Bizarre bubbles

In ordinary life, if we see an air bubble in a fluid, we expect it to rise.But in a paper presented this week at the American Physical Society’s Division of Fluid Dynamics in Portland, Oregon, Randy Ewoldt, a mechanical engineer from the University of Illinois, Urbana-Champaign, proved that under the right circumstances it’s possible to make bubbles sink.

What’s needed, he says, is a yield-stress fluid: one that only flows when pressures exceed a critical threshold. A good example is a hair gel.

Normally, bubbles in such fluids don’t rise but are trapped indefinitely. If you shake the container or bang it on a tabletop, though, these rapid accelerations can produce sufficient force for the bubbles to rise to the top.

That much is well known. But what would happen, Ewoldt wondered, if the process is reversed and the top of the container is banged into the underside of something like an overhanging shelf?

He tried it in his lab and lo and behold, the bubble defied gravity and moved downward with each successive rap. When he did the same with a steel ball in the liquid, it again defied gravity, rising with each rap.

“It’s the reverse of intuition,” he says.

There may be practical applications to this discovery, he says, “but at the moment I’m intrigued by how weird this is”. He’s especially taken by the fact that this is a simple effect nobody ever appears to have described before. “Who knows what else we’re missing?” he says.

Another scientist interested in bubbles is Justin Burton, a physicist at Emory University in Atlanta, Georgia, whose team presented a paper at the same meeting about giant soap bubbles. (The current world record bubble measured a staggering 96 cubic metres in volume, set by Gary Pearlman in Cleveland, Ohio last year. See him create a smaller but nonetheless impressive bubble below.)

Based on bubble-soap recipes posted on the website, Burton found that the formulas are surprisingly simple: nothing but water and soap spiked with an equally easily obtainable polymer such as guar gum, xanthan or polyethylene oxide.

It’s the polymer that does the trick, Burton said, by allowing bubbles to stretch smoothly and stably to enormous sizes.

He too notes that there are practical reasons for studying bubbles. Some types of pollution take the form of toxic foams.

The more we know about bubbles, he says, the more effectively we should be able to cope with these, possibly even shifting industrial processes to avoid the types of polymers that make for the most stable, long-lasting bubbles.