Is 64 a "magic" percentage that turns liquid to solid?

There's a large subfield in physics devoted solely to seeing how stuff behaves when it's squashed through other stuff. Those in that field may want to adopt 64 as their magic number, because it seems to be the percentage at which the behavior of matter changes in a peculiar way.

Materials science doesn't have the same sweeping glamor of Relativity, but it covers some very important ground. The predictable movement of fuel through gas tanks, or of grain through funnels, keeps our buses running and our food coming. It also turns out that those two kinds of flow - of liquids and solids - are connected by the number sixty-four.

For some time, scientists have studied the movement of material through passageways. Whether it was sand through an hour glass or rice through food processing equipment, a smooth, continuous flow was easily sustained. That is, as long as the material didn't pack into more than sixty-four percent of a given passageway. Below sixty-four percent capacity, grainy materials flowed like a liquid. Once the tube through which the 'liquid' moved was filled to sixty-four percent, things changed. The grains locked together until they behaved like a solid. Scientists referred to these solid obstructions as 'necklaces' because they form when a string of grains suddenly lock together in a chain.

It was assumed that this sort of thing happened only with rigid, solid, granular objects in air, until recently. A researcher blew nitrogen bubbles in water through a tube. At first the bubbles moved evenly through the tube, but as they reached sixty-four percent of the space, they jammed together and also behaved like a solid, albeit a fragile one. The bubbles were moving through a different medium, had different frictional interactions, and could squish and distend far more than solid material, but both suddenly transitioned from liquid to solid - behaviorally - when they filled 64 percent of the space they were in. This indicates that that percentage might be some kind of constant for behavioral phase transition. What once was a flowing liquid can now stand firm.

Via New Scientist and the Physics Review of Letters.