Comments on: Liquid Sand

By: the forces between grains of sand « 2point718

the forces between grains of sand « 2point718 — Mon, 13 Jul 2009 04:01:24 +0000

[...] by the Jaeger group at U. Chicago in Nature investigates (that Mark Trodden of Cosmic Variance summarizes) the interactions of sand particles. Jaeger’s group demonstrated the existence of surface [...]

By: My Time to Waste» Blog Archive » Jets!!

My Time to Waste» Blog Archive » Jets!! — Thu, 09 Jul 2009 03:22:24 +0000

[...] …from Discover Magazine. [...]

By: Josh Mitteldorf

Josh Mitteldorf — Wed, 08 Jul 2009 17:12:03 +0000

The forces that make falling water collect into droplets are surface tension forces, as the (polar) water molecules try to get as close to each other as they can. These forces are inapplicable in the case of sand. So the phenomenon – even though it looks similar to water droplets – must have an entirely different origin.

My guess is that it is about eddies in the airstream. Places where the sand is denser block from the wind resistance from the sand behind. So the sand behind “catches up” because it doesn’t have to fight air resistance. The sand in front of it is moving air out of the way and leaving a low pressure region in its wake. In fact, eddies could even create a downward airflow behind a sand ball, that tends to compress it and give it coherence.

By: Mark

Mark — Tue, 07 Jul 2009 01:43:00 +0000

No venkat – we DO understand something about QCD and we DO understand something about falling sand (a lot actually). Still, the fun is in what we do not understand, and the fact that we do not completely understand everyday systems would be more surprising to many people than the fact than we don’t understand the strong force completely. Sounds like you doin’t think that simple statement is true. I guess we’ll just have to disagree.

By: Ginger Yellow

Ginger Yellow — Fri, 03 Jul 2009 03:35:22 +0000

“Back when I was a kid, my Dad constructed a small factory for producing resin-coated sand. He used compressed air to fluidize the sand, making it easy to move it from place to place using just gravity”

I’ve no idea if it’s still there, but the Science Museum in London used to have a tub of sand fluidised in this way for kids to play with. It was very cool.

By: Robert Woodhead

Robert Woodhead — Fri, 03 Jul 2009 00:12:07 +0000

The videos are very pretty, but the effects are not surprising. Back when I was a kid, my Dad constructed a small factory for producing resin-coated sand. He used compressed air to fluidize the sand, making it easy to move it from place to place using just gravity — and even built a cooler for the finished product that fluidized several tons of coated sand at a time and ran it up and down and over a series of baffles with water cooling coils in them.

When air gets between the sand grains, they effectively act like a fluid, so you get effects like in the second video (the impact of the ball drives air into the surrounding sand, temporarily turning it into a fluid. You even get “surface tension” effects, I would suppose because of the pressure gradients (the trapped air has an easier time escaping at the surface of the sand).

Still, it’s a lovely demonstration of how subtle and wonderful nature is.

By: venkat

venkat — Thu, 02 Jul 2009 09:29:59 +0000

yes, we DO understand something about QCD. but we understand very little about falling sand and the other wierd properties of granular materials. in my opinion, that would make QCD simpler than falling sand…

By: Gouttes de sable « Dr. Goulu

Gouttes de sable « Dr. Goulu — Wed, 01 Jul 2009 20:35:42 +0000

[...] Trodden,“Liquid Sand“, 29 June 2009, Cosmic [...]

By: ruidh

ruidh — Wed, 01 Jul 2009 18:44:11 +0000

I have to think that simple acceleration due to gravity is sufficient to describe the sand clumping up to some degree. Two small segments of the falling stream don’t fall in lockstep. The lower segment has a higher velocity than the one above it at any point in time. That’s enough to get the sand being stretched out. Then it only takes small forces to get to clump.

By: Mark

Mark — Wed, 01 Jul 2009 14:33:22 +0000

venkat – the point is that almost everyone would say that falling sand is simple, but they would never say that about a supernova, or QCD. Further, you can make sand fall, or watch an impact on it right in your back yard if you like. I think it is worth pointing out that there is beautiful cutting edge physics to be found in systems like this.