Tuesday, September 25, 2007

Farewell from alpinekat

Well folks, this is it. I am leaving the APS for a temporary position at CERN, living among the mountains whose name I share. The alpinekat will become an endangered species on the Physics Buzz, but not quite extinct as I may make an occasional post about the goings-on with the new Large Hadron Collider.

In any case, thanks for reading (both of you).

Over and out!

Image credit: flickr's peuplier
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Thursday, September 20, 2007

A Knotty Problem: Curly Hair vs. Straight

My sisters always wanted curly hair, but they were cursed with fine, straight blond hair from the Scandinavian part of our ancestry.

But I've known lots of people with curly hair who wished their hair was straight. Overall, I assumed it was just one of those things where the grass is always greener on the other side.

Well it seems I was wrong. Apparently the grass is truly greener -- on the curly side of the fence (no matter what you curly-haired folks say).

According to research published in the August 2007 American Journal of Physics, curly hair is less likely to get tangled than straight hair.

Jean-Baptiste Masson of the Laboratory for Optics and Biosciences at the Ecole Polytechnique in France developed a theory to describe hair tangling and confirmed his results by asking French hairdressers to count tangles in different types of hair that they encountered at work. (I wonder how he managed to track down enough hairdressers in Paris . . . oh, wait a minute, never mind.)

Masson hopes his hair research will help him as he develops hairy products related to velcro. I'm just glad, after all these years of complaining about their hair, my sisters have finally been proven right (speaking as a big brother now, I believe it may be the first time they've been right about anything). As they always claimed -- Curly hair is better, at least as far as tangles are concerned.

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Friday, September 14, 2007

Pancakes with your nuclei?

A nucleus of protons and neutrons.
Luckily, the liquid drop model of atomic nuclei is not specific about the liquid represented. New research to be published in Physical Review Letters shows that while the protons and neutrons in a nucleus are ordinarily like a drop of water, they're more like syrup if you give them an energy boost.

The liquid drop model basically says that the protons and neutrons in a nucleus are like the particles in a drop of liquid. The particles at the edge of a droplet are subject to different forces than those on the inside, namely surface tension. This attractive force pulls nuclei and water droplets alike into spherical shapes.
Water droplets, held together by surface tension.

Viscosity is a measure of a liquid's resistance to flow. To bastardize a proverb, blood is more viscous than water. Luckily, the physicists who discovered the increasing viscosity of nucleons (protons and neutrons) chose a more pleasant viscous fluid.

Measuring the viscosity of something you can't even see is pretty tricky. The physicists, located at Oak Ridge National Laboratory, managed it by accelerating sulfur, titanium, and nickel at different speeds and slamming them into a thin layer of tungsten. The tungsten nuclei are about three times as massive as the bombarding nickel and nearly six times as massive as the sulfur nuclei.
The two added nuclei collide to make the sweaty misshapen nucleus in the center (they really are misshapen but maybe not so sweaty). Unlike the diagram, the whole nucleus splits into fission products (after the sweaty state).

When nucleus form the beam gets into a head-on collision with a target nucleus, the result is a compound nucleus with a lot of extra energy. There are a couple of ways of getting rid of that energy. One is the ejection of particles, namely neutrons in this case. The other is to emit gamma rays. The last is to fission, or split into two separate nuclei.

Waffles, fruit, and syrup. Mmmm.
Many of the nuclei did succumb to fission after a mere two "attoseconds" or 2 × 10-18 seconds, or a two billionths of a billionth of a second. But this was actually a little slow. Considering the particles and gamma rays spat out by the nuclei, the researchers calculated how fast the nuclei should have fissioned. The answer? Closer to 10-20 seconds, about a hundred times faster.

It's enough to make a physicist wonder what the hold-up is. The team asserts that the nuclei were moving like molasses because the extra energy makes the nucleons act like a more viscous fluid. Voila, a syrup-drop model for energized nuclei.
Anderson, J.U. et al. "Crystal Blocking Measurements of the Time Delay of Fission..." Physical Review Letters (forthcoming).

image credits:

Nucleus -- Cubic Awareness online
Droplets -- flickr's Blue Cubic Electron Syncrony
Compound nucleus -- Lawrence Berkeley Nat'l Lab image archives
Waffle and syrup -- flickr's kwei

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Physics (and firehoses) Levitates Car

OK, this is ridiculously cool. Click the video to see for yourself.

I liked it so much that I have nothing to say other than "Thank you, Isaac Newton."
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Tuesday, September 11, 2007

Scientists Study Structures in Steamy Springs

Say that title five times fast, I dare you. But seriously, folks, a pair of physicists from the University of Illinois recently published research concerning the formation of limestone structures in the thermal springs of Yellowstone National Park. While their discussion also touched on the terraces, I'm sticking to domes and stalactites. If you want to know more, the article will be published soon in Physical Review E.
Making Travertine

The particular type of limestone is a porous calcite called travertine. Water seeps downward through the soil, eventually coming into contact with hot gasses rising from a magma chamber. These gasses heat the water and infuse it with carbon dioxide (CO2). Combining with water (H2O), carbonic acid forms (H2CO3).

Rising up through limestone layers, the weakly acidic water dissolves the limestone and gathers calcium ions. The carbonic acid loses a positive hydrogen ion and begins to react with the calcium in the water, forming calcium carbonate (CaCO3), water, and carbon dioxide. When the hot water flows through a vent, it begins to release carbon dioxide, and the calcium carbonate forms a precipitate (falls out of the water), depositing itself on convenient surfaces.

Stalactites occur in caves, where water stuck to the ceiling through adhesive properties follows a slope to find a low point. There it deposits travertine as it waits for more water to collect until a droplet large enough to break the surface tension forms and falls. At Yellowstone, stalactites can grow as fast as an inch per year (fast for stalactites, but much slower than the domes and terraces).

Domes can form around the vents that spill hot spring water where the surrounding ground is flat. A thin layer of water deposits circular layers of travertine, building up into a dome shape. Near the edges, the water becomes too thin and begins to form rivulets, resulting in the fluting visible in the sides of the domes pictured. They grow as much as 1-5 mm a day.

Modeling Domes

Past research has focused on the small processes involving microbes and and crystal lattices. The work of Pak Yuen Chan and Nigel Goldenfeld tries to take in whole structures, investigating domes and stalactites macroscopically.

There are two ways to mathematically describe a system. One is a top-down method in which the scientists examine the system, gather data, and use statistics to make an equation that fits the data. The other is a ground-up method in which scientists use the equations that describe the processes that work together in the system. Then they figure out how to merge these processes into one big equation that describes the whole system.

The researchers had used the top-down method in the past and successfully described the building of domes and stalactites. Now, trying the ground-up method, the physicists did not find the fluting that occurs on the edges of domes. More problematic, they found that domes should be "unstable" structures, meaning that they should not arise spontaneously, and thus we should not find them in nature. Considering that the researchers probably wouldn't be studying the domes if they weren't found in nature, this is rather a serious problem. Why the wrong result?

Approximation. If physicists included every little detail of a system, the equations would be too large and unwieldy. Instead, they simplify or get rid of the factors that don't matter as much or would complicate the equation beyond usefulness. One of the factors that the scientists threw out, in calculating the formation of domes and stalactites, is surface tension. Unfortunately, surface tension turned out to be critical in the formation of the domes.

More Success with Stalactites

Making the same approximation in trying to describe the growth of stalactites, they found that stalactites should indeed arise naturally. Surface tension only affects the speed at which the stalactites build. Surface tension obviously doesn't affect domes and stalactites equally.

In domes, the spring water spreads out over a growing area. The layer of water becomes thinner and thinner until forces between the water molecules pull it into tiny rivulets. These rivulets cause the fluting on the edges of the domes. As you may see in the image, the "analytical" or ground-up model matches the actual dome up until the fluting occurs.

As water flows down stalactites, it covers a smaller and smaller surface. Since the layer grows thicker rather than thinning out, the water doesn't separate into rivulets. In fact, surface tension only matters at the tip where droplets of water accumulate and fall.

The moral of the story is, surface tension plays a critical role in the formation of the domes but not stalactites.

Image Credits:

Mammoth Springs -- Wendy Seltzer, flickr's wseltzer
Stalactites -- Tom Hodgkinson, flickr's hodgers
Dome images -- the researchers, Pak Yuen Chan and Nigel Goldenfeld

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Tuesday, September 04, 2007

The Art of Sliding

I say the "art" of sliding because I don't understand all the physics that goes into it, but here's a look at getting more speed on slides both wet and dry. If you understand more of the physics, feel free to enlighten us. I've been bothering my superiors with these questions all morning.

Last weekend, Greenbelt, MD held its annual Labor Day festival which was an interesting slice of small-town Americana in the midst of the sprawling DC suburbs. Still unable to resist the lure of the rides at the age of twenty-something, I acquired a wristband and rode. When some ten-year-old kid zipped past me on the Giant Slide, the manager at the bottom handing out the burlap bags advised me to lift myself onto my hands and heels, getting my rear off the bag. Lo and behold, it worked.

Why? Beginning mechanics courses teach us that friction doesn't depend on surface area, so shrinking my points of contact shouldn't make me go any faster. However, they always note that this is just an approximation. The folks here with more advanced knowledge than I couldn't offer a detailed theoretical explanation, but if you can, please share!

However, the change in contact area may be something of a red herring. Notice the image of the giant slide, particularly the brown spots where the slide has been worn down. Note also that these places are in the center, where my bum would slide, while the surface of the edges is still intact. The slide in Greenbelt was similarly worn, and I suspect that by lifting onto my hands and heels, I was keeping my weight off of the rough patches.

Buzz Skyline offers an alternative theory.

This is just a guess, but it may have something to do with the structure of the burlap. When you exert more pressure on a small area, the fibers probably deform more than they would if your weight is spread out. If you push gently on burlap, you can feel the prickly fibers sticking up. If you push hard, they bend over and the texture changes a little. In effect, it's like sliding on a different type of material depending on how much force you're exerting per square centimeter.

Now let's rewind to when I was about ten, atop a water slide at Illinois' Magic Waters with my father. Like any good dad, he offered advice about the best way to ride the slide -- on my heels and shoulder blades. Here we are again, reducing surface area! But it works, as many a water slide enthusiast will attest.

So, why? Well, according to a group of thrill-seekers trading tips on water sliding in a discussion thread, the reduction of contact area is unrelated to why this works. DerMorgan attests that his fastest ride was in Germany where baring one's rear is less taboo:
there were all these kids in front of me pulling down the back of their suits as they sat down on the slide, and i was like "what the fuh?". so, i asked one and he explained why. naturally, i tried it, and it's fast!

So in the case of water slides, the important point of the heels-and-shoulder-blades tactic is that it keeps the bathing suit off the slide. That said, remember that testing this hypothesis in the US could result in arrest for indecent exposure.

Unfortunately, I couldn't find hard evidence on either type of slide. If anyone treats these problems empirically or theoretically and publishes, drop me a line and I'll be sure to write about it ;-)

Image credits:
Giant Slide -- Athena Workman
Water Slide -- Brian "playhockeyeh"

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