Closing tabs

I can scarcely believe I just wasted an entire day reading stuff. "It never rains but pours" I guess...

1. Read Wells Tower's brilliant article about traveling with his dad in Iceland and Greenland. There are no satisfactory options re pagination, but the print version is the least bad. I won't excerpt anything because I can't decide what to, and because you really have no excuse for not reading the whole thing. NB Tower's prose is good but too heavy on obvious special effects. "Under a sky the color of..." appears at least twice, and various natural formations are compared to various kinds of candy, only once to possibly good effect:

Spilling from between a pair of russet crags, the dirty tongue of ice had a roasted look about it, like a charred marshmallow, pallid innards oozing forth.

2. Applied broetry: the Facebook terms of service in bro-speak.

3. Marina Warner on Tracey Emin (LRB). A fine lead-in:

Quilts used to be made from baskets of scraps; old clothes were cut up, the worn and stained bits discarded, the best parts kept for reuse. Every household where a woman lived had such a container – a midden of memories – and when the scraps had become a patchwork quilt, spotting this old dress or that old pair of curtains or that old cushion was part of the pleasure of the bed, a domestic pleasure. The quilt became history, the equivalent of an itinerant storyteller’s painted roll.

4. A nice exhibit on Palladio and his influence in Britain. Architecture is a little outside my usual limits but I have always been fond of Pope's epistle to Burlington on architecture. Exhibit includes some useful information about Burlington and his houses.

5. Fascinating article in Nature News about the search for chimpanzee culture:

Some chimps dance slowly at the beginning of rain showers, others don't; some use long sticks to dig up army ants; others use short sticks. In West Africa, some chimp groups hammer nuts with a stone or a piece of wood to open them. But east of the river Nzo-Sassandra, which cuts across Côte d'Ivoire, only one group has been seen cracking nuts. [...] Deciphering culture in the wild is difficult because researchers must ensure that behavioural differences between groups do not have other causes, such as variation in genetics or environmental conditions. "Why is it all chimps don't do everything? One solution is that there are hidden ecological differences between populations," says primatologist Richard Wrangham at Harvard University in Cambridge, Massachusetts. A behaviour could be linked to any number of variables such as amount of rainfall, the types of tree available, or the kinds of predator in the area, he says.

These influences can be subtle, as researchers found while studying how chimps use sticks to harvest army ants. Chimpanzees in Guinea sometimes use short sticks and sometimes use sticks up to twice as long. No reason for this was obvious until Tatyana Humle, an anthropologist at the University of Kent, UK, found that some ants are more aggressive, with longer legs and larger mandibles; they run up sticks quicker and bite harder⁵. This might explain why chimps elsewhere in Africa also choose tools of varying lengths to get at ants.

But researchers have not been able to find obvious explanations for other variations related to ant harvesting. Chimpanzees in Cote d'Ivoire sweep the ants off their sticks and into their palms before eating; in Guinea, only about 320 kilometres away, the animals stick the ant-laden sticks directly into their mouths. The same type of ant is present in both places.

6. Also in this week's Nature, presumably gated, an article about how the coffee-stain effect (i.e., the ring-like shapes of coffee stains, prev. posts here and here) does not exist for ellipsoidal (M&M shaped) colloidal particles [Nature 476, 308 (2011)]. I don't fully follow the argument but the basic idea is that repulsive interactions among the particles keep the solute from moving outward with the fluid.

7. Seventeenth-century drinking habits revisited, at the Awl. (See here for prev.)

Nor need it seem incredible, that common drunkards should drink thus, for they can disgorge themselves at pleasure, by only putting their finger to their throat, and they will vomit, as if they were so many live whales spewing up the ocean; which done, they can drink afresh.

Re spewing whales see also: ambergris, Simon Armitage. 

The fine structure of stains

I blogged last year about the Chicago group's work on why coffee-stains are ring-shaped, with a sharp outer edge fading as one moves in. A quick reminder:

(In other words, the edge of a droplet is stuck where it is; as the droplet evaporates, more and more of the water must move from the center to the edge, so that most of the water gets to the edge before it evaporates, so most of the evaporation and hence the deposition happens at the edge.)

There's a nice new article in PRL today (ungated) that takes a much closer look at the structure of a stain:

(c) is a blow-up [optical microscope] of the red square in (b) and (d) is a blow-up [electron microscope] of the red square in (c). The solute particles at the outermost edge of the stain are arranged in precise crystalline patterns; as you move further in towards the (relatively sparse) middle of the stain, the particles become randomly distributed. The physics of this turns out to be fairly simple, given what's already known about evaporation. To quote the paper:

The [solute] particle velocity increases dramatically in the last moments of the droplet’s life. We refer to this sudden change in speed as ‘‘rush hour.’’ The particles that arrive early, at a low deposition speed, form an ordered (square or hexagonal) structure. In contrast, particles that arrive during rush hour have a high speed and form a jammed, disordered phase.

[NB you could ask why there's a tendency for things to crystallize at all. In this case I think that's just electrostatic repulsion -- particles would like to be as far from each other as possible, i.e. in a crystal, but might not have any way to get there.] The authors also claim to have a theory of why one sees both hexagonal and square crystals in the ordered region [see part (d)] but I don't have the time right now to follow up that paper trail.

Update Here is the Physics blurb about this.

Viscous fingering, venom delivery, and other phenomena

1. Some entertaining PRL titles from Blogger Outage Day:

• Fluid mixing from viscous fingering
• Tears of Venom: Hydrodynamics of Reptilian Envenomation

Abstract of the venom article:

In the majority of venomous snakes, and in many other reptiles, venom is conveyed from the animal’s gland to the prey’s tissue through an open groove on the surface of the teeth and not through a tubular fang. Here we focus on two key aspects of the grooved delivery system: the hydrodynamics of venom as it interacts with the groove geometry, and the efficiency of the tooth-groove-venom complex as the tooth penetrates the prey’s tissue. We show that the surface tension of the venom is the driving force underlying the envenomation dynamics. In so doing, we explain not only the efficacy of the open groove, but also the prevalence of this mechanism among reptiles.

1'. Lovely high-speed video of how hummingbirds drink -- turns out it isn't capillary action after all. (Wired Science via Jeremy)

2. William Barnes was a delatinizer:

He called for the purification of English by removal of Greek, Latin and foreign influences so that it might be better understood by those without a classical education. For example, the word "photograph" (from Greek light+writing) would become "sun-print" (from Saxon). Other terms include "wortlore" (botany), "welkinfire" (meteor) and "nipperlings" (forceps).

3. R.S. Thomas's poem "In Church" is an unusually clear-cut example of the standard use of the linebreak in accentual verse (3 beats to the line here):

These are the hard ribs
Of a body that our prayers have failed
To animate. Shadows advance
From their corners to take possession
Of places light held
For an hour. The bats resume
Their business. The uneasiness of the pews
Ceases. There is no other sound
In the darkness but the sound of a man
Breathing, testing his faith
On emptiness, nailing his questions
One by one to an untenanted cross.

I will have more to say about this anon; I'm just posting the passage now in case I forget how it goes.

"It would be ok as long as the cans weren't stored upside-down."

I'd shared this preprint unread when it popped up on the arxiv but apparently it's good enough for Nature News, which has a good writeup. I particularly enjoyed the caveats.

The widget-free way to foamy stout

Stouts bubble less readily than lagers or other carbonated drinks when poured because they contain dissolved nitrogen as well as the carbon dioxide that drives the fizz. Adding nitrogen makes the beer less acidic, and gives a longer-lasting head with relatively small bubbles that are behind stouts' smooth, creamy 'mouth feel'.
But the addition also demands the use of the widget [a hollow sphere with a hole in it], which takes in gas and beer as it floats in the canned stout and, when the can is opened and the pressure drops, jets it out again through the hole, helping to create the foam.

The new study suggests that the same result could be achieved by coating part of the can's interior with cellulose fibres. [to increase the rate of bubbling, which is intrinsically low for stout: bubbles nucleate faster on rough surfaces]
[...]
However, [some guy] adds, the bubbles might fill with liquid while the can was in storage. Lee says the coating would be placed in the gap at the top of the can: "It would be ok as long as the cans weren't stored upside-down."

Andrew Alexander, a chemical physicist at the University of Edinburgh, UK, also believes that can coatings would be impractical compared with widgets. "Widgets are genius — they're cheap, work really well, are totally non-toxic and don't mess with the beer," he says. "Would a fibrous coating be cheaper than what is essentially a ping-pong ball with a hole in it? I don't think so."
But Lee says that using widgets slows down the process of canning stout. "Oxygen stuck in the widget can affect the beer's flavour, so you have to pump nitrogen in several times to remove it," he says. "The cellulose coating is an alternative worth investigating."
However, it is likely to be some time before fibre-lined stout cans appear on supermarket shelves. "We've spoken to brewers," says Lee, "but we're not sure if they're interested yet."

Fluid mechanics video dump

Adapted from an email I just wrote Kit, who wanted cool physics videos for a high school math class. These will be familiar to many -- from the feed -- but aren't archived anywhere, so it is perhaps worth collecting the links here.

1. Stephen Morris's fluid-mechanical sewing machine (maple syrup -- not really, but he is Canadian -- dropped onto a moving belt):

2. Stephen Morris, "chemical plumes" (Quicktime) and the "washboard road" effect (i.e., the fact that dirt roads go sinusoidal over time when driven on). Should note in passing that talking to David Grier about washboard road was one of the things that sold me on many-body physics as a prospective graduate student.

3. Sid Nagel's splashing-droplet videos (scroll down). The discovery that water doesn't splash at low atmospheric pressure is remarkable and not something I'd ever have expected. The other stuff on Nagel's website is pretty neat as well. (Either Nagel or someone introducing him described his lab as "where theory comes to die.")

4. The phenomenon of self-propelled Leidenfrost droplets moving uphill (U. of Oregon; see my recent post for context; this youtube video is a decent intro to the Leidenfrost effect).

Leidenfrost ratchets

This week's flood of recreational physics continues... Nature Physics has a new paper online (gated version here, no arxiv that I could find) on self-propelling Leidenfrost droplets. The Leidenfrost effect is the fact that a droplet [1] of water floats above a really hot skillet on a cushion of steam (generated, say, by the bottom of the droplet boiling -- see this youtube video). As steam is a bad conductor of heat, the floating droplet takes a long time to heat up and boil. It was discovered a few years ago [Linke et al., PRL 96, 154502 (2006), gated supplementary material has video] that droplets placed on "ratchets" -- i.e., surfaces with sawtooth-shaped serrations -- moved at about 10 cm/s "against the grain" of the serrations.

As the new Nature Physics paper says, various explanations are possible:

First, the base of the drop is deformed by the presence of the ratchet below, which induces a modulation of its curvature and consequent Laplace pressure gradients⁴. Second, a wave propagates from the trailing edge to the leading edge of the drop, making the transport of matter possible in the direction of its motion. Third, a Leidenfrost drop is likely to oscillate spontaneously²⁰; for each elementary rebound, part of the kinetic energy can be transferred from the vertical to the horizontal direction because of the slope of the teeth. Fourth, the Marangoni effect related to temperature differences might cause a displacement, as seen in Marangoni-levitating drops heated asymmetrically using a light source²¹. Fifth, as the drop loses material, this gas flow might provoke a motion provided it is made directional (or rectified) by the presence of the teeth.

The fifth explanation differs from the other four in that it doesn't rely on the droplet being fluid -- all the others depend on the deformability of the droplet. So the authors tested this by repeating the experiment with dry ice. Since dry ice sublimates, it too should levitate on a cushion of gas when dropped onto a "skillet" with a sufficient temperature gradient. But it isn't liquid, so it can't, e.g., "modulate its curvature."

In the event, a piece of dry ice propels itself exactly like a droplet of water; this establishes that gas flow is behind the self-propelling. How does this work?

As the gas moves towards the step, that is towards a sudden contraction in the fluid channel, the flow resistance is higher than in the reverse direction²³. As a consequence, the vapour will mainly escape along the smallest slopes of the texture, which propels the Leidenfrost body in the direction shown in Figs 1 and 2 (jet thrust). This interpretation was confirmed by forcing contact between the hot ratchet and a disk of dry ice, thus printing the tooth pattern on the bottom of the disk, and observing a similar motion with this textured disk on a hot flat solid.

In other words most of the evaporated carbon dioxide moves up the gradual ramps, i.e., "with the grain" of the ratchet, so (by Newton's third law) the levitating solid is pushed in the opposite direction.

In addition to the appeal of all simple phenomena that could have been discovered centuries ago, this work is a neat example of the scientific method in action, and esp. of the value of clever controls. This is an aspect of good scientific practice that doesn't get the attention it deserves; the original Freakonomics book is actually the only piece of popular writing about anything where I've seen it covered in any detail.

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[1] I don't know what the technical difference is between a droplet and a drop. No one ever seems to talk about drops of water in physics.

Recreational physics roundup

I have always enjoyed work on pattern formation on the everyday scale; it is heartening (a word I overuse, perhaps revealingly) to see that there is still so much in front of one's nose that bears closer inspection. (See here and here for previous local coverage.) The past few days have been abnormally rich on this front -- three worthwhile stories! -- and I wanted to blog about them, partly for ease of future reference.

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Bubbles and memory

There's a new story in Phys. Rev. Focus about hysteresis in soap bubbles. (Like most Focus articles it seems to have been written by a journalist. I also find it irritating that they publish Focus articles a while before the paper comes out, because you've typically forgotten all about the work by the time it's published.) Soap bubbles grown on a triangular-prism-shaped frame form dipyramids that either intersect at a triangle -- for a squat prism -- or are joined by a thin vertical strand of soapy water -- for a skinny prism. For a certain range of aspect ratios both solutions are possible, so as you stretch or contract the sides of the prism, the bubble can take either form depending on which way you were tuning the length (i.e., on the bubble's "past"). This is interesting primarily -- from a physics point of view -- as the most purely geometrical example of hysteresis that I know of: the films try to minimize their area, and in this range the two lowest-area configurations look substantially different.

It is also an excuse to replug a beautiful old paper by the Chicago group on a much more nontrivial example of memory in bubbles -- viz. how air bubbles blown through a nozzle "remember" irregularities in the shape of the nozzle.

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"Nonideal icicles"

Stephen Morris at the University of Toronto does a lot of beautiful work on pattern formation in systems that are "fluid" in some sense. (My favorite thing on his website is the fluid mechanical sewing machine, but irritatingly that link is broken.) Morris's group has a new paper out in Phys. Rev. E, featured in Physics, on the growth of icicles. They grew large numbers of icicles in their lab and studied how the quality of water, the wind speed, etc. affect the growth of icicles. A result that jumped out at me: lab-grown icicles often have bifurcated tips as in (c) and (d) of the figure, but this seems to happen most often when the fan in the experiment is turned off. The implication is that wind somehow straightens out icicles.

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Huddles of tetrahedra

This is somewhat older work that Ross McKenzie recently linked to. Here's an NYT piece on the race to find ever-closer packings of regular tetrahedra. I recommend reading the linked Nature article (journal link here, ungated arxiv here), which is pretty accessible. There is some back-story to this: Stanislaw Ulam conjectured a long time ago that it should be possible to pack any kind of convex shape more closely than hard spheres. As far as I know there is no proof of this, but it's plausible and known to be true for lots of shapes including M&M's (another NYT story) and more recently tetrahedra. How tetrahedra pack is of particular interest as there is a theory of glassiness (see an old post here) that depends rather crucially on the fact that you cannot tile three-dimensional space with tetrahedra. The theory is basically that particles in an incipient solid like to clump into tetrahedra (each particle is exactly as far away as it wants to be from every other) but the tetrahedra can't line up, so that on large scales you have a jammed amorphous mess: here is a PRB paper by David Nelson related to this theory. The Nature paper is prima facie a beautiful application of ideas from physics to solve a purely mathematical problem -- a project that's close to the formalist, interdisciplinary lump of matter I sometimes refer to as my heart -- but its really surprising finding is that one of the best ways to put the tetrahedra together is to have them form a quasicrystal. This actually makes it a little surprising that 3D quasicrystals aren't common in nature, unless tetrahedra are a much less central motif than the Kleman-Sadoc-Nelson line of thinking about glasses would suggest.

Quivering blobs, "captured newtons"

This is a remarkably ignorant video; as Flowing Data diplomatically puts it, "I suspect the creators behind the video [G.E.] didn't have a complete understanding behind the math and mechanics." It's not just the obvious howlers -- the nonsensical talk about "total captured newtons," as if forces were additive scalar things you could store -- but the fact that the video seems quite unrelated to its ostensible topic of "dynamic braking." But it's pretty:

For the record, dynamic braking (see Wikipedia) is when the inertial rolling of a train's wheels is used to generate electricity -- analogous to the motion of turbines. [Recall: a motor run backwards is a generator.] Because electrical energy is generated, the kinetic energy of the moving train must decrease and the train must slow down; however, the electrical energy can be stored or dissipated as heat in some other part of the train, whereas a straightforward frictional mechanism would turn most of the original kinetic energy to heat in the wheels and braking system. A mechanical analogy would be a "one-way" bowstring or trampoline that'd stay stretched until you pushed a button to release it... the blobs in these videos aren't anything of the sort, they're merely shock absorbers.