Fighting the frizz
My hair is slightly wavy. If I go swimming and it gets sodden, it becomes completely straight. But if I go out in fine, drizzly rain it forms tight curls. Why do the different types of wetness produce two very different styles?
鈥� The protein in your hair keeps its shape partly through hydrogen bonding. When you wet your hair these bonds break as water comes into contact with them. They then re-form when your hair dries.
When all of your hair is wet from swimming, then pretty much all of the hydrogen bonds will break. The weight of the water pulls the hair down and the bonds re-form in that position as your hair dries, leaving it straighter.
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When it rains, however, the water usually doesn鈥檛 come into contact with all of your hair, and so the hydrogen bonds break on only some parts of the hair, causing strands to curl slightly. Add to this the fact that your hair isn鈥檛 pulled downwards by the weight of the water, and this is why it is frizzy.
John Samuel
Shropshire, UK
鈥� I was a hairdresser for 35 years. From the fact that you say that your hair is slightly wavy, but it goes into tight curls if you go out into fine drizzle, I would suggest that your hair is naturally tightly curled, but the softer waves are the result of how your hair is handled as it dries.
Naturally curly hair has soluble hydrogen bonds and insoluble disulphide bonds that give it an oval cross section (only straight hair is round). When thoroughly wet hair absorbs water into its internal structure, the soluble linkages dissolve. The weight of water on and in the hair pulls it straighter than when it is dry, especially in longer hair. If you then brush or comb your hair as it dries, or set it on curlers with a larger diameter than that of your natural curl, then instead of taking up its natural curl it will be pulled into the soft waves you mention as the linkages reform into the new shape.
When you go outside into fine drizzle your hair is not soaked. Instead it absorbs enough water to dissolve the soluble linkages, but not enough to weigh it down and straighten it. As your hair subsequently dries the linkages re-form in an unstressed way and your hair then takes on its natural curl, which is the tighter frizz.
The curls in your hair can be altered by making more permanent linkages (disulphide bonds) with an ammonium thioglycolate or 鈥減ermanent wave鈥� solution. These bonds are not affected by water.
It is quite useful to have the two types of bonds in your hair: the soluble bonds let you style your hair in different ways to suit the occasion, and the insoluble bonds stop your hairstyle from melting away in the rain or when you wash it.
David Newman
Gosforth, Tyne and Wear, UK
Titanic explosions
I recently attended the 鈥淭itanic鈥� exhibition at London鈥檚 Science Museum. One of the exhibits informed me that great care had to be taken when bringing cast-iron objects to the surface from 4 kilometres down on the seabed, because when they emerge from the water they can explode. Why do these objects do this and how is the problem counteracted?
鈥� There are several phenomena involved. One is that cast iron invariably contains small gas cavities or blowholes that are formed well beneath its surface. Another is that it has quite low ductility, and will fracture rather than deform. Thirdly, it is a very heterogeneous material, containing about 4.5 per cent carbon and significant amounts of silicon and manganese, together with phosphorus and sulphur. The principal phases that are present are graphite, argentite and ferrite.
When immersed in an electrolyte such as seawater, electrolytic corrosion starts up at the surface of the casting. One of the products of this corrosion is hydrogen in an ionic or atomic state. In this state it can diffuse through the ferrite lattice and find its way to the gas cavities. There it re-forms as molecular hydrogen, increasing the pressure in the cavities.
Because this electrolytic process takes place at great depth and pressure, the pressure build-up in the gas cavities reaches equilibrium with the external water pressure. Raising the cast-iron object from the deep seabed removes the external pressure on the iron, so the gas in the cavities creates very high stresses. At best, the iron will develop cracks. At worst, the casting will shatter.
C. C. Hanson
Metallurgist
Farnham St Martin, Suffolk, UK
鈥� New 杏吧原创 (11 May 2002, p 10) has reported that old cannon balls brought up from the sea sometimes explode after being handled. This happens under special circumstances, when sulphate-reducing bacteria that are common in ocean sediments colonise the minute cracks and crevices in the iron. The bacteria use sulphates in the seawater as a source of oxygen and excrete the resulting reduced sulphur species. In the presence of iron, the soluble sulphur species react to form iron disulphide (pyrite) or iron monosulphide minerals.
Iron sulphides, thermodynamically stable under the reducing conditions on the seafloor, commence oxidation as soon as they are brought to the surface. This reaction is highly exothermic, produces acid, and involves a considerable increase in volume. Substantial oxidation can occur within hours, perhaps even faster. Within confined spaces, the rapid volume change of brittle objects during oxidation can result in potentially explosive break-up.
Jeff Taylor
Principal environmental geochemist
Earth Systems
Kew, Victoria, Australia
This week鈥檚 questions
Size matters
In discussions on family planning and HIV, one occasionally sees statistics such as 鈥渃ondoms work only x per cent of the time鈥�, where x is usually between 85 and 99. How is this number measured, and what sort of sample size is used?
Rolf Andreassen
Bergen, Norway
Dexterity dilemma
Why are some people left-handed and others right-handed?
Leila Gabasova (aged 12)
Moscow, Russia