No flow. A drop of superfluid helium (left, with its mirror image underneath) won't slide down a cesium surface. In the upper right, another drop hangs from the 0.4 millimeter dropper.

A Stay-at-Home Superfluid

Superfluids are immune to many of the forces that constrain ordinary liquids. Because they have no internal resistance to flow, ultracold helium-4 or helium-3 slips through microscopic holes, flows effortlessly uphill, and flouts efforts to contain for study. Now for the first time, researchers have made and photographed a superfluid drop that resolutely sits still--even when its perch is tilted. The research, appearing in tomorrow's issue of Science, has physicists scratching their heads.

The attractive forces between helium molecules are so weak that the molecules would prefer to stick to just about any surface than to each together. And when helium molecules are chilled to a few degrees above absolute zero, they become what's known as a Bose-Einstein condensate and all of the atoms fall into the same quantum state--dramatically lowering the chances that the fluid will be slowed by friction on a surface. Put a drop of superfluid on most surfaces, and it spreads out into a thin layer, rather than beading like water on glass. But theorists predicted that one surface might have a shot at capturing a drop of superfluid: a sheet of cesium. The large orbit of the loosely bound outer electron of cesium atoms would repel the negative charge of the helium atom--overcoming the van der Waals forces that normally help spread superfluid helium over a surface.

After months of fiddling, Peter Taborek and two colleagues at the University of California, Irvine, proved the theorists right. They deposited drops of helium-4 on a cesium substrate kept at 1.16 kelvin. As expected, the superfluid drops formed. What's more, the drops seemed stuck to the surface. The researchers shook the apparatus until waves formed in the drop, even tilted it 10 degrees, but "the drop hangs on by its toenails," says Taborek. It took tilting the sheet at large angles or adding additional liquid to coax the drop down the incline.

Milton Cole of Pennsylvania State University in University Park thinks that tiny variations in the crystalline structure of the cesium could somehow provide a foothold for the superfluid. Cole thinks the cesium sheet may provide new ways to study friction, as it removes the many complications of friction experiments with warm fluids. "This could open up a whole new area of research," he says.