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Trickling sand: how an hourglass ticks.

The familiar image of sand gradually trickling from one chamber into the other of an hourglass has long represented the inexorable passage of time. But new experiments reveals that sand in an hourglass doesn't always flow smoothly.

A team of researchers has discovered that under certain conditions, the flow of particles in an hourglass actually stops and starts at regular intervals. In other words, the hourglass "ticks."

Xiao-lun Wu of the University of Pittsburgh, Daniel Bideau of the University of Rennes in France, and their co-workers describe their findings in the Aug. 30 PHYSICAL REVIEW LETTERS.

The researchers performed their experiments using uniform glass beads, ranging from 41 to 160 microns in diameter, in a specially modified hourglass. Its lower chamber has an opening so that beads could flow into a cup resting on a balance, allowing the researchers to record how steadily the mass increases.

The group found that the beads flow continuously only for a certain range of values of the ratio between the bead diameter and the width of the neck linking the hourglass' two chambers. The flow becomes intermittent when the particles are less than one-twelfth or more than one-half the neck's width.

The researchers suggest that a tiny difference in air pressure causes this surprising behavior of small particles. Gravity pulls the beads down, forcing them against each other to form a tenuous network of arch-like structures. As beads pass through the neck of an hourglass, they carry air with them. The air pressure in the upper chamber thus falls slightly below that in the lower chamber. This pressure difference -- typically one ten-thousandth of an atmosphere --exerts an upward push that stabilizes the arches in the bead-packing. The flow comes to a halt. It begins again when the difference in air pressure diminishes.

As evidence supporting their explanation, the researchers note that by opening a cap on the upper chamber, they can bring it to the same pressure as the lower chamber. Under these conditions, the flow becomes continuous.

Moreover, the occurrence of intermittent flow appears extremely sensitive to vibrations, temperature changes, and air currents. "Indeed, sealing the lower chamber of the hourglass and simply holding it, thus slightly warming the lowe chamber, is enough to stop the flow entirely," the researchers report.

These findings may help elucidate the mechanics of the flow of fine powders, such as cement or certain drugs, where trapped air often plays a significant role.
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Title Annotation:flow of glass beads in hourglass stops and starts at regular intervals
Author:Peterson, Ivars
Publication:Science News
Date:Sep 11, 1993
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