Sand, or other granulated material, seems to pour and flow just like a liquid. There are even some animals that have adapted to these properties, and can “swim” through the sands of deserts. However, it doesn’t always work that way. From the New Scientist:
[…] 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.
This lead Rémi Lespiat, a researcher with the University of Paris-East to wonder if bubbles behaved in a similar fashion. To test the behavior of bubbles, he shot nitrogen gas through a water tank creating bubbles. He then watched their progress through a tube, observing their movements. When the number of bubbles was low, they flowed quite normally, but as soon as they occupied 64% of the tube, the flow jammed to a halt.
Though it’s very preliminary research, it is quite surprising to see such startlingly similar results. In fact, it may be suggestive of a hard and fast rule for flowing bodies, and could prove useful for designing systems were a continuous flow of objects is important.