Ah, the ticking of the hourglass.
Wait a minute. Ticking hourglass? Clocks tick. Hourglasses flow. Are we mixing our metaphors?
Not necessarily, according to physicists who study the flow of small particles. Sometimes the sand in an hourglass can actually stop and start at regular intervals, making the hourglass "tick."
To study this, researchers built a special hourglass with a scale in the bottom chamber to measure the accumulation of sand. The sand they used was special also in that it contained tiny glass beads of uniform size. They found that if the size of the beads was less than a twelfth the diameter of the hourglass's neck the sand would start ticking.
What causes this is air pressure. As each clump of sand passes through the neck, it carries a small amount of air with it. This is enough to raise the pressure in the lower chamber ever so slightly. This tiny increase in pressure, pushing up on the sand above, was just enough to hold it back until the pressure equalized and there was another tick.
This is all well and good, but you might be wondering, so what? After all, why should we care whether sand flows evenly in an hourglass or not? Is this study just a case of scientists with too much time on their hands?
Not at all. These findings are important, and not just for hourglass makers. Many industrial activities, from mixing cement to manufacturing certain drugs, depend on carefully measuring the flow of fine powders. Studying the hourglass's ticks might help ensure you get exactly the right dosage the next time you fill a prescription.