Monday, June 20, 2016

Vapor Explosions: Magic and Metallurgy

It is New Year's Eve and, somewhere in Scandinavia, a family sits around a small table, illuminated by candlelight, speaking to one another in subdued tones. On the table, an ornate spoon rests in a small silver stand, its head sitting above an open candle flame. Next to it, a stainless steel bowl of cool water seems to be full of shadow in the dim and directional light coming from the candles. As a light snow begins to fall outside the windows, an ingot of metal is placed in the spoon, and a small child stands on his chair to watch it melt while the rest of the family looks on with an air of pleasant expectancy. Before long, the ingot is a small molten pool of lead and tin in the spoon. In this family, tradition dictates that the youngest goes first. With gentle encouragement from the rest, the child reaches out to grab the spoon by its handle. His mother's hand hovers around his, not touching but following, ready to grab the handle in case he slips or loses his grip, but his hand is steady, suspending the spoon over the water basin. He begins to pour, hesitantly at first, then upends the spoon, dumping the rest of the molten metal into the water all at once. It hangs together, sliding easily and completely off the spoon as a single large droplet, but when it hits the water it deforms with a quick hiss into a pitted and bubbled structure, shaped by steam at the same time that it hardens and solidifies thanks to the water's absorption of its heat.  The child's grandmother reaches into the bowl and retrieves the tiny lump of metal, holding it up between the candle flame and the wall, where it casts a bizarre and enlarged shadow. The grandmother, well-practiced at this divination, rotates the metal piece, bringing a long protrusion into view near the bottom of the image. She pauses, shifts the shape along another axis, and its shadow takes on the vague appearance of a horse in profile.
"A horse!" She exclaims in Finnish, and the family erupts in peals of laughter.
"A new car for the little one, this year!"

Small ingots of lead or tin are melted and quenched in water in
the European holiday tradition of molybdomancy.
Image Credit: Micha L. Rieser, CC BY-SA 3.0
This is the European practice of molybdomancy, an old and unusual holiday tradition of foretelling the future with molten metal. While the art doesn't necessarily produce reliable results vis-à-vis predicting the future, there's deep physics in the way the shapes coalesce in response to the water. Now, with a paper published Friday in Physical Review E, a team at the King Abdullah University of Science and Technology is laying the groundwork to understand what happens in that cloud of steam and chaos.

"The quenching of molten metals in water is not only the stuff of myth and wizardry, it is also of practical importance in many critical situations, especially when the metal drop is at very high temperature and a thin unstable vapor layer forms between the metal and the pool," explains Nadia Kouraytem, grad student at King Abdullah and lead author of the paper.

The study utilized Field's Metal (not to be confused with the Fields Medal), an alloy of bismuth, tin, and indium, which has an unusually low melting point—around 60C. This low melting point means, conversely, a relatively low solidification temperature. As a result, if the molten metal is heated to a very high temperature, it can transfer a great deal of energy before dropping to a low enough temperature to solidify again. When the molten metal hits the water, the water evaporates almost instantly, forming a thin layer of steam that rapidly expands up to 1000 times its original volume. The rapid expansion drives the metal away in a phenomenon known as a "vapor explosion", scattering the metal into millions of tiny droplets, which then cool rapidly into beads.

"The 1000-fold increase in the volume of water-to-vapor phase change at the interface while penetrating into the molten metal causes these so-called vapor explosions," says Kouraytem.
"This occurs for example in fuel-coolant interaction during accidents in nuclear power plants and leads to phreatomagmatic eruptions when lava flows over wetlands."

For the everyday person, this work might translate into better safety protocols at nuclear power plants, or perhaps new techniques for the manufacture of micro- or nano-sized particles. Like traditional molybdomancy, however, the most satisfying takeaway is some really cool visuals: the group took high-speed footage of their work, and was kind enough to supply it for your viewing pleasure.

Tin at 250C added to water, filmed at 25kfps, played back at 30fps.
Image Credit: Kouraytem, Lee, & Thoroddsen, 2016.
Field's Metal at 350C added to water. Recorded at 33kfps, played back at 30 fps.
Image Credit: Kouraytem, Lee, & Thoroddsen, 2016.
Field's Metal at 400C added to water. Recorded at 33kfps, played back at 15fps.
Image Credit: Kouraytem, Lee, & Thoroddsen, 2016.
Field's Metal at 450C added to water. Recorded at 33kfps, played back at 15fps.
Image Credit: Kouraytem, Lee, & Thoroddsen, 2016.
Field's Metal at 500C added to water. Recorded at 33kfps, played back at 15fps.
Image Credit: Kouraytem, Lee, & Thoroddsen, 2016. 
Field's Metal at 550C added to water. Recorded at 33kfps, played back at 30fps.
(Then played backwards, just for kicks!)
Image Credit: Kouraytem, Lee, & Thoroddsen, 2016.

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