Cushions for drops of levitated helium.

Water spilling onto a hot skillet shatters into droplets that skate and bounce across the sizzling surface. The drops ride on layers of water vapor generated by the pan's intense heat. Researchers have now observed a similar effect at temperatures near absolute zero. As magnetically levitated drops of liquid helium are gradually chilled toward temperatures at which the liquid turns into a superfluid, drops can come into contact with each other yet fail to coalesce because a thin layer of evaporated helium gas separates them.

"We didn't expect to see this," says physicist George M. Seidel of Brown University in Providence, R.I. He and his colleagues describe their finding in the Dec. 2 Physical Review Letters.

The researchers discovered the effect in the course of developing a powerful magnetic trap for levitating drops of helium in both its ordinary liquid and its superfluid states. "We were able to maintain drops [as large as 2 centimeters in diameter] in the trap indefinitely," the researchers report.

When two drops of liquid helium were introduced into the trap simultaneously, the Brown team noticed that the drops would appear to come into contact but would not combine into a single drop, as most liquids would. They even observed drops bouncing off each other before coming to rest.

Seidel and his colleagues propose that, as the temperature is lowered, slow evaporation from the drops creates a layer of gas that keeps the liquid surfaces from making direct contact with each other.

The drops coalesce as soon as the helium gets cold enough to turn into a superfluid, a state in which no further evaporation occurs.