Granular jamming explains why materials like sand can suddenly behave like a rigid solid instead of a flowing fluid, even though the grains occupy only a fraction of the total volume. The key variable is packing fraction: below a critical density grains can rearrange freely, but above it they lock together and jam, which is why a shaken hourglass sometimes needs a jiggle to keep the sand flowing. A 2003 Physical Review E paper, 'Jamming at zero temperature and zero applied stress,' worked out the statistics of this transition, using packing fraction as the control parameter with no thermal jiggling and no external force applied.
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The 2003 paper's title specifies 'zero temperature' (no thermal jiggling) and 'zero applied stress' (no external push), isolating packing fraction as the sole driver of jamming.
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Liquid-filled hourglasses rarely jam because the fluid lowers the effective packing ratio and its small turbulent currents help dislodge stuck particles.
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The discussion closes by pointing to a 2010 paper from a Cornell group that applied jamming physics to build a gripping robot, setting up the next segment.
436 words · auto-generated from the episode video
1:51:04>> This is interesting. Yeah. I know this is good. Um so in 2023 um sorry 2003 they wanted to start understanding jamming cuz this they've sort of introduced this idea of jamming and fluids. So what does jamming actually do? So imagine this is what jamming actually means. Like have you ever held those hourglasses where sometimes the sand gets stuck. >> Mhm. >> Now it's sand. It should flow like a fluid, but sometimes the sand gets stuck and you got to jiggle it a little bit to have the sand fall from the top of the hourglass to the bottom of the hourglass. The question he's asking is why, right? If the spheres occupy only a small fraction of the volume, then why is it jamming? Well, that's exactly the
1:51:46case. If the spheres are occupying only a small fraction of the volume, then they can be re rearranged like it's nobody's business. Okay? >> But if the packing fraction of how much sphere there is versus how much empty space there is, if that is higher than some certain amount there's going to be jamming. And he quantified exactly what the statistics of that jamming is. The the idea being we have in an hourglass there's a narrower space where the jamming arises and so the actual dynamics of the amount of space those smaller particles are in there's some >> XY
1:52:27>> smaller space higher jamming potential >> and we now have a effectively the formula to understand >> at the rate at which jamming >> increases. >> Yeah. And that's why if you have hourglasses that have a fluid in it, you almost never have jamming because the fluid creates a lower packing ratio one and the fluid has tiny bits of turbulence that'll like move around the jamming. Right? This is the kind of physics that he was focused on in this next paper that um we're talking about here. It's in the physical review e in 2003. Jamming at zero temperature and zero applied stress, the epitome of disorder. Zero temperature meaning no jiggling. >> Yep. and zero applied stress, meaning
1:53:08you're not like pushing it in any way. How does the jamming actually happen? And the control knob happens to be the packing fraction. Now, what are the practical uses? This is one of the craziest things that I've seen actually, and this is actually quite old. Um, this this next paper that I'm about to show you, 2010 pen out of Cornell University, they used this idea to create a robot that can pick anything up.
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