Thursday, June 28, 2012

Want to win £1000?

I have a piece in the Guardian online on the Mpemba effect and the RSC’s £1000 prize for explaining it. The article is largely unchanged from what I wrote, but here it is anyway.

The aim here was to stimulate suggestions from readers of how this thing can be explained – or even if there’s a real effect to be explained, though few seem to question that. (One does so with amusing literalism, thinking it implies that hot water will always freeze first whatever the temperature difference. All the same, this reinforces Charles Knight’s point that the phenomenon is too ill defined.) I like too the cute popular notion of “heat loss momentum” – check out Newton’s cooling law, please.

But I’m certainly not going to mock the many confused or just plain wrong suggestions put forward, since the whole point of the exercise is to get people engaged, not to laugh at their errors. However, I can’t help being struck by the inevitable one or two who say, apparently in all seriousness, that the answer is just obvious and everyone but them has been too stupid so far to see it. One chap dispenses with all of the additional ‘mysteries’ in the article this way too. Why can all arms of a snowflake sometimes be identical? “The symmetry comes from the initial nucleation of the crystal. It starts symmetrically and keeps growing symmetrically. And computer simulations have shown this.” I can only assume he/she (probably he) has seen some simulated flakes and failed to read the warning that symmetry was imposed on all six arms. He certainly didn’t think it worth bothering to check out the link to Ken Libbrecht’s page, which makes it clear that (as I said in the piece) the side-branches are, according to the standard theory of dendritic growth, amplified randomness. So the entire form of any given flake is somehow inherent in its initial nucleus? Please. I couldn’t help smiling too at the apparent belief of some readers that the Brazil-nut effect was actually discovered in muesli (leading to a discussion on how muesli gets packaged). Anyway, the comments thread provides a nice little cross-section of how folk think about science. And, I think, somewhat encouraging at that, despite the misconceptions.


If you can explain, before the end of July, why hot water freezes faster than cold, you could bag £1000. That’s what the Royal Society of Chemistry (RSC) is offering for “the most creative explanation” of this phenomenon, known as the Mpemba effect. They say that submissions should be “eye-catching, arresting and scientifically sound”, and may use any media, including film.

At the end of the month the problem will also be put to an international summer school for postgraduate science students called Hermes 2012, convened at Cumberland Lodge in Windsor Great Park to present research in materials science and imbue the participants with skills in science communication. The event, organized by Imperial College and sponsored by the RSC, is timed to coincide with the opening of the Olympic Games as a kind of scientific Olympiad. A presentation of the top entries to the RSC’s competition, alongside the efforts of the meeting attendees, will form a highlight of the event on 30 July.

All good fun – except that the Mpemba effect seems at first encounter to be scientific nonsense. Let’s have that again: “why hot water freezes faster than cold”. How can that be? In order to freeze, hot water has to lose more heat than cold, so why would that happen faster? Even if the cooling of hot water somehow catches up with that of the colder water, why should it then overtake, if the two have at that point the same temperature?

Yet this effect has been attested since antiquity. Aristotle mentions it, as do two of the fathers of modern science, Francis Bacon and René Descartes in the seventeenth century. The effect is today named after a Tanzanian schoolboy, Erasto Mpemba, who was set the project of making ice cream from milk in the 1960s. The pupils were supposed to boil their milk, let it cool, then put it in the fridge to freeze. But Mpemba worried about losing his space in the fridge, and so put in the milk while it was still hot. It froze faster than the others.

When Mpemba learnt a few years later that this seemed to contradict the theory of heat transfer devised by Isaac Newton, he recalled his experiment and asked his teacher to explain it – only to receive a mocking reply. Undeterred, he carried out his own experiments, and asked a visiting university professor from Dar es Salaam, D. G. Osborne, what was going on. Osborne was more open-minded – he asked his technician to repeat the experiment, and found the same result. In 1969 Osborne published the result in a physics education journal. Coincidentally, that same year a physicist in Canada described the same result, saying that it was already folk wisdom in Canada that a car should be washed with cold water in winter, because hot water froze more quickly.

Yet no one really knows if the Mpemba effect is real. You’d think it should be easy to check, but it isn’t. Ice specialist Charles Knight of the National Center for Atmospheric Research in Boulder, Colorado, says that the claim that “hot water freezes faster than cold” is so ill-defined that it’s virtually meaningless. Does it mean when ice first starts to appear, or when the last bit of water is frozen? Both are hard to observe in any case. And there are so many things you could vary: the amount of water, the shape of the containers, the initial temperature difference, the rate of cooling… Do you use tap water, distilled water, de-aerated water, filtered water? Freezing is notoriously capricious: it can be triggered by tiny scratches on the sides of the flask or suspended dust in the liquid, so it’s almost impossible to make truly identical samples differing only in their starting temperature. For this reason, even two samples starting at the same temperature typically freeze at different times. If such ‘seeding’ sites are excluded, water can be ‘supercooled’ well below freezing point without turning to ice – but here experiments are conflicting. Some find that initially hotter water can be supercooled further, others that it can be supercooled less before it freezes.

There is one trivial explanation for Mpemba’s observations. Hot water would evaporate faster, so if there was no top on the flasks then there could have been less liquid left to freeze – so it would happen faster. Tiny gas bubbles in solution could also act as seeds for ice crystals to form – and hot water holds less dissolved gas than cold.

All this means that a single experiment won’t tell you much – you’ll probably have to do lots, with many different conditions, to figure out what’s important and what isn’t. And you’ve only got a month, so get cracking.

Other mysteries to solve at home:

1. Why do the Brazil nuts gather at the top of the muesli? There’s no complete consensus on the cause of the so-called Brazil nut effect, but current explanations include:
- shaken grains in a tall box circulate like convection currents while the big bits get trapped at the top, excluded from the narrow descending current at the sides
- little landslides in the void that opens up temporarily under a big grain as it is shaken upwards ratchet it ever higher
- it's all to do with the effect of air between the grains

The problem is made harder by the fact that, under some conditions, the big grains can sink to the bottom instead – the ‘reverse Brazil nut effect’.

2. Does the water in a bathtub spiral down the plughole in opposite directions in the Northern and Southern Hemisphere? Cyclones rotate counterclockwise in the north and clockwise in the south, a consequence of the Earth’s rotation called the Coriolis effect. But is the effect too weak to govern a plughole vortex? In 1962 an American engineer named Ascher Shapiro claimed that he consistently observed counterclockwise plughole vortices in his lab, but this result has never been verified. The problem is that it’s really hard to rid a bathtub of water of any residual currents that could bias the outcome.

3. Why are all six arms of a snowflake sometimes (but not always) identical? How does one arm know what the other is doing? The standard theory of snowflake formation explains the ornate branching patterns as amplifications of random bumps on the sides of needle-like ice crystals. But if they’re random, how can one arm look like another? One suggestion is that they listen to one another: acoustic vibrations in the ice crystal set up standing-wave patterns that dictate the shape. But this doesn’t seem to work. Most snowflakes aren’t actually as symmetrical as is often supposed – but the fact that some are is still unexplained.

Thursday, June 21, 2012

Society is a complex matter

It is a book? A booklet? A brochure? Search me, but my, er, tract on social complexity is now published by Springer. This 70-page item was commissioned to explain the case for considering society as a complex system, along the lines envisaged in the FuturICT project (currently one of the contenders for the $1 bn pot offered for the EU’s Flagship initiative), in a way that offers an introduction and primer to folks such as policy-makers. I’d like to think that it amounts to somewhat more than an extended puff piece for FuturICT, although it provides an unabashed summary of that project at the end (written by the project leader Dirk Helbing of ETH) – an initiative whose time has surely come, regardless of whether it will achieve its grand goals. I dearly hope that it is selected for the full Flagship funding some time this year.

In any event, this book(let) could also be seen as a brief, non-comprehensive progress report on the subject broached in my book Critical Mass.

I also have an editorial in a special issue of ChemPhysChem on the subject of nanobubbles. It’s available for free online. I am preparing a feature on this controversial topic for Chemistry World.

Friday, June 15, 2012

Silly names

Here’s my Crucible column for the June issue of Chemistry World. Incidentally, since the magazine chose to illustrate its regular contributors using artwork rather than photos, I have become the man with the narrowest shoulders in chemistry.

This is because I provided them with a photo without shoulders showing, so they drew them in. Andrea Sella, meanwhile, has become an uber-geek –

- which I suspect he would merrily concede anyway. I hope to see him making some bangs and flashes at Cheltenham tomorrow - after this.
In his new novel The Chemistry of Tears, author Peter Carey displays his deep interest in science. The book alludes to the mechanical inventiveness that sustained the Industrial Revolution, and makes much reference to Charles Babbage’s proto-computer the Difference Engine and to modern scientific analysis of historical artifacts. (The chemistry of tears is given a fleeting nod too.) Having met Carey recently, I know how keen he is to get the science correct. So I hope he won’t be too mortified to discover that he has used ‘silicon’ when he clearly meant ‘silicone’.

Did I feel the thrill of superiority at noticing this? On the contrary, I felt cross on Carey’s behalf, for what could be more idiotic and confusing than giving a chemical compound a name that is all but indistinguishable in spelling and sound from a chemical element? This has long topped my list of Bad Chemical Names. It’s not even as though there is much excuse. When Frederick Kipping, an excellent synthetic chemist at the University of Nottingham, began making organosilicon compounds, including polymeric siloxanes, using Grignard reagents around the turn of the century, he was keen to develop an analogy between carbon compounds and what seemed to be their silicon analogues. For that reason he called the long-chain diphenylsiloxane Ph2SiO that he made in 1901 a ‘silicone’ because its molecular formula was analogous to that of the ketone benzophenone Ph2CO. Yet it was already clear that the silicon compound was polymeric and that there was no chemical analogy to ketones [1]. Now we’re lumbered with Kipping’s confounding name.

It’s not hard to find other examples, and drugs are some of the worst offenders. This, however, is a tough one. There are so many to give names to, and you don’t want them to be totally arbitrary. Yet do they have to be quite so un-euphonious? I don’t hold out any prospect that my wife will ever pronounce ibuprofen properly, and the only reason I do so is that I have acquired the chemist’s pedantry for complex nomenclature. I know that the chemical reasoning here is sounder: it is a contraction of isobutyl propanoic phenolic acid. But it makes you wonder whether some people still get their analgesia from aspirin simply because they can pronounce it at the pharmacist’s.

The same goes for countless other generic drug names: clopidogrel, lansoprazole, bevacizumab, venlafaxine. The brand names are hardly masterful neologisms, sounding more like characters in a bad science fantasy novel, but at least they tend to roll off the tongue: Crestor, Effexor, Valtrex. Of course, the IUPAC names of these compounds are far worse for the lay person, but they are never intended for lay use, and they are tightly constrained by their information content. For generic names, we do have some choice.

These aren’t totally arbitrary, however – there is some method in the madness. A drug’s generic (or ‘international nonproprietary’) name is ultimately fixed by the World Health Organization, but many are first prescribed by the US Adopted Names (USAN) Council, which has its own guidelines. In particular, the stem of the name denotes the effect or mode of action: -axine drugs are antidepressants that inhibit uptake of particular neurotransmitters, -vir means an antiviral, -mab is a monoclonal antibody, and so on. Some prefixes and other syllables have particular meanings – lev- and dex- are clearly stereoisomers, for example – but most don’t. USAN aims to avoid names that non-English speakers will find hard to pronounce, as well as ones that turn out to have obscene connotations in other languages, so there are no equivalents of the wonderful arsole.

Despite this logic, is it really just familiarity that separates codeine and paracetamol from atorvastatin and montelukast? Faced with these agglomerations of semi-arbitrary syllables, one has to wonder if half a system is worse than no system at all. The proliferation of brand names only makes matters worse; I had no idea until I looked that taxol is also known as onxol, abraxane and cryoxet. I shouldn’t even call it taxol, of course, since Bristol-Myers Squibb got very upset when its brand name was used instead of the generic paclitaxel. While plenty of brand names have become common names through familiarity –(think of Kevlar), this was the reverse: taxol was widely used by chemists before Bristol-Myers Squibb registered it as a trademark, provoking dismay at their appropriation of established usage [2].

Whatever the legal issues, the fact is that taxol works as a name: anyone can say it without effort and not confuse it with something else. With graphene, fullerenes and dendrimers, scientists have shown that they can sometimes master the trick of balancing euphony, descriptiveness and specificity in chemical naming. But there’s still cause for entreating that, before you christen your compound, think how it will sound in Boots.

1. K. L. Mittal & A. Pizzi (eds). Handbook of Sealant Technology, p. 13. CRC Press, Bocan Raton, 2009.
2. N. White & S. Cohen, Nature 375, 432 (1995).

Wednesday, June 13, 2012

New website articles

I've just put two new pdfs on my website: extended versions of a piece on Turing patterns published in the June issue of Chemistry World, and of a piece on animal photonics and physical coloration published in the May issue of Scientific American. Both have considerably more information than is in the published versions, because you're worth it. You'llfind them in the Selection of Articles, under the sections "Patterns" and "Other". And I have an article on "radiofrequency biology" in the 9 June issue of New Scientist, which I fear is not available online without subscription. What, you want that too? OK, give me a moment.

Tuesday, June 12, 2012

Green fireworks

An edited version of this piece has just gone up on the BBC Future site.


Some environmental activists have been called killjoys for seeking to ban firework displays. They are concerned that, as one campaigner put it, “fireworks shows spray out a toxic concoction that rains down quietly into lakes, rivers and bays.” But there may be a solution that doesn’t spoil the fun: green fireworks. A team of scientists at the US Army’s Pyrotechnics Technology and Prototyping Division at Picatinny Arsenal in New Jersey, USA, has found more eco-friendly replacements for one of the troublesome chemical components of fireworks, the so-called oxidizer that sets off the explosion.

As you might imagine, the researchers, led by Jared Moretti and Jesse Sabatini, are concerned less with the civilian pyrotechnics unleashed on 4 July in the US, 5 November in the UK, or at every conceivable opportunity in China, and more with military applications such as battlefield flares, which tend to use similar chemical formulations. But Moretti says that their new formulations also “have tremendous potential for civilian fireworks applications.”

Oxidizers are chemical compounds rich in oxygen, which they can relinquish to set the mixture burning. The most common types are nitrates and chlorates or perchlorates. Potassium nitrate is the ‘saltpetre’ used in old recipes for gunpowder, while sodium chlorate is a herbicide notorious for its use in homemade ‘sugar/weed-killer’ bombs. Many civilian and military pyrotechnic devices now use either potassium perchlorate or barium nitrate as the oxidizer. Both of these have environmental drawbacks. The US Environmental Protection Agency (EPA) are scrutinizing the use of perchlorate because it can substitute for iodide in the thyroid gland, disrupting the production of hormones. It can also cause growth abnormalities in embryos. The strict limits placed on perchlorate levels in drinking water by the EPA has hampered military training in the US and threatens to cause problems for civilian firework displays too.

Barium is a health hazard too: it can interfere with heart function and cause constriction of the air passages in breathing. Aside from flares, both potassium perchlorate and barium nitrate are currently used by the US Army in an incendiary mixture called IM-28, which is added to armour-piercing bullets so that the impact creates a bright flash that marks the impact point. Finding a replacement incendiary oxidizer for this application was the immediate motivation for the research by Moretti and colleagues.

Among the alternatives considered already are nitrates that don’t contain barium, in particular sodium nitrate. However, that – as well as another candidate, strontium nitrate – has a different problem: it readily absorbs water vapour from the atmosphere (that is, it is hygroscopic), because the compound is quite soluble in water. This means that the substance is liable to become damp if the pyrotechnic device is stored for a long time, and so it won’t ignite.

Moretti and colleagues have now identified alternatives that don’t seem to have any of the health risks of current oxidizers nor suffers from moisture-sensitivity. This isn’t just a question of finding another compound that will cause ignition. It also has to produce a bright flash, ideally of white light (different metals, in particular, tend to generate different colours), and should not be so exotic as to be unaffordable. It had also better not be set off too easily: one doesn’t want flares and fireworks detonating in the box if they get too warm.

The researchers find that sodium and potassium periodate (pronounced “per-eye-oh-date”) seem to fulfil all these requirements. These are analogous to perchlorates, with the chlorine atoms replaced with iodine. That’s a crucial difference from the point of view of thyroid toxicity. It seems likely that perchlorate ions can nudge out iodide ions in the thyroid because they have a similar size. But periodate ions are considerably too big to substitute for iodide in the same manner.

Yet isn’t it a bit odd to talk at all of ‘green’ military technology – stuff that is used in combat, perhaps lethally, but doesn’t harm the environment? The apparent irony is not lost on the researchers engaged in such work. But it’s hardly cynical to say that, since armed conflicts do occur whether you like it nor not, one would rather not pollute the environment afterwards for civilians.

With that in mind, making military armaments greener has become a significant concern. The US Department of Defense issued a ‘statement of need’ last October calling for research proposals for ‘environmentally advantaged submunitions’ – basically, ‘green’ explosives. For example, the ‘primer’ that sets off the bullet-propelling explosive in small arms typically contains lead, which lingers in firing ranges and accumulates alarmingly in the blood of trainee soldiers and police officers.

High explosives are problematic too. TNT is a carcinogen, although rarely used now in military applications, while the most common alternatives, compounds called HMX and RDX, can cause neurological and reproductive problems. In 1984 a child was hospitalized with epileptic seizures after chewing on a piece of RDX plastic explosive stuck to the clothes of its mother, a munitions worker (and you thought your parenting was irresponsible?). The army is worried about how much of this stuff is left lying around ranges and battlegrounds in unexploded dud shells, which constitute 3-4% of those supplied to troops. Hundreds of thousands of duds were dropped as cluster bombs in the 1991 Gulf War, for example.

The new green incendiary oxidizers represent another facet of this general trend – and they have the added appeal of benefitting peaceful pyrotechnics too.

Reference: J. D. Moretti, J. J. Sabatini & G. Chen, Angewandte Chemie advanced online publication, doi: 10.1002/anie.201202589.