Droplet Levitates On Electric Blue Plasma

A blue glow emanates from beneath a levitating droplet of weak hydrochloric acid. Cedric Poulain et al./CEA.

A team of scientists made a discovery that sparked their interest. After giving some hydrochloric acid an electric shock, the researchers were also shocked at what they saw: A pale blue streak of light was separating the droplet from the table. The blue glint came from plasma, similar to the substance that lightning is made from.

The effect is comparable to the Leidenfrost effect. This happens when water is dropped onto a scalding surface. Instead of boiling and fizzling away as a gas, the water drops are levitated ever so slightly above the surface and bead up to form drops of water. This is because the surface, which is much hotter than the boiling point of water, vaporizes a layer of water molecules that are in contact with the surface. This creates an insulating layer of gas between the liquid water droplet and the hot surface, trapping the liquid water molecules together as a coalesced droplet. You may have seen this effect at home over the surface of an electric hob.

But rather than using heat, the scientists used a jolt of electricity in this new research. This created a layer of eerie, glowing blue plasma between the liquid and the solid surface. The electricity turned some of the liquid into a vapor that then ionized into a plasma. Their results can be seen in Applied Physics Letters.

The team opted to test hydrochloric acid in this experiment since its acidic nature makes it a good conductor of electricity. The hydrochloric acid drop was suspended atop a metal plate with a voltage applied across it. As the drop fell toward the plate, it started to undergo an epic transformation from hydrochloric liquid to hydrogen and oxygen gas. 

As the scientists increased the voltage to 50 volts, they were surprised to see the intrepid droplet starting to spark and emit its own light. The droplet levitated, floating above the surface of the plate. The tiny separation between the drop and the plate was highlighted with a delicate, blue glow from the plasma. 

Some of the researchers were suspicious that this glimmer wasn't plasma at all but actually a cushion of hydrogen gas. However, further tests disproved this idea. The brilliant cushion was mostly made of vaporized water. 

In the long-term, there is the potential to create a dense plasma with relatively little voltage. More work needs to be done before this effect can see further applications: the plasma created is composed of two different types of plasma and isn't yet well understood. The team's next goal is to study these layers as well as to figure out exactly what is going on at the moment the falling droplet taps the electrified surface and creates a spark.

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