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Loosely packed spheres.

Loosely packed spheres

Uniform ball bearings poured into a large pitcher would most likely fill between 60 and 63 percent of the space inside the container. Those limits, originally determined by experiment, represent the range over which random packings of uniform spheres can rest in stable arrangements. Such random packings have long been of interest as models for the arrangement of atoms in simple liquids and glasses.

George Y. Onoda and Eric G. Liniger of the IBM Thomas J. Watson Research Center in Yorktown Heights, N.Y., have now taken a closer look at the lower limit on packing density. Their experiments show that the fraction of space occupied by randomly packed spheres in arrangements that can still support a load can be as low as 55.5 percent -- if the gravitational force acting on the spheres is in effect zero. Their results appear in the May 28 PHYSICAL REVIEW LETTERS.

To minimize the effects of gravity. Onoda and Liniger immersed glass spheres, about 250 microns in diameter, in a liquid with a density close to that of the spheres. After letting the spheres settle gently in the liquid-filled, 500-milliliter graduated cylinder, they measured the fraction of space that the randomly placed spheres took up within the container. The fraction they obtained corresponds to a continuous, rigid network of touching spheres.

The researchers also demonstrated that this packing fraction seems to determine the onset of dilatancy, an important property of granular materials such as sand. Dilatancy occurs when particles must spread apart in order to move past each other in response to a shear force applied to the material. That causes the overall volume occupied by packed particles to increase. The research results show that the packing fraction must be higher than 0.555 for dilatancy to occur.
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Title Annotation:random packings of uniform spheres
Publication:Science News
Date:Jun 16, 1990
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