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Streams of falling water tend to clump into droplets as surface tension attracts globules of liquid in midair. Even though solid objects are presumed not to have surface tension, the same phenomenon has been observed with grains or sand or tiny beads. But so far, no one has been able to explain exactly what and how much force is at work drawing these objects together.

A new high-speed video outfit, designed by University of Chicago physics graduate student John Royer, has been able to capture these nanoscale forces for the first time, reports a paper published today in Nature (Scientific American is part of Nature Publishing Group).

Using an atomic force microscope, Royer and the research team were also able to measure the attracting force and found that even though the falling solids might look like tumbling liquid, surface tension that attracts the pieces is as much as 100,000 times less than liquid surface tension.

Watch the video for a demonstration.

Image and video courtesy of Helge F. Gruetjen (Supported by Germany-U.S. Fulbright Scholar Program), John R. Royer, Scott R. Waitukaitis, and Heinrich M. Jaeger, The University of Chicago

7 Comments

1. 1. RDH 03:30 PM 6/24/09

   I assume they have ruled out gravitational forces between the tiny beads. It would be quite small given the (I assume) small mass of each bead.

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2. 2. CM 04:42 PM 6/24/09

   To form a sand 'drop': Under the assumption of fixed boundaries, what determines whether each tiny bead clusters with a group in front (faster falling beads), behind (slower falling beads), or not at all (outliers)? Rather, the law of attraction is a force within the force of gravity?

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3. 3. tdgibbs 05:52 PM 6/24/09

   I presume this procedure was conducted in a vacuum. If not, particles might aggregate due to aerodynamics. Consider the V-wedge that geese form when in flight. The path of least resistance is behind another bird. That's also why motor racers often draft within the wake of another car.

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4. 4. CM in reply to tdgibbs 11:17 AM 6/26/09

   Dear T,
   What you say is true with regard to resistance and that certainly influences speed. Except when we address the source of the natural force, say, without obvious acceleration, deceleration, and/or navigation (steering) capacities (ignoring the order a bead has in "line" as a starting position) - What then drives the bead to aggregate with a particular cluster?

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5. 5. asteday 06:32 PM 6/28/09

   RDH, you bring up an increasingly interesting point. Could the attraction be in fact the manifestation of gravity as a seemingly different interaction? The beads could be of an infinitesimal mass, but relatively speaking, couldn't their masses be enough so they would attract? Whatever the explanation is, I hope enough research will bring the solution into light.

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6. 6. shamoon 07:05 AM 6/29/09

   could it be that the tiny differences in the masses of the beads are resisting gravity at different rates, and the clustered beads are of exactly the the same mass ?????

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7. 7. Porzitsku 01:26 PM 7/14/09

   The mass of each particle would have to be insignificant compared with the mass of the earth, making gravity inconsequential as far as attraction goes, but maybe a factor in the rate of acceleration as each particle falls. More likely is an electromagnetic attraction.

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