Woah: Droplets levitate over blue light cushion

Researchers from the French Alternative Energies and Atomic Energy Commission have found a new way to levitate liquid droplets, a discovery that could lead to a new way to generate freely movable microplasma (and it also comes with a rather colorful side-effect).

As physicist Cedric Poulain and his co-authors explained in the latest edition of Applied Physics Letters, their new technique also resulted in the creation of a tiny light show: the liquid droplet was observed sparkling while it floated above a faint blue glowing gap.

“We were interested in a better understanding of the boiling mechanism,” Poulain told redOrbit via email, “Namely, the formation of bubbles (nucleation), as well as what happens at high heat flux when suddenly all the bubbles coalesce, leading to the well-known film boiling.”

Results similar to the Leidenfrost levitation effect

The floating effect is described as similar to a phenomenon known as Leidenfrost levitation, in which droplets essentially dance on a hot vapor cushion. However, by using electricity to make the curtain instead of heat, the research team found that they were able to ionize the gas, creating plasma that glowed with a soft blue light.

In a press release, Poulain explained that whether a hot plate or an electrode is used to heat a liquid, the differences in temperature result in a flux of electrons. At low voltages, this flux occurs “in a conductive or convective manner,” he said, but at higher voltages, bubbles of gas or vapor are created. Upping this voltage to even higher levels results in the Leidenfrost levitation effect.

The researchers believe that the deformability of a liquid drop could allow them to create a new machine capable of moving the plasma created in this manner along a surface. However, Poulain said in a statement that they weren’t thinking of a device when they first came up with the idea for this experiment; they were focused on exploring the limits of the analogy between boiling and water electrolysis, or the breakup of water into elemental gases using electricity.

“The initial goal was rather fundamental,” he told redOrbit. “We wanted to better understand the boiling phenomenon, and the process of nucleation in general. Our findings emphasize the value of exploring a phenomenon (like bubble production) through two different processes which often are studied by two different communities.”

Sending voltage through hydrochloric acid

The researchers created a system designed to send electricity through conductive droplets, while filming the behavior of the droplets at high speeds. They took a tiny drop of weak hydrochloric acid (a conductor) and suspended it above a metal plate and applied a voltage across the drop. As the drop touched the plate, electricity started flowing, causing water in the acid to break down into hydrogen and oxygen gas.

At 50 volts, the bottom of the droplet started sparking and levitated, rising over the surface of the plate. This caused a faint blue glow to emanate from the gap, and while the researchers initially believed that the drop might be resting on a cushion of hydrogen gas from the breakup of water, they eventually found that the gaseous cushion was made primarily out of water, which had been vaporized by energy from the electric current.

The research may provide new insights into fundamental questions of physics, and Poulain noted that the method used in the experiment could provide an easy, original, and inexpensive new way to create plasma. Next, they intend to analyze the composition of this plasma layer, as it appears to be a superposition of two different types of plasma.

As for that blue light given off during the experiments, the team said that the small gap between the droplet and the metal plate likely caused the highly electric field required in order to generate a long-term, dense plasma with little energy.

(Image credit: Cedric Poulain, et al/CEA)

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