit: F. Pacheco-Vázquez, et al. 2011 American Physical Society

(PhysOrg.com) -- When a meteor impacts a planet or a moon, it always stops at a relatively shallow depth, even when impacting at high speeds. Until now, researchers have assumed that all objects impacting a granular medium ? such as sand or beads ? rapidly lose energy and stop at a shallow depth. But in a new study, researchers have demonstrated that weighted ping-pong balls impacting a 600-cm (20-ft)-long tube filled with polystyrene beads can reach a terminal velocity, which allows the balls to continue sinking endlessly to an infinite depth.

The team of researchers, Felipe Pacheco-Vázquez, et al., from institutions in Mexico and Cuba, has published their study on the surprising result in a recent issue of Physical Review Letters. As the scientists explain, their study shows that some granular matter is ?no longer safe to walk on.?

?One may safely walk on sand, and also on sugar, rice, and a myriad of dry granular materials,? Pacheco-Vázquez of CINVESTAV-Monterrey in Nuevo Leon, Mexico, told PhysOrg.com. ?In this paper, we warn that not all such materials are safe to walk on. In fact, under certain conditions, one can be endlessly swallowed by the medium. We show that, when a spherical ?intruder? is deposited on the free surface of a silo of expanded polystyrene beads, it plunges into the ?granular sea,? eventually stopping at a certain depth or falling forever, depending on the weight of the intruder.?

As the researchers explain in their study, the possibility of an object reaching a terminal velocity while sinking through a granular medium has never before been observed or predicted, although terminal velocity is well-documented for objects traveling through fluids. Researchers have assumed that all objects impacting a granular medium should eventually come to a stop due to the inelastic properties of granular materials (i.e., the grains do not easily move under an applied stress). In contrast, an object falling in a fluid can reach a terminal velocity because molecules in a fluid move much more easily under an applied stress.

Here, the researchers performed several experiments in which they launched 4-cm-diameter ping-pong balls into a long cardboard tube, which had an inner diameter of 45 cm and was filled with polystyrene beads. Each of the ping-pong balls had a small hole through which the researchers could add steel particles to obtain a desired mass, and then seal the hole. The researchers prepared 18 ping-pong balls with masses ranging from 15 to 182 g. Using a launching pad above the tube, the researchers released the ping-pong balls and tracked their trajectories with a high-speed video camera and a wireless accelerometer as they fell through the tube.

The results showed that the lighter ping-pong balls slowed down and stopped in the middle of the tube, while ping-pong balls with masses greater than about 86 g sank all the way to the bottom. Further, the velocities of the balls that reached the bottom seemed to level off, suggesting that they had reached a constant velocity.

This video is not supported by your browser at this time.

This video shows the trajectory of a projectile with sufficient mass to reach a terminal velocity as it falls through a tube filled with granular matter. Image credit: F. Pacheco-Vázquez, et al. 2011 American Physical Society. http://prl.aps.org/abstract/PRL/v106/i21/e218001

Through calculations and simulations, the researchers showed that, if the length of the tube were extended, the heavier balls would continue to sink indefinitely. The reason is that the balls reached a terminal velocity when the upward drag force and the downward force of gravity (which depends on the object?s mass) were balanced. At this time, the object?s acceleration is zero, so that it continues sinking at a constant speed.

From a physics perspective, the only way for an object to achieve terminal velocity in a granular medium is if the depth-dependent force of friction felt by the sinking object becomes saturated (constant) before it matches the gravitational force dependent on the object?s weight. Although this reasoning sounds logical, the researchers explain that it goes against intuition, since one would expect the projectile to be stopped due to its high energy dissipation with the grains.

The researchers noted that the two conditions needed for an object to reach a terminal velocity are a critical mass and a complete saturation of the object?s initial pressure. This finding implies that, even if an object has an extremely high initial velocity, it won?t achieve terminal velocity if it does not have the critical mass. So even for a bullet shot from a gun, if the bullet?s mass is less than the critical value, it will eventually come to a stop.

On a final note, the researchers emphasized that the critical mass differs for different granular media due to the different densities of the media. For example, for a ping-pong-ball-sized object to achieve a terminal velocity when impacting sand, the object would have to have a mass of about 14 kg (for a density of 400 g/cm3). No known material on Earth has such a large density.

In the future, the scientists plan to investigate what happens when several objects impact a granular medium simultaneously.

?We would like to study the penetration of a group of projectiles with a mass greater than the critical mass in this particular medium, because maybe we can observe an infinite cooperative dynamics,? Pacheco-Vázquez said. The scientists have previously investigated similar cooperative dynamics in an earlier study.

More information: F. Pacheco-Vázquez, et al. ?Infinite Penetration of a Projectile into a Granular Medium.? Physical Review Letters 106, 218001 (2011). DOI:10.1103/PhysRevLett.106.218001

Copyright 2011 PhysOrg.com.

All rights reserved. This material may not be published, broadcast, rewritten or redistributed in whole or part without the express written permission of PhysOrg.com.

Smallest periodic table fits on the side of human hair (w/ Video)

Smallest periodic table fits on the side of human hair (w/ Video)

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Scientists at The University of Nottingham have written what they believe is the world's smallest periodic table -- on the side of a human hair. The table is so small that a million of them could be replicated on a typical post-it note.

Experts from the University’s Nottingham Nanotechnology and Nanoscience Centre used a sophisticated combination of ion beam writer and electron microscope to carve the symbol of all 118 elements into the strand of hair taken from the head of Professor Martyn Poliakoff, an expert in Green Chemistry. You can see how it was done:

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Professor Poliakoff said: “Although the application was lighthearted I felt that it enabled us to show people how such nano writing is done. Our microscopist, Dr Mike Fay, made the whole operation seem so simple and really demystified it in a most appealing way.”

Professor Poliakoff has become one of the stars of the Periodic Table of Videos. What is thought to be the world’s tiniest Periodic Table was presented to him as a birthday present.

To demonstrate just what the very latest technology is capable of nano technologists Dr Fay and Dr Chris Parmenter took advantage of the freezing weather to engrave the words Merry Christmas into a snowflake. You can see them in action:

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The video showing the Christmas message being carved onto a shard of snow was made for the physics and astronomy project Sixty Symbols.

Philip Moriarty, Professor of Physics, said: “Although writing on a snowflake is on one hand a bit of seasonal fun, it’s also a neat demonstration of the powerful capabilities of the tools that scientists use in the lab on a day-to-day basis. It’s great that, via Sixty Symbols, we have the opportunity to show a wide audience just how exciting academic research can be.”

Working at the most unimaginable scale, the team feature in the latest videos made by the University’s film maker in residence Brady Haran. Presented by experts from the University his short films take a behind the scenes look at science with the aim of making science fun and unravelling some of the mysteries surrounding the technology and symbols used by scientists.

Brady said: “Making videos like this shows that scientists are like everyone else — they do things for fun at Christmas. But light-hearted videos like these have real educational value. They've already been watched tens of thousands of times and a wide audience has gained a better understanding of how electron microscopes work and what they can do. It is just another example of how learning can be quite good fun.”

The University of Nottingham has a broad research portfolio but has also identified and badged 13 research priority groups, in which a concentration of expertise, collaboration and resources create significant critical mass. Key research areas at Nottingham include energy, drug discovery, global food security, biomedical imaging, advanced manufacturing, integrating global society, operations in a digital world, and science, technology & society.

Through these groups, Nottingham researchers will continue to make a major impact on global challenges.

What a ride! Researchers take molecules for a spin (w/ Video)

What a ride! Researchers take molecules for a spin (w/ Video)

Kolomeisky and Rice graduate student Alexey Akimov have taken a large step toward defining the behavior of these molecular whirligigs with a new paper in the American Chemical Society's Journal of Physical Chemistry C. Through molecular dynamics simulations, they defined the ground rules for the rotor motion of molecules attached to a gold surface.

"This is no cartoon. It's a real molecule, with all the interactions taking place correctly," said Anatoly Kolomeisky as he showed an animation of atoms twisting and turning about a central hub like a carnival ride gone mad.

Kolomeisky, a Rice University associate professor of chemistry, was offering a peek into a molecular midway where atoms dip, dive and soar according to a set of rules he is determined to decode.

Kolomeisky and Rice graduate student Alexey Akimov have taken a large step toward defining the behavior of these molecular whirligigs with a new paper in the American Chemical Society's Journal of Physical Chemistry C. Through molecular dynamics simulations, they defined the ground rules for the rotor motion of molecules attached to a gold surface.

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An animation of a rotor simulation

It's an extension of their work on Rice's famed nanocars, developed primarily in the lab of James Tour, Rice's T.T. and W.F. Chao Chair in Chemistry as well as a professor of mechanical engineering and materials science and of computer science, but for which Kolomeisky has also constructed molecular models.

Striking out in a different direction, the team has decoded several key characteristics of these tiny rotors, which could harbor clues to the ways in which molecular motors in human bodies work.

The motion they described is found everywhere in nature, Kolomeisky said. The most visible example is in the flagella of bacteria, which use a simple rotor motion to move. "When the flagella turn clockwise, the bacteria move forward. When they turn counterclockwise, they tumble." On an even smaller level, ATP-synthase, which is an enzyme important to the transfer of energy in the cells of all living things, exhibits similar rotor behavior -- a Nobel Prize-winning discovery.

Understanding how to build and control molecular rotors, especially in multiples, could lead to some interesting new materials in the continuing development of machines able to work at the nanoscale, he said. Kolomeisky foresees, for instance, radio filters that would let only a very finely tuned signal pass, depending on the nanorotors' frequency.

"It would be an extremely important, though expensive, material to make," he said. "But if I can create hundreds of rotors that move simultaneously under my control, I will be very happy."

The professor and his student cut the number of parameters in their computer simulation to a subset of those that most interested them, Kolomeisky said. The basic-model molecule had a sulfur atom in the middle, tightly bound to a pair of alkyl chains, like wings, that were able to spin freely when heated. The sulfur anchored the molecule to the gold surface.

While working on a previous paper with researchers at Tufts University, Kolomeisky and Akimov saw photographic evidence of rotor motion by scanning tunneling microscope images of sulfur/alkyl molecules heated on a gold surface. As the heat rose, the image went from linear to rectangular to hexagonal, indicating motion. What the pictures didn't indicate was why.

That's where computer modeling was invaluable, both on the Kolomeisky lab's own systems and through Rice's SUG@R platform, a shared supercomputer cluster. By testing various theoretical configurations -- some with two symmetrical chains, some asymmetrical, some with only one chain -- they were able to determine a set of interlocking characteristics that control the behavior of single-molecule rotors.

First, he said, the symmetry and structure of the gold surface material (of which several types were tested) has a lot of influence on a rotor's ability to overcome the energy barrier that keeps it from spinning all the time. When both arms are close to surface molecules (which repel), the barrier is large. But if one arm is over a space -- or hollow -- between gold atoms, the barrier is significantly smaller.

Second, symmetric rotors spin faster than asymmetric ones. The longer chain in an asymmetric pair takes more energy to get moving, and this causes an imbalance. In symmetric rotors, the chains, like rigid wings, compensate for each other as one wing dips into a hollow while the other rises over a surface molecule.

Third, Kolomeisky said, the nature of the chemical bond between the anchor and the chains determines the rotor's freedom to spin.

Finally, the chemical nature of rotating groups is also an important factor.

Kolomeisky said the research opens a path for simulating more complex rotor molecules. The chains in ATP-synthase are far too large for a simulation to wrangle, "but as computers get more powerful and our methods improve, we may someday be able to analyze such long molecules," he said.

Mechanical engineering at the molecular level: Self-assembly of nano-rotors (w/ Video)

Mechanical engineering at the molecular level: Self-assembly of nano-rotors (w/ Video)

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Rods of sexiphenyl-dicarbonitrile spontaneously form three-bladed rotors. Driven by thermal energy, they start rotating in their honeycomb-cage. Credit: Graphic: Dirk Kuehne, TUM

German scientists from the Technische Universitaet Muenchen have managed to direct the self-assembly of rod-shaped molecules into rotors only few nanometers in size. The tiny systems serve the study of forces that act on molecules on surfaces and in cage-like structures. Their findings are published in the current online issue of the Proceedings of the National Academy of Sciences.

In the nanoworld many things are different. Scientists only recently started unveiling and harnessing the underlying laws and principles. A team associated with Professor Johannes Barth from the Physics Department of the TU Muenchen have now succeeded in capturing rod-shaped molecules in a two-dimensional network in such a way that they autonomously form small rotors that turn in their honeycomb-like cages.

Nature itself provides the role model for such self-organizing systems. This is how proteins bring reactants so close together that reactions can take place – reactions that are possible only in very close proximity. These effects are put to use in catalysts: surface reactants find their way to each other on the surface of these facilitators. However, the coveted dream of using self-organization effects in such a way that nano machines assemble themselves is still a thing of the future.

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Driven by thermal energy, the nano-rotors start rotating. Credit: Video: Florian Klappenberger, TUM

The rotors developed in Garching are an important step in this direction. First, the physicists built up an extensive nano lattice by allowing cobalt atoms and rod-shaped molecules of sexiphenyl-dicarbonitrile to react with each other on a silver surface. This results in a honeycomb-like lattice of extreme regularity with astonishing stability. Just like graphene, for which its discoverers were awarded the Nobel Prize only a few weeks ago, this lattice is exactly one atom thick.

When the researchers added further molecular building blocks, the rods spontaneously gathered, typically in groups of three, in a honeycomb cell while neighboring cells remained empty. The chummy molecules must have had a reason for organizing themselves in threesomes. Under a scanning tunneling microscope the scientists were able to recognize why. The three molecules oriented themselves in such a way that the nitrogen ends each faced a phenyl-ring hydrogen atom. This triple-bladed rotor arrangement is so energetically advantageous that the molecules maintain this structure even when thermal energy drives it to rotation.

Because the honeycomb-cell is not round, but hexagonal, there are two different positions for the rotors that can be distinguished as a result of the interactions between the outer nitrogen atoms and the hydrogen atoms of the cell wall. Furthermore, the three molecules arrange in a clockwise and a counter-clockwise manner. In experiments at various carefully controlled temperatures the physicists were able to "freeze" all four states and examine them closely. They could thus determine the energy of these thresholds from the temperature at which the rotation resumed.

"We hope that in future we will be able to extend these simple mechanical models to optical or electronic switching," says Professor Johannes Barth. "We can set a specific cell size, we can specifically bring in further molecules and study their interaction with the surface and the cell wall. These self-organizing structures hold enormous potential."

Smallest periodic table fits on the side of human hair (w/ Video)

Smallest periodic table fits on the side of human hair (w/ Video)

Enlarge

Scientists at The University of Nottingham have written what they believe is the world's smallest periodic table -- on the side of a human hair. The table is so small that a million of them could be replicated on a typical post-it note.

Experts from the University’s Nottingham Nanotechnology and Nanoscience Centre used a sophisticated combination of ion beam writer and electron microscope to carve the symbol of all 118 elements into the strand of hair taken from the head of Professor Martyn Poliakoff, an expert in Green Chemistry. You can see how it was done:

You need Flash installed to watch this video

Professor Poliakoff said: “Although the application was lighthearted I felt that it enabled us to show people how such nano writing is done. Our microscopist, Dr Mike Fay, made the whole operation seem so simple and really demystified it in a most appealing way.”

Professor Poliakoff has become one of the stars of the Periodic Table of Videos. What is thought to be the world’s tiniest Periodic Table was presented to him as a birthday present.

To demonstrate just what the very latest technology is capable of nano technologists Dr Fay and Dr Chris Parmenter took advantage of the freezing weather to engrave the words Merry Christmas into a snowflake. You can see them in action:

You need Flash installed to watch this video

The video showing the Christmas message being carved onto a shard of snow was made for the physics and astronomy project Sixty Symbols.

Philip Moriarty, Professor of Physics, said: “Although writing on a snowflake is on one hand a bit of seasonal fun, it’s also a neat demonstration of the powerful capabilities of the tools that scientists use in the lab on a day-to-day basis. It’s great that, via Sixty Symbols, we have the opportunity to show a wide audience just how exciting academic research can be.”

Working at the most unimaginable scale, the team feature in the latest videos made by the University’s film maker in residence Brady Haran. Presented by experts from the University his short films take a behind the scenes look at science with the aim of making science fun and unravelling some of the mysteries surrounding the technology and symbols used by scientists.

Brady said: “Making videos like this shows that scientists are like everyone else — they do things for fun at Christmas. But light-hearted videos like these have real educational value. They've already been watched tens of thousands of times and a wide audience has gained a better understanding of how electron microscopes work and what they can do. It is just another example of how learning can be quite good fun.”

The University of Nottingham has a broad research portfolio but has also identified and badged 13 research priority groups, in which a concentration of expertise, collaboration and resources create significant critical mass. Key research areas at Nottingham include energy, drug discovery, global food security, biomedical imaging, advanced manufacturing, integrating global society, operations in a digital world, and science, technology & society.

Through these groups, Nottingham researchers will continue to make a major impact on global challenges.