Metal explosions 'driven by charge'

Image copyright P Jungwirth / Nature Chemistry

Using super-fast video footage and computer simulations, scientists have revised our understanding of explosions well known from high-school chemistry.

Chemists in the Czech Republic and Germany captured images of the alkali metals sodium and potassium exploding on contact with water.

They saw a flash of bluish purple and "spikes" of metal shooting outwards.

This suggests that the reaction is kick-started by positive charges repelling each other.

Writing in the journal Nature Chemistry, the researchers say their results explain how an explosion can be sparked, if the conditions are just right, despite there being very little contact area between the water and the metal.

"If you want to have an explosive reaction... you need a lot of contact between the reactants," said senior author Prof Pavel Jungwirth, from the Academy of Sciences of the Czech Republic.

"And this is exactly what we don't have here."

When a small nugget of potassium or sodium is dropped into water, the interface between metal and water is only small - and is rapidly obscured by a layer of gas that the reaction produces.

"So the reaction, as it is explained in the text books, should be self-quenched - it shouldn't lead to this runaway, explosive behaviour," Prof Jungwirth told the BBC.

He and his colleagues set about solving this conundrum by studying the very beginning of the reaction - less than a millisecond (0.001 seconds) - when it either becomes explosive or simply fizzes along.

"We needed another physical mechanism that would ensure the mixing of the metal and the water," explained Dr Phil Mason, who conducted most of the experiments.

But to capture the reaction in enough detail, the team had to secure the loan of a high-speed videocamera - which costs about $100,000.

"When you say you want to film some explosions with these cameras... people usually don't want to rent them to you," Prof Jungwirth said.

After many trials, the team settled on a method for reliably producing a small explosion, which they could observe in fine detail.

They used an alloy of sodium and potassium, which is a liquid at room temperature, to make sure that a nice clean drop of metal would make rapid, clean contact with the water - and to stop the metal oxidising on the way, they dropped it in a tube full of argon, rather than air.

"We went through quite a lot of kit before we worked out how to do it safely," Dr Mason said.

Finally, in collaboration with a laboratory from the Braunschweig University of Technology in Germany, the chemists captured the footage they needed.

Two tell-tale pieces of evidence from the video suggested a new interpretation of how these reactions become explosive.

Spikes of metal could be seen shooting very rapidly out into the water, within less than half a millisecond of the two surfaces coming into contact. And there was a flash of a bluish purple colour at about the same time.

These two observations are consistent with a "Coulomb explosion", where the metal becomes positively charged and those charges push each other outward, causing the spikes to form.

Prof Jungwirth explained this phenomenon has been known since the 19th century, but never spotted in this context.

"If you put enough charge on [a droplet], the repulsion between the charges will be larger than the cohesion energy of the droplet... and it will disintegrate," he said.

"And the way it disintegrates is that the droplet shoots out these spikes, sometimes called Raleigh columns."

In this case, the metal droplet becomes positively charged when electrons flood away from it in the very early stages of the reaction.

"The electrons are very light, so they almost immediately leave the metal and move into water," Prof Jungwirth said.

This is consistent with the blue flash, he added: "That's actually the absorption of light by the electrons."

The team also built computer simulations of the reaction that match their observations, explaining how repulsion between the positive charges left in the metal can produce the spikes, which in turn expand the surface area of the metal and accelerate the reaction - creating the explosion.

"That's what also makes us feel more confident," Prof Jungwirth said. "On a microscopic scale, when we do the calculations, the atoms really behave as we see on the macroscopic scale, on the cameras."

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