# Chemical energy!

Suppose a typical modern family car does about 40 miles to the gallon or, in metric terms, 100 km for every 7 litres of fuel. That means if you have a teaspoon of petrol (about 0.004 litres), it contains enough energy to roll your car about 60 m (200 ft), or roughly 15 times the car’s own length. Consider how hard it is to push a car, even once you’ve got it going from a standstill, and I’m sure you’ll agree that’s quite remarkable. The simple fact is that petrol is absolutely chock full of energy: short of uranium (nuclear fuel), it’s just about the most energy-rich material there is. […]

That’s from Chris Woodford in his article, Why Your Car is a Chemistry Lab on Wheels (“What makes cars one of the most successful inventions of all time? The answer lies in science”).

Fabulous stuff!

# Materials in Apple Watch

Here’s a fine piece —

How Apple Makes the Watch — on the materials (specidfically, gold, stainless steel and aluminum) and processes which go into making several key (but non-electronic) components of Apple watch. It uses publicity videos from Apple as a starting point, and describes some of the processing steps (and perhaps the science behind it) seen in those videos.

# Strongest Natural Material?

David Schultz in ScienceShhot: Spider silk dethroned as nature’s toughest fiber:

Spider silk is famous for its amazing toughness, and until recently a tensile strength of 1.3 gigapascals (GPa) was enough to earn it the title of strongest natural material. However, researchers report online today in the Journal of the Royal Society Interface that the record books need to be updated to properly recognize the incredible strength of the limpet teeth. … [T]he limpets’ teeth boast a tensile strength of between 3 and 6.5 GPa, researchers report.

# Volleyballene!

One more theoretical prediction about a supermolecule with 20 selenium atoms and 60 carbon atoms whose architecture resembles that of a volleyball (arXiv link to the paper):

The simulation gives a remarkably detailed picture of the properties of the new molecule. Jing and co have simulated the character of the bonds that hold it together, their binding energy, their vibrational frequencies, and the stability of the structure.

And they say volleballene is clearly the most stable of all the structures that Sc20C60 can form. The tam’s vibrational analysis suggests that volleyballene should be stable when heated and remarkably stable chemically too.

In other words, volleyballene is a molecule waiting to be synthesized.

# A short profile of Prof. Harry Bhadeshia

Over at Forbes India. He hits an interesting note (and I have heard him say this in a seminar at IISc many years ago, so this must be one of his favourites) when he says:

… Bhadeshia explored the world of steel, which was complicated and at the same time had an “unbelievable variety” that remains unseen by the world at large. Explains Bhadeshia: “Every year, 1.3 billion tonnes of steel is produced, but there is no need for the outside world to understand it…it is a product made in an extremely sophisticated and controlled environment…it is so reliable that no one needs to worry about it. On the other hand, everyone needs to worry about the operating software in their computers as these are not very well developed products!”

Prof. Bhadeshia’s website offers a wealth of information on many, many aspects of materials science and engineering in general, and physical metallurgy of steels in particular.

# A Peep into a Jet Engine

Jacob O’Neal’s animations of the inner workings of a jet engine are a visual treat!

Check O’Neal’s website for a hi-res animation.

Some of the jet engine components (especially those in the turbine immediately behind the combustor) are also an excellent example of extreme materials as they withstand extremely high temperatures for hours and hours!

# Is Spider Silk tough enough and strong enough to stop a metro train?

I’m sure you remember this scene in Spiderman 2:

How realistic is this scenario — is it actually possible for a web of spider silk to halt a speeding metro train? Apparently, the paperit is.

Assuming that the ‘web’ has eight strands of silk, each with a diameter of about 5mm, the stress it has to withstand is about 1.3 GPa. Apparently, some spider silks do have such high strengths.

Hat tip: Discovery News.

Now, I guess someone will step forward to ‘prove’ that it is actually possible for Spiderman’s arms to hold things up right until the end!

# NPR Materials?

The previous post was on NLC materials that expand along some (but only some!) directions under an applied pressure. NLC, of course, stands for negative linear compressibility. The expansion along some directions is more than compensated by a shrinkage along the other directions, so that the overall volume does indeed decrease with pressure — a mandatory condition demanded by thermodynamic (specifically, mechanical) stability of the system.

A related property is Poisson’s ratio. Under a squeezing action, most materials shrink along the (axial or longitudinal) direction of squeezing , and expand along the two perpendicular (transverse) directions. In technical jargon, the negative of the ratio of the transverse strain to the longitudinal strain is called $\nu$, the Poisson’s ratio.

For most materials, Poisson’s ratio — as defined above — is positive. But it could be negative! There’s nothing fundamentally wrong with it, because the material is stable as long as its Poisson’s ratio is greater than -1 (i.e., $\nu > -1.0$. At the high end, $\nu$ has an upper limit of 0.5). Let’s call them NPR materials, with NPR standing for “Negative Poisson’s Ratio”.

To be sure, NPR materials are rarer than the “normal” kind. There has been quite a bit of effort to understand why some materials may exhibit NPR — see this short Nature article, for example; this article’s author also gave these materials a name: “auxetic materials”. I am not sure how popular this name is, though.

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Just a final aside. Cork has a Poisson’s ratio which is pretty close to zero. Here’s how Wikipedia explains its use as a wine stopper:

The use of cork as a stopper for wine bottles is the result of the fact that cork has a Poisson ratio of practically zero. This means that, as the cork is inserted into the bottle, the upper part which is not yet inserted will not expand as the lower part is compressed. The force needed to insert a cork into a bottle arises only from the compression of the cork and the friction between the cork and the bottle. If the stopper were made of rubber, for example, (with a Poisson ratio of about 1/2), there would be a relatively large additional force required to overcome the expansion of the upper part of the rubber stopper.

# How do materials respond to hydrostatic pressure?

Thermodynamic stability demands that the compressibility, $\beta = - V^{-1} \left( \partial V / \partial P \right)$, be positive. This means that the volume should decrease with increasing pressure.

For fluids (gases and liquids), the volume decrease is accomplished by an equal decrease in linear dimensions along all directions. This is because of their lack of internal structure (specifically, translational or lattice periodicity), which makes them isotropic.

Crystalline solids do have an internal structure, and atoms and molecules within them have all kinds of bonds (with different strengths) with their neighbours. This could make their response quite anisotropic (direction-dependent) under an imposed pressure. Most materials still exhibit a contraction along all directions, their anisotropy implies that the level of contraction (strain) could be different along different directions.

Are there materials that respond to pressure by decreasing in size along some directions and increase along some others? Apparently, there are, and such materials have been dubbed NLC materials, with NLC standing for “negative linear compressibility.” But, remember, this property should not violate the thermodynamic condition that the overall volume should decrease with pressure.

All this is to link to this Nature Materials commentary on a recent discovery of an NLC material reported in the December 2012 issue of the same journal.