Science: How to make an hourglass tick

 作者:封篦     |      日期:2019-03-02 10:12:04
Sand flowing through the neck of an hourglass can be made to ‘tick’, dropping intermittently but regularly, with a well-defined frequency. This discovery, by researchers investigating the flow of granular materials, raises the whimsical possibility of an hourglass that ticks once a second. More practically, it has implications for industrial processes. The research was carried out at the University of Pittsburgh, Pennsylvania, and the University of Rennes, France. Instead of sand, scientists used glass beads with diameters of 41, 58, 81, 115 and 160 micrometres. The beads flowed smoothly only when passing through a hole between 2 and 12 times the size of the beads. For particles that are large relative to the hole, the flow halts briefly as the particles repeatedly form an arch that blocks the flow; when the arch collapses, the flow resumes. This was already known. But for smaller particles, the team, headed by Xiao-lun Wu, discovered the previously unknown ticking. The time between the ‘ticks’ ranged from 1 to 10 seconds, the longer intervals being associated with smaller beads. The flow is controlled by the difference in air pressure between the two halves of the hourglass. When a small avalanche of particles falls into the lower chamber, it causes a drop in air pressure in the chamber above. The pressure difference between the two chambers stops the particles flowing downwards, until air seeps back, the pressure equalises and another avalanche falls. ‘The effect is extremely sensitive to the pressure gradient across the neck,’ Wu said, adding that he has measured the difference at about 10-4 atmospheres (Physical Review Letters, vol 71, p 1363). The ticking hourglass may yet become an executive toy. But some executives will be more interested in what the study has to say about the flow of powders in bulk – especially the managers of the cement works in Germany where, the researchers report, several thousand tonnes of cement poured out of a silo and covered the site,