Liquid Sand

By Mark Trodden | June 29, 2009 5:57 am

One of the more fun physics stories that I’ve seen recently is from an area of research quite removed from my own, but that I have found fascinating for a while now. I have been fortunate to have excellent condensed matter colleagues at both my recent institutions, and quite a number of them are interested in soft condensed matter – classical physics that describes the behavior of large numbers of particles, far from equilibrium, often when entropic considerations dominate the dynamics.

The field covers such diverse systems as the behavior of biological membranes and the dynamics of grain in silos, and contains many examples in which nontrivial geometry and topology lead to the possibility of discovering new phenomena that, unlike in my own field, can increasingly often be checked in a laboratory experiment designed and built in a relatively short time.

The story that caught my eye (via Wired Science) recently concerns the behavior of a system that is so simple that you would think we know all that there is to be known about it – falling sand.

In the video above, a stream of sand is allowed to fall over several feet, and is filmed using a high speed video camera that falls at the same speed as the sand. The result, as you can see, is that the sand forms “droplets” just as water would, even though most people would not think of granular materials as anything like a liquid. The work was performed by Heinrich Jaeger‘s group at the University of Chicago, and published in the current issue of Nature, which also deemed it worthy of a News and Views article and an Editor’s Summary (subscription required for all these things, unfortunately).

Interestingly, this system is still not fully understood – although it is clearly displaying liquid-like characteristics, the scales of the droplets and the forces involved are very different from the traditional regimes in which liquids are described – so there’s still work to be done. You can see many other examples of behavior like this on Jaeger’s granular materials page, with even more videos. The one I liked best is this granular jet one

Remember – that ball isn’t falling into a thick liquid – that’s sand!

There are apparently all kinds of applications of this kind of work. but I just think it’s beautiful all on its own.

  • cope

    Those are fun to watch, thanks.

    Given that the free-falling sand is weightless, would it be reasonable to scale up the clumping of the grains (by whatever processes or forces) to the initial aggregation of fine-grained material in the formation of planetesimals/planets?

  • bigjohn756

    What is the real time for these videos, especially the ball dropping one?

  • The Science Pundit

    Have they ever done any sand experiments on the space station? That just seems like the logical next step to me.

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  • Dennis

    I once had some grout powder in a can that, when you tapped the side of the can, the surface of the powder rippled exactly like water. Unfortunately, I don’t know the composition of that particular example, but the powder was extremely fine.

  • cmt

    Just to underscore the weirdness of the movie of the sand pearling, the same forces that drive pearling in water also tend to result in spherical droplets so as to minimize surface area. Those drops, once formed, do not appear to change shape at all to me.

    Anyway, thanks for highlighting an area of physics that often gets too little attention, and too few interested students.

  • Toiski

    Is the sand in the first video falling through a void or through air? Would it make any difference?

  • coolstar

    Both videos are very interesting, though the second isn’t all that surprising and is probably well known to people who study hypervelocity collisions, the first certainly is surprising. One has to wonder how electrostatic charges on the grains matter in the first one and if you’d see the same effect in a vacuum.

  • chowder

    It seems to me that the droplets must be very delicate; if there were crosswinds of any kind, the whole effect would be ruined. I don’t think sand is as sticky as water.

  • gopher65

    My first guess at this is that it’s caused by airflow. The sand falls, and as it does, it chaotically gets pushed into regions with the least flow resistance (lowest air pressure as seen in an area cross section of any particular point in the stream of sand).

    In the first few milliseconds this would be a small effect, as there would only be minor differences between the air pressure in one region and another, but as time went on you’d see more and more sand being forced into smaller regions, forming “droplets”.

    That’s a completely different mechanism from the way water or mercury forms droplets though, which has to do with cohesion and surface tension.

  • Florine

    A longer video with some explanation is over here:

  • Mark

    Thanks Florine!

  • venkat

    what made you imagine that falling sand is a simple system?!

  • Mark

    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.

  • ruidh

    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.

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  • venkat

    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…

  • Robert Woodhead

    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.

  • Ginger Yellow

    “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.

  • Mark

    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.

  • Josh Mitteldorf

    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.

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About Mark Trodden

Mark Trodden holds the Fay R. and Eugene L. Langberg Endowed Chair in Physics and is co-director of the Center for Particle Cosmology at the University of Pennsylvania. He is a theoretical physicist working on particle physics and gravity— in particular on the roles they play in the evolution and structure of the universe. When asked for a short phrase to describe his research area, he says he is a particle cosmologist.


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