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Flying circus of physics

Chap 2 (fluids) archived stories part C

Friday, February 06, 2009

For Chapter 2, here is part C of the new stories and also the updates to the items in the book, including many video links and journal citations. If you want all the video links (hundreds) and journal citations (thousands) for this chapter, go to

First, a list
2.85  Water-walking insects and the Cheerios effect
2.89  Pub trick --- Tia Maria wormy action
2.91  Coffee stain rings
2.102  Walking on a beach
2.102  Quicksand
2.102  Escape from mud
2.103  Nimitz Freeway collapse
2.103  Liquefaction
2.108  Pub trick --- beer enhanced Brazil nut effect
2.109  Avalanche balloon
2.116  Rockfalls
2.120  Glug-glug
2.120  Pub trick --- exchanging water and whiskey
2.120  Pub trick with Red Bull
2.120  Pub trick --- Red Bull can stuck to your hand
2.120  Pub trick --- yard of ale and beer boot
2.120  Pub trick --- using glug-glug to clear beer foamglug
2.125  Pub trick --- lifting rice with a rod
2.140  Stress dip below a sandpile
2.141  Chladni singing
2.144  Ball floating in an air stream
2.150  Pub trick --- collecting grains of black pepper 

Reference and difficulty dots
Dots · through ··· indicate level of difficulty
Journal reference style: author, title, journal, volume, pages (date)
Book reference style: author, title, publisher, date, pages

Now the stories:

2.85  Water-walking insects and the Cheerios effect
Jearl Walker
September 2006   As discussed in the book, the Cheerios effect is the tendency of individual floating grains of breakfast cereal (such as Cheerios) to aggregate because of their distortions of the water surface. The water tends to rise up along the side of each Cheerio, and when two of them are near, the curved water surface between them pulls them together. A high school student told me about this effect just as I was writing the first edition of The Flying Circus of Physics. More recently, researchers have written several very nice papers about the effect.
   Some water-walking insects may use the Cheerios effect to climb out of the water next to, say, a log. A water-walking insect avoids sinking by causing the water surface below its legs to indent, which creates upward forces to support it. The forces are generally attributed to the surface tension of the water, that is, the forces along a water surface due to the mutual attraction of the water molecules. The water surface near a log curves upward --- the water molecules are attracted to the log and to each other, and so the water is pulled a short distance upward onto the log. A water-walking insect should have a difficult time negotiating this curvature (it would be like you trying to climb up a hill of ideally slippery ice), but a least one type meets the challenge by pulling upward on its front legs to decrease the indentation at the front and pushing down on its rear legs to increase it at the rear. The result is that the surface tension along the curved surface helps pull the insect to the log, which it can then grab to climb out.    Description and videos (including "robostrider," the mechanical water strider that surely is the nightmare of any water strider) Video of striders  A demonstration of floating via surface tension

· · ·  Singh, P., and D. D. Joseph, “Fluid dynamics of floating particles,” Journal of Fluid Mechanics, 530, 31-80 (2005)
· · ·  Vella, D., and L. Mahadevan, “The ‘Cheerios effect,” American Journal of Physics, 73, No. 9, 817-825 (September 2005) Go to and then click on the button that offers a PDF file.
· · ·  Hu, D. L., J. W. M. Bush, “Meniscus-climbing insects,” Nature, 437, 733-736 (September 2005)

Want more references? Use the link at the top of this page.

2.89 Pub trick --- Tia Maria wormy action
Jearl Walker
Dec 2009 The liqueur Tia Maria (see the image here, by ralph&dot) is often served with several millimeters of cream on the top and sipped through a straw. If the drink is left to stand for several minutes and if the cream is not too runny or too thick, vigorous movement appears on the surface, with the surface forming cells or worm-like tubular patterns.

As the (very nice) video explains, we get a circulation process that is driven by two effects: (1) variations in density of the fluid and (2) variations in the surface tension.

Density: The mixture of water and alcohol in the Tia Maria liquid is lighter than pure water. As the mixture reaches the surface and the alcohol partially evaporates, the mixture becomes denser and sinks.

Surface Tension: Surface tension is due to the mutual attraction of molecules along the surface of any liquid. The insects called water striders, for example, can stand on water because of this mutual attraction of water molecules.

In one or more regions of the Tia Maria drink, alcohol diffuses (slowly passes) up through the cream, decreasing the surface tension in the cream due to mutual attraction of the molecules on the surface. The alcohol–cream liquid (with weak surface tension) is then pulled along the surface into the remaining regions of cream (still having strong surface tension). More alcohol rises to replace the liquid that is removed, and so on. For complex reasons, the presence of the cream (specifically, its resistance to motion) triggers circulation patterns of rising and descending liquid that can form into either isolated cells when the cream layer is thick or worm-like tubular rolls when the cream layer is thin. New Scientist video about the Tia Maria effect

Dots · through ··· indicate level of difficulty
Journal reference style: author, title, journal, volume, pages (date)
Book reference style: author, title, publisher, date, pages
··· Slavtchev, S., M. Hennenberg, J.-C. Legros, and G. Lebon, “Stationary solutal Marangoni instability in a two-layer system,” Journal of Colloid and Interface Science, 203, 354-368 (1998)
· Cartwright, J. H. E., O. Piro, and A. I. Villacampa, “Pattern formation in solutal convection: vermiculated rolls and isolated cells,” Physica A, 314, 291-298 (2002)
· Cartwright, J., O. Piro, and A. Villacampa, (letter) “Cream on,” in “The Last Word,” New Scientist, 177, No. 2379, inside back cover (25 January 2003)
2.91  Coffee stain rings
Jearl Walker 
Oct 2008   Here is something that sounds deadly dull: Pick up some sandwiches and a cup of coffee. Then spill a small amount of coffee onto a table and watch it evaporate as you eat the sandwiches. Now, a normal person would complain bitterly about doing this, seeing no sense in it. But a physicist is like Sherlock Holmes, who would have surely sat for hours watching the coffee evaporate and who would have reported the strange pattern that develops. The stain left behind by each small pool of spilt coffee has a prominent brown ring that marks the perimeter of the pool either initially or at various stages during the evaporation, as the pool pulled inward. You can see the rings in my photos here.


People have been drinking coffee and leaving such stain rings for 1000 years, but the rings were not studied until 1997 when researchers at the James Franck Institute in Chicago effectively said: The coffee should be forming uniformly brown stains across the pool as the pool evaporates --- what gives with these rings?

Here is the explanation of the rings but, as the researchers suggest, it may not be complete. When coffee (largely just water) is spilt onto a table, the extent of its spreading depends on the attraction between the water molecules and the table molecules. More attraction produces more wetting, that is, more spreading. As the water begins to evaporate, it tends to pull its perimeter inward, to reduce its surface area. However, the perimeter can become pinned at points on the table where the surface tension of the water is unable to move the perimeter inward.

The inward migration of the perimeter halts at those points, but the water evaporation does not. So, as water evaporates from a pinned perimeter, additional coffee must gradually flow in to replace it. The evaporation leaves behind the coffee grains and any dyes and oils from those grains. Thus, these materials tend to accumulate at the pinned perimeter, creating the stain ring we see. If the perimeter goes through an alternating series of pinning and movement, the stain will have several nested rings. The width of each ring depends on the rate of evaporation as the ring is being formed. The width also seems to depend on the curvature of the perimeter --- a concave perimeter generally has a narrower ring, suggesting a slower rate of evaporation there than in the convex regions. The difference in the rate is most likely due to the probability that a water molecule which has left the liquid happens to run randomly back into the liquid and be recaptured.

You can see similar ring structure if you live in a region where salt is used to melt ice on sidewalks and roadways during winters. As a pool of salt water evaporates, a white ring of salt outlines the original border of the pool. You might also see ring structure in a variety of other situations involving evaporating drops, such as with watercolors and drops from colored popsicles.

Here is a twist to the story. In one of the monthly articles that I used to write, I describe a curious pattern that would develop overnight in a “watch glass” (a shallow glass container with gently sloped sides) holding Middle Eastern coffee. Such coffee is brewed with sugar and very fine coffee grains (more a silt than visible grains). When the coffee is left for several hours in a container with a shallow slope, a serrated pattern of short brown fingers and short clear fingers develops along the perimeter. A drawing of what I saw is included here. (The drawings here are by Michael Goodman, who was the artist for the articles I wrote.)

I wrote the article in May 1983 but still have never found any reference to such a pattern. My explanation is that the water evaporation along the shallow perimeter of the coffee concentrates the sugar there, causing the heavy sugar water to gradually slide down the glass surface of the watch glass. The sliding clears a short path (one of the clear fingers) through the coffee silt. To replace this liquid at the perimeter, water with both sugar and silt gradually flows up to the perimeter, producing brown fingers of silt.

What I cannot explain is the spacing of the fingers along the perimeter. The evaporation creates an unstable situation in which concentrated (heavy) sugar water lies higher in the watch glass than less concentrated (lighter) sugar water. Heavy liquid lying above lighter liquid can produce periodic patterns in many other situations, but with the Middle Eastern coffee, I don’t know what factors determine the width of the fingers and thus the periodicity (the repetition distance) in the pattern. (Physicists go nuts over patterns and the factors that determine their periodicities.)

If you would like to read my original article, take the citation listed below to your local library. Instead, you can buy a CD containing all the Amateur Scientist articles ever written by using one of the links in the Store here at this FCP site --- do you see the button in the menu at the left side of the screen? Let me know if you find similar patterns in other evaporating liquids, have another explanation of the patterns, or know of references that explain the coffee patterns.

The mantra of The Flying Circus of Physics, both the book and this web site, is that “physics is everywhere.” Indeed, curious physics even lies in puddles of evaporating coffee. This web site of the James Franck Institute researchers contains a lot of information and photos.

Dots ·  through · · ·  indicate level of difficulty
Journal reference style: author, title, journal, volume, pages (date)

·  Walker, J., "What causes the 'tears' that form on the inside of a glass of wine?" in “The Amateur Scientist,” Scientific American, 248, No. 5, 162-170 (May 1983)
· · ·  Deegan, R. D., O. Bakajin, T. F. Dupont, G. Huber, S. R. Nagel, and T. A. Witten, “Capillary flow as the cause of ring stains from dried liquid drops,” Nature, 389, 827-829 (23 December 1997)
Walker, G., “The thrill of the spill,” New Scientist, 156, No. 2105, 34-35 (25 October 1997)
·  Nagel, S. R., “Klopsteg Memorial Lecture (August, 1998): Physics at the breakfast table --- or waking up to physics,” American Journal of Physics, 67, No. 1, 17-25 (January 1999)
· · ·  Deegan, R. D., “Pattern formation in drying drops,” Physical Review E, 61, No. 1, 475-485 (January 2000)
· · ·  Deegan, R. D., O. Bakajin, T. F. Dupont, G. Huber, S. R. Nagel, and T. A. Witten, “Contact line deposits in an evaporating drop,” Physical Review E, 62, No. 1, 756-765 (July 2000)
· · ·  Hosoi, A. E., and J. W. M. Bush, “Evaporative instabilities in climbing films,” Journal of Fluid Mechanics, 442, 217-239 (2001)
· · ·  Zhang, N., D. F. Chao, and W. J. Yang, “Convective instability in transient evaporating thin liquid layers,” Journal of Non-Equilibrium Thermodynamics, 27, 71-89 (2002)
· · ·  Torii, S., and W.-J. Yang, “Evaporation-induced Benard convection in a thin liquid layer,” International Journal of Energy Research, 27, 255-264 (2003)
·  Lin, Z., and S. Granick, “Patterns formed by droplet evaporation from a restricted geometry,” Journal of the American Chemical Society, 127, 9, 2816-2817 (9 March 2005)
·  Li, F-I., S. M. Thaler, P. H. Leo, and J. A. Barnard, “Dendrimer pattern formation in evaporating drops,” Journal of Physical Chemistry B, 110, 25838-25843 (2006)
· · ·  Xu, J., J. Xia, S. W. Hong, Z. Lin, F. Qiu, and Y. Yang, “Self-assembly of gradient concentric rings via solvent evaporation from a capillary bridge,” Physical Review Letters, 96, No. 6, article # 066104 (4 pages) (17 February 2006)
· · ·  Hu, H., and R. G. Larson, “Marangoni effect reverses coffee-ring depositions,” Journal of Physical Chemistry B Letters, 110, 7090-7094 (2006)
· · ·  Belyi, V. A., D. Kaya, and M. Muthukumar, “Periodic pattern formation in evaporating drops,” available at
·  Ueno, I., and K. Kochiya, “Effect of evaporation and solutocapillary-driven flow upon motion and resultant deposition of suspended particles in volatile droplet on solid substrate,” Advances in Space Research, 41, 2089-2093 (2008)

2.102  Walking on a beach
Jearl Walker
June 2011  If you step onto wet sand (not so wet that the grains swirl) and then lift your foot, why is the sand within your footprint relatively dry and why does it become wet again within a few minutes?

Before you step on the sand, the grains are about as closely packed as they can get, and water fills the intermediate spaces. The sand looks wet because you see reflections off the water along the sand surface. When you step on the sand, you shear the sand by making parts move over or across other parts. This motion must increase the spacing between the grains. (The sand is said to be dilatant because shearing increases its volume from the initially closely packed state.) Water soon drains from the sand surface into the increased space between grains, leaving the sand surface relatively dry. Within a few minutes, either the grains slide back into closer packing or additional water comes in from the surrounding or underlying sand, and then the sand surface looks wet again.

If you had a squeezable bottle of sand and water, you could collapse the bottle a bit with a gentle, slow squeeze, which would allow the grains to move slowly out of their closely packed arrangement and also allow water to seep into the new spaces to lubricate the grains. However, an abrupt squeeze would attempt to move the grains too quickly, without the necessary water lubrication. The friction among the grains would be so great that you could not collapse the bottle at all.


Dots · through ··· indicate level of difficulty
Journal reference style: author, title, journal, volume, pages (date)
· Walker, J., "Serious fun with Polyox, Silly Putty, Slime and other non-newtonian fluids," in “The Amateur Scientist,” Scientific American, 239, No. 5, 186-196 (November 1978)
··· Gudehus, G., “On the onset of avalanches in flooded loose sand,” Philosophical Transactions of the Royal Society of London A., 356, 2747-2761 (1998)

2.102 Quicksand
Jearl Walker
Sep 2011  Is it real or just something made up for those old grade B jungle movies? And if it is real, why is it any different from the wet sand you run across as the waves sweep over a common beach? And, more important, if it is real, how can you escape from it?

There are many videos on the web pretending to show you quicksand, but what they actually show is a big mud pit, where a stream or garden hose has produced a fairly deep layer of viscous mud. Playing in mud is always fun, of course, but a mud hole is not a quicksand pit.

To get quicksand, you need a sand bed with a water influx, such as from a natural spring under the sand. The influx moves the grains apart somewhat and lubricates them so that they can slide over one another. If you step onto this arrangement, you can sink through the lubricated sand. If you then struggle by trying to move your legs upward quickly, the quicksand suddenly becomes rigid and you cannot move your legs. The trouble is that your legs can move only if the spacing between the adjacent grains increases, but that requires the sliding of grains against grains. There is so much friction resisting that sliding, all around your legs, that moving your legs might be impossible.

Water is a common newtonian fluid, which means that its viscosity does not change when stress (pressure) is applied to it. In contrast, quicksand is a nonnewtonian fluid because its viscosity changes when it is stressed. For some types of nonnewtonian fluids (such as ketchup), the viscosity decreases under stress. For quicksand, it dramatically increases.

Quicksand is a dense fluid and you may not sink enough to drown. In an idealized situation, you might be able to lie down on it by bending at the waist, and then you could crawl by thrusting your hands along the surface, slowly extracting your legs. The emphasis here is on “slowly” because you want to avoid stressing the quicksand as much as possible, to keep its viscosity as low as possible. Here are two videos in which Bear Gryllus of the Discovery Channel escapes quicksand by bending over, slowly working his legs free, and then “monkey crawling” off the quicksand.

As Gryllus explains in the videos, the obvious danger of getting stuck in quicksand is that you could then be exposed too long to the Sun (as in a desert) or go too long without food and water.

However, other wilderness experts point out that quicksand in wild country can be far more subtle in danger because it is likely to be hidden from view under a layer of standing or running water. So if you are wading through water that is, say, waist deep and happen to step onto quicksand, you could sink enough to put your head below the water surface. With your legs trapped in place, you would then drown.

Suppose that the layer of water is not deep. You can argue that because you float higher in quicksand than in water, you should still be able to breathe if you step into the quicksand. However, the floating level is helpful only if you actually float at that level. If you fall into quicksand, you fall through your floating level. In water, you would just bob back up to that level, but not in quicksand. You are just stuck.

An even more subtle danger is that as you sink into the quicksand, you might divert the water flow that makes the sand quick, and then the sand around you becomes firm. The situation is then hopeless.

Experts advise that the only sure way to escape quicksand, especially when it lies underwater, is to be prepared for an escape. When quicksand is a possibility, you should have a bowline tied under your arms and around the chest, and someone at the other end of the rope should be ready to pull hard should you fall into quicksand.
I explained the physics of quicksand on the award-winning television show "Newton's Apple." The video is available at the FCP (public) Facebook site: 
Click on the sixth video (I am wearing a jungle jacket in the thumbnail image). The host graciously sinks into the quicksand as I smugly look on.  

Physics is everywhere and even lurks in jungles and deserts.

Dots · through ··· indicate level of difficulty
Journal reference style: author, title, journal, volume, pages (date)
· Matthes, G. H., “Quicksand,” Scientific American, 188, No. 6, 97-102 + 124 (June 1953)
· Walker, J., "Serious fun with Polyox, Silly Putty, Slime and other non-newtonian fluids," in “The Amateur Scientist,” Scientific American, 239, No. 5, 186-196 (November 1978)
· Kruszelnicki, K., “The Earth did swallow them up!” New Scientist, ??, 27-29 (21/28 December 1996)
··· Gudehus, G., “On the onset of avalanches in flooded loose sand,” Philosophical Transactions of the Royal Society of London A., 356, 2747-2761 (1998)
·· Khaldoun, A., E. Eiser, G. H. Wegdam, and D. Bonn, “Liquefaction of quicksand under stress,” Nature, 437, 635 (29 September 2005)
2.102  Escape from mud
Jearl Walker
January 2016  In Thailand, a Norwegian couple wandered onto a mud bank to take photographs. The bank obviously had recently been flooded and had an extensive area of deep mud. The couple apparently stepped into an especially deep layer and were so entrenched that they could not pull their legs out. A very kind Thai fisherman left his boat and went to their rescue. However, he could not pull them out. Then he became clever with the physics:

The mud (a slurry of sand and dirt) was like quicksand. If you sink into quicksand and then try to pull out a leg, the quicksand holds onto you tighter because the motion increases the viscosity. When the man tried to pull his right leg out of the mud, the mud just hung onto him harder. So also when the fisherman tried to pull the man out.

The fisherman then realized that he needed to lie down behind the woman to allow her to escape.

In doing so, he spread out his weight and then also the woman’s weight over a much larger area and thus reduced the pressure on the mud. That limited his sinking into the mud. By pushing her hands down on the fisherman’s back, the woman managed to free her legs and then walk away from the deep area. The fisherman then used the same technique for the man to escape.

If you sink into quicksand or mud, the best thing to do is to lie down, either in the backward direction if you are sunk up to your knees or the forward direction if you are sunk up to your waist. The horizontal portion of your body will then be spread over enough area that it will not sink in very far. You then effectively have a platform of support and can pull your legs out.

Here is another escape:

Dots · through ··· indicate level of difficulty
Journal reference style: author, title, journal, volume, pages (date)
· Matthes, G. H., “Quicksand,” Scientific American, 188, No. 6, 97-102 + 124 (June 1953)
· Walker, J., "Serious fun with Polyox, Silly Putty, Slime and other non-newtonian fluids," in “The Amateur Scientist,” Scientific American, 239, No. 5, 186-196 (November 1978)
· Fisher, D., J. E. Cullis, and J. Ring, “Sinking feeling,” in “The Last Word,” New Scientist, search “quicksand” at ??
· Kruszelnicki, K., “The Earth did swallow them up!” New Scientist, ??, 27-29 (21/28 December 1996)
··· Gudehus, G., “On the onset of avalanches in flooded loose sand,” Philosophical Transactions of the Royal Society of London A., 356, 2747-2761 (1998)
·· Khaldoun, A., E. Eiser, G. H. Wegdam, and D. Bonn, “Liquefaction of quicksand under stress,” Nature, 437, 635 (29 September 2005)

2.103  Nimitz Freeway collapse
Jearl Walker
October 2014  In 1989, minutes before the start of the third game of the baseball World Series in Oakland, California, an earthquake of magnitude 7.1 erupted 100 kilometers away. When the seismic waves reached the Oakland-San Francisco area, collapsing structures killed 67 people and did extensive damage. Perhaps the most dramatic damage occurred on a long stretch of the Nimitz Freeway: an upper deck fell onto a lower deck, crushing vehicles and motorists. Obviously the upper deck collapse was due to the severe oscillations produced by the seismic waves, but why was the collapse of the upper deck restricted to that certain length of the freeway, with no collapse along the rest of the freeway which was almost identical in construction? Warning: some of the video is explicit in showing injuries.

The Nimitz Freeway collapse was confined to the stretch built on a loosely structured mudfill, which underwent liquefaction (or fluidization) during the shaking. That is, when the particles in the mudfill were shaken, the average distance between them increased and the mudfill became more fluid (it could flow) than solid. With the mudfill in a fluid state, the seismic waves had a much greater effect than in the surrounding regions where the freeway was anchored in rock deposits.

We can measure how devastating seismic waves can be in at least two ways.

Maximum speed of oscillations. We can calculate the maximum speed that the waves gave to the ground particles as they made the particles oscillate. In the mudfill regions, the maximum speed was at least five times what it was in the rock-deposit regions. Thus, the ground shake was much more severe in the mudfill regions than in the rock-deposit regions.

Resonance. We can determine the resonant frequencies of the deck sections of the bridge. Those sections can oscillate both horizontally (due to longitudinal waves) and vertically (due to transverse waves) in certain frequency ranges. If the sections are shaken at one of those frequencies, the oscillation of the section builds. You do something similar with a child in a playground swing. The swing will swing with a certain frequency of oscillation. If you repeatedly push on the child with that frequency, the extent of swinging (the amplitude) increases. The frequency matching and resulting increase in amplitude is said to be resonance. For the bridge sections, the resonant frequencies for horizontal oscillations ranged from 1.6 to 4.5 hertz and the resonant frequency for vertical oscillations was 2.5 hertz. Seismograph recordings of the seismic waves that hit the region showed that the greatest ground oscillations were in the frequency range of 2 to 5 hertz. Thus, the bridge sections were shaken in resonance, and the oscillation amplitudes quickly built up to the point where the support structures failed.

For both reasons, greater maximum oscillation speed and resonance, the bridge sections were shaken so severely that they collapsed.

Dots · through ··· indicate level of difficulty
Journal reference style: author, title, journal, volume, pages (date)
· Hough, S. E., P. A. Friberg, R. Busby, E. F. Field, K. H. Jacob, and R. D. Borcherdt, “Sediment-induced amplification and the collapse of the Nimitz Freeway,” Nature, 344, 853-855 (26 April 1990)
2.103 Liquefaction
Jearl Walker
May 2011 A heavy object can sink into the ground during an earthquake if the shaking causes the ground to undergo liquefaction, in which the soil grains experience little friction as they slide over one another. The ground is then effectively quicksand. The possibility of liquefaction in sandy ground can be predicted in terms of the void ratio for a sample of the ground. That ratio is equal to the volume of the voids (empty spaces between the grains) to the volume of the sand grains. If the ratio exceeds 0.80, liquefaction can occur during an earthquake, and then buildings can tilt and fall or simply sink into the ground.

If the ground is saturated with water, the oscillations can move the grains faster than the water can flow, which increases the water pressure. Then, for a while after the shaking, the higher water pressure can force the water to the surface of the ground, with it gurgling out of the ground.

Here is a video that demonstrates the liquefaction that occurred during the earthquake in Christchurch, New Zealand, in September, 2010. First notice the garden which underwent liquefaction. The man talking on the video has loaded some of garden soil into a wheelbarrow. When the wheelbarrow is then rolled over a cobblestone drive, the oscillations caused by the cobblestones liquefy the soil as we watch it.

Here are three demonstrations of liquefaction with models, the first coming from the Exploratorium in San Francisco.

The first of the following videos had been the best filmed example of liquefaction until modern times when handheld video cameras became so widespread. The other videos show liquefaction effects at Christchurch. best example of liquefaction aftermath at Christchurch New Zealand Sep 2010 aftermath also


2.108  Pub trick --- beer enhanced Brazil nut effect
Jearl Walker
July 2012 The Brazil nut effect is the name given to the tendency of a large object (say, a Brazil nut) to move up through a container of smaller objects (say, peanuts or small nuts) when the container is vertically oscillated. This motion occurs in some packages of food products, causing the larger objects to separate from the smaller ones, which is not desirable if you intend to pour mixed contents from the top.

Two main theories have been advanced for the motion.

(1) The oscillations set up a circulation within the container. Because of the relative freedom of motion in the center compared to the restricted motion and rubbing along the wall, there is a gradual upward flow in the center, which causes a gradual downward flow along the walls. The larger object can be brought from near the bottom to the top by the flow and then be left there.

(2) The larger object can be ratcheted up to the surface. During an upward shake, it and the smaller objects are tossed upward. During the fall back down, a few of the smaller objects fall under the larger object, preventing it from returning to its initial height. Thus, the larger object is gradually brought to the surface.

Lots of researchers have investigated how the Brazil nut effect depends on relative sizes and densities of the objects and on the role of air within the mixture. Recently C. P. Clement, H. A. Pacheco-Martinez, M. R. Swift, and P. J. King of the University of Nottingham, England, reported on experiments in which a steel ball (the Brazil nut) is in a container of smaller glass beads that was vertically oscillated. With air between the objects, the steel ball either remained in place or very gradually climbed. However, when water was in the container, the ball moved upward rather quickly. The researchers attributed the enhanced Brazil nut effect to the flow of the water during the oscillation. During an upward stroke, the ball pushed beads and also water out of its way. That water flow enhanced the removal of beads above the ball. Also during the upward stroke, water flowed into the space left under the ball, bringing beads into the space. Thus the ball was ratcheted upward.

I tried a similar experiment. I partially filled a narrow drinking glass with dry-roasted, unsalted peanuts, with a Brazil nut buried at the center of the peanuts. Here you can see the placement of the nut:

And here is a photo of the full glass:

With my palm closing the top of the glass, I shook the glass until the Brazil nut reached the surface. The results varied between 50 and 100 shakes.

Next I poured beer into another glass, waited until the head of foam disappeared, and then poured enough into the arrangement of peanuts and buried Brazil nut until the beer surface was just below the peanut surface.

I repeated my shaking procedure to see how many shakes were now required to bring the Brazil nut from midway to the top surface. The results varied between 4 and 10. Thus, I was clearly seeing a beer-enhanced Brazil nut effect.

By the way, beer-soaked peanuts are pretty tasty but peanut-flavored beer is not so good.

Dots · through ··· indicate level of difficulty
Journal reference style: author, title, journal, volume, pages (date)
··· C.P. Clement, H. A. Pacheco-Martinez, M. R. Swift, and P. J. King, “The water-enhanced Brazil nut effect,” European Physics Letters, 91, article #54001 (6 pages) (September 2010)


2.109  Avalanche balloon
Jearl Walker
November 2013   An avalanche balloon is used by some skiers caught in a snow avalanche. The balloon, which is carried in a backpack, is initially folded in a backpack. If an avalanche approaches, the skier pulls on a rip-cord device to inflate the balloon with nitrogen gas from a cylinder; the nitrogen flow draws in air from outside the balloon. As the skier and balloon are caught up in the avalanche, they tend to move to the top of the flow rather than be buried down within the flow. Thus the skier has a better chance of survival. The question is, of course, “Why does the skier move to the top of the flow?” Here are two videos:
shows actual avalanche and partial burial
camera mounted on head of snowboarder who is caught in avalanche and partially buried

With an avalanche balloon attached to the back, a skier caught in flowing snow is effectively a Brazil nut caught in a shaken container of peanuts. In the Brazil-nut effect, the nut is placed at the bottom of jar and then the jar is filled with peanuts, which are smaller. If the closed jar is then vertically shaken, the nut climbs up through the peanuts until it reaches the surface. One explanation is that when the jar is in a downward part of the shaking, the nut and peanuts are somewhat in free fall. As the peanuts near the bottom of the nut land on more firmly packed peanuts, they have a chance of ricocheting underneath the nut. Thus, the nut tends to land a bit higher with each toss.

A common explanation for the avalanche balloon involves the change in density of the skier-backpack system when the balloon is inflated. The density change is certainly important. The balloon provides an upward buoyancy force, because the gas it holds is less dense than the flowing snow. However, that force is insufficient to lift the skier to the top of the snow. Instead, the skier is lifted by the increased volume due to the inflated balloon: The Brazil nut (skier and inflated balloon) is much larger than the surrounding peanuts (moving and oscillating clumps of snow).
long but good explanation of surviving an avalanche and use of the balloon


Dots · through ··· indicate level of difficulty
Journal reference style: author, title, journal, volume, pages (date)
· Radwin, M. I., C. K. Grissom, “Technological advances in avalanche survival,” Wilderness and Environmental Medicine, 13, No. 2, 143-152 (summer 2002)
2,116  Rockfalls
Jearl Walker
June 2015  A rockfall is where a rock or a collection of rocks fall from the side of a mountain, usually the face of a cliff. If you are in or near the path, you suddenly feel small and defenseless. Here are three examples:

Near the beginning of this first video, watch the top of the mountain --- part of it moves! And that is the part that ends up hitting the road.

There was not quite enough energy left in the boulder for its center of mass to come up over its contact point on the road, and so the boulder rotated back down onto the road instead of down onto the car. Very lucky physics there.

In this next one, the cameraman was knocked down but hopefully was not hurt.

This next video is tough to watch because as the people escape their vehicles, one person is hit and then needs help escaping. Note that toward the end, the windshield in front of the video camera is broken. one person hit

Most rockfalls are due to weathering of bedrock: (1) Fissures that collect water can be widened and extended when the water freezes, because the water expands. (2) The rock is weakened by chemical weathering, especially when moisture is available. Although any rock can undergo weathering, rockfalls occur only when the bedrock face is steep and when a section continues to be supported while fissures are driven into the face. At some point, the section’s attachment or support is overwhelmed, and the rock breaks free.

Depending on circumstances, the freed rock can fall through the air, bounce down a fairly steep slope, tumble down a moderate slope, or slide down a shallow slope. It can also shatter into smaller rocks. With any of these outcomes, it loses much of its energy in the collision. It can also lose energy if it collides with trees, and thus a stand of trees is often grown as a rockslide barrier.

When a rockfall involves many rocks of various sizes, the rocks can become segregated along a slope because the larger rocks reach the bottom of the slope whereas smaller rocks tend to catch in the low points (the crannies) along the slope. In general, material along a slope tends to vary from fine grade at the top to coarse grade at the bottom. The foremost rock in some rockfalls ends up at a surprisingly large distance from the other rocks, presumably because it gains energy as other rocks hit it from the rear during the sliding and rolling portion of the fall. Here is an example: damage to house, long trails

Other videos: rock wall gradually falls news video helicopter causes rockfall earthquake rockfall

Dots · through ··· indicate level of difficulty
Journal reference style: author, title, journal, volume, pages (date)
· Porter, S. C., and G. Orombelli, “Alpine rockfall hazards,” American Scientist, 69, No. 1, 67-75 (January-February 1981)
· Scheidegger, A. E., “A review of recent work on mas movements on slopes and on rock falls,” Earth-Science Reviews, 21, No. 4, 225-249 (1984)
· Uhrhammer, R. A., (abstract) “Seismic analysis of the Yosemite rock fall of July 10, 1996,” EOS, 77, 508 (1996)
· Wieczorek, G. F., and S. Jager, “Triggering mechanisms and depositional rates of postglacial slope-movement processes in the Yosemite Valley, California,” Geomorphology, 15, 17-31 (1996)
· Day, R. W., “Case studies of a rockfall in soft versus hard rock,” Environmental & Engineering Geoscience, 3, No. 1, 133-140 (spring 1997)
· Peila, D., S. Pelizza, and F. Sassudelli, “Evaluation of behaviour of rockfall restraining nets by full scale tests,” Rock Mechanics and Rock Engineering, 31, No. 1, 1-24 (1998)
·· Morrissey, M. M., and W. Z. Savage, “Air blasts generated by rockfall impacts: analysis of the 1996 Happy Isles event in Yosemite National Park,” Journal of Geophysical Research, 104, No. B10, 23189-13198 (10 October 1999)
·· Matsuoka, N., and H. Sakai, “Rockfall activity from an alpine cliff during thawing periods,” Geomorphology, 28, 309-328 (1999)
· Wieczorek, G. F., J. B. Snyder, R. B. Waitt, M. M. Morrissey, R. A. Uhrhammer, E. L. Harp, R. D. Norris, M. I. Bursik, and L. G. Finewood, “Unusual July 10, 1996, rock fall at Happy Isles, Yosemite National Park, California,” Geological Society of American Bulletin, 112, No. 1, 75-85 (January 2000). Photos 1 and 2 in Fig. 11 are reversed; correction on page 959 (June 200)
··· Okura, Y., H. Kitahara, T. Sammori, and A. Kawanami, “The effects of rockfall volume on runout distance,” Engineering Geology, 58, 109-124 (2000)
·· Dorren, L. K. A., “A review of rockfall mechanics and modelling approaches,” Progress in Physical Geography, 27, No. 1, 69-87 (2003)

2.120  Glug-glug
Jearl Walker
October 2007
The continuing theme of this web site and The Flying Circus of Physics book is that physics is everywhere. How about pouring water from an inverted bottle? Geez, that certainly sounds boring, and yet the physics is quite curious and the mathematics quite challenging.

Suppose that you held an inverted bottle partially filled with water, with the cap still in place and then magically made the cap disappear. The water really should not come out because of air pressure. Oh, the water will slump somewhat, but that movement increases the space for the air that is trapped in the bottle above the water. With more space, the air pressure decreases, and then there is normal air pressure below the water and reduced air pressure above it. That is, there is greater push on the water from the air below the water than above, and that difference should support the water. However, you probably know from embarrassing experience that an inverted beverage will spill out onto your clothes, perhaps on that first date when you were trying to make such a good impression.

As I explain in The Flying Circus of Physics, the trouble with the inverted arrangement is that it is unstable to the waves that are created by chance disturbances to the water surface at the bottle opening. A slight tremor of your hand or a slight breeze is enough to send waves over the surface. Most of the waves may be irrelevant but some have about the right wavelength to cause an upward deflection to the surface at one spot and a downward deflection at another spot. The upward deflection allows air to move slightly upward and the downward deflection allows water to move slightly downward.

If the bottle opening is small enough, the surface tension of the water (the mutual attraction of water molecules) might correct the deflections and stability might be reestablished. However, if the opening is somewhat wider, a large bubble of air can pinch off at the opening, rise through the water column, and add to the trapped air. The resulting increase in pressure in the trapped air ruins the support provided by the pressure difference between top and bottom of the column, and a slug of water can then fall free of the bottle.

The bottle may then alternate between having a large bubble flow upward and a slug of water fall free, until the bottle is drained. The process produces a sound that gives the process its name: glug-glug.

The glug-glug of a bottle is but one example of an instability situation that has long fascinated scientists and mathematicians. In normal circumstances, a dense fluid should descend through a less dense fluid. (Molasses will sink to the bottom of glass of water.) However, with the inverted bottle of water, a dense fluid (the water) lies over a less dense fluid (air), an arrangement that is often called a Rayleigh-Taylor instability after the two men who worked out how waves from chance disturbances can upset the arrangement. If the opening is small, the unstable arrangement is maintained because the waves cannot overcome the self-correcting surface tension. If the opening has moderate width, the waves very quickly lead to glug-glug.

If the opening is even larger, the waves on the water surface allow air to flow almost continuously up one side of the opening and water to flow almost continuously down the opposite side. The bottle then drains rather quickly in a rapid glug-glug.

A study of glug-glug has just been published by Masahiro I. Kohira (of Japan Space Forum), Nobuyuki Magome (of Nagoya Bunri College), Hiroyuki Kitahata, and Kenichi Yoshikawa (both of Kyoto University and Spatiotemporal Project). They used a plastic bottle instead of a glass bottle so that the flexibility of the sides better allowed glug-glug. The opening to the bottle consisted of a tube. By changing the tube, they could change the width of the opening.

When the tube was narrow, the water remained in the bottle. For a moderately wide tube, glug-glug occurred, with the time between glugs gradually increasing as the bottle drained. (The researchers video recorded the flow so that they could measure the timing of the glugs.) For a wider tube, the almost continuous flow of air and water occurred.

The researchers also experimented with two inverted plastic bottles, with a short tube connecting the pockets of trapped air. When the openings were identical and of moderate width, the glug-gluging (not a real word but it sounds so good when I say it.) of the bottles was in phase (in step). That is, the upward bubble flows in the bottles occurred simultaneously and the downward water flows occurred simultaneously. The action was in phase because the connected tube allowed any change in air pressure in one bottle to be immediately felt in the other bottle, and so they effectively had one big bottle.

However, when the tube was U shaped and a bit of water lay in the bottom of the U, the glug-gluging was out of phase (out of step). Apparently the sluggishness (another of my favorite words) was enough to delay an air pressure change in one bottle from being felt in the other bottle.

Physicists love this kind of stability analysis, but how about you? Is there any way to put this stuff to work for you? Here is one way. Suppose you are at a fancy-dress dinner with dignitaries when, out of nervousness, you knock a glass of ice water into the lap of the hostess. Instead of just running from the room and discarding your career, you can look her straight in the eyes and say, “Oh, sorry, I was just doing an experiment. Here, let me tell you about Rayleigh-Taylor instability.” Well, it might be worth a try.

· · · Clanet, C., and G. Searby, “On the glug-glug of ideal bottles,” Journal of Fluid Mechanics, 510, 145-168 (2004)
· Kohira, M. I., N. Magome, H. Kitahata, and K. Yoshikawa, “Plastic bottle oscillator: Rhythmicity and mode bifurcation of fluid flow,” American Journal of Physics, 75, No. 10, 893-895 (October 2007)

Want more references? Use the link at the top of this page.

2.120  Pub trick---exchanging water and whiskey
Jearl Walker
April 2009    You have a shot glass of water and a shot glass of whiskey (or some similar fluid). Can you exchange the water and the whiskey without using a third container (not even a straw or your mouth)? Here is one of the video links where the answer to this very old pub trick is demonstrated:

We see the performer invert the glass of water by using a thin card (such as an ID or driver’s license) and then positioning it over the glass of whiskey. By carefully sliding the card sideways, a narrow gap is opened between the two glasses. Because water is denser than whiskey, it naturally pours down into the lower glass, forcing the whiskey into the upper glass. The replacement is complete because water and whiskey do not mix, that is, they do not spread uniformly through one another. Otherwise, we would end up with the same water-whiskey mixture in the two glasses.

There is a subtle point involved in the trick, one that we have seen in other pub tricks. Yes, the water is denser than the whiskey, but it cannot descend unless the water-whiskey interface is sufficiently unstable.

This arrangement of denser fluid lying over lighter can be stable if the gap is narrow enough.

Chance disturbances tend to send waves across the interface between the two fluids, with some portions of the interface moving up and other portions moving down. The upper motion tends to allow the whiskey to flow up into the water, and the downward motion tends to allow the water to flow down into the whiskey.

However, all this can occur only if waves from the disturbances can fit into the width of the gap. That is, the wavelengths of the waves must be smaller than the gap width. If the gap is narrow enough, it is too narrow to allow any of the waves that the disturbances would produce. With no waves on the interface, the interface is stable and the trick does not work. If we widen the gap, it is eventually wide enough that some waves can occur and then we get the exchange of whisky and water. In the following video, you can see the whiskey rise into the water at certain points along the gap --- those are the points where a wave initially moved the interface upward. At other points, water flows down into the whiskey --- those are the points where the wave initially moved the interface downward.

Here are some more links: very old movie footage of legendary magician Harry Blackstone Sr. (this is indeed an old pub trick)

2.120  Pub trick with Red Bull
Jearl Walker
Nov 2008   
If you invert a pint of beer or any other common liquid, you are just going to make a mess, not the kind of thing that will win you the favors of a crowd in a pub. However, you can invert an open can of the “energy drink” Red Bull if you take a certain precaution, as seen in this video:

As the video shows, you open the can by using the tab release and then, before you invert the can, you rotate the tab until it covers the hole through which you normally would drink the liquid.

But wait. As you see in the photo here, the tab has a hole in it and does not even fully cover the drinking hole. So, why doesn’t the liquid just pour out through the tab’s hole and the open spaces around the tab? When you rotate the tab over the drinking hole, you certainly do not seal the can.

Here is an even more general question --- when you invert a cylindrical glass of water, why does the water pour out? Oh yes, I know that gravity tends to pull it out, but what I am getting at is this: how does gravity pull it out? If you invert a jar of jam, gravity pulls on the jam just as it pulls on the water, but the jam does not slide out. Of course, water is liquid and can flow, but why should that matter? Well, as a matter of fact, if the glass container is fairly narrow, the water will not pour out.

To see why, let’s start with a drinking straw. If you dip the straw into water and then seal the top of the straw with a finer, you can pull the straw out of the water while keeping water in the straw. Of course, if you remove your finger and open the top of the straw, the enclosed water pours out. But back up. Why did the water initially stay in the straw? After all, the gravitational force on the water is the same before and after you remove your finger.

When your thumb is in place, the water does tend to pour out of the straw but as it begins to slump, the space of the air trapped between the water and your finger begins to increase in volume. That means that the pressure in that air decreases and is then lower than the air pressure on the water at the bottom of the straw. The pressure difference (normal at the bottom of the water column and lower at the top) provides an upward force that supports the water in the straw, countering the gravitational force. When you remove your finger, the air pressure above the water quickly increases to its normal value, the pressure difference disappears, and the water pours out.

If you repeat this demonstration with progressively wider tubes, you eventually reach a width for which the water pours out even when your finger (or thumb or hand) seals the top of the tube. In the photographs here, I can hold (red-dyed) water in a plastic tube but not in a wider tube. Why does the width matter?

The answer has to do with the stability of the bottom water surface as it is disturbed by your small involuntary hand shake, a chance air breeze, the small room vibrations, or even the sound waves of someone talking. Any such disturbance sends waves over the water surface, just as waves are sent over the surface of coffee if you gently shake the coffee container. Each such wave tends to send the surface upward in some regions while sending the surface down in other regions.

On the bottom water surface, the upward-going regions tend to move air up into the tube and the downward-going regions tend to move water down out of the tube. However, surface tension, which is due to the mutual attraction of the water molecules, opposes such curved regions of the water surface and tends to smooth out the surface.

When the tube is narrow, the waves sent over the water surface must have short wavelengths (to fit into the width) and the upward intrusion of air and the downward slumping of water is easily overcome by the surface tension. That is, the surface tension smoothes out the surface and the water stays in the tube.

When the tube is wider, longer wavelengths will fit across the water surface. Two factors are now important. One, chance disturbances are more likely to produce these longer wavelengths than the shorter wavelengths. Second, surface tension is less likely to smooth out these waves. So, an upward-going region keeps on moving upward, carrying air to the top of the water column, and a downward-going region allows water to descend out of the tube. Because the air above the water column is increasing, the air pressure there increases and the pressure difference between the top and bottom of the column is no longer enough to stop the slump of water.

The resulting flow of water is called glug-glug because of the sound made as periodically air goes up one side of the tube and water slumps out the other side, time after time until all the water has fallen out of the tube.

Now we can explain the Red Bull trick. The drinking hole is too wide for surface tension to prevent the waves with longer wavelengths from setting up glug-glug. When the tab is rotated and fixed in place, the remaining holes are small enough to prevent glug-glug or at least greatly decrease it. The trick won’t work with the normal can of pop or beer because the drinking hole is large, as you can see in the photograph here.

You might already know of a similar pub trick, now commonly shown in classrooms. If you place a square of paper across a drinking glass partially filled with water and, with one hand, press the paper against the rim, you can, with the other hand, quickly invert the glass without spilling the water. You can then remove the hand pressing against the paper. Water will remain in the glass. The usual explanation for this trick is that the paper is held to the rim by surface tension as the water molecules cling to the rim and to the paper. However, the paper serves another role --- it decreases or even eliminates waves on the water surface, not allowing air to move up one side of the glass.

The beauty of the Red Bull trick is that if it works, you can explain about the waves on the liquid surface, and if it does not work, you can still explain about the waves. Of course, you will then have to walk through the pub with wet pants. I don’t have a physics story to help you out with that.

·  Grimvall, G., “Questionable physics tricks for children,” Physics Teacher, 26, 378-379 (September 1987)
· ·  Soltzberg, L. J., “Far from equilibrium---the continuous-flow bottle,” Journal of Chemical Education, 64, No. 2, 147-152 (February 1987)
· · ·  Sasaki, T., “Parameters affecting whether water will flow out from an inverted open bottle,” Journal of Chemical Education, 66, No. 12, 1005-1006 (December 1989)
· · ·  Glaister, P., “Some observations on the inverted bottle problem,” Journal of Chemical Education, 68, No. 7, 623-624 (July 1991)
· ·  O’Connell, J., “Boyle saves a spill,” in “String & Sticky Tape Experiments,” R. D. Edge, editor, Physics Teacher, 36, 74 (February 1998)
· · ·  Clanet, C., and G. Searby, “On the glug-glug of ideal bottles,” Journal of Fluid Mechanics, 510, 145-168 (2004)
·  Subramaniam, R., and K. A. Toh, “’Magic’ cup defies the laws of physics,” Physics Education, 39, 334 (2004)
·  Kohira, M. I., N. Magome, H. Kitahata, and K. Yoshikawa, “Plastic bottle oscillator: Rhythmicity and mode bifurcation of fluid flow,” American Journal of Physics, 75, No. 10, 893-895 (October 2007)
· · ·  Huppert, H. E., and M. A. Hallworth, “Bi-directional flows in constrained systems,” Journal of Fluid Mechanics, 578, 95-112 (2007)
·  Subramaniam, R., and Y. K. Hoh, “’Magic’ cup illustrates surface tension,” Physics Education, 43, 251-252 (May 2008)
·  Subramaniam, R., and J. P. Riley II, “Physics trick gets students interested,” Physics Education, 43, No. 4, 355-356 (July 2008)

2.120  Pub trick --- Red Bull can stuck on your hand
Jearl Walker
August 2013  Red Bull is a liquid (a so-called energy drink) that comes in a fairly narrow can. The challenge this week is to stick the bottom of the can to your palm such that you can move the can around to various orientations and then pop open the can and, while the can is still stuck to the palm, pour the contents into a glass.

Here is a video that shows all this is done:

Now, anyone can do the trick but the real challenge is to explain it.

Wet your palm with a modest amount of water. Then position the bottom of the can on one side of the palm and, while pressing it downward into the palm’s flesh, slide the can to the center of the palm. By pressing downward, you squeeze out much of the air in the gap initially between the can and the palm. When you then release the pressure, that gap tends to expand but the expansion reduces the air pressure in the air remaining in the gap. With a dry hand, air can seep into the gap at various points around the gap’s edge. But with a water layer there, the water seals the edge because it sticks to both the skin and the can.


2.120  Pub trick --- yard of ale and beer boot
Jearl Walker
Sep 2009   As the name implies, a yard of ale is a beer container that stands a yard tall (about 1 meter), with a spherical bulb at the lower end of a long slender neck. The volume varies from about 2 pints to as much as 3.5 pints (about 0.2 liter to about 1.7 liters). Shorter versions (half a yard) can also be found in some pubs. Here is a photo of me with a half-yard glass of stout:

Regardless of length or volume, the challenge is to drink the beer from a yard of ale without becoming soaked. Initially, drinking from it is just like drinking from a mug --- the flow is easily controlled by the tilt of the container in a way that we learned as children. As the fluid pours out of the mug and air flow into it, you bring the lower end upward, until the side of the mug is horizontal and all the fluid has poured out.

Initially, the same goes with the yard of ale, because the beer in the bulb at the far end is initially trapped there. It can come out only if air goes into the bulb to replace it. If we vigorously shook the bulb, we might be able to dislodge some of its beer, but the dislodged beer would be pushed right back into the bulb by the air pressure on the open surface of the beer in the neck.

Air cannot move into the bulb until the yard of ale is almost horizontal. By then the air in the neck has reached the bulb and suddenly air can bubble into the bulb, allowing a gush of beer to escape from the bulb. This exchange of air and beer may occur in a quick, alternating series of bubbles and gushes, in a process known as glug-glug from the sound that it produces. The point is that the gushing suddenly puts a lot of beer into the neck. Because the yard of ale is approximately horizontal, this flood of beer quickly runs to the open end and drenches the drinker.

In the pub challenge to avoid becoming drenched, here is one solution. As the yard of ale nears the horizontal orientation, the drinker quickly raises the far end and then quickly lowers it, allowing only a small amount of air to bubble into the bulb. Thus, only a small amount of beer comes out of the bulb and, because the far end is then again lower, this extra beer in the neck cannot simply rush to the open end. The drinker than drinks more of the beer until air is again on the verge of bubbling into the bulb. Then the quick up-and-down motion is repeated to allow another small amount of beer to leave the bulb without rushing to the open end. This process is repeated until the beer level in the bulb is lower than the height of the opening into the bulb. Thereafter, drinking the beer is just like drinking from a mug. Here is a video example:

A second, more subtle approach is used in this next video:

As the drinker brings the yard of ale up close to the horizontal, he uses both hands to quickly rotate the neck, either back and forth (clockwise and counterclockwise) or continuously in one direction, so as to swirl the beer in the neck. The swirling causes the beer to slosh at the opening into the bulb. By chance, the sloshing sometimes set up a brief passageway by which a small amount of air can move into the bulb, allowing a small amount of beer to move out of the bulb. However, the beer does not gush out because the passageway is almost immediately closed by the continued sloshing.

A beer boot is similar to a yard of ale and, as the name implies, is a large container in the shape of a boot. Here too the challenge is to drink the beer without getting drenched when the air reaches the lower end of the container, as you can see in this video:

If the toe of the boot points up,

the drinker can use either the quick up-and-down technique or the twisting technique as with a yard of ale. However, there is another solution with the boot because the lower end is not spherical like the bulb in a yard of ale. To avoid being drenched, the person can merely rotate the boot so that the toe points to the left or right or down.

Then drinking from the boot is exactly like drinking from a normal beer mug, as you can see in this video:

Someone long ago, for his own strange purposes, decided to drink beer from a yard-high container or a glass mug in the shape of a boot (or even from an actual boot, as legend would have it). Surely people laughed at him, especially when he ended up wearing much of the beer. But even stranger, he continued to drench himself until he hit upon the correct physics to control the flow. See, there is a big advantage of physics --- you can avoid making a wet fool of yourself in a pub. Well… maybe. I suppose you can know all the physics in the world and still make a wet fool of yourself if you drink all the beer in a yard of ale or a beer boot. 

More links: yard of ale yard of ale, some twisting

2.120  Pub trick--- using glug-glug to clear beer foam
Jearl Walker
Feb 2010   This trick is more theatrical than useful, but it is still impressive. When beer is poured into a glass from a bottle, the glass is usually titled so that the beer hits its clear interior wall so that it flows gently down into the beer already in the glass. The idea is to avoid any splashing, which releases the carbon dioxide (or, in the case of Guinness, the nitrogen) that is dissolved in the liquid. Any such release produces bubbles of the gas, which form the “head” on the beer. Many beer drinkers prefer a slower release of the gas to keep the beer from going “flat,” which dulls the taste.

Here is another approach in reducing the head but it maximizes the splashing. With the glass upright, invert the bottle so that the beer gushes from it into the glass. The beer emerges in pulses that are separated by intervals in which air rushes into the bottle. The process is called glug-glug because of the sound the bottle makes. Now here is the theatrical part: As the foam develops on the top of the beer already in the glass, keep the open end of the bottle within the foam.


When air is pulled up into the bottle, it captures some of the foam and carries it up into the bottle. Thus, less foam is left in the glass.

When all the liquid has flowed out of the bottle, turn the bottle right side up and then either rock the bottle side to side or roll the bottle between your hands. Either action causes the liquid in the walls of the bubbles to drain, thus thinning the walls until the bubbles collapse. After a few seconds of this motion, most of the bubbles have collapsed and their liquid is then ready to pour from the bottle. Here are three videos showing the technique. The third one is performed by either a robot or a controlled mechanical arm, showing that even a machine enjoys good theater when pouring a beer.

2.125 Pub trick --- lifting rice with a rod
Jearl Walker
Dec 2010 The challenge this month at first seems impossible. You are at a wedding reception being held in a pub (yes, a pub) and have an open container of uncooked rice to throw on the bride and groom (as is the custom in America and perhaps elsewhere). Because the bride and groom are slow to arrive at the pub, someone challenges you: Can you pick up the container by using only a common wood dowel (round rod)? You are otherwise not allowed to touch the container.

Here is a video in which I show the pub trick to Jay Ingram, the host of The Daily Planet television show on Discovery Channel Canada. Click on the November 30 video on the Facebook site for The Flying Circus of Physics.

The secret is to drive the rod into the rice. Significant force is required as the rod moves deeper into the rice. Once the rod is in place, tap or gently shake the container for a few minutes. When you then pull upward on the exposed end of the rod, you can pick up the entire container of rice.

Anyone can do this pub trick, however, the real challenge is to explain it.

As I point in The Flying Circus of Physics book, this effect was first noticed long ago when merchants and customers would thrust poles into bags of grain to check the contents. As a rod is pushed into rice, for example, the friction on the rod increases for two reasons. (1) More grains press against the rod. (2) The deeper grains, which support the weight of higher grains, press harder. Thus, the total friction resisting the rod’s motion increases with the rod’s depth.

The friction increases even faster as the rod nears the bottom of the container. The effect is not well understood, but a reasonable guess is that great resistance to rearrangement of the grains comes from grain arches: Grains become locked into arches that resist the rod’s motion. (Just as architectural arches can be strong, so can grain arches.)

When the container is tapped or shaken after the rod is inserted, the rice becomes closely packed. In particular, it is closely packed against the rod. When you pull up on the rod, the friction between the grains and the rod holds the rod tightly. Also, grains surrounding the rod are closely packed and hold tightly onto one another. And the grains next to the container wall hold tightly onto the wall. So, everything—rod, grains, and container—is locked together. However, if the rod or container is very slippery or if the rice is not tapped down in place, you will have a lot of rice to pick up from the floor.

If you were to pull hard enough to extract the rod in a slow, controlled way, you would probably find that the force on it from the rice varies in a periodic way. Although this variation is not well understood, it is probably due to the formation and collapse of arches near the rod.

Dots · through ··· indicate level of difficulty
Journal reference style: author, title, journal, volume, pages (date)
Book reference style: author, title, publisher, date, pages
· Reynolds, O., “Experiments showing dilatancy, a property of granular material, possibly connected with gravitation,” in Papers on Mechanical and Physical Subjects, Vol. II, Cambridge University Press, pages 217-227
··· Cowin, S. C., and L. E. Trent, “Force of extraction for a cylinder buried in sand,” Journal of Applied Mechanics, Transactions of the ASME, 47, 969-971 (December 1980)
·· Horvath, V. K., I. M. Janosi, and P. J. Vella, “Anomalous density dependence of static friction in sand,” Physical Review E, 54, No. 2, 2005-2009 (1996)
· Stone, M. B., D. P. Bernstein, R. Barry, M. D. Pelc, Y.-K. Tsui, and P. Schiffer, “Getting to the bottom of a granular medium: A surprising resistance would be put up by sand grains hiding a buried treasure chest,” Nature, 427, (5 February 2004)
··· Arroyo-Cetto, D., G. Pulos, R. Zenit, and M. A. Jimenez-Zapata, “Compaction force in a confined granular column,” Physical Review E, 68, article # 051301 (4 pages) (2003)Energy is x
2.140  Stress dip below a sandpile
Jearl Walker
March 2007   What is something you really enjoyed as a child but now find utterly boring? Among many possible answers is playing with sand. Building sandpiles was loads of fun years ago but not now. However, many physicists are still fascinated by sandpiles and have managed to build their careers on them. The reason is that a sandpile is a treasure trove of puzzles that figure into the general science of granular flow (that is the flow of grains of anything from powder to apples, in environments from the cosmetic industry to the dunes on a desert). Here is one puzzle.
    If you measure the stress (or pressure) under a sandpile from the outer edge to the center, the stress should be greatest under the highest point, where the supporting surface must support the most weight. However, measurements show that although the stress increases toward the center of the pile, it actually decreases in the region of the highest point. That decrease is called the stress dip, and it just should not be there.
    As explained in The Flying Circus of Physics, the stress dip is most likely due to the formation of arcs of sand grains that are produced when the sand is poured to make the pile. Such arcing creates force chains, which are lines of support among the grains that form a skeletal structure hidden from view within the pile. This generation of force chains shifts support away from the center of the pile.
     Recently, I. Zuriguel and T. Mullin of the University of Manchester and J. M. Rotter of the University of Edinburgh recently described a method to make the force chains visible in a two-dimensional granular pile. The “grains” are cut from a photoelastic polymer layer that is birefringent. That is, if the layer is placed between polarizing sheets, the stress in the polymer material shows up as a pattern.
    A narrow rectangular container was made to hold the grains. The front and back were made of Perspex plates that were held 7 millimeters apart. The grains were carefully poured into the container. A polarizing sheet was mounted on the rear Perspex and a second sheet was mounted on the front Perspex, with the polarizing directions of the sheets perpendicular to each other. When light is sent through the apparatus, the light becomes polarized by the first sheet and should not be passed by the second sheet because of the perpendicular arrangement. However, the stressed regions in the polymer grains rotate the polarization of the light, so that in some regions the light gets through the second polarizing sheet, revealing where the grains are stressed.

    When the polymer grains are a mixture of circular grains with two different diameters, the grain pile has a slight stress dip under the highest section. However, when the grains are elliptical and identical, the pile has a dramatic stress dip under the highest section. Presumably, the elliptical shape helps the grains form the supporting arcs that shifts support away from the center.

· · ·  Metzger, P. T., and C. M. Donahue, “Elegance of disordered granular packings: A validation of Edward’s hypothesis,” Physical Review Letters, 94, article #148001 (4 pages) (15 April 2005)
· · Atman, A. P. F., P. Brunet, J. Geng, G. REydellet, G. Combe, P. Claudin, R. P. Behringer, and E. Clement, “Sensitivity of the stress response function to packing preparation,” Journal of Physics: Condensed Matter, 17, S2391-S2403 (2005)
· · ·  Atman, A. P. F., P. Brunet, J. Geng, G. Reydellet, P. Claudin, R. P. Behringer, and E. Clement, “From the stress response function (back) to the sand pile ‘dip’,” European Physical Journal E, 17, 93-100 (2005)
· · ·  Krimer, D. O., M. Pfitzner, K. Brauer, Y. Jiang, and M. Liu, “Granular elasticity: general considerations and the stress dip in sand piles,” Physics Review E, 74, article # 061310 (10 pages) (2006)
· · ·  Zuriguel, I., T. Mullin, and J. M. Rotter, “Effect of particle shape on the stress dip under a sandpile,” Physical Review Letters, 98, article # 028001 (12 January 2007)
· · ·  Zuriguel, I., and T. Mullin, “The role of particle shape on the stress distribution in a sandpile,” Proceedings of the Royal Society A, 464, 99-116 (2008)

Want more references? Use the link at the top of this page.

2.141 Chladni singing

Jearl Walker

June 2011 Patterns called Chladni figures are formed by sand on a horizontal metal plate when it is made to oscillate more or less continuously. In the common classroom demonstration, the oscillations are set up by a bow drawn across one edge or by placing the plate on an upright speaker cone driven by an oscillator. However, Meara O’Reilly produces Chladni figures by singing. In the following video, you see her sprinkle salt on a plate and then sing into a microphone.

The amplified signal is then fed to the plate, causing the plate to oscillate. From O’Reilly’s website

here is a photo of the plate when she sings a C note.

At some points (called antinodes), the metal plate oscillates most, and at some points (called nodes) the plate does not oscillate at all. The antinodes can be adjacent and form lines across the plate; the nodes also can be adjacent and form their own lines. Any sand grains initially located at an antinode are thrown into the air, away from the lines, and tend to collect in the node lines. Once collected, the sand grains reveal the node lines and form one of the Chladni figures.

Here are a few more video links and some journal references to Chladni pattern physics. To find many more, go to

and scroll down to item 2.141 Powder patterns. more Chladni singing, Yerevan Waldorf school, 7th grade. Chladni patterns on large vibrating plate Chladni pattern on top plate of violin Chladni pattern reveals the oscillations on a plate at various frequencies Chladni patterns on plate that is bowed

Chladni patterns on a drum oscillated at different rates Chladni patterns Subwoofers in a car Chladni patterns part 1 Chladni patterns part 2 Chladni patterns part 3

Chladni patterns part 4


2.144  Ball floating in an air stream
Jearl Walker
July 2007
In the Flying Circus of Physics book, I describe a fairly common physics demonstration, one that you might see in science museums: A ball is placed in an upward stream of air, where it is seemingly trapped in spite of the appreciable jostling. Whenever it starts to leave the stream in any given direction, the air flow pulls it back to the center of the stream. Here is an example of the demonstration:
Explanations for the stability sometimes involve the Bernoulli principle, a rule about how the air pressure in a stream changes when the speed changes. Even I have used that explanation. However, the truth is that I have never understood how that argument could possibly work with the ball. (Indeed, I suspect that whenever I have faced a fluid flow effect that I did not understand, I probably bluffed my way by saying, “Oh, the effect must be due to the Bernoulli principle.”)

A much clearer explanation is that when the ball begins to leave the stream toward, say, the left, the stream then flows along only the right side. There the stream comes around the curved surface and then leftward across the top of the ball, where at some point it breaks free of the ball. Thus, the stream no longer leaves the ball by moving upward. It now leaves the ball by moving leftward. That is, the curvature of the ball directs the air stream to the left, effectively pulling the stream to the left. As a result the ball is pulled to the right. (You can think of the pull to the right as being the reaction to the action on the air stream, if you like that old-fashioned wording of Newton’s third law of motion). Thus the ball is brought back to the center position. Of course, you can hit the ball hard enough to knock it from the stream, but otherwise the ball is trapped.

Recently Paul Gluck of the Israel Academy of Science and Arts in Jerusalem described a more arresting arrangement. He places a golf ball in the lower part of the stream and then places three or even four balloons higher in the stream, separated from one another. He cautions that the balloons must be spherical because any projection will catch the air stream and ruin the stability. He inflates a spherical balloon, ties off the stem, cuts off the portion sticking outward from the knot, pushes the knot into the interior of the balloon, and the covers the spot with tape.

Floating a ball in an air stream seems like magic to anyone seeing the demonstration for the first time. Just think how surprising four floating balls will be. Hey, come to think of it, armadillos roll up into balls. I wonder …. Video of golf ball floating in air stream  beach ball beach ball  humorous ball in water stream ping pong ball Pop bottle floating Ball held in a water stream The ghost juggler two balls classroom

· Gluck, P., “A delicate balance: hovering balloons in an air stream,” Physics Teacher, 44, 574-575 (December 2006)

Want more references? Use the link at the top of this page. And then scroll down to item 2.144.

2.150 Pub trick --- collecting grains of black pepper
Jearl Walker
Oct 2011 This is a common classroom demonstration but also works in a pub, especially if the pub customers have spent their adult lives in the pub, avoiding any science classroom. Partially fill a bowl with clean water and then sprinkle black pepper grains over the water. The grains stay wherever they randomly land. Dip your finger into the water. Except for the very spot where your finger enters, the grains ignore your finger. Is there any way you can get the grains to vigorously move away from your finger, collecting near the edge of the water surface?

Here are two videos that demonstrate the trick.

As the videos properly explain, the trick is to coat your finger with a bit of liquid soap. You could do this out of sight of the pub customers if you have a small vial of liquid soap. When the soap molecules reach the water in the bowl, the molecules spread out over the water surface. Each molecule has an end that is attracted to water molecules (hydrophilic) and an opposite end that is not attracted to water molecules (hydrophobic). Thus, the soap molecules arrange themselves so that hydrophilic ends are downward.

In clean water, adjacent water molecules strongly attract one another. We refer to that mutual attraction as being a surface tension. Thus, clean water has a large surface tension. The molecules near the edge of the water pool pull on the molecules slightly farther from the edge, which pull on the molecules even farther, and so. The whole surface is under tension. Any one of the surface molecules is pulled in all directions equally and thus is stationary.

When soap molecules spread across the surface, they separate otherwise adjacent water molecules, and then there is no longer an attraction between those molecules. As a result, the surface tension of the soapy water is less than that of the clean water.

The soap molecules spread from the point where your finger enters the water. That means that the surface tension in that region is suddenly decreased. The water farther from your finger is still clean and still has a large surface tension. Any water molecule at the border between the clean water and the soapy water is then pulled strongly toward the clean water and more weakly toward the soapy water. The stronger pull wins and thus the border water molecules move away from the soapy region. As that water moves, the floating pepper grains are carried along.

When I clean my electric shaver, I see something similar. To clean the head, I brush out the hundreds of small lengths of hair (the clippings) that have collected in the head. These clippings fall into the water in the toilet, but instead of staying where they land, they immediately move away. However, here a different effect governs the motion. The clippings are naturally oily. When they land in the water, the oil spreads over the water surface (the oil is lighter than the water and thus floats). This spreading oil carries the clippings away from the landing site. This probably should not be considered as a pub trick unless you are male and just happen to shaving in the pub. (That sounds a bit sad, don’t you think?)

Dots · through ··· indicate level of difficulty
Journal reference style: author, journal, volume, pages (date)
· Edge, R. D., “Surface tension,” in “String & Sticky Tape Experiments,” Physics Teacher, 26, No. 9, 586-587 (December 1988)


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