Having returned from exhorting my geoscientific colleagues to converse with the physicists who are exploring the bizarre behaviours of granular materials, I am now endeavouring (hopelessly) to catch up on the piles of journals and magazines that were awaiting me in London. I have just been perusing the April editions of the New Scientist, and came upon the following short report, titled “Bubbles exhibit weird sandy behaviour”:
A SQUIRT of bubbles can act like a liquid or a solid depending on its density - a feat thought unique to grainy materials such as sand.
Pour sand, seeds or powder down a chute and they can flow like a liquid. But if the grains are packed so that they fill 64 per cent or more of the chute, they jam up and behave like a solid. The grains are thought to start moving with their neighbours, forming temporary "necklaces" that resist flow, although it is unclear why the transition occurs at this point.
To investigate whether bubbles behave in a similar way, Rémi Lespiat and his colleagues at the University of Paris-East in France squirted nitrogen gas into a chamber of water and monitored the resulting bubbles' passage through a tube.
At low densities, the bubbles flowed. But when they filled 64 per cent of the tube, they jammed just like the grains (Physical Review Letters, DOI: 10.1103/PhysRevLett.106.148302). The result suggests there may be a universal rule that kicks in when objects fill 64 per cent of a space.
"It's surprising that they get the same figure," says Randall Kamien at the University of Pennsylvania in Philadelphia. He suggests using the insight to evenly mix bubbles of gases for medical applications.
My exhortations in Milan included references to the phenomenon of jamming, the analogies between jammed materials (such as glass), and traffic flow (living in Jakarta this has particular resonance), and, for that particular audience, the poorly understood behaviour of sand in their well-bores. The reference in the New Scientist piece to “necklaces” reminded me of an illustration that I had used in my talk – the bridging (and therefore jamming) of granular materials as they flow through an orifice (think, for example, of an ineffectively designed sandglass). Thanks to the wonders of the ability of photo-elastic materials to show us the distribution of stress (and the formation of “force chains”) during flow through a chute (and thanks to the researchers at Duke University who revealed this), the illustration at the head of this post shows exactly such a necklace, bridging and jamming flow.
And now it seems that bubbles do exactly the same thing – what a strange and wonderful world we live in.